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Q:H COACTIVATION RATIOS DURING EXERCISE
QUADRICEPS TO HAMSTRINGS COACTIVATION RATIOS DURING CLOSED
CHAIN, HIGH VELOCITY EXERCISE IN HEALTHY, RECREATIONALLY
ACTIVE ADULTS
________________________________________________________________________
Independent Research
Presented to
The Faculty of the College of Health Professions and Social Work
Florida Gulf Coast University
In Partial Fulfillment
Of the Requirement for the Degree of
Doctor of Physical Therapy
________________________________________________________________________
By
Maci Hatch
Keisha Sollie
2015
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Q:H COACTIVATION RATIOS DURING EXERCISE
APPROVAL SHEET
This independent research is submitted in partial fulfillment of
the requirements for the degree of
Doctor of Physical Therapy
_____________________________
Maci Hatch
_____________________________
Keisha Sollie
Approved: May 2015
_____________________________
Dr. Arie van Duijn, EdD, PT, OCS
Committee Chair/Advisor
_____________________________
Dr. Eric Shamus, PhD, DPT, CSCS
Committee Member
The final copy of this independent research has been examined by the signatories, and we
find that both the content and the form meet acceptable presentation standards of scholarly
work in the above mentioned discipline.
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Q:H COACTIVATION RATIOS DURING EXERCISE
Acknowledgements
We would like to express the appreciation to our committee chair Dr. Arie van
Duijn and committee member, Dr. Eric Shamus for their time, support, and
encouragement throughout the duration of our Independent Research Study. In addition,
we would like to thank Dr. Shawn Felton from the Human Performance Department for
his time and help with our study. Lastly, we would like to thank all of the participants
who voluntarily participated in our research. Without the help of each of these
individuals, we would not have been able to successfully complete this study.
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Q:H COACTIVATION RATIOS DURING EXERCISE
Table of Contents
Abstract ................................................................................................................................6
Introduction ........................................................................................................................10
Coactivation Ratios ........................................................................................................12
Mechanism of Injury ......................................................................................................19
Plyometric Training ........................................................................................................20
Purpose of the Study ..........................................................................................................22
Research Questions ............................................................................................................23
Methodology ......................................................................................................................23
Study Design ..................................................................................................................23
Participants .....................................................................................................................24
Inclusion Criteria ............................................................................................................24
Equipment and Preparation ............................................................................................24
Procedures ......................................................................................................................26
High Velocity, Closed Chain Exercise ...........................................................................28
Barrier jump front to back ..........................................................................................29
Barrier jump side to side.............................................................................................29
Lateral bounding .........................................................................................................30
Scissor jump ...............................................................................................................30
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Squat jump ..................................................................................................................30
Data Sampling and Reduction/Data Analysis ................................................................31
Statistical Analysis .........................................................................................................32
Results ................................................................................................................................33
Q:H Coactivation Ratios ................................................................................................33
Peak EMG Flexion Angles for Each Muscle During All Jumps ....................................36
Peak EMG Flexion Angles for Each Muscle Within Each Exercise .............................45
Discussion ..........................................................................................................................54
Q:H Coactivation Ratios ................................................................................................54
Peak EMG Flexion Angles for Each Muscle During All Jumps ....................................56
Peak EMG Flexion Angles for Each Muscle Within Each Exercise .............................57
Recommendations and Limitations ................................................................................60
Conclusions ........................................................................................................................62
References ..........................................................................................................................63
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List of Figures and Tables
Figure 1. Qualisys© soft marker placement for biomechanical assessment .....................25
Figure 2. Noraxon© SEMG dual electrodes .....................................................................26
Figure 3. Noraxon© SEMG Electrode Placement Figure .................................................27
Figure 4. Barrier jump front to back ..................................................................................29
Figure 5. Barrier jump side to side.....................................................................................29
Figure 6. Lateral bounding .................................................................................................30
Figure 7. Scissor jump .......................................................................................................30
Figure 8. Squat jump ..........................................................................................................31
Table 1. Calculated Quadriceps: Hamstrings Coactivation Ratios for Each Plyometric
Exercise (Max+SD) ...........................................................................................................34
Table 2. Pairwise Comparisons Between Exercises ..........................................................35
Table 3. Multivariate Tests with Jump as the Within Subject Variable and Gender as the
Between Subject Variable ..................................................................................................36
Table 4. EMG Channel 1 (VM) Peak Flexion Angle Differences Among Exercises .......37
Table 5. Effect of Jump on Peak Flexion Angle VM Muscle Activation ..........................38
Table 6. EMG Channel 2 (VL) Peak Flexion Angle Differences Among Exercises ........38
Table 7. Effect of Jump on Peak Flexion Angle VL Muscle Activation ...........................39
Table 8. Pairwise Comparisons of VL Muscle Activation for Each Exercise ...................40
Table 9. EMG Channel 3 (MH) Peak Flexion Angle Differences Among Exercises .......41
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Table 10. Effect of Jump on Peak Flexion Angle MH Muscle Activation ........................41
Table 11. Pairwise Comparisons of MH Muscle Activation for Each Exercise ................42
Table 12. EMG Channel 4 (BF) Peak Flexion Angle Differences Among Exercises .......43
Table 13. Effect of Jump on Peak Flexion Angle BF Muscle Activation .........................43
Table 14. Pairwise Comparisons of BF Muscle Activation for Each Exercise .................44
Table 15.Peak EMG Flexion Angles of all Muscles During Barrier Jump Front to Back
............................................................................................................................................46
Table 16. Effect of Barrier Jump Front to Back on Peak EMG Flexion Angle for Each
Muscle ................................................................................................................................46
Table 17. Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During
Barrier Jump Front to Back ................................................................................................47
Table 18. Peak EMG Flexion Angles of all Muscles During Barrier Jump Side to Side .48
Table 19. Effect of Barrier Jump Side to Side on Peak EMG Flexion Angle for Each
Muscle ................................................................................................................................48
Table 20. Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During
Barrier Jump Side to Side ..................................................................................................49
Table 21. Peak EMG Flexion Angles of all Muscles During Lateral Bounding ...............50
Table 22. Effect of Lateral Bounding on Peak EMG Flexion Angle for Each Muscle .....50
Table 23. Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During
Lateral Bounding ...............................................................................................................51
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Table 24. Peak EMG Flexion Angles of all Muscles During Scissor Jump ......................52
Table 25. Effect of Scissor Jump on Peak EMG Flexion Angle for Each Muscle ............52
Table 26. Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During
Scissor Jump ......................................................................................................................53
Table 27. Peak EMG Flexion Angles of all Muscles During Squat Jump ........................54
Table 28. Effect of Squat Jump on Peak EMG Flexion Angle for Each Muscle ..............54
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Abstract
Purpose
The anterior cruciate ligament (ACL) has been reported as one of the most
commonly injured ligaments of the knee. A high incidence of ACL injuries are non-
contact injuries that occur during high velocity, closed chain movements and quick
changes in motion, such as accelerating, decelerating, cutting, and pivoting (Noyes &
Barber-Westin, 2012). There is paucity in the current literature regarding quadriceps to
hamstrings (Q:H) coactivation ratios during closed chain, high velocity exercises. These
exercises may be useful to prevent future knee injury by increasing the dynamic stability
of the knee joint and its surrounding structures.
The primary purpose of this study was to determine the functional Q:H
coactivation ratios during high velocity, closed chain knee movements in healthy,
recreationally active adults. A secondary purpose of this research was to determine the
knee flexion angles at which the maximum EMG activity occurred for each muscle
examined. Previous research has focused on the Q:H coactivation ratios during open
chain isokinetic knee motion, as well as low velocity, closed chain knee motion. This
study investigated the following research questions: What are the Q:H coactivation ratios
during closed chain, high velocity exercises including squat jump, barrier jump side to
side, barrier jump front to back, scissor jump, and lateral bounding in recreationally
active adults? At what angle of knee flexion does the maximum EMG activity occur of
the vastus medialis (VM), vastus lateralis (VL), medial hamstrings (MH), and biceps
femoris (BF)?
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Number of Subjects
Convenience sampling was utilized to recruit 20 healthy, recreationally active
college students (12 men, 8 women) between the ages of 18-30 years old within the
Department of Physical Therapy and Human Performance at Florida Gulf Coast
University.
Materials/Methods
This was a descriptive study of cross-sectional design with repeated measures in
which the participants performed 8 repetitions of 5 high velocity, closed chain exercises
on the selected lower extremity. Data collection was performed utilizing Noraxon©
surface electromyography (EMG) measurements of the vastus medialis, vastus lateralis,
medial hamstrings, and biceps femoris, in addition to Qualisys© Motion Capture System
to measure the joint angles and planes of motion during the exercises. Normalized EMG
amplitude levels were used to derive Q:H coactivation ratios for each of the exercises.
Ratios were calculated by dividing the sum of the peak quadriceps EMG activity (VM,
VL) by the sum of the peak hamstrings EMG activity (MH, BF):
(VM + VL)/(MH + BF) = Q:H coactivation ratio.
A one way repeated measures analysis of variance (ANOVA) to identify
differences in Q:H coactivation ratios among exercises. A multivariate analysis was used
to identify the effect of the jump between subjects. In addition, a one way repeated
measures analysis of variance (ANOVA) was used to identify differences in peak muscle
activity for each of the four muscles during all five exercises and to identify differences
in peak muscle activity for each of the five exercises. A multivariate analysis was used to
identify the effect of jump on peak EMG flexion angle for each EMG channel (each
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muscle) and to identify the effect of jump on peak muscle activity within each exercise.
SPSS was used to perform all statistical analysis.
Results
Statistically significant differences (p<0.05) were found between the Q:H ratios of
lateral bounding and the scissor jump (mean=-1.069), 95% CI [-2.135, -0.004]) and
between lateral bounding and the squat jump (mean=-0.694), 95%CI [-1.288, -0.100). In
addition, there was a statistically significant difference (F4,14=37.963, p<0.001) in vastus
lateralis activation during lateral bounding when compared to the other four exercises.
There was a statistically significant difference (F4,14=3.22, p<0.05) in peak flexion medial
hamstrings activation during bounding when compared to the barrier jump front to back,
barrier jump side to side, and the scissor jump. There was also a statistically significant
difference (F4,14=5.728, p<0.05) in peak flexion biceps femoris activation for lateral
bounding when compared to barrier jump side to side, scissor jump, and squat jump.
Furthermore, there were statistically significant differences found during the barrier jump
front to back (F3,15=10.561, p<0.001), barrier jump side to side (F3,15=14.810, p<0.001),
lateral bounding (F3,15=3.533, p<0.05, and scissor jump (F3,15=13.216, p<0.001).
Conclusion
We evaluated the Q:H coactivation ratios among five high velocity, closed chain
plyometric exercises, as well the knee flexion angles that coincide with peak muscle
activity. Results of our study identified that the barrier jump front to back, barrier jump
side to side, and scissor jump facilitated earlier activation of the hamstrings in relation to
the quadriceps suggesting that these exercises provide the most stability to the posterior
aspect of the knee, thus protecting the ACL. In contrast, lateral bounding facilitates
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earlier quadriceps activation and therefore should be used with caution in the early stages
of ACL rehabilitation due to the anterior shear force placed on the ACL from the
quadriceps. In conclusion, having knowledge of both the overall Q:H ratios as well as
the timing of peak muscle contraction allows for better exercise prescription and
progression and could also be used in injury prevention programs to decrease the
likelihood of ACL injury or re-injury.
Clinical Relevance
This study identified exercises that facilitate hamstring activation and
stabilization, as well as exercises that should be used with caution during ACL
rehabilitation. Clinicians can use the results of this study to guide their exercise
prescription with the ACL rehabilitation and prevention population.
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Introduction
Anterior cruciate ligament (ACL) tears are the most common ligamentous injuries
that occur in the knee joint with over one million injuries occurring worldwide every year
(Noyes & Barber-Westin, 2012). Of these injuries, it is estimated that approximately
250,000 physically active young adults sustain ACL tears annually (Salmon, Russel,
Musgrove, Pinezewski, & Refshauge, 2005). The majority of those who sustain ACL
injuries are athletes under 25 years of age who are involved in high school, collegiate, or
league sports (Noyes & Barber-Westin, 2012). At least two-thirds of ACL injuries occur
during non-contact situations while an athlete is accelerating, decelerating, cutting, or
pivoting, all motions which occur frequently in athletes who play sports or are
recreationally active (Noyes & Barber-Westin; Begalle, DiStefano, Blackburn, & Padua
2012). In addition, it is common for an ACL injury to be paired with a meniscus tear,
which is a risk factor for tibiofemoral osteoarthritis in later years (Meunier, Odensten, &
Good, 2007). ACL injuries result in both physical impairments as well as high economic
costs for athletes who undergo ACL reconstructive surgery (Gottlob & Baker, 2000).
Salmon et al. (2005) found that 12% of patients who underwent ACL reconstruction
suffered a recurrent ACL injury within 5 years of surgery. Furthermore, young, active
adults who return to activities that require lateral side stepping, cutting, and jumping have
up to a ten-fold increased likelihood of repeated ACL injury (Salmon et al., 2005).
When examining the overall stability of the knee, it is essential to understand the
antagonistic relationship between the hamstrings and the quadriceps. To provide
dynamic stability to the knee joint, the actions of the quadriceps and hamstrings must be
coordinated and coactivated to assist in protecting the joint. Thus, the coactivation of
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both muscle groups is necessary to prevent motions that may predispose the ACL to
injury (Withrow, Huston, Wojtys, & Ashton-Miller, 2008). During knee extension the
quadriceps contract concentrically while the hamstrings contract eccentrically.
Conversely, the hamstrings contract concentrically and the quadriceps contract
eccentrically during knee flexion (Coombs & Garbutt, 2002). The hamstrings function
synergistically with the ACL to counteract the force of the quadriceps contraction, and
ultimately the anterior translation of the tibia on the femur (Draganich, Jaeger, & Kralj,
1989, Aagaard et al., 2000). Moreover, the hamstring muscles are activated by
mechanoreceptors in the ACL when the ligament is placed under stress (Myer, Ford, &
Hewett, 2005). The ACL provides 87% of restraining force at 30 degrees of knee flexion
and 85% at 90 degrees of knee flexion making it the main restraint against anterior tibial
translation during knee movements (Noyes & Barber-Westin, 2012).
In order to understand the agonist-antagonist relationship between the quadriceps
and hamstrings during knee movements, specific muscle activation ratios are utilized.
When describing knee extension, the ratio used is concentric quadriceps to eccentric
hamstrings contraction (Qcon/Hecc). Conversely, during knee flexion, the ratio is
described as eccentric quadriceps contraction to concentric hamstrings contraction
(Qecc/Hcon) (Coombs & Garbutt, 2002). A significant over activation of either muscle
group during knee flexion or knee extension can result in excessive anterior or posterior
translation of the tibia on the femur causing added stress on the knee ligaments (Beynnon
& Fleming, 1998). An imbalanced Q:H ratio, particularly a quadriceps dominant ratio,
causes an anterior translation of the tibia on the femur(Cheung, Smith, & Wong, 2012).
Because the primary function of the ACL is to limit this anterior translation of the tibia,
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an individual’s ACL may be predisposed to injury as the result of an unbalanced Q:H
ratio, with an excessive anterior tibial shear force from the over-activity of the quadriceps
(Markolf et al., 1995; Renstrom et al., 1986). One study found that a vigorous
quadriceps contraction has actually been shown to cause ACL rupture in cadavers
(DeMorat, Weinhold, Blackburn, Chudik, & Garret, 2004). The hamstrings provide
posterior stabilization to the knee by counteracting the anterior shear force produced by
the quadriceps, thus reducing the amount of force placed on the ACL (Noyes & Barber-
Westin, 2012; Renstrom et al., 1986). Reduced activation of the hamstrings relative to
the quadriceps causes an increase in restraint force to be placed on the ACL (Beynnon &
Fleming, 1998; Croisier et al., 2008). In addition, weakness of the hamstrings may
increase the risk of an ACL rupture by contributing to a greater ground reaction force
being transmitted to the knee joint upon landing (Hewett, Myer, & Ford, 2006).
Therefore, decreased activation and strength of the hamstrings relative to the quadriceps
may increase one’s susceptibility to ACL injury (Boden, Griffin, & Garret, 2000, Myer et
al., 2009, and Chappel, Creighton, Giuliani, Yu, & Garret, 2007). Consequently, a more
balanced ratio between the hamstrings and the quadriceps assists in stabilizing the knee,
as well as prevents the likelihood of ligamentous instability and injury.
Coactivation Ratios
The quadriceps to hamstrings (Q:H) coactivation ratio has been evaluated to
determine the muscle balance surrounding the knee joint during various activities. This
ratio is not only position dependent, but can vary greatly based on the velocity of the
motion (Rosene, Fogarty, & Mahaffey, 2001). The conventional concentric H:Q strength
ratio (Hcon/Qcon) is frequently described as the hamstrings to quadriceps peak moment
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ratio. It is calculated by dividing the maximal concentric hamstrings moment by the
maximal concentric quadriceps moment during a specific joint angular velocity. There is
a lack of consensus of a normative value for this H:Q ratio; however, 0.6 has frequently
been cited in previous research (Baltzopoulos & Brodie, 1989; Kannus, 1994). The
limitation to using this conventional ratio is that concentric muscle contraction cannot
occur simultaneously in antagonistic muscle groups (Coombs & Garbutt, 2002). It has
recently been suggested that the agonist-antagonist relationship for knee extension and
flexion may be better described by a more functional Q:H ratio of concentric quadriceps
to eccentric hamstrings muscle activation (Qcon/Hecc) (Aagaard, Simonsen, Magnusson,
Larsson, & Dyhre-Poulson, 1998). Functional knee joint movement only allows
eccentric hamstring muscle contraction to be paired with concentric quadriceps muscle
contraction during knee extension or concentric hamstring muscle contraction paired with
eccentric quadriceps muscle contraction during knee flexion. This functional Qcon/Hecc
ratio may be used to indicate the extent to which the hamstring muscles are capable of
activating to counteract the anterior tibial shear force produced by maximal concentric
quadriceps contraction. Aagaard, Simonsen, Magnusson, Larsson, and Dyhre-Poulson
(1998) found a functional H:Q ratio of 1.00 for fast isokinetic open-chain knee extension,
indicating a significant capacity of the hamstrings to provide dynamic joint stabilization
during active, open-chain knee extension. On the other hand, lower values of 0.30 have
been reported for functional H:Q ratios representative of fast isokinetic open-chain knee
flexion. This suggests that that hamstrings have a reduced capacity for dynamic knee
joint stabilization during forceful open-chain knee flexion movements paired with
simultaneous eccentric quadriceps contraction (Aagaard, Simonsen, Magnusson, Larsson
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& Dyhre-Poulson, 1998). Although these values are useful, it is necessary to evaluate the
Q:H ratio at various velocities as the velocity of the movement has a large impact on the
overall ratio at the knee.
An issue that arises when trying to quantify the Q:H ratio is that the ratio is
velocity and position dependent. Therefore, it is imperative that Q:H ratios are evaluated
at multiple velocities to determine what those differences are. Since there is a lack of
research on high velocity closed chain Q:H ratios, previous literature regarding open
chain isokinetic Q:H ratios at varying velocities will be examined. Evaluation of
isokinetic eccentric antagonistic hamstring strength relative to concentric agonist
quadriceps strength may provide a relationship of value in describing the maximal
potential of the hamstring muscle group (Coombs & Garbutt, 2002). Hence, isokinetic
measurements are commonly used to measure the hamstrings to quadriceps strength
ratios, and can provide a quantitative measurement of the agonist and antagonist
contraction at the knee joint (Rosene et al., 2001). These values can then be used to
determine the moment-velocity patterns for both muscle groups, and allows the muscle
balance and functional ability at the knee to be derived. Rosene, Fogarty, & Mahaffey
(2001) assessed isokinetic Q:H ratios at three different speeds for both male and female
intercollegiate athletes and found that as the velocity increased, the Q:H ratio also
increased. The researchers found the Q:H ratios of 49.8% at 60°·s-1
, 53.6% at 120°·s-1
,
and 58.6% at 180°·s-1
for men and 50.3% at 60°·s-1
, 56.1% at 120°·s-1
, and 58.9% at
180°·s-1
for women. An additional research study also examined the ratio with varying
velocities and joint angles, and found that as the leg is extended, the ratio changes from
0.48 to 1.29, indicating a greater ability of the hamstrings to counteract the overactive
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quadriceps near terminal extension (Coombs & Garbutt, 2002). The aforementioned
research is able to provide evidence that Q:H ratios are velocity dependent when
performing open-chain knee flexion and extension. In addition, instability of the knee is
well compensated for at slow speeds, but compensation decreases as the speed increases
(Rosene et al., 2001). However, there is a gap in the current literature examining high
velocity closed chain Q:H ratios. Thus, Q:H ratios must be measured during high
velocity closed chain movements to examine the impact of velocity on hamstring and
quadriceps activity. Once determined, these ratios can be utilized to determine the
functional stability of the knee at higher velocities, which more closely mimic athletic or
recreational activities.
In order to understand the coactivation ratios of the hamstrings during maximal
knee extension, electromyography (EMG) readings are taken during active knee
movements. Although closed chain Q:H coactivation ratios are the focus of this research,
current literature has reported surface EMG data during open-chain isokinetic knee
movements as well as low velocity closed chain exercises. Aagaard et al (2000) had their
subjects perform two types of knee extension moments, maximal concentric quadriceps
contractions and maximal eccentric hamstrings contractions. Based on the moment and
EMG recorded, the relationship between hamstring EMG and the consequent flexor
moment was established for all joint angles through the ROM. The researchers then used
the hamstring antagonist EMG values and converted them into antagonist moment, based
on the EMG-moment relationships determined in both trials (Aagaard et al., 2000). From
this data the researchers found substantial hamstring coactivation during quadriceps
(agonist) contraction. When they examined the range of 30-10° from full knee extension,
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the antagonist hamstring moment corresponded to 30-75% of the measured knee extensor
moment (Aagaard et al). The researchers presented the hamstring coactivation data,
averaged in 10° intervals through the active ROM. They determined that hamstring
coactivation is greater toward full knee extension (10-30°) than in midrange of joint
movement (40-60°), and conversely the quadriceps are less active toward full knee
extension (Aagaard et al., 2000). This research study found higher coactivation ratios
than previous studies based on the way that the EMG data was normalized. In previous
research, antagonist hamstring EMG was normalized relative to the EMG of the
concentric hamstring agonist contraction (Aagaard et al., 2000). As a result, eccentric
antagonist hamstring moments were estimated from concentric EMG-moment
relationships, resulting in a significant underestimation of the antagonist hamstring
moments. However, this study normalized the antagonist EMG relative to the EMG
recorded during agonist contraction of the exact same type, eccentric hamstrings and
concentric quadriceps, which resulted in much higher coactivation ratios. These methods
are advantageous due to the greater ability to predict the antagonist moments because it is
based on real measurements rather than using mathematical assumptions to predict the
coactivation data. This study found that there is substantial antagonist coactivation of the
hamstring muscles during slow isokinetic knee extension, causing a flexor moment at the
knee (Aagaard et al., 2000). These moments have the ability to counteract the anterior
tibial translation induced by the quadriceps near full knee extension. This further supports
the notion that coactivation of the hamstrings during knee extension can act
synergistically to assist the ACL in preventing the anterior translation of the tibia (Baratta
et al., 1988).
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Although the isokinetic assessments provide clear data for the Q:H ratios, these
isolated contractions are not functional motions for athletes participating in a sport or
recreational activities. Therefore, the state in which athletes are commonly found playing
and practicing in must also be evaluated to determine the Q:H ratios for active play.
Closed kinetic chain (CKC) exercise is commonly used during ACL rehabilitation and
lower extremity injury prevention due to the added joint compression and stability that
they provide to the knee. Closed chain exercises vary greatly from open chain exercise as
they reduce the amount of strain on the ligaments of the knee, namely the ACL, and they
encourage the coactivation of the quadriceps and hamstrings to provide stability to the
knee during movement (Beynnon et al., 1995). In addition, closed chain exercises
closely resemble functional activities of daily living as well as recreational activities.
Open chain exercises also encourage the over-activity of the quadriceps, an issue that is
to be avoided to decrease the risk of ligamentous injury to the knee (Begalle et al., 2012).
An issue that arises when examining closed chain exercises is that the Q:H ratio can vary
greatly depending on the exercise performed. As stated prior, a Q:H ratio of 1.0 indicates
a perfect coactivation of the hamstrings and quadriceps, which is optimal for dynamic
joint stabilization to protect the knee during athletic movements. The differences in ratios
are important for clinicians to be aware of, as the risk of injuring the knee increases
greatly with specific exercises.
Begalle et al. (2012) set out to determine the coactivation ratios of commonly
prescribed lower extremity rehabilitation exercises as well as those used in injury
prevention programs. Rather than looking specifically at strength ratios, they presented
the data in coactivation ratios, to guide rehabilitation specialists with exercise
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prescription and progression. They determined the exercises with the most balanced
ratios to be the single-limb deadlift, lateral hop-to-balance, transverse hop-to-balance,
and lateral band-walk exercises. These exercises all had the lowest levels of quadriceps
activation (45-68% MVIC), with midrange hamstrings activation (10-18% MVIC).
Although these exercises have the most balanced ratios, they are all still quadriceps
dominant and do not produce enough hamstring activation to actually strengthen the
muscles (Begalle et al., 2012). Thus, when examining closed chain exercise for lower
extremity rehabilitation and injury prevention it is necessary to be mindful of the over-
activity of the quadriceps and to isolate the strengthening of the hamstrings in order to
achieve the desired functional muscle balance at the knee.
The Q:H ratio has also been examined as a possible screening tool for
predisposition to injury. When the knee is injured, the Q:H ratio is often used as a
rehabilitative goal due to the importance of the flexor-extensor strength balance in overall
knee stabilization. Reduced function of the antagonist hamstrings due to activities that
emphasize loads on the knee extensors may result in muscular imbalances between the
hamstrings and quadriceps, thereby possibly predisposing recreationally athletes to injury
(Begalle et al., 2012; Rosene et al., 2001). This predisposition may be due to the
surrounding ligamentous structures supporting most of the imposed load and decreased
antagonist hamstrings coactivation during extension loads (Rosene et al., 2001). One
research study determined that six weeks of hamstring strength training was sufficient to
increase the functional Q:H ratio to greater than 1.0, which is recommended for
prevention of non-contact ACL injuries (Holcomb, Rubley, Lee, & Guadagnoli, 2007).
As physical therapists, it is necessary to use the Q:H ratio data from all sources of
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measurement including isokinetic, low velocity closed chain, and high velocity closed
chain, to determine the best type of injury prevention as well as rehabilitation program
for athletes. Although isokinetic and low velocity closed chain exercises have been
heavily researched, high velocity closed chain activity has yet to be examined.
Mechanism of Injury
The quadriceps, as anterior cruciate ligament (ACL) antagonists, may contribute
to ACL injury. Numerous investigators have reported that quadriceps contraction
increases ACL strain between 10° and 30° of knee flexion (Renstrom et al., 1986; Arms,
Pope, & Johnson, 1984). Because most noncontact ACL injuries occur with the knee
close to full extension, it is likely that the quadriceps play an important role in ACL
disruption (Boden, Griffin, & Garret, 2000).
Previous research has found that most non-contact ACL tears occur during quick,
high velocity movements when the knee angle is moving towards full extension (Cheung,
Smith, & Wong, 2012; Noyes & Barber-Westin, 2012). Several studies show that ACL
loading increases as the knee flexion angle decreases. Arms et al. (1984) studied the
biomechanics of ACL rehabilitation and found that quadriceps muscle contraction
significantly strains the ACL from 0-45 degrees of knee flexion, but did not strain the
ACL when knee flexion is greater than 60 degrees. Similarly, Beynnon et al. (1995)
measured ACL strain during rehabilitation exercises and found that isometric quadriceps
muscle contraction resulted in a significant increase in ACL strain at 15 and 30 degrees
of knee flexion, while it resulted in no change in ACL strain relative to the relaxed
muscle condition at 60 and 90 degrees of knee flexion. Furthermore, Li et al. (1999)
investigated the quadriceps and hamstrings muscle loading in relation to ACL loading
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and showed that ACL loading increased as knee flexion angle decreased, while the
quadriceps were loaded regardless of the hamstring muscle loading conditions.
A study done by Colby et al. (2000) used surface EMG to measure hamstring and
quadriceps muscle activation in male and female collegiate athletes during eccentric
motions including sidestep cutting, crosscutting, single leg stopping, landing, and
pivoting at various knee flexion angles. The percentage of muscle activation during these
motions was determined based off of a maximum isometric contraction of both the
quadriceps and the hamstrings. The results found that peak quadriceps muscle activation
occurred between 39-53 degrees of knee flexion and averaged 161% of maximum
isometric quadriceps contraction during quick, stopping motions. In contrast, the
minimal hamstring muscle activation occurred between 21-34 degrees of knee flexion
and averaged between 14-40% of maximum isometric hamstring contraction during
stopping and cutting motions. The high level of quadriceps activity paired with the low
level of hamstring activity along with the low angles of knee flexion during these motions
can result in significant anterior translation of the tibia on the femur (Noyes & Barber-
Westin, 2012).
Plyometric Training
Plyometric training consists of high velocity eccentric to concentric muscle
loading, reflexive reactions, and functional movements. Movements are characterized by
rapid eccentric contraction in which the muscle lengthens immediately followed by a
concentric contraction of the same muscle in which it shortens (de Villarreal, Requena, &
Newton, 2010). Plyometric training has frequently been utilized in sport-specific training
as well as in ACL rehabilitation and prevention programs due to its ability to train the
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muscles, connective tissue, and nervous system to react quickly, while maintaining
proper technique and body mechanics (Hewett, 2007). Ultimately, this prepares the
athlete for high velocity situations that require rapid starting and stopping movements or
quick changes in direction (Kisner & Colby, 2012). This dynamic neuromuscular
training has been demonstrated to reduce gender related differences in force absorption,
active joint stabilization, muscle imbalances, and functional biomechanics (Rahimi,
Arshadi, Behpur, Boroujerdi, & Rahimi, 2006). Researchers have concluded that
plyometric training has been shown to be one of the most effective tools to reduce non-
contact ACL injuries when compared to other prevention programs that solely focus on
resistance or balance training (Alentorn-Geli et al., 2009).
A study by Hewett, Stroupe, Nance, & Noyes (1996) focused on the effectiveness
of plyometrics to increase hamstring strength as well as decrease landing forces by
teaching neuromuscular control of the lower limb in both male and female athletes.
Researchers found that after a six week plyometric training program, both male and
female participants experienced decreased landing forces, which translates to less force
being placed on the knee joint and associated ligaments. Additionally, the jump training
program brought the female athletes from a Q:H ratio that was significantly lower than
the male subjects up to an equivalent value (Hewett, Stroupe, Nance, & Noyes, 1996).
Similarly, Myer, Ford, Palumbo, and Hewett (2005) conducted a study to examine the
effects of a comprehensive neuromuscular training program on measures of performance
and lower limb movement biomechanics in female athletes. The subjects participated in
a six week training program that included plyometrics, resistance training, and speed
training. The trained group demonstrated increased lower extremity strength, single leg
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hop distance, and speed as well as decreased knee valgus and varus torques during
landing when compared to the untrained group. In addition, Heidt et al (2000)
implemented a seven week preseason training program that consisted of strength training,
plyometrics, and sport specific cardiovascular exercise for 300 female soccer players.
Results of this study found that the trained group of females experienced a lower
percentage (2.4%) of ACL injuries compared to the untrained group (3.1%), suggesting
that preseason conditioning that includes plyometric training can have an influence in
preventing ACL injury.
Purpose
The primary purpose of this study was to determine the functional Q:H
coactivation ratios during high velocity, closed chain knee movements in healthy,
recreationally active adults. A secondary purpose of this research was to determine the
knee flexion angles at which the maximum EMG activity occurred for each muscle
examined. Previous research has focused on the Q:H coactivation ratios during open-
chain isokinetic knee motion, as well as low velocity, closed chain knee motion.
However, the high incidence of ACL injuries in the recreationally active population occur
during high velocity, closed chain movements and quick changes in motion, such as
accelerating, decelerating, cutting, and pivoting (Noyes & Barber-Westin, 2012). Due to
paucity in the current literature, high velocity, closed chain motions require further
examination.
Prior literature has found that plyometrics are effective in reducing torque on the
knee joint during landing, as well as strengthening and increasing activation of the
quadriceps and hamstrings (Potteiger et al., 2005). Therefore, these exercises may be
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useful to prevent future knee injury by increasing the dynamic stability of the knee joint
and its surrounding structures. Previous researchers have noted that a more balanced
functional Qcon/Hecc ratio (1.0) is ideal because it puts less anterior tibial shear force on
the knee joint and thus, places less strain on the ACL (Coombs & Garbutt, 2002). One
research study determined that six weeks of hamstring strength training was sufficient to
increase the functional Q:H ratio to greater than 1.0, which is recommended for
prevention of non-contact ACL injuries. However, a ratio of 1.0 which constitutes no net
movement is not possible during active knee movements.
Research Questions
Due to paucity in the current literature regarding Q:H coactivation ratios during
closed chain, high velocity exercise, this study investigated the following research
questions: What are the Q:H coactivation ratios during closed chain, high velocity
exercises including squat jump, barrier jump side to side, barrier jump front to back,
scissor jump, and lateral bounding in recreationally active adults? At what angle of knee
flexion does the maximum EMG activity occur of the vastus medialis, vastus lateralis,
medial hamstrings, and biceps femoris?
Methods
Study Design
This research was a descriptive study of cross-sectional design with repeated
measures, in which all participants performed the selected exercises. Data collection was
performed utilizing Noraxon© surface electromyography (EMG) measurements for the
quadriceps and hamstrings in addition to Qualisys© Motion Capture System to measure
the joint angles and planes of motion during the exercises.
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Participants
The participants initially included 20 healthy men and women (12 men, 8 women)
between 18-30 years old who were recreationally active, which was defined as 60
minutes of physical activity at least three days per week. However, due to technical issues
during data collection, only 18 subjects were included in the final data analysis. Subjects
were gathered using convenience sampling from the student body of Florida Gulf Coast
University's Department of Physical Therapy and Human Performance. All subjects
demonstrated the ability to perform the required exercises without pain and with proper
form. The data was collected during a single testing session. All participants provided
written informed consent, and the study was approved by the Institutional Review Board
of Florida Gulf Coast University.
Inclusion Criteria
The inclusion criteria included: participants must fall within the age range 18-30
years old, be currently enrolled in the Department of Physical Therapy and Human
Performance at Florida Gulf Coast University, have no history of surgery to the tested
knee within the last year, and no history of knee injury to the tested knee within the last
six months at the time of testing. All subjects were required to perform the selected
exercises without pain and with proper form.
Equipment and Preparation
The laboratory utilized for data collection was equipped with a 10-camera Oqus
300 1.3MP infrared motion capture system (Qualisys© Gothenburg, Sweden) and a
portable Noraxon© surface EMG system (Scottsdale, Arizona). Reflective markers as
well as rigid marker sets (RMS) were placed on bilateral landmarks throughout the body,
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with the exception of the RMS placed on the center of the lower back, to document the
spatial position of the ankle, knee, hip, and pelvis throughout the motion (see Figure 1).
Each RMS consisted of a rigid black shell with four reflective markers attached. Elastic
wraps (SuperWrap; Fabrifoam Products, Exton, PA) were utilized to provide an
attachment point for each RMS. The individual reflective markers were placed on the
appropriate reference points through anatomic palpation. The system was calibrated to
each individual with the subject in a static position as well as while performing a
dynamic movement prior to each subject’s performance of the exercises. The EMG
signals were recorded on the participant’s tested limb throughout all exercises and MVIC
collection. The selected muscles included vastus lateralis, vastus medialis, medial
hamstrings, and biceps femoris.
Figure 1. Qualisys© soft marker placement for biomechanical assessment.
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Procedures
Before any measurements were recorded, participants performed a five minute
jogging warm-up at submaximal speed. The participants wore standard workout attire
consisting of athletic shorts, shirts, and tennis shoes. Prior to testing, the participants
were verbally and physically taught the chosen test exercises and given time to practice
the movements until they felt comfortable performing them correctly.
After the participants completed the five minute jogging warm-up, the surface
EMG dual electrodes (see Figure 2) were placed on the skin over the quadriceps and
hamstrings.
Figure 2. Noraxon© SEMG dual electrodes.
Source: Noraxon USA | Noraxon Dual EMG Electrode. Retrieved from
http://www.noraxon.com/products/accessories/noraxon-dual-emg-electrode/
The electrodes were taped onto the skin and reinforced with fitted spandex shorts to
ensure optimal attachment during the exercises. The skin was prepped with alcohol prior
to placement. The EMG electrodes were positioned in parallel along the appropriate
muscle bellies to record the muscle activity during the selected exercises. The specific
electrode placement was based on the Noraxon suggestions as follows: vastus lateralis:
lateral, anterior surface of the distal one-third of the thigh; vastus medialis: medial,
anterior surface of the distal one-third of the thigh; medial hamstrings: medial, posterior
surface of the thigh approximately midway between the ischial tuberosity and the
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popliteal fossa; biceps femoris: lateral, posterior surface of thigh approximately midway
between the popliteal fossa and the ischial tuberosity.
Figure 3. Noraxon© SEMG Electrode Placement Figure. The muscles utilized in the
study include vastus medialis, vastus lateralis, semitendinosus (medial hamstrings), and
biceps femoris. Source: Noraxon© Muscle Map: Guide for Electrode Placement. Noraxon© USA Inc. Retrieved from
http://www.health.uottawa.ca/biomech/courses/apa4311/applications.html
Once the electrodes were in place, three separate maximum voluntary isometric
contractions (MVIC’s) were performed against manual resistance provided by a single
investigator for muscles of the quadriceps and the hamstrings, including vastus lateralis,
vastus medialis, medial hamstrings, and biceps femoris to normalize muscle activation
data recorded during the exercises. Vastus medialis MVIC testing was performed with
the participant seated at the edge of a mat table with the hips and knees flexed to 90
degrees while the investigator manually resisted knee extension, with external rotation of
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the tibia. Vastus lateralis MVIC testing was performed in the same position as the vastus
medialis testing, with the exception that the tibia was placed in internal rotation during
resisted knee extension. Medial hamstrings MVIC testing was performed with the
participant lying in prone with the hip positioned in neutral, tibia in external rotation,
knee flexed to 90 degrees, and the investigator manually resisted knee flexion. Biceps
femoris MVIC testing was performed in the same position as the medial hamstrings
testing, with the exception that the tibia was placed in internal rotation during resisted
knee flexion.
The procedures of the study followed the protocol described by Begalle et al.
(2012) in which EMG readings for closed chain kinetic exercises were recorded. They
followed the manuscript of DiStefano et al. (2009) that looked at the gluteal muscle
activation for common therapeutic exercises. Begalle et al. (2012) focused on quadriceps
and hamstring coactivation in low velocity, closed chain exercises only.
The focus of this study was on closed chain exercises at higher velocities to
mimic the motions in which ACL tears occur. The EMG data was collected while
participants completed eight repetitions of each of the exercises in a randomized order
with a two minute rest period between exercises.
High Velocity, Closed Chain Exercise
According to Nyland et al. (1999), knee rehabilitation and injury prevention
programs should be focused on lower extremity closed kinetic chain tasks, which would
include mini squats, single leg vertical and horizontal hopping, lateral shuffles in a mini
squat position, back pedaling, and quick multidirectional movement responses.
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The following exercises were chosen based on their prevalence in injury prevention and
sport specific training for athletes. All exercises are adapted from Noyes & Barber-
Westin (2012) and are described as follows:
Barrier jump front to back. A cone approximately 6-8” in height was placed on
the floor. The participants started in an upright position with the knees deeply flexed and
were instructed to jump in front of the cone to behind the cone, keeping the feet together.
They were instructed to land on both feet at the same time with the same amount of knee
flexion as the starting position. Participants were instructed to land softly on the balls of
the feet and rock back to the heels to control the landing.
Figure 4. Barrier jump front to back Figure 5. Barrier jump side to side
Barrier jump side to side. A cone approximately 6-8” in height was placed on
the floor. The participants started in an upright position with the knees deeply flexed and
were instructed to jump from one side of the cone to the other, keeping the feet together.
They were instructed to land on both feet at the same time with the same amount of knee
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flexion as the starting position. Participants were instructed to land softly on the balls of
the feet and rock back to the heels to control the landing.
Lateral bounding. The participants began by assuming a half squat position leg
with the knees slightly flexed, and eyes looking forward. The participants were
instructed to shift weight onto the outside leg and immediately push off and extend
through the outside leg attempting to bound to the opposite side, landing on the opposite
leg and remaining in this position for 3 seconds.
Figure 6. Lateral bounding Figure 7. Scissor jump
Scissor jump. Participants began in a lunge position with the non-dominant knee
bent directly over the ankle. They were instructed to push off with the front leg and jump
straight up in the air while landing with the opposite leg bent in front.
Squat jump. Participants started in a fully crouched position with the hands
touching the ground on the outside of the heels. The participants were instructed to point
the knees and feet forward while keeping the upper body upright with the chest open.
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They were also instructed to keep the knees under the hips and to keep the knees and
ankles shoulder width apart. Participants were instructed to jump up and raise the arms
as high as possible and return to the starting position with the hands reaching back
towards the heels.
Figure 8. Squat jump
Participants utilized a metronome to perform each exercise and they were
required to stabilize in the landing position for three seconds (equivalent of three beats of
the metronome). They were observed during all practice and recorded repetitions to
ensure correct performance of the exercise.
Data Sampling and Reduction/Data Analysis
Preamplified active surface EMG electrodes with an interelectrode distance of
10mm, and amplification factor of 10,000 (20-500 Hz), and a common mode rejection
ratio of more than 80 dB at 60 Hz was used to measure activation of the quadriceps and
hamstrings. Data was collected and processed utilizing Qualisys© and Visual 3D
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software (C-motion, Germantown, MD, USA). Electromyographic signals were pre-
amplified at the interface and sampled at 1500 Hz. Utilizing Visual3D software, the EMG
signals were band-pass filtered at 20 to 500 Hz and full-wave rectified before a linear
envelope was created with a 10-Hz low-pass, phase-corrected Butterworth filter.
Qualisys© software was utilized to identify the beginning and end of the middle 4
repetitions for each exercise, and the peak EMG signal amplitudes for the quadriceps and
hamstrings were calculated and averaged.
One MVIC value was obtained for each muscle by averaging the three means.
The mean EMG amplitudes for each exercise were normalized to these reference values
and expressed as percentages of MVIC’s.
Normalized EMG amplitude levels were used to derive Q:H coactivation ratios
for each of the exercises. Ratios were calculated by dividing the sum of the peak
quadriceps EMG activity (VM, VL) by the sum of the peak hamstrings EMG activity
(MH, BF).
(VM + VL)/(MH + BF) = Q:H coactivation ratio
Balanced or equal coactivation calculated by this method resulted in a
coactivation ratio of 1.0, whereas ratios greater than 1.0 indicated greater quadriceps than
hamstrings activation. Similarly, ratios less than 1.0 indicated greater hamstrings than
quadriceps activation.
Statistical Analysis
Although the participants performed eight repetitions of each exercise, only the
middle four repetitions were used for data analysis. Descriptive statistics of the Q:H
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coactivation ratios were generated by dividing the sum of the peak quadriceps EMG
activity (VM, VL) by the sum of the peak hamstrings EMG activity (MH, BF).
(VM + VL)/(MH + BF) = Q:H coactivation ratio
A one way repeated measures analysis of variance (ANOVA) to identify
differences in Q:H coactivation ratios among exercises. A multivariate analysis with
jump as the within subject variable and gender as the between subject variable was used
to identify the effect of the jump between subjects. A one way repeated measures
analysis of variance (ANOVA) was used to identify differences in peak muscle activity
for each of the four muscles during all five exercises and to identify differences in peak
muscle activity for each of the five exercises. A multivariate analysis was used to
identify the effect of jump on peak EMG flexion angle for each EMG channel (each
muscle) and to identify the effect of jump on peak muscle activity within each exercise.
SPSS version 22 was used to perform all statistical analysis.
Results
Q:H Coactivation Ratios
Calculated Q:H coactivation ratios with standard deviations are displayed in Table
1 for each plyometric exercise. The Q:H coactivation ratios were greatest (quadriceps
dominant activation pattern) during the barrier jump front to back and the barrier jump
side to side, displaying approximately five times more quadriceps than hamstrings
activation in these plyometric exercises. The Q:H ratios were smaller during the lateral
bounding exercise and the squat jump. The lateral bounding exercise (3.08+2.61)
resulted in the smallest Q:H coactivation ratio (most balanced quadriceps and hamstrings
activation). Pairwise comparisons between jumps are displayed in Table 2. Statistically
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significant differences (p<0.05) were found between the Q:H ratios of lateral bounding
and the scissor jump (mean=-1.069), 95% CI [-2.135, -0.004]) and between lateral
bounding and the squat jump (mean=-0.694), 95%CI [-1.288, -0.100). No between
subjects effect was noted. A statistically significant difference (F4,13=3.651, p<0.05) was
noted with the overall multivariate test of jump (see Table 3). Between subjects analysis
was not significant, so no post hoc analysis was run.
Table 1
Calculated Quadriceps: Hamstrings Coactivation Ratios for Each Plyometric Exercise
(Max + SD)
Exercise Q:H Coactivation Ratio
Barrier Front to Back: Max 5.05+7.33
Barrier Side to Side: Max 4.51+5.29
Lateral Bounding: Max 3.08+2.61
Scissor: Max 4.10+3.24
Squat Jump: Max 3.70+3.02
Note. SD=standard deviation; Q:H=quadriceps to hamstrings
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Table 2
Pairwise Comparisons Between Exercises
(I) jump (J) jump
Mean
Difference
(I-J)
Std.
Error Sig.b
95% Confidence Interval for
Differenceb
Lower
Bound Upper Bound
1 Barrier
jump front
to back
2 .481 .565 .407 -.716 1.678
3 1.862 1.773 .309 -1.896 5.620
4 .792 1.358 .568 -2.086 3.671
5 1.168 1.742 .512 -2.525 4.861
2 Barrier
jump side
to side
1 -.481 .565 .407 -1.678 .716
3 1.381 1.256 .288 -1.283 4.044
4 .311 .826 .711 -1.440 2.062
5 .687 1.214 .579 -1.886 3.260
3 Lateral
bounding
1 -1.862 1.773 .309 -5.620 1.896
2 -1.381 1.256 .288 -4.044 1.283
4 -1.069* .503 .049* -2.135 -.004
5 -.694* .280 .025* -1.288 -.100
4 Scissor
jump
1 -.792 1.358 .568 -3.671 2.086
2 -.311 .826 .711 -2.062 1.440
3 1.069* .503 .049* .004 2.135
5 .376 .500 .464 -.685 1.437
5 Squat
jump
1 -1.168 1.742 .512 -4.861 2.525
2 -.687 1.214 .579 -3.260 1.886
3 .694* .280 .025* .100 1.288
4 -.376 .500 .464 -1.437 .685
Note. Based on estimated marginal means; 1=barrier jump front to back; 2=barrier jump
side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump
bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 3
Multivariate Tests with Jump as the Within Subject Variable and Gender as the Between
Subject Variable
Value F
Hypothesis
df Error df Sig.
Partial Eta
Squared
Pillai's trace .529 3.651a 4.000 13.000 .033* .529
Wilks' lambda .471 3.651a 4.000 13.000 .033* .529
Hotelling's trace 1.123 3.651a 4.000 13.000 .033* .529
Roy's largest
root 1.123 3.651
a 4.000 13.000 .033* .529
Peak EMG Flexion Angles for Each Muscle During All Jumps
EMG signals for each individual muscle were examined to determine if there
were statistically significant differences in peak EMG flexion angles during each of the
five exercises. Vastus medialis peak flexion angles were examined and the values are
listed in Table 4. There were no statistically significant differences for vastus medialis in
peak EMG flexion angles when comparing all five exercises (see Table 5). Vastus
lateralis peak flexion angle values are listed in Table 6. There was a statistically
significant difference (F4,14=37.963, p<0.001) in vastus lateralis activation during lateral
bounding when compared to the other four exercises (see Tables 7 & 8). Medial
hamstrings peak flexion angle values are listed in Table 9. There was a statistically
significant difference (F4,14=3.22, p<0.05) in peak flexion medial hamstrings activation
during bounding when compared to the barrier jump front to back, barrier jump side to
side, and the scissor jump (see Tables 10 & 11). In addition, when examining lateral
Note. Each F tests the multivariate effect of jump. These tests are based on the linearly
independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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bounding, the peak medial hamstrings muscle activity occurred at a greater flexion angle
(58.94˚) when compared to the peak medial hamstrings EMG during the other exercises.
Biceps femoris peak flexion angle values are listed in Table 12. There was a statistically
significant difference (F4,14=5.728, p<0.05) in peak flexion biceps femoris activation for
lateral bounding when compared to barrier jump side to side, scissor jump, and squat
jump (see Tables 13 & 14). Overall, the peak EMG for all muscle groups occur at
smaller flexion angles for lateral bounding than all other exercises examined. The one
exception to this pattern is the medial hamstrings peak flexion angles during the lateral
bounding exercise.
Table 4
EMG Channel 1 (VM) Peak Flexion Angle Differences Among Exercises
Jump Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Barrier jump front to back 63.453 4.621 73.203 53.704
Barrier jump side to side 66.591 4.041 75.116 58.066
Lateral bounding 53.470 2.403 58.541 48.399
Scissor jump 64.018 4.981 74.528 53.508
Squat jump 61.038 6.358 74.452 47.625
Note. Mean flexion angle values are measured in degrees
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Table 5
Effect of Jump on Peak Flexion Angle VM Muscle Activation
Effect Value F
Hypothesis
df Error df Sig.
jump Pillai's Trace .416 2.495b 4.000 14.000 .090
Wilks' Lambda .584 2.495b 4.000 14.000 .090
Hotelling's Trace .713 2.495b 4.000 14.000 .090
Roy's Largest
Root .713 2.495
b 4.000 14.000 .090
Note. Each F tests the multivariate effect of jump. These tests are based on the linearly
independent pairwise comparisons among the estimated marginal means.
bExact statistic
*p<0.05. **p<0.01. ***p<0.001
Table 6
EMG Channel 2 (VL) Peak Flexion Angle Differences Among Exercises
Jump Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Barrier jump front to back 66.702 3.345 73.759 59.645
Barrier jump side to side 68.337 3.251 75.197 61.478
Lateral bounding 49.798 2.186 54.410 45.186
Scissor jump 64.592 3.563 72.108 57.076
Squat jump 66.270 4.710 76.207 56.333
Note. Mean flexion angle values are measured in degrees
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Table 7
Effect of Jump on Peak Flexion Angle VL Muscle Activation
Value F
Hypothesis
df Error df Sig.
Pillai's trace .916 37.963a 4.000 14.000 .000*
Wilks' lambda .084 37.963a 4.000 14.000 .000*
Hotelling's trace 10.847 37.963a 4.000 14.000 .000*
Roy's largest
root 10.847 37.963
a 4.000 14.000 .000*
Note. Each F tests the multivariate effect of jump. These tests are based on the linearly
independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 8
Pairwise Comparisons of VL Muscle Activation for Each Exercise
(I) jump (J) jump
Mean
Difference
(I-J)
Std.
Error Sig.b
95% Confidence Interval for
Differenceb
Lower
Bound Upper Bound
1 Barrier
jump
front to
back
2 1.635 5.364 .764 9.681 12.952
3 16.904* 2.467 .000*** 22.109 11.698
4 2.110 3.114 .507 8.680 4.460
5 .432 5.285 .936 11.581 10.718
2 Barrier
jump side
to side
1 1.635 5.364 .764 12.952 9.681
3 18.539* 3.592 .000*** 26.118 10.960
4 3.745 4.799 .446 13.870 6.380
5 2.067 3.897 .603 10.289 6.154
3 Lateral
bounding
1 16.904* 2.467 .000*** 11.698 22.109
2 18.539* 3.592 .000*** 10.960 26.118
4 14.794* 2.664 .000*** 9.174 20.414
5 16.472* 3.929 .001** 8.182 24.761
4 Scissor
jump
1 2.110 3.114 .507 4.460 8.680
2 3.745 4.799 .446 6.380 13.870
3 14.794* 2.664 .000*** 20.414 9.174
5 1.678 4.611 .720 8.050 11.405
5 Squat
jump
1 .432 5.285 .936 10.718 11.581
2 2.067 3.897 .603 6.154 10.289
3 16.472* 3.929 .001** 24.761 8.182
4 1.678 4.611 .720 11.405 8.050
Note. Based on estimated marginal means; 1=barrier jump front to back; 2=barrier jump
side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump; mean flexion angle
values are measured in degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 9
EMG Channel 3 (MH) Peak Flexion Angle Differences Among Exercises
Jump Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Barrier jump front to back 26.790 6.999 41.557 12.023
Barrier jump side to side 43.558 4.707 53.489 33.628
Lateral bounding 58.935 6.721 73.116 44.754
Scissor jump 34.976 5.934 47.496 22.455
Squat jump 48.123 8.173 65.366 30.879
Note. Mean flexion angle values are measured in degrees
Table 10
Effect of Jump on Peak Flexion Angle MH Muscle Activation
Value F
Hypothesis
df Error df Sig.
Pillai's trace .479 3.220a 4.000 14.000 .045*
Wilks' lambda .521 3.220a 4.000 14.000 .045*
Hotelling's trace .920 3.220a 4.000 14.000 .045*
Roy's largest
root .920 3.220
a 4.000 14.000 .045*
Note. Each F tests the multivariate effect of jump. These tests are based on the linearly
independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 11
Pairwise Comparisons of MH Muscle Activation for Each Exercise
(I) jump (J) jump
Mean
Difference
(I-J)
Std.
Error Sig.b
95% Confidence Interval for
Differenceb
Lower
Bound Upper Bound
1 Barrier
jump
front to
back
2 16.768 9.377 .092 3.017 36.553
3 32.145* 9.411 .003** 12.289 52.000
4 8.186 8.175 .331 9.061 25.433
5 21.333 10.465 .057 .747 43.412
2 Barrier
jump side
to side
1 16.768 9.377 .092 36.553 3.017
3 15.377* 6.908 .040* .803 29.951
4 8.582 6.807 .224 22.944 5.779
5 4.564 8.916 .615 14.246 23.375
3 Lateral
bounding
1 32.145* 9.411 .003** 52.000 12.289
2 15.377* 6.908 .040* 29.951 .803
4 23.959* 7.104 .004** 38.947 8.971
5 10.812 9.233 .258 30.291 8.667
4 Scissor
jump
1 8.186 8.175 .331 25.433 9.061
2 8.582 6.807 .224 5.779 22.944
3 23.959* 7.104 .004** 8.971 38.947
5 13.147 8.864 .156 5.555 31.849
5 Squat
jump
1 21.333 10.465 .057 43.412 .747
2 4.564 8.916 .615 23.375 14.246
3 10.812 9.233 .258 8.667 30.291
4 13.147 8.864 .156 31.849 5.555
Note. Based on estimated marginal means; 1=barrier jump front to back; 2=barrier jump
side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump; mean flexion angle
values are measured in degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 12
EMG Channel 4 (BF) Peak Flexion Angle Differences Among Exercises
Jump Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Barrier jump front to back 48.136 6.516 61.884 34.388
Barrier jump side to side 62.791 3.486 70.147 55.436
Lateral bounding 36.089 6.066 48.888 23.291
Scissor jump 56.256 5.381 67.609 44.903
Squat jump 61.761 5.267 72.874 50.648
Note. Mean flexion angle values are measured in degrees
Table 13
Effect of Jump on Peak Flexion Angle BF Muscle Activation
Value F
Hypothesis
df Error df Sig.
Pillai's trace .621 5.728a 4.000 14.000 .006**
Wilks' lambda .379 5.728a 4.000 14.000 .006**
Hotelling's trace 1.636 5.728a 4.000 14.000 .006**
Roy's largest
root 1.636 5.728
a 4.000 14.000 .006**
Note. Each F tests the multivariate effect of jump. These tests are based on the linearly
independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 14
Pairwise Comparisons of BF Muscle Activation for Each Exercise
(I) jump (J) jump
Mean
Difference
(I-J)
Std.
Error Sig.b
95% Confidence Interval for
Differenceb
Lower
Bound Upper Bound
1 Barrier
jump
front to
back
2 14.655 7.369 .063 .893 30.202
3 12.047 7.893 .145 28.700 4.605
4 8.120 6.991 .262 6.630 22.869
5 13.625 7.480 .086 2.157 29.406
2 Barrier
jump side
to side
1 14.655 7.369 .063 30.202 .893
3 26.702* 5.172 .000*** 37.615 15.789
4 6.535 5.017 .210 17.119 4.049
5 1.030 3.961 .798 9.388 7.328
3 Lateral
bounding
1 12.047 7.893 .145 4.605 28.700
2 26.702* 5.172 .000*** 15.789 37.615
4 20.167* 6.738 .008** 5.950 34.383
5 25.672* 6.131 .001** 12.737 38.607
4 Scissor
jump
1 8.120 6.991 .262 22.869 6.630
2 6.535 5.017 .210 4.049 17.119
3 20.167* 6.738 .008** 34.383 5.950
5 5.505 5.130 .298 5.318 16.328
5 Squat
jump
1 13.625 7.480 .086 29.406 2.157
2 1.030 3.961 .798 7.328 9.388
3 25.672* 6.131 .001** 38.607 12.737
4 5.505 5.130 .298 16.328 5.318
Note. Based on estimated marginal means; 1=barrier jump front to back; 2=barrier jump
side to side; 3=lateral bounding; 4=scissor jump; 5=squat jump; mean flexion angle
values are measured in degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Peak EMG Flexion Angles for Each Muscle Within Each Exercise
EMG signals for each muscle were examined to determine if there were
statistically significant differences in peak EMG flexion angles of the four muscles
during barrier jump front to back, barrier jump side to side, lateral bounding, scissor
jump, and squat jump. Peak EMG flexion angles for the barrier jump front to back are
listed in Table 15. There was a statistically significant difference (F3,15=10.561, p<0.001)
between the flexion angles of the combined hamstrings when compared to the combined
quadriceps. The peak EMG flexion angles for the hamstrings occurred at a smaller
flexion angle than the quadriceps (see Tables 16 & 17). Peak EMG flexion angles for the
barrier jump side to side are listed in Table 18. There was a statistically significant
difference (F3,15=14.810, p<0.001) between the flexion angle of the medial hamstrings
compared to the other muscles. The peak EMG flexion angle (43.56˚) for the medial
hamstrings was significantly smaller than the biceps femoris, vastus lateralis, and vastus
medialis (see Tables 19 & 20). Peak EMG flexion angles for lateral bounding are listed
in Table 21. There was a statistically significant difference (F3,15=3.533, p<0.05)
between the flexion angle of the medial hamstrings compared to the other muscles. The
peak EMG flexion angle (58.94˚) for the medial hamstrings was significantly larger than
the biceps femoris, vastus lateralis, and vastus medialis (see Tables 22 & 23). Peak EMG
flexion angles for the scissor jump are listed in Table 24. There was a statistically
significant difference (F3,15=13.216, p<0.001) between the flexion angles of the combined
hamstrings compared to the combined quadriceps (see Tables 25 & 26). The peak EMG
flexion angles for the combined hamstrings were significantly smaller than the peak
EMG flexion angles for the combined quadriceps. Peak EMG flexion angles for the
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squat jump are listed in Table 27. There were no statistically significant differences
between peak EMG flexion angles amongst the muscles during this exercise (see Table
28).
Table 15
Peak EMG Flexion Angles of all Muscles During Barrier Jump Front to Back
Flexion angle Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Vastus medialis 63.453 4.621 73.203 53.704
Vastus lateralis 66.702 3.345 73.759 59.645
Medial hamstrings 26.790 6.999 41.557 12.023
Biceps femoris 48.136 6.516 61.884 34.388
Note. Mean flexion angle values are measured in degrees
Table 16
Effect of Barrier Jump Front to Back on Peak EMG Flexion Angle for Each Muscle
Value F
Hypothesis
df Error df Sig.
Pillai's trace .679 10.561a 3.000 15.000 .001**
Wilks' lambda .321 10.561a 3.000 15.000 .001**
Hotelling's trace 2.112 10.561a 3.000 15.000 .001**
Roy's largest
root 2.112 10.561
a 3.000 15.000 .001**
Note. Each F tests the multivariate effect of flexion angle. These tests are based on the
linearly independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 17
Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Barrier Jump
Front to Back
(I) flexion angle
(J) flexion
angle
Mean
Difference
(I-J)
Std.
Error Sig.b
95%
Confidence
Interval for
Differenceb
Lower
Bound
1 Vastus medialis 2 3.248 4.955 .521 7.205
3 36.663* 8.972 .001** 55.593
4 15.317* 6.465 .030* 28.957
2 Vastus lateralis 1 3.248 4.955 .521 13.702
3 39.912* 7.720 .000*** 56.199
4 18.566* 6.783 .014* 32.877
3 Medial
hamstrings
1 36.663* 8.972 .001** 17.734
2 39.912* 7.720 .000*** 23.624
4 21.346 10.422 .056 .642
4 Biceps femoris 1 15.317* 6.465 .030* 1.678
2 18.566* 6.783 .014* 4.254
3 21.346 10.422 .056 43.334
Note. Based on estimated marginal means;1=vastus medialis; 2=vastus lateralis;
3=medial hamstrings; 4=biceps femoris; mean flexion angle values are measured in
degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 18
Peak EMG Flexion Angles of all Muscles During Barrier Jump Side to Side
Flexion angle Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Vastus medialis 66.591 4.041 75.116 58.066
Vastus lateralis 68.337 3.251 75.197 61.478
Medial hamstrings 43.558 4.707 53.489 33.628
Biceps femoris 62.791 3.486 70.147 55.436
Note. Mean flexion angle values are measured in degrees
Table 19
Effect of Barrier Jump Side to Side on Peak EMG Flexion Angle for Each Muscle
Value F
Hypothesis
df Error df Sig.
Pillai's trace .748 14.810a 3.000 15.000 .000***
Wilks' lambda .252 14.810a 3.000 15.000 .000***
Hotelling's trace 2.962 14.810a 3.000 15.000 .000***
Roy's largest
root 2.962 14.810
a 3.000 15.000 .000***
Note. Each F tests the multivariate effect of flexion angle. These tests are based on the
linearly independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 20
Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Barrier Jump
Side to Side
(I) flexion angle
(J) flexion
angle
Mean
Difference
(I-J)
Std.
Error Sig.b
95%
Confidence
Interval for
Differenceb
Lower
Bound
1 Vastus medialis 2 1.747 3.200 .592 5.005
3 23.032* 3.330 .000*** 30.057
4 3.800 4.384 .398 13.048
2 Vastus lateralis 1 1.747 3.200 .592 8.498
3 24.779* 4.953 .000*** 35.229
4 5.546 4.222 .206 14.453
3 Medial
hamstrings
1 23.032* 3.330 .000*** 16.008
2 24.779* 4.953 .000*** 14.329
4 19.233* 5.766 .004** 7.067
4 Biceps femoris 1 3.800 4.384 .398 5.449
2 5.546 4.222 .206 3.360
3 19.233* 5.766 .004** 31.399
Note. Based on estimated marginal means;1=vastus medialis; 2=vastus lateralis;
3=medial hamstrings; 4=biceps femoris; mean flexion angle values are measured in
degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 21
Peak EMG Flexion Angles of all Muscles During Lateral Bounding
Flexion angle Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Vastus medialis 53.470 2.403 58.541 48.399
Vastus lateralis 49.798 2.186 54.410 45.186
Medial hamstrings 58.935 6.721 73.116 44.754
Biceps femoris 36.089 6.066 48.888 23.291
Note. Mean flexion angle values are measured in degrees
Table 22
Effect of Lateral Bounding on Peak EMG Flexion Angle for Each Muscle
Value F
Hypothesis
df Error df Sig.
Pillai's trace .414 3.533a 3.000 15.000 .041*
Wilks' lambda .586 3.533a 3.000 15.000 .041*
Hotelling's trace .707 3.533a 3.000 15.000 .041*
Roy's largest
root .707 3.533
a 3.000 15.000 .041*
Note. Each F tests the multivariate effect of flexion angle. These tests are based on the
linearly independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 23
Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Lateral
Bounding
(I) flexion angle
(J) flexion
angle
Mean
Difference
(I-J)
Std.
Error Sig.b
95%
Confidence
Interval for
Differenceb
Lower
Bound
1 Vastus medialis 2 3.672 2.262 .123 8.445
3 5.465 6.898 .439 9.089
4 17.381* 5.993 .010* 30.024
2 Vastus lateralis 1 3.672 2.262 .123 1.101
3 9.137 7.304 .228 6.274
4 13.709* 6.418 .048* 27.250
3 Medial
hamstrings
1 5.465 6.898 .439 20.019
2 9.137 7.304 .228 24.548
4 22.846* 8.833 .019* 41.482
4 Biceps femoris 1 17.381* 5.993 .010* 4.738
2 13.709* 6.418 .048* .167
3 22.846* 8.833 .019* 4.209
Note. Based on estimated marginal means;1=vastus medialis; 2=vastus lateralis;
3=medial hamstrings; 4=biceps femoris; mean flexion angle values are measured in
degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Table 24
Peak EMG Flexion Angles of all Muscles During Scissor Jump
Flexion angle Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Vastus medialis 64.018 4.981 74.528 53.508
Vastus lateralis 64.592 3.563 72.108 57.076
Medial hamstrings 34.976 5.934 47.496 22.455
Biceps femoris 56.256 5.381 67.609 44.903
Note. Mean flexion angle values are measured in degrees
Table 25
Effect of Scissor Jump on Peak EMG Flexion Angle for Each Muscle
Value F
Hypothesis
df Error df Sig.
Pillai's trace .726 13.216a 3.000 15.000 .000***
Wilks' lambda .274 13.216a 3.000 15.000 .000***
Hotelling's trace 2.643 13.216a 3.000 15.000 .000***
Roy's largest
root 2.643 13.216
a 3.000 15.000 .000***
Note. Each F tests the multivariate effect of flexion angle. These tests are based on the
linearly independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
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Table 26
Pairwise Comparisons of Peak EMG Flexion Angle of All Muscles During Scissor Jump
(I) flexion angle
(J) flexion
angle
Mean
Difference
(I-J)
Std.
Error Sig.b
95%
Confidence
Interval for
Differenceb
Lower
Bound
1 Vastus medialis 2 .574 5.594 .919 11.228
3 29.042* 5.009 .000*** 39.611
4 7.762 6.818 .271 22.146
2 Vastus lateralis 1 .574 5.594 .919 12.377
3 29.616* 5.738 .000*** 41.722
4 8.336 5.756 .166 20.481
3 Medial
hamstrings
1 29.042* 5.009 .000*** 18.473
2 29.616* 5.738 .000*** 17.510
4 21.280* 7.617 .012* 5.210
4 Biceps femoris 1 7.762 6.818 .271 6.623
2 8.336 5.756 .166 3.809
3 21.280* 7.617 .012* 37.350
Note. Based on estimated marginal means;1=vastus medialis; 2=vastus lateralis;
3=medial hamstrings; 4=biceps femoris; mean flexion angle values are measured in
degrees bAdjustment for multiple comparisons: Least Significant Difference (equivalent to no
adjustments).
*p<0.05. **p<0.01. ***p<0.001
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Q:H COACTIVATION RATIOS DURING EXERCISE
Table 27
Peak EMG Flexion Angles of all Muscles During Squat Jump
Flexion angle Mean
Std.
Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Vastus medialis 61.038 6.358 74.452 47.625
Vastus lateralis 66.270 4.710 76.207 56.333
Medial hamstrings 48.123 8.173 65.366 30.879
Biceps femoris 61.761 5.267 72.874 50.648
Note. Mean flexion angle values are measured in degrees
Table 28
Effect of Squat Jump on Peak EMG Flexion Angle for Each Muscle
Value F
Hypothesis
df Error df Sig.
Pillai's trace .161 .963a 3.000 15.000 .436
Wilks' lambda .839 .963a 3.000 15.000 .436
Hotelling's trace .193 .963a 3.000 15.000 .436
Roy's largest
root .193 .963
a 3.000 15.000 .436
Note. Each F tests the multivariate effect of flexion angle. These tests are based on the
linearly independent pairwise comparisons among the estimated marginal means. aExact statistic
*p<0.05. **p<0.01. ***p<0.001
Discussion
Q:H Coactivation Ratios
The primary purpose of this study was to assess Q:H coactivation ratios during
high velocity, closed chain exercises in healthy, recreationally active adults. We found
differences in the levels of Q:H coactivation during the five selected exercises.
Interpretation of our results may offer insight into the potential effectiveness of these
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Q:H COACTIVATION RATIOS DURING EXERCISE
exercises in terms of muscle activation between the quadriceps and hamstrings. Our
study provides information that may assist clinicians in selecting high velocity, closed
chain exercises that are most appropriate for their current rehabilitation goals and
throughout their exercise progression. Exercises and their implications for rehabilitation
will be discussed in the order of the smallest (most balanced) Q:H coactivation ratios to
the largest coactivation ratios (most quadriceps dominant).
The two most balanced Q:H coactivation ratio was observed during the lateral
bounding exercise (3.08+2.61) and the squat jump (3.70+3.02). Of the five selected
exercises, these two exercises appear to be driven by less quadriceps activity than the
scissor jump, barrier jump front to back, and barrier jump side to side. This data implies
that these exercises may be safer to utilize in ACL rehabilitation due to more balanced
activation between the quadriceps and hamstrings.
The largest Q:H coactivation ratio was observed during the barrier jump front to
back (5.05+7.33). This exercise appears to be driven primarily by quadriceps activity
with minimal hamstring activation to counteract the quadriceps. Previous research has
shown that an imbalance in muscle activation between the quadriceps and hamstrings,
particularly overactive quadriceps muscle activation, causes an anterior translation of the
tibia on the femur (Cheung, Smith, & Wong, 2012). Because the primary function of the
ACL is to limit this anterior translation of the tibia, an individual’s ACL may be
predisposed to injury as the result of muscular imbalance between the quadriceps and
hamstrings, with an excessive anterior tibial shear force from the over-activity of the
quadriceps (Markolf et al., 1995; Renstrom et al., 1986). These results suggest that the
large Q:H ratio seen during the barrier jump front to back could cause an increase in
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Q:H COACTIVATION RATIOS DURING EXERCISE
anterior tibial translation and greater ACL loading, thus predisposing the ACL to injury
or re-injury. This information should be carefully considered when deciding whether to
incorporate the barrier jump front to back into an ACL rehab program due to the
increased quadriceps activation that it promotes.
Peak EMG Flexion Angles for Each Muscle During All Jumps
Based on our results, there were no significant differences in the peak EMG
flexion angles of the vastus medialis when comparing all five exercises. However,
significant differences in peak EMG flexion angles of the remaining muscles were
observed. We found that the peak EMG flexion angles of the vastus lateralis and biceps
femoris during the lateral bounding exercise differed significantly from all other
exercises performed in that a smaller flexion angle was observed during this exercise.
Conversely, the medial hamstrings behaved in the opposite manner during the lateral
bounding exercise by having the largest peak EMG flexion angle. These results lead the
researchers to conclude that the medial hamstrings are the last muscle to activate during
the lateral bounding exercise. As a result, the medial hamstrings are unable to provide
stabilization to the posterior aspect of the knee to offset the earlier activation of the
combined quadriceps.
Previous research has found that the hamstrings provide posterior stabilization to
the knee by counteracting the anterior shear force produced by the quadriceps, and thus
reducing the amount of force placed on the ACL (Noyes & Barber-Westin, 2012;
Renstrom et al., 1986). Reduced activation of the hamstrings relative to the quadriceps
causes an increase in restraint force to be placed on the ACL (Beynnon & Fleming, 1998;
Croisier et al., 2008). In addition, weakness of the hamstrings may increase the risk of an
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Q:H COACTIVATION RATIOS DURING EXERCISE
ACL rupture by contributing to a greater ground reaction force being transmitted to the
knee joint upon landing (Hewett, Myer, & Ford, 2006). Therefore, decreased activation
and strength of the hamstrings relative to the quadriceps may increase one’s susceptibility
to ACL injury (Boden, Griffin, & Garret, 2000, Myer et al., 2009, and Chappel,
Creighton, Giuliani, Yu, & Garret, 2007).
This information is imperative to consider when deciding which exercises are the
safest and most appropriate for ACL rehabilitation. Although our initial results
demonstrated that lateral bounding had the most balanced Q:H coactivation ratio, further
data analysis was required to determine the flexion angles that the maximal muscle
contractions for each muscle were occurring. In order to safely prescribe these exercises,
the clinician must have knowledge of the timing of the maximum quadriceps activation in
relation to the maximum hamstrings activation. This information can be used to
determine exercises that promote earlier hamstring activation to provide posterior
stabilization to the knee and thus, protect the ACL.
Peak EMG Flexion Angles for Each Muscle Within Each Exercise
Based on our results, there were statistically significant differences between the
peak EMG flexion angles during the barrier jump front to back, barrier jump side to side,
and the scissor jump. The peak EMG flexion angles of the hamstrings were smaller than
the peak EMG flexion angles of the quadriceps, meaning that the MH and BF activate at
a smaller flexion angle to provide posterior stabilization to the knee before the VL and
VM activate. Numerous investigators have reported that quadriceps contraction increases
ACL strain at smaller angles (0°-30°) of knee flexion (Arms et al. 1984; Beynnon et al.
1995; & Li et al. 1999). In addition, most non-contact ACL injuries occur when the knee
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Q:H COACTIVATION RATIOS DURING EXERCISE
is moving towards terminal extension making it imperative that the hamstrings activate at
a smaller flexion angle in order to provide posterior stabilization to the knee joint and
decrease the amount of restraint force placed on the ACL (Renstrom et al., 1986; Arms et
al., 1984; Boden et al., 2000). This information can be used to guide the clinician in
choosing exercises that promote earlier hamstring activation and thus, more protection of
the ACL. Our results suggest that the barrier jump front to back, barrier jump side to
side, and the scissor jump could be a safe exercises to use when beginning to incorporate
dynamic activities into an ACL rehabilitation program as they promote earlier activation
of the hamstrings and thus, increased stabilization of the knee joint.
In contrast to the barrier jump side to side, barrier jump front to back, and scissor
jump where the peak EMG flexion angle of the medial hamstrings was smaller than the
other muscles, during the lateral bounding exercise, the peak EMG flexion angle of the
medial hamstrings was significantly larger than the other three muscles. This means that
the combined quadriceps activate at a smaller flexion angle than the medial hamstrings
activate during this exercise. As previously discussed, several studies have shown that
ACL loading increases as the knee flexion angle decreases (Arms et al. 1984; Beynnon et
al. 1995; Li et al. 1999; Renstrom et al., 1986; & Boden et al., 2000). Furthermore,
Colby et al. (2000) compared quadriceps to hamstrings activation at various knee flexion
angles during dynamic activities including side step cutting, cross-cutting, stopping, and
landing and found that the peak quadriceps activity occurred at 22˚ of knee flexion
suggesting that increased strain is placed on the ACL at this angle. Since the quadriceps
are ACL antagonists, over-activation and earlier activation of the quadriceps in relation to
the hamstrings can result in increased strain and greater likelihood of ACL injury.
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Careful consideration should be used when determining when and/or whether to
incorporate bounding into an ACL rehabilitation program as it facilitates earlier
activation of the quadriceps and thus, increased strain and risk of injury to the ACL.
Finally, there were no statistically significant differences in peak EMG flexion
angles during the squat jump. The peak EMG flexion angles during this exercise are
similar between the quadriceps and hamstrings with peak muscle activation for both
muscle groups occurring at approximately 62˚ of knee flexion. As previous research has
found, most non-contact ACL injuries occur while an athlete is accelerating, decelerating,
cutting side to side, or pivoting (Noyes & Barber-Westin; Begalle, DiStefano, Blackburn,
& Padua 2012). Since the squat jump does not encourage any lateral motion during the
exercise, it appears that this could be the safest exercise to utilize earlier on in ACL
rehabilitation as it could facilitate a more balanced coactivation pattern between the
quadriceps and hamstrings throughout the exercise.
Based on our findings, it appears that the barrier jump front to back, barrier jump
side to side, and scissor jump may be safe exercises to utilize early on in an ACL
rehabilitation program as they facilitate earlier activation of the hamstrings, which is
necessary to provide posterior stabilization to the knee. In addition, the squat jump
facilitates peak muscle activity at similar knee flexion angles between the quadriceps and
hamstrings, suggesting that this exercise could also be safe to use in ACL rehabilitation.
Overall, it is important for athletes to master performing front to back motions, such as
the barrier jump front to back before attempting to perform lateral motions that require
more dynamic stability to protect the knee. Exercises such as lateral bounding should be
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used with caution, especially during the early stages of rehabilitation due to the earlier
activation of the quadriceps in relation to the hamstrings.
Recommendations and Limitations
Based on our initial findings, the most balanced (smallest) coactivation ratios
were observed during the squat jump and lateral bounding exercises. The largest (most
quadriceps-dominant) Q:H ratios were observed during the barrier jump side to side and
barrier jump front to back exercises. However, examining these ratios in isolation does
not provide the clinician with enough information to determine which exercises are the
most appropriate at various stages of the rehabilitation program. Although examining the
coactivation ratios was the primary purpose of this study, upon further examination of the
data, we determined that the timing of the peak muscle activity is a fundamental factor to
consider when prescribing these exercises.
Based on the coactivation ratio data, lateral bounding appeared to be the safest
exercise due to the smallest (most balanced) ratio. However, when examining the timing
of the peak quadriceps and hamstrings contractions, we determined that the hamstrings
are unable to stabilize the posterior aspect of the knee prior to the quadriceps contraction.
Therefore, it is our recommendation that this exercise should be used with caution due to
the potential for increased strain placed on the ACL. In contrast, the barrier jump front to
back had the highest (least balanced) ratio, but had earlier hamstring peak contraction
which provided stability to the knee. Overall, our data presents one of the challenges in
prescribing safe and effective exercise for ACL prevention and rehabilitation. Although
coactivation ratios are an important component to exercise prescription, without
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examining the timing of peak muscle contractions, the clinician cannot choose the
optimal exercise to retrain the muscles while also protecting the ligaments of the knee.
We examined muscle activation in a healthy, recreationally active population with
a small sample size of 18 subjects. Thus, a limitation of this study is that the results
cannot be generalized to the overall active population. In addition, because our
population was recreationally active, defined as 60 minutes of exercise at least 3 days a
week, the results cannot be generalized to other specific athletic populations, such as
collegiate athletes. Future studies should include a greater number of subjects with the
potential to study specific cohorts of athletes, such as soccer players, basketball players,
or volleyball players. In addition, there was some variability between the subject’s ratios.
A small number of subjects had ratios that were significantly larger (Q:H ratio=31.85:1)
than the mean rations, which could indicate that these subjects are more quadriceps
dominant, and thus more prone to ACL injury. Another limitation of this study was that
it only included 5 exercises. Although we tried to pick exercises that moved in all planes
of motion, playing sports such as soccer and basketball require an athlete to move in a
multitude of different directions quickly and our exercises may not cover every motion
associated with playing these sports. Finally, a limitation of our study was the method of
MVIC collection. The MVIC collection was conducted at the beginning of the study and
it was limited by the amount of manual force provided by the researcher. Future studies
should utilize the BioDex system for MVIC collection to ensure that the amount of force
provided is equivalent between subjects.
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Conclusions
We evaluated the Q:H coactivation ratios among five high velocity, closed chain
plyometric exercises, as well the knee flexion angles that coincide with peak muscle
activity. Results of our study identified that the barrier jump front to back, barrier jump
side to side, and scissor jump facilitated earlier activation of the hamstrings in relation to
the quadriceps suggesting that these exercises provide the most stability to the posterior
aspect of the knee, thus protecting the ACL. In contrast, lateral bounding facilitates
earlier quadriceps activation and therefore should be used with caution in the early stages
of ACL rehabilitation due to the anterior shear force placed on the ACL from the
quadriceps. Having knowledge of both the overall Q:H ratios as well as the timing of
peak muscle contraction allows for better exercise prescription and progression and could
also be used in injury prevention programs to decrease the likelihood of ACL injury or
re-injury.
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