Impact of prior hamstring strain injury & biofeedback on eccentric & isometric knee flexor strength Casey K.E Sims B. Exercise & Movement Science Master of Applied Science (research) Submitted in fulfilment of the requirement for the degree of Master of Applied Science (research) School of Exercise and Nutrition Sciences Faculty of Health Queensland University of Technology 2019
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Impact of prior hamstring strain injury & biofeedback on eccentric &
isometric knee flexor strength
Casey K.E Sims B. Exercise & Movement Science
Master of Applied Science (research)
Submitted in fulfilment of the requirement for the degree of
Master of Applied Science (research)
School of Exercise and Nutrition Sciences
Faculty of Health
Queensland University of Technology
2019
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3
Abstract
Hamstring strain injury (HSI) is the most prevalent non-contact injury in running-based
sports, and there have been considerable efforts to understand the aetiology of primary and
recurrent injuries. Rehabilitation and prevention programs have emerged from these
investigations with eccentric biased exercises, particularly the Nordic hamstring exercise
(NHE), showing powerful effects on injury rates, hamstring muscle architecture and eccentric
strength. The NHE is utilised reasonably widely in elite sport, however, there is minimal
understanding of the most effective way to prescribe the exercise. Successful randomised
controlled trials have employed as many as 3 sets of 10-12 repetitions per training session,
although it is uncommon to employ so many repetitions per set in elite sport and it is possible
that lower repetition schemes may induce less strength loss and allow more work to be
performed. Furthermore, the effects of a prior hamstring injury and performance feedback
(biofeedback) on the forces produced during the NHE are not known. Finally, a range of
strength tests has been employed to assess knee flexor strength and the torque-joint angle
relationship after a hamstring injury and this series of studies aimed to assess the value of
various bilateral and unilateral tests of dynamic and isometric strength.
Study 1 aimed to compare knee flexor forces and medial and lateral hamstring surface
electromyography (sEMG) during the performance of 30 repetitions of the NHE performed as
either 3 sets of 10 repetitions or 5 sets of 6 repetitions with knee flexor forces recorded via load
cells at the ankles. Fourteen recreationally active males participated in two sessions with
randomised presentation order of the two set and repetition schemes. There was no difference
between the average of the peak forces across 30 repetitions for the two prescriptions (mean
difference = 0.77N, 95% CI = -5.90 to 5.74; p = 0.97). Peak forces declined slightly between
the first and last two repetitions of both the 5x6 (mean change = -85.1N or 11.3%, 95%CI = -
4
166.24 to -3.957N; p = 0.04) and the 3x10 scheme (mean change = -69.3N or 9.3%, 95% CI =
-131.23 to - 7.38N; p = 0.03) but there was no significant difference between the extent of these
changes. Normalised BF sEMG was significantly lower during the 3x10 than the 5x6
prescription (mean difference = 0.15, 95% CI = 0.164 to 0.125; p <0.01).
The aims of Study 2 were to: 1) assess the effects of previous HSI on knee flexor force
production during 30 repetitions of the NHE (3 sets of 10 repetitions); 2) To assess the effects
of real-time visual force biofeedback on the knee-flexor forces and medial and lateral hamstring
sEMG; and, 3) To assess biceps femoris long head (BFlh) muscle architecture in previously
injured limbs. Twelve males with a history of unilateral HSI presented to the laboratory on
three occasions. All had reported to be fully recovered from their injuries as determined by a
full return to sport and training. The first visit was to scan the BFlh via 2D ultrasound. The
second and third visits involved the performance of 3 sets of 10 repetitions of the NHE, once
with real-time visual biofeedback of knee flexor forces at each ankle presented throughout the
session and once without feedback of any sort. Previously injured BFlh muscles had shorter
fascicles (mean difference = -0.74cm, 95% CI = -0.3 to -0.011; p = 0.0001) and larger pennation
angles (mean difference = 1.110, 95% CI = 0.43 to 1.84; p = 0.005) than uninjured contralateral
muscles. There were no significant effects of previous injury (mean difference = -2.5N, 95%
CI = -46.7 to 41.7; p = 0.910) or biofeedback (-9.91N, 95% CI = -72.48 to 52.64; p= 0.751) on
the average of the peak knee-flexor force outputs across the 30 repetitions. There was no
significant effects of feedback (mean difference = -9.91N, 95% CI = -72.48 to 52.64; p = 0.751)
or previous injury (mean difference = -2.50N, 95% CI = -46.7 to 41.7; p = 0.910) on sEMG;
however, the average normalised lateral hamstring sEMG across the 30 repetitions was
significantly lower than that of the medial hamstrings (mean difference = 28.30%, 95% CI = -
49.51 to -7.10; p= 0.01).
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Study 3 aimed to: 1) determine the association between a prior HSI and knee flexor
joint angle of peak torque (JAPT) and the sEMG – knee angle relationship, or muscle
architecture; and, 2) to compare knee flexor strength and sEMG during the NHE, Razor Curl
and bilateral and unilateral isometric maximal voluntary contractions. Eight recreationally
active males with a unilateral history of HSI visited the laboratory on three occasions. The first
visit was to acquire BFlh architecture measures via 2D ultrasound. The second visit involved
isometric testing (to construct a torque-joint angle relationship for the knee flexors) on an
isokinetic dynamometer and the final visit involved a testing battery of the NHE, Razor curl
and prone unilateral and bilateral isometric contractions with maximum forces determined via
loads cells as per the previous studies. Muscle architecture measures showed significantly
shorter BFlh muscles fascicles (mean difference = -0.74cm, 95% CI = -0.27 to -1.23; p= 0.008)
and greater pennation angles (mean difference = 1.470, 95% CI = 1.0 to 1.93; p = <0.001) in
previously injured than uninjured limbs. Previously injured limbs did not differ in terms of
normalised isometric torque output in the seated isometric tests compared to the contralateral
uninjured limbs across the five angles tested on the dynamometer (mean difference = -0.03Nm,
95% CI = -0.34 to 0.29; p = 0.854) and no significant between-limb differences in force were
noted at any joint angle (p > 0.7 for all comparisons). The previously injured limbs were not
weaker than uninjured limbs in the NHE (mean difference = -7.4N, 95% CI = -28.8 to 14.1; p
= 0.44)), Razor curl (mean difference = 27.6N, 95% CI = 12.6 to -67.7; p = 0.149). The
difference between bilateral and unilateral strength was significant for the uninjured limbs
(mean difference = 29.7N, 95% CI =10.6 to 48.8; p = 0.008), but not the previously injured
limbs (mean difference = 13.0N, 95% CI = -5.2 to 31.1; p = 0.136).
6
Despite the force output being higher in the Razor curl than the NHE, sEMG was higher
in the NHE for both the lateral (mean difference = 27.5%, 95% CI = 17.9 to 27.0; p = <0.001)
and medial hamstrings (mean difference = 25.3%, 95% CI = 12.4 to 38.2; p = 0.002). The
medial hamstrings were significantly more active than the lateral in both exercises (NHE mean
difference = 32.4%, 95% CI = 7.0 to 57.9; p = 0.02; Razor curl mean difference = 34.6, 95%CI
= 8.1 to 61.2; p = 0.018).
The program of research has presented some novel data that contributes to evolving
knowledge of HSI. For individuals familiar with the NHE, there appears to be small and similar
levels of force loss, as determined by peak forces across an exercise session, regardless of
whether 30 repetitions are arranged in sets of 6 or 10 repetitions. Limbs with a history of
relatively mild hamstring strains appear to have no significant eccentric or isometric strength
deficits, no increase in strength loss during 30 repetitions of the NHE exercise and no
differences in the knee-flexor JAPT despite exhibiting differences in muscle architecture by
comparison with contralateral uninjured limbs.
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Table of Contents Abstract ............................................................................................................................... 3
List of Figures...................................................................................................................... 9
List of tables ...................................................................................................................... 11
List of abbreviations ......................................................................................................... 12
STATEMENT OF ORIGINAL AUTHORSHIP ............................................................. 13
CHAPTER 4: The Effects of Biofeedback and Previous Hamstring Injury on Eccentric Strength During a Nordic Hamstring Training Session .................................................. 63
4.1 Research Design ........................................................................................................ 63
CHAPTER 5: The Effect of Previous Hamstring Strain Injury on Isometric and Dynamic Strength Profiles of the Knee-Flexors ............................................................... 85
5.1 Research Design ........................................................................................................ 85
FIGURE 15: Knee-Flexor Eccentric Force Across Time for NHE ..................................... 73
FIGURE 16: Average summed eccentric knee-flexor force per repetition of the NHE ........ 74
FIGURE 17 Comparison between feedback conditions for all 30 repetitions combined ..... 75
FIGURE 18: Averages of prone knee-flexor isometric contractions pre and post NHE exercise .......................................................................................................................................... 76
FIGURE 19: The effect of feedback on normalised sEMG for injured vs uninjured and medial
vs lateral hamstrings .......................................................................................................... 78
FIGURE 20: Normalised sEMG for medial and lateral hamstrings during 30 NHE repetitions
with and without feedback. ................................................................................................ 73
Table 3: Architectural characteristics of biceps femoris long head muscles in previously injured and uninjured limbs. ............................................................................................. 71
Table 4: Pre and Post Exercise Isometric Contractions – Descriptive Statistics ............... 77
Table 5: Hamstring injury history information for all participants. ................................... 89
Table 6: Architectural characteristics of biceps femoris long head muscles in previously injured and uninjured limbs. ............................................................................................. 90
12
List of abbreviations
AFL Australian Football Leauge
BF Biceps Femoris
BFlh Biceps Femoris long head
BFsh Biceps Femoris short head
CI Confidence Interval
EMG Electromyography
EMGBF Electromyography Biofeedback
H:Q Hamstrings to Quadriceps
HIS Hamstring Strain Injury
IAAF
International Amateur Athletics
Federation
JAPT Joint Angle of Peak Torque
MH Medial Hamstring
MRI Magnetic Resonance Imagery
MVC Maximal Voluntary Contraction
MVIC
Maximal Voluntary Isometric
Contraction
NFL National Football League
NHE Nordic Hamstring Exercise
ROM Range of Motion
RR Relative Risk
sEMG Surface Electromyography
UEFA Union of European Football Associations
VL Vastus lateralis
VMO Vastus medialis oblique
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STATEMENT OF ORIGINAL AUTHORSHIP
The work contained in this thesis has not been previously submitted to meet requirements
for an award at this or any other higher education institution. To the best of my knowledge and
belief, the thesis contains no material previously published or written by another person except
where due reference is made.
Signature: QUT Verified Signature
Date: February 2019
ACKNOWLEDGMENTS
Firstly I’d like to thank my main supervisor Associate Professor Tony Shield for his
unwavering patience and support for the duration of this research program. From reading
countless drafts to answering even the smallest questions, he was there and always willing to
help. In addition associate supervisor Dr Geoffrey Minett for his timely and extremely helpful
feedback and direction to steer onto the right path. I’d also like to thank the Hamstring Strain
Injury Research Team for all their help in the laboratory and guidance in completing this thesis.
Lastly, a thank you to the participants for their contribution to this work and without them, this
wouldn’t have been possible.
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Chapter 1: INTRODUCTION
Sporting injuries are common and considerable efforts have been made to quantify their
effects on team success and finances (Eirale, Tol, Farooq, Smiley, & Chalabi, 2013; Ekstrand,
1984). Increasing the VMO/VL sEMG ratio between the aforementioned muscles is proposed
to reduce symptoms of patellofemoral pain, reduce the risk of re-injury after ACL
reconstruction and arthroscopic meniscectomy (Ingersoll & Knight, 1991; Kirnap et al., 2005).
FIGURE 1: Changes in VMO/VL sEMG ratio for an exercise group and an exercise + biofeedback group in patellofemoral pain sufferers with and without sEMG biofeedback. There was a significant improvement only in the exercise + biofeedback group (Ng et al., 2008).
Kinrap and colleagues (2005) added EMG biofeedback of VMO and VL muscles to the
post arthroscopic meniscectomy rehabilitation for 20 patients. The feedback group (n=20)
exhibited a significant increase in maximum isometric strength and increased scores on the
Tegner-Lysholm knee function test compared to the traditional rehabilitation control group
(n=20). Patellofemoral pain patients also benefited from biofeedback correction of voluntary
41
activation levels between VMO to VL (Ng et al., 2008). The pre- and post-testing involved
collecting EMG data from participants as they conducted six hours of activities of daily living.
The biofeedback group displayed a significantly greater VMO/VL ratio compared to baseline,
while the non-biofeedback group had insignificant changes as seen in Figure 1.
The results of Ng et al. (2008) and Kinrap et al. (2005) are consistent with others aiming
to selectively increase muscle activation or strength in affected lower limbs to aid in the
rehabilitation process (Akkaya et al., 2012; Draper & Ballard, 1991; Ingersoll & Knight, 1991;
Jeyanthi, Natesan, & Manivannan, 2014; Wise et al., 1984). Nevertheless, there is some
evidence to refute the efficacy of biofeedback in a clinical setting with further research with
larger sample sizes are required to determine the efficacy of biofeedback in rehabilitation
(Giggins et al., 2013; Lepley et al., 2012). Furthermore, the application of biofeedback to HSI
is limited or non-existent.
Biofeedback has not been limited to rehabilitation of injured limbs. Healthy subjects
have also used biofeedback to increase strength, power and muscle activation (Croce, 1986;
SD = Standard deviation CoV = Coefficient of variance
There were no observed differences in the average of summed peak eccentric knee-
flexor forces across 30 repetitions between the two exercise protocols, (mean difference = -
50
0.77N, 95% CI = -5.90 to 5.74; p= 0.97) as seen in Figure 5. There were significant declines in
strength between the first repetitions (1 & 2) and the last repetitions (29 & 30) with both
protocols, with a mean decline of -85.1N (11.3%), (95% CI = -166.24 to -3.957; p = 0.04)
during the 3x10 protocol and -69.3N (9.3%), (95% CI = -131.23 to -7.38; p = 0.03) during
the 5x6 protocol.
FIGURE 4: Average of summed eccentric knee-flexor forces for all 30 repetitions for the 3x10 and 5x6 protocols. Bars represent means and error bars represent the standard deviation.
0
100
200
300
400
500
600
700
800
900
Sum
med
Ecc
entr
ic K
nee-
Flex
or F
orce
(N)
Protocol
3 x 10 repetitions5 x 6 repetitions
51
FIGURE 5: Eccentric knee flexor forces in the first two and last two repetitions of the 3x10 and 5x6 protocols. Bars represent means and error bars represent the standard deviation. * p = 0.04, ** p = 0.03 for the differences between the forces generated in the first and last two repetitions for each protocol.
* **
52
FIGURE 6: Averaged summed eccentric knee-flexor force per repetition for each exercise prescription Bars represent group means, error bars represent the standard deviation (SD)
The summed knee flexor forces reached their peaks at 40o ± 15o and 45o ± 12o in the first
two repetitions of the 5x6 and 3x10 protocols, respectively. The corresponding knee angles in
the final two repetitions were 47o ± 15o and 49o ± 15o of the 5x6 and 3x10 protocols,
respectively. There was no significant main effect for protocol on knee angles of peak force
(mean difference = 3o; 95% CI = -3o to 9o, p = 0.345). There was no main effect for time on
knee angle of peak force (mean difference = 5o; 95% CI = -1o to 11o, p = 0.091). However, for
the final two repetitions of the 5x6 protocol, knee angle of peak force was significantly more
flexed than the first two reps (mean difference = -7o, 95% CI = -11o to -2o, p = 0.013). No
significant difference in knee flexion angle was observed between the first and last two
repetitions of the 3x10 protocol (mean difference = -3o, 95% CI = -13o to 6o, p = 0.438).
FIGURE 7: Averaged knee angle at peak summed force for each exercise prescription Bars represent group means, error bars represent the standard deviation (SD) * = 0.013
0
10
20
30
40
50
60
70
First Repetitions Last Repetitions
Kne
e Ang
le a
t Sum
med
pea
k fo
rce
(deg
rees
/seco
nd) 3x10
Repetitions
5x6Repetitions
*
54
FIGURE 8: Averaged knee angle velocity at peak summed force for each exercise prescription - Bars represent group means, error bars represent the standard deviation (SD) * = 0.001 ** = 0.004
Knee angle velocity at peak summed force
The summed knee flexor forces reached their peaks whilst falling at a velocity -20o.s-1±
16o.s-1 and -17o.s-1 ± 8o.s-1 in the first two repetitions of the 5x6 and 3x10 protocols,
respectively. The corresponding velocities in the final two repetitions were -40o.s-1 ± 14o.s-1 and
-38o.s-1 ± 38o.-1 of the 5x6 and 3x10 protocols, respectively. There was no significant main
effect for protocol on knee angles of peak force (mean difference = 2o; 95% CI = -13o to 17o,
p = 0.722). There was a main effect for time on the velocity at peak summed force (mean
difference = 20o.s-1; 95% CI = 8o.s-1 to 33o.s-1, p = 0.004). More specifically, the 5x6 protocol
showed a significant increase in the velocity at peak summed force between the first two
repetitions and the last two repetitions (mean difference = 20o.s-1; 95% CI = 10o.s-1 to 31o.s-1,
p = 0.001). However, despite it being very similar in size, the increase in velocity observed
-55
-45
-35
-25
-15
-5
First Repetitions Last RepetitionsVe
loci
ty a
t Sum
med
Pea
k Fo
rce
(deg
rees
/seco
nd)
3x10Repetitions
5x6Repetitions
** *
55
within the 3x10 protocol was not statistically significant (mean difference = 21o.s-1; 95% CI =
-0.1o.s-1 to 42o.s-1, p = 0.051).
Surface Electromyography
Normalised sEMG, as an average across all 30 repetitions, was not significantly
different for the medial hamstrings between the 3x10 (0.92 ± 0.05) and 5x6 (0.94 ± 0.05)
protocols (mean difference = 0.02, 95% CI = -0.048 to 0.001; p = 0.054). In contrast, the
normalised BFlh EMG was significantly lower for the 3x10 (0.71 ± 0.03) compared to 5x6
(0.85 ± 0.04) protocol across the 30 repetitions (mean difference = 0.14, 95% CI = 0.164 to
0.125; p <0.01) (see Figures 9 and 10).
56
FIGURE 9: Average normalised sEMG for all 30 repetitions of the 3x10 and 5x6 protocols. Bars represent means and error bars represent standard deviation. MH = Medial Hamstrings, BF = Biceps Femoris. * p < 0.01.
0
0.2
0.4
0.6
0.8
1
1.2
Bflh nEMG MH nEMG
nEM
G
3x10 repetitions
5x6 repetitions
*
57
FIGURE 10: Normalised Surface EMG for 30 repetitions for the 3 x 10 and 5 x 6 protocols, for the BF and MH. Error bars are omitted for the sake of clarity.
Warren et al., 2002). Conventional fatigue from metabolic stress is proposed to be the largest
contributor to impaired performance in repeated concentric and isometric contractions whereas
muscle damage is observed to be the main contributor to impaired performance after repeated
eccentric contractions (Warren et al., 2002). The proposed mechanism behind muscle damage
59
induced fatigue post eccentric activity is the disruption and consequential dysfunction of the
excitation-contraction coupling mechanism (Warren et al., 2002). Strength losses caused by
fatiguing concentric or isometric activities are observed during and immediately post exercise,
and they tend to recover rapidly upon cessation of activity. By contrast, the loss in maximal
torque generating capacity caused by repeated eccentric contractions may be small to moderate
acutely and develop later and last significantly longer, even up to 96 hours (McNeil et al.,
2004). In conventional exercise involving significant concentric or isometric contractions, the
work to rest ratios significantly influence muscle fatigue (Bigland-Ritchie, Rice, Garland, &
Walsh, 1995; McNeil et al., 2004). However, eccentric contractions seem to be largely
unaffected by rest periods between the repeated efforts, further reinforcing that muscle damage
is the main contributor to observed strength loss or lack thereof (McNeil et al., 2004; Teague
& Schwane, 1995). In the current study, which employed participants who were familiar
(within the previous month) with the performance of the NHE, the decline in peak forces was
very modest, and this suggests that either exercise session induced minimal muscle damage.
The results of this study provide novel insight into the effects of fatigue on performance
of the NHE. The two protocols used, 3 sets of 10 repetitions and 5 sets of 6 repetitions, had no
significantly different effects on strength loss. This suggests the possibility that neither protocol
has obvious advantages over the other, although the higher repetition option would allow 30
repetitions to be performed in a shorter time frame.
Prior to this work no study had assessed the strength losses incurred during NHE
without confounding effects from testing protocols such as repeated eccentric or concentric
efforts. One group has investigated the acute fatiguing effects of the NHE by having
participants perform 6 sets of 5 repetitions and assessing strength between sets with concentric
and eccentric efforts. Marshall and colleagues (2015), found after the first set of NHE, a
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significant 7.9 - 17% reduction in eccentric torque with no further significant reductions for
the subsequent sets (Marshall, Lovell, Knox, Brennan, & Siegler, 2015). The authors did note
that significant differences from pre-exercise values were found during eccentric actions in the
later 60 degrees of the range of motion and throughout concentric actions. As only eccentric
forces were measured in the current study, it is not possible to know whether the participants
experienced a loss of concentric strength.
In the current study, peak knee flexor forces declined to a small extent across the 30
maximal repetitions of the NHE while the velocity at which these forces were reached changed
more significantly. Given the shape of the force-velocity curve for slow and medium speed
eccentric actions, it seems likely that reductions in force generating capacity were counteracted
by increases in knee angle velocity. Lengthening actions at approximately -20o.1 are typically
weaker than those performed at -40 to -60o.1, and these velocities were observed in the current
investigation. This effect of velocity may lead to under estimation of the strength loss incurred
during the NHE (Westing, Serger, Kalson, Ekblom, 1998).
One statistically significant difference between the two exercise protocols was the level
of normalised BFlh sEMG. While the normalised MH sEMG did not differ between protocols,
there was a lower BFlh EMG in the 3 sets of 10 protocol than the 5 sets of 6 protocol. The
practical significance of this difference, given that forces were not different between protocols,
is unclear. However, this may suggest some potential advantages to performing fewer
repetitions per set in this exercise, but more investigation is required before this can be stated
with certainty. Any potential advantages of a lower repetition prescription should be
investigated because 3 sets of 10 are prescribed in weeks 5 to 10 of the hamstring prevention
programs employed in the Nordic hamstring RCTs (Petersen et al., 2011; van der Horst et al.,
2015b). It is thought that at least some of the benefits of eccentric training are mediated by
61
architectural changes such as the addition of in-series sarcomeres (and resulting lengthening of
fascicles) within the BFlh (Bourne et al., 2016). Higher levels of BFlh activation with the lower
repetition range employed in this study might, on their own suggest some advantages for injury
prevention programs but more work is needed to substantiate this, and the lack of a difference
in force outputs is difficult to reconcile with the differences in sEMG.
The optimal placement of hamstring injury prevention exercises within training
sessions isn’t known, but some injury prevention programs such as the FIFA 11+ prescribe
NHE in addition to 11 other exercises during the warm-up period (Bizzini et al., 2013;
Impellizzeri et al., 2013). While the FIFA 11+ plus program doesn’t prescribe 30 repetitions,
it does call for up to 12 repetitions in a single set. If there is an element of hamstring fatigue or
weakness from the NHE in addition to the other exercises performed, it is possible that these
muscles may be placed at risk in the following training session. The BFlh goes through the
greatest amount of strain of all hamstring muscles (Chumanov et al., 2011; Schache et al., 2012;
Thelen et al., 2005) during the late swing phase of gait with the hamstring muscle group acting
eccentrically to decelerate the forward swinging shank (Chumanov et al., 2011; Dolman et al.,
2014; Thelen et al., 2005). If the hamstring muscle group is weakened, it would potentially
reduce the capacity to decelerate the shank. Reduced deceleration capacity may stretch muscle
fibres and their sarcomeres beyond optimal lengths, resulting in an accumulation of
microscopic tears which could potentially accumulate and then result in HSI. The current
results suggest, however, that strength loss from a single set of 10 repetitions or two sets of six
repetitions is minimal in athletes who are accustomed to performing this exercise.
Having the capacity to measure eccentric strength during the NHE and the muscle
activation via sEMG, in real-time, has provided some insight into the acute fatigue effects of
the NHE on the hamstring muscle group. However, there are some limitations to this
62
investigation that should be acknowledged. The current study utilised sEMG to attain insight
into muscle activation and this method is prone to cross-talk so that some of the signal is
actually ‘noise’ from adjacent muscles. Functional magnetic resonance imaging (fMRI) would
allow a measure of muscle activation with better spatial resolution, but this technique does not
provide a measure of muscle activation changes across time in the way that sEMG can. Another
limitation arises from the recruitment of recreationally active participants because it is not clear
whether these results would translate to elite sporting populations. However, the current
participants were accustomed to performing this exercise, and they had similar or even slightly
better eccentric strength than elite athletes in previous studies from this group (Opar et al.,
2105; Timmins et al., 2016).
While further investigation into the appropriate prescription and programming of the
NHE is needed, the current findings suggest that strength loss is minimal in recreational athletes
who are familiar with the exercise and that the prescription of 6 or 10 repetitions does not
appear to significantly influence the knee flexor forces or the degree of strength loss exhibited
during exercise sessions.
LINKING PARAGRAPH
We now know the effects of the NHE on healthy recreational athletes without a history
of HSI. However, it is not clear whether athletes with a history of HSI will display the same
responses. Previous history of HSI has been shown to increase the degree of force loss in bouts
of concentric knee-flexor exercise (Lord, Ma'ayah, & Blazevich, 2018), but we do not currently
know the effect of prior HSI on eccentric strength decline during exercise such as the NHE.
63
CHAPTER 4: The Effects of Biofeedback and Previous Hamstring Injury on Eccentric Strength During a Nordic Hamstring Training Session
4.1 RESEARCH DESIGN
4.1.2 OBJECTIVES
1) To assess the effects of previous HSI on knee-flexor force production and hamstrings
sEMG during the performance of 30 repetitions of the NHE.
2) To assess the effects of real-time visual force biofeedback on knee-flexor forces and
hamstring muscle activation during the performance of 30 repetitions of the NHE.
3) To assess the biceps femoris architecture (fascicle length, fascicle length, and
pennation angle) in previous injured compared to the uninjured contralateral limb.
4.1.3 PARTICIPANTS
Fourteen recreationally active adult males with a history of unilateral HSI were
recruited for this investigation. Two participants were excluded from data collection after
familiarisation because of injuries sustained during participation in sporting activities unrelated
to this study. The sample size was chosen based upon previous and similar research conducted
in the laboratory. The participants gave informed consent to participate in the study which was
approved by the Queensland University of Technology Human Research Ethics Committee
(approval number 1600000767).
4.1.4 METHODOLOGY
Participants visited the laboratory on three occasions with the first and second visit
separated by one week and the second and third visit separated by three weeks. During the first
64
session, participants had ultrasound images taken midway along the longitudinal axis of the
BFlh muscle belly to assess fascicle length and pennation angle.
Images were taken using two-dimensional B-mode ultrasound (frequency, 12 MHz;
depth, 8 cm; field of view, 14×47 mm) (GE Healthcare Vivid-i, Wauwatosa, USA). The
scanning site was determined as the halfway point between the ischial tuberosity and the knee
joint fold, along the line of the BFlh. All architectural assessments were performed with
participants in a prone position and the hip neutral following at least five minutes of inactivity.
Following the ultrasound scans, participants warmed up by cycling at a submaximal pace for
five minutes. Participants then performed three to six repetitions of the NHE (NHE), under the
instruction of the investigator, to familiarise themselves with the appropriate technique
requirements of the exercise.
On the second and third visits to the laboratory, participants performed three sets of ten
repetitions of the NHE, once with (Figure 10) and once without visual feedback (Figure. 9) of
force production at the ankle. The order of presentation was randomised. The exercise
prescription of three sets of ten repetitions was chosen from the study in Chapter 3. The
prescription showed minimal changes in force out puts comparative to the alternate prescription
and is a common prescription observed in RCTs previously conducted by Mjolsnes et al.,
(2014) and Petersen et al., (2015). Upon arrival at the laboratory, participants initially
performed a warm-up consisting of five minutes of sub-maximal ad self-paced cycling.
Participants then performed two isometric maximal voluntary contractions of the knee-flexor
muscles in a prone position with one-minute rest between repetitions. Three maximal
repetitions of the Nordic Hamstring Curl were then performed followed by a five-minute rest,
after which participants completed three sets of ten repetitions of the NHE with one minute
rests between sets. Two prone, maximal isometric voluntary contractions of the knee-flexors
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were performed after the NHE, one immediately after the final repetition of the NHE and the
other one minute later.
FIGURE 11: Nordic hamstring exercise on hamstring testing device. Participants pull against ankle restraints as they resist falling.
FIGURE 12: Nordic hamstring exercise with feedback displayed on a laptop in front of participant performed on hamstring testing device. Participants pull against ankle restraints as they resist falling.
An electronic goniometer (PRV6 5K potentiometer) fixed to the left knee measured
knee angle throughout the exercise sessions. The knee-flexor forces at the ankles were
measured with uniaxial load cells (MLP-1K, Transducer Techniques, CA, USA) attached in-
66
series to ankle straps. The middle of each ankle strap was aligned with the lateral malleoli, and
the ankle restraints were vertical and perpendicular to the shank.
Surface electromyographic (sEMG) information was collected from the lateral and medial
hamstrings via bipolar pre-gelled Ag/AgCl sEMG electrodes (10 mm diameter, 20 mm inter-
electrode distance) placed over the appropriate muscle bellies halfway between the ischial
tuberosity and tibial epicondyles. The reference electrode was placed on the ipsilateral head of
the fibula.
All sEMG, force and joint angle data were sampled at 1000 Hz via a 16-bit PowerLab
26T AD unit (amplification = 1000; common mode rejection ratio = 110 dB) and analysed
using LabChart 7 (ADInstruments, New South Wales, Australia). Raw sEMG data were filtered
using a Bessel filter (frequency bandwidth=10–500 Hz) and then full-wave rectified and
smoothed over 100ms windows. Knee angle data were low pass filtered at 4 Hz. All data was
then transferred to a personal computer.
During the feedback session, participants were shown force traces from their left and
right legs superimposed in a single channel in real time (Figure 11) on a 1 7inch monitor for
easier reading. The overlay trace was derived from the real time left and right force traces
superimposed upon each other in the Labchart 7 software (ADInstruments, New South Wales,
Australia). Participants were instructed to “activate the injured limb to match its force trace to
your uninjured limb, without sacrificing the strength of your uninjured limb”. The force data
was presented to the participants at eye level via a computer screen.
67
FIGURE 13: Left leg (red in the top trace), right leg (blue in the middle trace) and left and right leg forces both shown within the one trace (bottom) for three consecutive repetitions of the Nordic hamstring exercise.
4.1.5 DATA ANALYSIS
Strength Measures
The peak forces (N) for each repetition of the NHE for each participant were derived
by taking the maximum value for the left and right limb at the peak of summed force for each
repetition. The data was then exported into Microsoft Excel and SPSS for statistical analysis.
Data for the prone maximal isometric contractions (MVIC) was the average force obtained
from the 0.5 seconds of data surrounding the peak force (0.25 seconds either side of the peak)
from each contraction. All isometric contractions were performed twice with the results
averaged. For the purposes of reviewing changes in force across the 30 repetitions of the NHE,
the first two repetitions and the last two repetitions of each set were averaged and are denoted
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by S1 (start or first two repetitions of set 1), E1 (end or last two repetitions of set 1), S2, E2,
S3 and E3.
Between limb asymmetry in force was derived from the point of peak summed force.
The injured limb’s force was compared to the uninjured limb’s force as a raw value and as a
percentage value.
Surface Electromyography
sEMG data were averaged across the 0.5 seconds of data before the peak in all dynamic
exercise efforts (NHE). The sEMG data obtained from the NHE was then normalised to that
obtained from the pre-exercise maximal isometric contractions. For all prone isometric
contractions, sEMG data were averaged across the 0.5 second period surrounding the peak
force. All isometric contractions were performed twice, and the average of the two contractions
was used for analysis.
Muscle Architecture
The MicroDicom software (Version 0.7.8) was used for analysing the ultrasound
images collected. Due to the limited view of the ultrasound probe a muscle fascicle of interest
was outlines and the muscle fascicle length was determined using the equation: FL=sin
(AA+90°) x MT/sin(180°-(AA+180°-PA)). Muscle Thickness (MT was defined by the as the
distance between the superficial and intermediate aponeuroses of the BFLH). The ultrasound
technician was blinded to all participants’ previous injury data.
4.1.6 STATISTICAL ANALYSIS
Strength Measures
Statistical analysis was performed using SPSS version 23.0.1 (IBM Corporation,
Chicago IL). The normality of the data was assessed with the Shapiro-Wilks test. Sphericity
69
was assessed via Mauchly's test and when sphericity was not observed the Huynh-Feldt
correction was used.
A repeated measures time (start of set 1 (S1), end of set 1 (E1), S2, E2, S3, E3) by leg
(injured v uninjured) by feedback condition analysis of variance (ANOVA) was employed to
assess whether there were differences in knee flexor force production.
Prone isometric strength was analysed via a time (Pre 1, Pre 2, Post 1, Post 2), limb
(injured vs uninjured) and condition (feedback vs no feedback) repeated measures analysis of
variance (ANOVA). When significant main effects were detected, post hoc t-tests with
Bonferroni corrections were employed for pair-wise comparisons. The mean differences were
reported with their 95% confidence intervals (CIs).
Surface Electromyography
Analysis of the prone isometric exercises was performed by a limb (injured vs
uninjured) by time (Pre-, post 0 and post 1 minute) by muscle (lateral and medial) repeated
measures analysis of variance (ANOVA).
Smoothed rectified sEMG data was averaged over 0.5 seconds before the peak forces
in all dynamic exercise efforts (NHE). The sEMG data obtained from the NHE was then
normalised to that obtained from the prone maximal isometric contractions at the knee angle
of 00. For all prone isometric contractions, sEMG data was averaged over the 0.5 seconds
surrounding the peak force of each contraction, (0.25 seconds either side of the peak).
When significant main effects were detected from ANOVAs, post hoc t-tests with
Bonferroni corrections were employed for pair-wise comparisons. The mean differences were
reported with their 95% confidence intervals (CIs).
70
Muscle Architecture
Measures of muscle architecture (fascicle length, muscle thickness, and pennation
angle) were analysed by comparing the injured to uninjured limbs via paired sample t-tests.
The mean differences were reported with their 95% confidence intervals (CIs).
4.2 RESULTS
Participants
All participants (age 23.5 ± 1.8 years, height 179.2 ± 5.7cm, body mass 83.1 ± 6.8 kg)
had a history of unilateral HSI to their right limb only, within the past 24 months. The average
time since the most recent insult was 8.3 ± 6.4 months with an average recovery time of 4.8 ±
2.9 weeks. Details of the participant’s injury information can be found in Table 2.
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Table 3: Hamstring injury history information for all participants
Participant Injured Limb
Muscle injured
Number of HSIs
Months Since last HSI
Grade of last HSI
Rehabilitation time (weeks)
1 Right ST 2 18 2 10
2 Right BFlh 1 2 1 5
3 Right BFlh 2 2 1 5
4 Right BFlh 1 2 1 2
5 Right BFlh 3 12 2 4
6 Right BFlh 1 2 1 4
7 Right BFlh 3 18 1 4
8 Right BFlh 1 5 1 12
9 Right BFlh 2 12 1 3
10 Right BFlh 3 6 2 5
11 Right BFlh 1 18 1 3
12 Right BFlh 2 8 1 2
BFlh: Biceps Femoris Long Head ST: Semitendinosus
Rehabilitation time was defined by the length of time that it took to return to full training or
competition. The severity (grade) was determined by the American Medical Association's
guidelines (Craig, 1973)
72
Biceps femoris long head architecture
BFlh fascicles were significantly shorter (mean difference = -0.74 cm, 95% CI = -0.3
to -1.13; p = 0.001) and pennation angles significantly higher (mean difference = 1.11o, 95%
CI = 0.43 to 1.84; p = 0.005) in previously injured than the uninjured limbs. Muscle thickness
was not significantly different between limbs (mean difference = 0.014cm, 95% CI =-0.09 to
0.12; p = 0.769) (see Table 3).
Table 4. Architectural characteristics of biceps femoris long head muscles in previously injured and uninjured limbs.
There was no observed effect of previous injury on the knee-flexor force output across
the 30 repetitions of the NHE according to the mean forces obtained from the 30 repetitions
(Figure 13; mean difference = -2.50N, 95% CI =-46 .7 to 41.7; p = 0.910). There were
significant reductions in the mean knee-flexor force output over with time, although these did
not differ between injured and uninjured limbs. The mean force output declined from time point
1 (repetitions 1 & 2) to time point 4 (repetitions 19 & 20) with a mean difference of 27.8N
(95% CI = 17.8 to 41.3; p = <0.001). A statistically significant difference in mean force output
was observed between the 1st time point and time points 5 (repetition 21 & 22) (mean
difference = 35N, 95% CI= 17.8 to 52.1; p = <0.001) and 6 (repetitions 29 & 30) (mean
difference = 43.6N, 95% CI = 28.8 to 58.3; p = <0.001), respectively. There was also a
significant reduction in knee-flexor force between time point 3 (repetition 11 & 12) and 4
(repetitions 19 & 20) (mean difference = 15.2N (95% CI = 6.8 to 24.3; p = <0.001).
FIGURE 14: Knee flexor strength comparison between limbs (N) of all NHE repetitions performed. Bars represent group means, error bars represent standard deviation (SD).
0
50
100
150
200
250
300
350
400
Kne
e-Fl
exor
For
ce (N
)
UninjuredInjured
74
FIGURE 15: Knee-flexor eccentric force across time for NHE Bars represent group means, error bars represent standard deviation (SD) * = P <0.001
0
50
100
150
200
250
300
350
400
450
Time Point 1 (Rep 1&2)
Time Point 2 (Rep 9& 10)
Time Point 3 (Rep11 & 12)
Time Point 4 (Rep19 &20)
Time Point 5 (Rep21 &22)
Time Point 6 (Rep29 & 30)
Ecc
entr
ic K
nee-
Felx
or F
orce
(N)
No Feedback
Uninjured
No Feedback Injured
Feedback Uninjured
Feedback Injured
Total
*
*
*
*
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FIGURE 16: Average summed eccentric knee-flexor force per repetition of the NHE Bars represent group means, error bars represent standard deviation (SD)
There was no observed effect of real-time feedback throughout the NHE protocol on
peak knee-flexor forces across the 30 repetitions in the previously injured (mean difference =-
9.918N, 95% CI = -72.48 to 52.64; p = 0.751) or uninjured limbs (mean difference = 2.47N,
95% CI = -60.08 to 65.42; p = 0.937).
FIGURE 17: Comparison between feedback conditions for all 30 repetitions combined. Bars represent group means, error bars represent standard deviations (SD)
0
50
100
150
200
250
300
350
400
Ecce
ntri
c K
nee-
Flex
or F
orce
(N)
No FeedbackFeedback
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Table 4: Pre and Post Exercise Isometric Contractions – Descriptive Statistics
Isometric knee-flexor strength immediately post-exercise was not significantly
different from pre-exercise values (mean difference = -21.9, 95% CI -59.7 to 15.9; p=0.539).
However, there was a small and significant increase in isometric force between the two post-
exercise contractions (mean difference 20.8N, 95% CI = 4.5 to 37.2; p= 0.011). More
specifically, the uninjured limb showed a significant increase between the second and first post-
exercise contractions (mean difference = 26.5N, 95% CI = 10.5 to 42.6; p=0.002), while the
injured limb did not (mean difference = 15.2, 95% CI = 6.1 to 36.5; p= 0.259). The use of
feedback had no observed effect the knee-flexor isometric forces averaged across the four tests
(Pre 1, Pre 2, Post 1, Post 2) performed (mean difference -8.3N, 95% CI = -35.9 to 19.4; p=
0.523).
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FIGURE 18: Averages of prone knee-flexor isometric contractions pre and post NHE exercise. Bars represent group means, error bars represent standard deviation (SD)
Surface Electromyography
When averaged across the entire 30 repetitions, the normalised sEMG activity from the
lateral hamstring muscles was significantly lower than the medial hamstring muscle group
(mean difference = -28.30, 95% CI = -49.51 to -7.10; p = 0.01). Previous injury had no observed
effect on the normalised sEMG in either the lateral (mean difference = -1.13, 95% CI =-22.43
to 20.15; p = 0.90) or medial (mean difference = -18.49, 95% CI =-49.13 to 12.14; p = 0.90)
hamstring muscles, in either feedback condition. The use of feedback had no effect on the
normalised sEMG on either lateral (mean difference = -4.46, 95% CI = -29.58 to 20.65; p =
0.70) or medial hamstrings for all repetitions (mean difference = -2.13, 95% CI = -29.73 to
20.65; p = 0.703).
FIGURE 19: The effect of feedback on normalised sEMG for injured vs uninjured and medial vs lateral hamstrings. Bars represent group means, Error bars represent standard deviation (SD)
80
0
20
40
60
80
100
120
140
160
Repetition 1&2 Repetition 9&10 Repetition11&12
Repetition19&20
Repetition21&22
Repetition29&30
norm
alise
d sE
MG
(%)
FIGURE 18: Normalised sEMG for medial and lateral hamstrings during 30 NHE repetitions with and without feedback.
LL Lateral No FeedbackLL Medial No FeebackRL Lateral No FeedbackRL Medial No FeedbackLL Lateral FeedbackLL Medial FeedbackRL Lateral FeedbackRL Medial Feedback
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4.3 DISCUSSION
The use of the NHE is now relatively common in strength and conditioning programs
(Oakley, Jennings, & Bishop, 2017). The NHE has proven effective in reducing the incidence
and recurrence rate of HSIs in football populations (Arnason et al., 2008a; Mjølsnes et al.,
2004; Petersen et al., 2011; van der Horst et al., 2015b), although this exercise is typically
performed with a partner holding the exerciser’s ankles. As a consequence, the movement is
typically performed without performance feedback, and this may potentially limit the value of
the exercise, particularly if athletes under-perform or if previously injured athletes rely
predominantly on their uninjured limb. Biofeedback has been employed in rehabilitation for
other lower limb injuries such as ACL rupture, patellofemoral pain, and knee arthroscopy;
however, as far as the author is aware, it has not been applied in hamstring injury rehabilitation.
The results of the current study revealed no significant effects of visual biofeedback on
strength or muscle activation during a single exercise session in recreational athletes with a
unilateral history of HSI. However, the current literature on the use of feedback predominantly
focuses on its use in longitudinal rehabilitation programs (Giggins et al., 2013), which could
potentially allow participants more opportunity to learn how to utilise biofeedback and
consequently alter their muscle activation patterns. The acute nature of the current study
potentially did not allow participants to learn how to simultaneously perform the exercise while
also interpreting and applying the information they received through feedback despite the
participants completing a familiarisation and having a larger 17 inch screen in front of them to
make the force traces easy to read whilst participating in the protocol. While no study has
specifically targeted previous HSIs through feedback, most studies examining the knee
extensors performed their protocols on an isokinetic dynamometer (Croce, 1986; Ekblom &
Eriksson, 2012; Kirnap et al., 2005; Ng et al., 2008; Randell et al., 2011). The seated
dynamometer would allow for no upper body movement and potentially make reading,
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interpreting and applying biofeedback easier for participants, compared to the constant upper
body movement associated with the NHE.
Another limitation of the current study is the lack of significant strength deficit in
previously injured limbs. Previous injury had no effect on force output compared to the
contralateral uninjured limb across the 30 repetitions. The lack of strength deficit in the injured
limb is inconsistent with previous studies which have shown reduced eccentric strength and
muscle activation in the previously injured compared to uninjured contralateral limbs (Bourne,
Opar, Williams, Al Najjar et al., 2015; Opar, Williams et al., 2014b; Timmins, Bourne, Shield,
Williams, Lorenzen et al., 2015). Differences in injury severity may explain the results, as
previous studies typically recruited participants with grade two or three injuries compared to
the less severe grade one injuries that nine of the 12 participants had experienced in this study.
The participants did provide their injury history via recall due to their sub-elite status and lack
of uniform reporting procedures from a professional such as a team doctor or physiotherapist.
Screening procedures ensured that all participants who were involved in the study had injuries
diagnosed by a medical professional and sought subsequent treatment. There is a possibility
that grade one injuries suffered by the current participants did not evoke a great neuromuscular
inhibitory response. Significant deficits in fascicle length and increased pennation angles in
previously injured BFlh muscles have been reported after more severe injuries (Timmins,
Bourne, Shield, Williams, Lorenzen et al., 2015; Timmins, Shield et al., 2015) and it has been
assumed by some that injury may have caused these, probably because this has been proposed
previously (Brockett et al., 2004). Unfortunately, it is not possible to know whether the shorter
fascicles predate initial injuries or occur as a consequence of them.
Lower fascicle lengths suggest a reduction in the number of in-series sarcomeres and
this does not directly influence maximum force generating capacity when isometric
measurements are employed as observed within this study. Furthermore, if there was any
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reduction in muscle PCSA within this previously injured muscle (the BFLH) it should be
considered that there are numerous other knee flexors that generate torque (eg semitendinosus,
semimembranosus, biceps femoris short head, gracilis, sartorius and gastrocnemius). Mild
losses in BFLH PCSA would therefore have limited effects on knee flexor torque and there is
the possibility that other muscles may have exhibited compensatory hypertrophy since injury,
as has been reported previously (Silder et al., 2009)
The present results suggest that visual biofeedback of force output has no impact on the
strength loss during an exercise session involving 30 repetitions of the NHE. With or without
biofeedback, the significant finding from the NHE protocol performed here was the reduction
in knee-flexor force output over time, suggesting the protocol has a fatiguing effect on the
knee-flexors. The presence of strength loss across the repetitions is consistent with findings of
Marshall and colleagues (2015) who investigated 30 repetitions of the NHE in soccer players
and measured strength between sets of the NHE with eccentric and concentric efforts. The
findings of this study demonstrated strength loss during the performance of the NHE without
the potential confounding effects of other eccentric and concentric efforts. Additionally the
current investigation did examine the repetitions and subsequent variables at the peak of
summed force as opposed to the segments of the repetition as a whole. Whilst the peak summed
force knee angle was low amongst our participants it only captured information a specific point
of time and not across knee angles associated with HSI.
The EMG results highlighted the heterogeneity of muscle activation during the NHE,
with the preferential recruitment of the medial hamstrings, similar to that observed in study one
in this thesis and by Bourne and colleagues (2017) using fMRI. Despite preferentially targeting
the medial hamstrings, which are not the most commonly injured hamstring muscles during
high-speed running, the exercise still elicits high levels of activation in the lateral muscles, and
this supports its place within injury prevention programs. The author acknowledges there are
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limitations to this study. The lack of training in using feedback could have prevented the
participants from engaging with the information presented and utilising it to their greatest
capacity. The acute nature of the study didn’t allow participants the chance to learn and adapt
to having feedback while also completing the exercise. By contrast with the significant and
persistent strength deficits that have been published after more severe HSIs, the injuries in the
participants in this study, may have been more readily rehabilitated or simply recovered
spontaneously. It remains to be seen whether the use of feedback during the NHE over longer
periods of time would be more or less beneficial for more severely injured athletes.
4.4 CONCLUSION
Mild hamstring injuries do not appear to result in any lasting weakness or an
exaggerated loss of eccentric strength across 30 repetitions of the NHE. Furthermore,
biofeedback of force does not result in any acute changes in performance during the
performance of a NHE exercise session.
LINKING PARAGRAPH
The bilateral and eccentric NHE test is now used relatively widely in elite level sports.
However, it is unclear whether or not this test is more or less sensitive to the effects of prior
injury than unilateral and isometric tests of the knee flexors. It is also possible to perform a
razor curl on the same device designed to assess the NHE, and it is not known whether this
popular exercise is more sensitive to the effects of previous injury. The next study is designed
to assess the potential value of other isometric and bilateral tests of hamstring strength in
addition to changes in the isometrically derived knee flexor torque-joint angle curve after HSI.
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CHAPTER 5: The Effect of Previous Hamstring Strain Injury on Isometric and Dynamic Strength Profiles of the Knee-Flexors
5.1 RESEARCH DESIGN
5.1.1 OBJECTIVES
The aims of this investigation were to;
1. Confirm whether or not prior hamstring strain injury is associated with a shift in the
JAPT.
2. Determine whether this shift is associated with surface EMG activity of the hamstrings
at long muscle lengths.
3. To compare knee-flexor strength and medial and lateral hamstrings surface EMG
activity during the NHE, the razor curl and bilateral and unilateral isometric maximal
voluntary contractions in previously injured and uninjured limbs.
4) To assess the biceps femoris architecture (fascicle length, fascicle length, and
pennation angle) in previous injured compared to the uninjured contralateral limb.
5.1.2 PARTICIPANTS
Eight recreationally active adult males with a history of unilateral HSI were recruited
for this study. Participants completed a cardiovascular risk screening form and an injury
questionnaire with the information provided detailing the injury location, grade and
rehabilitation period of their most recent hamstring strains occurring within the past 18 months
(7.7 ± 7.2 months). All had made a full return to competition and training and were free from
any other lower limb pathologies. Participants gave informed consent to participate in the study
86
which was approved by the Queensland University of Technology Human Research Ethics
Committee (approval number 1600000479).
5.1.3 METHODOLOGY
Participants completed a familiarisation session in which they performed isometric
contractions while seated on a dynamometer. During the first session, participants had
ultrasound images taken midway along the longitudinal axis of the BFlh muscle belly to assess
fascicle length and pennation angle.
Images were taken using two-dimensional B-mode ultrasound (frequency, 12 MHz;
depth, 8 cm; field of view, 14×47 mm) (GE Healthcare Vivid-i, Wauwatosa, USA). The
scanning site was determined as the halfway point between the ischial tuberosity and the knee
joint fold, along the line of the BFlh. All architectural assessments were performed with
participants in a prone position and the hip neutral following at least five minutes of inactivity.
Following the ultrasound scans, participants warmed up by cycling at a submaximal pace for
five minutes. Participants then performed three to six repetitions of the NHE (NHE), Razor
curl (RC), under the instruction of the investigator, to familiarise themselves with the
appropriate technique requirements of the exercise.
Seven days after familiarisation participants visited the laboratory for testing. The
remaining participants did not complete a familiarisation session due to previous exposure to
the testing methods in the laboratory. After completing 5 minutes of submaximal and self-
paced pedalling on a cycle ergometer, participants were then seated on a dynamometer (Biodex
System 3, New York, USA) to perform maximal voluntary isometric contractions of the knee-
87
flexor and extensors at knee angles of 10, 30, 50, 70 and 90 degrees from full knee extension
(Figure 20).
FIGURE 21: Isometric knee-flexor dynamometry.
Two knee extensor and two knee-flexor contractions were performed at each joint angle
with one minute rest between efforts and a randomised presentation order of joint angles. For
all dynamometer testing, surface electromyographic (sEMG) information was collected via
bipolar pre-gelled Ag/AgCl sEMG electrodes (10 mm diameter, 20 mm inter-electrode
distance) placed on the posterior thigh half way between the ischial tuberosity and tibial
epicondyles. The reference electrode was placed on the ipsilateral head of the fibula. The lower
limbs were tested in randomised order.
Seven days after the first testing session participants returned to the laboratory for
further eccentric and isometric hamstring strength tests, this time on a device developed by the
research group. After a five minute cycling warm-up, the participants performed two maximal
voluntary isometric contractions of the knee-flexors in the prone position with two minutes rest
between contractions. They then performed unilateral maximal voluntary isometric
88
contractions on each leg with a two minute rest period between repetitions. Finally, they
performed three maximal repetitions of NHE with no rest between repetitions, followed by a
two-minute rest and then three maximal repetitions of a razor curl, again without rest between
repetitions. Knee-flexor forces at the ankles were measured with uniaxial load cells (MLP-1K,
Transducer Techniques, CA, USA) attached in-series to ankle straps. The middle of each ankle
strap was aligned with the lateral malleoli, and the ankle restraints were vertical and
perpendicular to the shank. Surface electromyographic (sEMG) data was collected from lateral
and medial hamstrings as described for the first testing session and knee angle measurements
were made using a custom-made electronic goniometer (PRV6 5K potentiometer) fixed to the
left knee.
FIGURE 22: Nordic hamstring exercise on hamstring testing device. Participants
pull against ankle restraints as they resist falling
FIGURE 23: Razor curl test on hamstring testing device. Participants pull against ankle restraints as they resist falling while attempting to fully extend their knees and hips with their faces held close to the ground.
89
All sEMG, force and joint angle data were sampled at 1000 Hz via a 16-bit PowerLab
26T AD unit (amplification = 1000; common mode rejection ratio = 110 dB) and analysed
using LabChart 7 (ADInstruments, New South Wales, Australia). Raw sEMG data were filtered
using a Bessel filter (frequency bandwidth=10–500 Hz) and then full-wave rectified and
smoothed over 100ms windows. Knee angle data were low pass filtered at 4 Hz. All data was
then transferred to a personal computer.
5.1.4 DATA ANALYSIS
Strength Measures
The peak forces (N) for each repetition of the NHE and razor curl for each participant
were derived by taking the maximum value for the left and right limb at the moment of the
peak of summed force for each repetition. This process was repeated for all individual
repetitions for each participant. The data was then exported into Microsoft Excel and SPSS for
statistical analysis.
For both seated and prone maximal isometric contractions (MVIC), data were obtained
from the 0.5 seconds of data surrounding the instant of peak force for each contraction (0.25
seconds either side of the peak). All isometric contractions were performed twice with the
results averaged for each contraction angle. For MVICs performed on the dynamometer,
torques for each participant were normalised to those obtained at 900 from full extension.
Surface Electromyography
Surface EMG (sEMG) data was derived by taking and averaging the 0.5 seconds of
data before the peak forces in all dynamic exercises (NHE, and Razor Curl). For all isometric
contractions, both seated and prone, sEMG data was derived by sampling the 0.5 seconds
surrounding the peak force. Surface EMG for the Nordic, razor curl, and prone isometric tests
were normalised to the MVIC contraction values in the prone position. All isometric
90
contractions were performed twice at each position and knee angle; as such the average of the
two contractions was used for analysis. For MVICs performed on the dynamometer, sEMG
values for each participant were normalised to that obtained at 900 from full extension as they
cannot be normalised to the prone positioning due to the change in muscle length, and hip
position.
Muscle Architecture
The MicroDicom software (Version 0.7.8) was used for analysing the ultrasound
images collected. Due to the limited view of the ultrasound probe a muscle fascicle of interest
was outlines and the muscle fascicle length was determined using the equation: FL=sin
(AA+90°) x MT/sin(180°-(AA+180°-PA)). Muscle Thickness (MT was defined by the as the
distance between the superficial and intermediate aponeuroses of the BFLH). The ultrasound
technician was blinded to all participants’ previous injury data.
5.1.5 STATISTICAL ANALYSIS
Statistical analysis was performed using SPSS version 23.0.1 (IBM Corporation,
Chicago IL,). When appropriate, the normality of the data was assessed with the Shapiro-Wilks
test. Sphericity was assessed via Mauchly's Test of sphericity. When sphericity wasn’t
observed, the Huynh-Feldt correction was used.
For the seated dynamometry torques and normalised sEMG results, joint angle (10, 30,
50, 70, 900) by limb (injured vs uninjured) repeated measures analyses of variance (ANOVA)
were used. When significant main effects were detected, post hoc t-tests with Bonferroni
corrections were employed for pair-wise comparisons. Prone isometric results were analysed
via a laterality (bilateral vs. unilateral) by leg limb (injured vs uninjured) repeated measures
ANOVA. The force outputs for the injured versus the uninjured limb in both the Nordic
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Hamstring and Razor curl tests were compared via an exercise (NHE vs. Razor Curl) by limb
(injured vs uninjured) repeated measures ANOVA. When significant main effects were
detected, post hoc t-tests with Bonferroni corrections were employed to determine the
significant pair-wise comparisons. The mean differences were reported with their 95%
confidence intervals (CIs).
Muscle Architecture
Measures of muscle architecture (fascicle length, muscle thickness, and pennation
angle) were analysed by comparing the injured to uninjured limbs via paired sample t-tests.
The mean differences were reported with their 95% confidence intervals (CIs).
5.2 RESULTS
All participants (age 23.5 ± 2.1 years, height 178.2 ± 5.8cm, and body mass 84.6 ± 7.3
kg) had a history of unilateral HSI within the past 18 months. The average time since the most
recent insult was 7.7 ± 7.2 months with a mean return to play time of 6 ± 3.6 weeks. Details of
the participant’s injury information can be found in Table 4.
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Table 5: Hamstring injury history information for all participants.
Participant Injured Limb
Muscle injured
Number of HSIs
Months Since last HSI
Grade of last HSI
Rehabilitation time (weeks)
1 Right ST 2 18 2 10
2 Right BFlh 1 2 1 5
3 Right BFlh 2 2 1 5
4 Right BFlh 1 2 1 2
5 Right BFlh 3 12 2 4
6 Right BFlh 1 2 1 4
7 Right BFlh 3 18 1 4
8 Right BFlh 1 5 1 12
BFlh: Biceps Femoris Long Head ST: Semitendinosus
Rehabilitation was defined by a return training and match. The severity (grade) was determined by the American Medical Association's guidelines (Craig, 1973).
Biceps femoris long head architecture
BFlh fascicles were significantly shorter (mean difference = -0.74 cm, 95% CI = -0.27
to -1.23; p = 0.008) and pennation angles significantly higher (mean difference = 1.47o, 95%
CI = 1.0 to 1.93; p <0.001) in previously injured than the uninjured limbs. Muscle thickness
was not significantly different between limbs (mean difference = -0.04cm, 95% CI = -0.18 to
0.10; p = 0.546) (see Table 5).
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Table 6. Architectural characteristics of biceps femoris long head muscles in previously injured and uninjured limbs.
Previously injured limbs were not weaker than the contralateral uninjured limbs in
normalised isometric torque across the five angles tested on the dynamometer (mean difference
= -0.03Nm, 95% CI = -0.34 to 0.29; p = 0.854) and no significant between-limb differences in
force were noted at any joint angle (p > 0.7 for all comparisons).
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FIGURE 24: Normalised Torque – knee joint angle data for previously injured and uninjured limbs as measured by isometric dynamometry.
Surface electromyography joint-angle relationship
Surface EMG for the isometric contractions performed on the dynamometer was
normalised to the result obtained from the test at 900 of knee flexion. Comparisons between
previously injured and uninjured limbs showed no significant main effect of injury history
(mean difference = 3.7%, 95% CI = -11.8 to 19.2; p = 0.512). When compared across joint
angles, the medial hamstrings were significantly more active, in terms of normalised sEMG,
than lateral hamstrings (mean difference = 23.2%, 95%CI = 12.1 to 34.3, p = 0.002), although
this effect was statistically significant in the uninjured limbs (mean difference = 30.8%, 95%
CI = 12.5 to 49.1; p = 0.005) but not the previously injured limbs (mean difference = 15.5%,
95%CI = -0.8 to 31.9; p = 0.060).
0
0.5
1
1.5
2
2.5
10 30 50 70 90
Nor
mal
ised
Torq
ue (N
m)
Angle (degrees)
InjuredUninjured
95
FIGURE 25: Normalised sEMG during isometric seated knee flexor dynamometry Error bars are omitted for the sake of clarity.
Dynamic strength tests
Previously injured limbs were not weaker than contralateral uninjured limb in either the
NHE (mean difference = -7.4N, 95% CI = -28.8 to 14.1; p = 0.444) or the razor curl test (mean
difference = 27.6N, 95% CI= 12.6 to -67.7; p = 0.149).
0
20
40
60
80
100
120
140
10 30 50 70 90
Nor
mal
ised
EMG
(%)
Degrees
InjuredLateral
InjuredMedial
UninjuredLateral
UninjuredMedial
96
FIGURE 26 Eccentric knee-flexor force for NHE and Razor Curl Bars represent group means, error bars represent standard deviation (SD)
Dynamic sEMG measures
Previous injury had no effect on the normalised sEMG during either the NHE or razor
curl (mean difference = 9.8%, 95% CI = -23.3 to 43.0; p = 0.506). Normalised sEMG was
higher in the NHE than the razor curl for both lateral (mean difference = 27.5%, 95% CI = 17.9
to 27.0; p = <0.001) and medial hamstrings (mean difference = 25.3%, 95% CI = 12.4 to 38.2;
p = 0.002) and the medial hamstrings were significantly more active than the lateral in both
exercises (NHE mean difference = 32.4%, 95% CI = 7.0 to 57.9; p = 0.02; Razor curl mean
difference = 34.6, 95% CI = 8.1 to 61.2; p = 0.018).
0
100
200
300
400
500
600
NHE Razor Curl
Forc
e (N
)
UninjuredInjured
97
FIGURE 27: Comparison between exercises for normalised sEMG for injured vs uninjured and medial vs lateral hamstrings.Bars represent group means, error bars represent standard deviations (SD. * p = 0.001, *** p =0.02, **** p= 0.001
Bilateral and unilateral prone isometric strength measures
From the maximal isometric testing in a prone position, the combined force generated
from the bilateral test was compared to the summed forces of the two unilateral tests. A bilateral
deficit was observed with unilateral knee-flexor forces 21.3N (4%) (95% CI = 5.1 to 37.6; p =
0.017) higher than the bilateral forces. The difference between bilateral and unilateral strength
was significant for the uninjured limbs (mean difference = 29.7N, 95% CI =10.6 to 48.8; p =
0
20
40
60
80
100
120
140
160
180
200
NHE Razor Curl
Nor
mal
ised
sEM
G (%
)
UninjuredLateral
UninjuredMedial
InjuredLateral
InjuredMedial
**
**** *** **** ****
98
0.008), but not the previously injured limbs (mean difference = 13.0N, 95% CI = -5.2 to 31.1;
p = 0.136).
FIGURE 28: Prone isometric force during unilateral and bilateral contractions. Bars represent group means, error bars represent standard deviation (SD). * p = 0.001
Bilateral and unilateral prone isometric sEMG measures
There was no significant difference in normalised sEMG between bilateral and
unilateral isometric contractions (mean difference = 2.6%, 95% CI = -4.8 to 10.1; p = 0.427).
However, there was a main effect of injury on unilateral normalised sEMG with lower levels
of activity in previously injured limbs (mean difference = 11.0%, 95% CI = 1.1 to 20.9; p =
0.035). There was a significant exercise by limb by muscle interaction for normalised sEMG
351322
0
50
100
150
200
250
300
350
400
450
Unilateral Bilateral
Forc
e (N
)
Exercise
UninjuredInjured
*
99
for lateral hamstrings with uninjured limbs exhibiting significantly higher activity than
previously injured limbs during the unilateral test (mean difference = 14.6%, 95% CI = 4.5 to
24.8; p = 0.011).
FIGURE 29: Comparison between isometric tests for normalised sEMG for injured vs uninjured and medial vs lateral hamstrings. Bars represent group means, error bars represent standard deviations (SD). * p= 0.035, ** p = 0.011.
0
20
40
60
80
100
120
140
Bilateral Test Unilateral Test
Nor
mal
ised
sEM
G (%
)
UninjuredLateral
UninjuredMedial
InjuredLateral
InjuredMedial
**
*
100
5.3 DISCUSSION
To this author’s knowledge, this is the first study to investigate JAPT in athletes with a
history of HSI via isometric measurements. One goal of this study was to examine whether
prior HSI was associated with a shift in the knee-flexor angle of peak torque. Such a shift has
been reported in the literature, although a total of only ten participants across two studies have
been shown to exhibit such changes (Brockett et al., 2001, 2004; Brughelli et al., 2009). The
results from the current isometric tests show that previously injured, and contralateral limbs
were both stronger at 10 degrees from full extension than at any other tested angle and, unlike
There are some limitations to the current investigation. Firstly, the number of participants,
while comparable to a number of similar studies (Brockett et al., 2001, 2004; Brughelli et al.,
104
2009) was low, and this raises the question of how reproducible the findings are. This study
had a small participant cohort due to the stricter criteria on previous lower limb pathologies.
This study required participants to only exhibit a unilateral hamstring injury in the absence of
all other lower limb pathologies. Timmins and colleagues (2016), when reviewing the JAPT
literature highlighted the limitation in assuming that changes in one muscle contribute to larger
deficits in knee flexion torque despite multiple muscles contributing to this movement. As such,
investigating those with only a hamstring strain would help with the current understanding
surrounding the impact of HSI on the JAPT without confounding factors. Between limb
differences in hamstring force outputs were compared to previous studies which had a minimal
detectable change of 76N and 60N for the left and right limbs, respectively, in healthy
participants. As highlighted earlier, the participants could have had their strength sufficiently
rehabilitated and or not suffered a severe enough injury for neuromuscular inhibition to occur.
However, there is minimal literature investigating the effects of low grade hamstring injuries
to which the current results can be compared. The author acknowledges that there is a
likelihood of a type II error due to the low statistical power of the current study and this
reinforces the need for further research with larger cohorts to understand the effect of previous
injury on the JAPT.
Another limitation was the self-reporting of injuries from participants and the lack of
detailed information regarding their rehabilitation. A small proportion of hamstring strains in
sport are associated with negative findings on MRI and it is possible that some participants in
the current study may have fitted into this category (Gibbs, Cross, Cameron, & Houang, 2004).
The use of sEMG is also problematic in determining levels of muscle activity due to potential
crosstalk from neighbouring muscles. This effect is minimised by placing recording electrodes
for the one muscle close together, and the inter-electrode distances of 20mm employed in the
current study are likely to minimise crosstalk. It should be noted, however, that the current
105
results for the NHE are consistent with those from fMRI studies which show preferential
activation of the semitendinosus over the biceps femoris long head (Bourne et al., 2015 and
2017).
The retrospective nature of this study does not allow determination of whether the
deficits observed in injured limbs, such as those for fascicle lengths, were present before the
initial injury or were caused by it. There is evidence that BFlh fascicle length is an independent
risk factor for HSI (Timmins, Shield et al., 2015; Timmins et al., 2016) but there is no current
evidence that fascicle lengths decrease as a result of HSI. In the future, it may be of value to
follow, for extended periods of time, large cohorts of previously uninjured athletes to determine
the impact of injuries as they occur.
5.4 CONCLUSION
Isometric testing in seated and prone positions in people with a previous low-grade
hamstring strain injury showed no significant shifts in the torque-joint angle relationship or any
strength deficits in previously injured limbs when compared to the contralateral limbs.
Similarly, the dynamic NHE and razor curl tests showed no strength deficits in previously
injured limbs. This study suggests that previous low-grade injury is associated with BFlh
fascicle length deficits and elevated pennation angles without any effects on isometric or
eccentric strength.
106
CONCLUSION
This program of research aimed to improve our understanding of the acute effects of eccentric
and isometric contractions on knee-flexor force output and sEMG and to ascertain whether
prior low grade injury effects these measures and if the use of biofeedback can alter measures.
Study 1 further enforced the need to investigate the appropriate prescriptions of
rehabilitation exercises. Despite the 30 repetitions of the NHE showing no differences in torque
output regardless of prescription (3x10 vs. 5x6). In a practical setting this suggests that neither
protocol has an obvious advantage over the other, although the 3x10 repetition protocol can be
performed in a shorter time frame compared to the 5x6 protocol. Differences in the BFlh sEMG
did highlight the needs to prescribe exercises such as the NHE appropriately, to retain its shown
benefits but without putting the muscle at subsequent risk of injury in the following sessions.
The effects of force loss observed in Study 1 were still unknown to affect previously
injured muscles in the same fashion. Study 2 aimed to investigate the effect that previous injury
has on the performance of the NHE at a higher volume. In addition, Study 2 aimed to assess
the use of biofeedback during the exercise and its subsequent effects on performance. While
previous injury and feedback did not affect the NHE performance. The structural differences
in the muscles bring to attention an avenue of future enquiry into enhancing rehabilitation
programs that ameliorate the functional deficits but the structural deficits such as those
observed in Study 2.
Finally, Study 3 aimed to evaluate the value of other tests for hamstring strength testing
of varying contraction modes and laterality in addition to muscle architecture. Muscle
architecture measures exhibited significant alterations to fascicle length and pennation angle
when comparing the previously injured limb to the uninjured limb. Despite alterations to the
107
structure of the muscle, the function as measured by strength appears to be unaffected. The
strength test results were mixed, but the differences between injured to uninjured limbs and
between the lateral and medial muscles again highlighted the need to create and implement
effective rehabilitation and preventative strategies that target the muscles capacity activate and
enhance the structure of the muscle as well as the architecture.
Moving forward, this program of research has added some insight into the acute effects
of lower grade hamstring has on ability to perform various hamstring tests and exercise
protocols in the presence of structural deficits. Future research should aim to elaborate on these
findings and look forward into enhancing current hamstring protocols to ensure both structure
and function are restored to pre-injury levels and reduce the risk of future HSI for athletes.
Additionally, further research will need to investigate if the same protocols and tests effect
those in elite sport similarly or differently to the sub-elite populations tested within this
research programme.
Overall the findings from this program of research have provided some novel insights
that have contributed to the existing knowledge of hamstring strain injury. Low-grade
hamstring strain showed no lasting deficits on JAPT, eccentric, and or isometric strength
despite significant between limb differences in muscle architecture when measured by fascicle
length and pennation angle. The findings provided insight into the acute effects the respective
testing and exercise protocols have on the hamstring muscle group and may inform
practitioners moving forward when making decisions for their programming needs.
108
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