Validity and reliability of different kinematics methods used for bike fitting

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Validity and reliability of different kinematics methodsused for bike fittingBorut Fondaab, Nejc Sarabonbc & François-Xavier Liaa School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham,Birmingham, UKb Laboratory for Motor Control and Motor Behavior, S2P, Science to Practice, Ltd., Ljubljana,Sloveniac Science and Research Centre, Institute for Kinesiology Research, University of Primorska,Koper, SloveniaPublished online: 06 Feb 2014.

To cite this article: Borut Fonda, Nejc Sarabon & François-Xavier Li (2014) Validity and reliability of different kinematicsmethods used for bike fitting, Journal of Sports Sciences, 32:10, 940-946, DOI: 10.1080/02640414.2013.868919

To link to this article: http://dx.doi.org/10.1080/02640414.2013.868919

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Validity and reliability of different kinematics methods used for bikefitting

BORUT FONDA1,2, NEJC SARABON2,3, & FRANÇOIS-XAVIER LI1

1School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, Birmingham, UK, 2Laboratory for MotorControl and Motor Behavior, S2P, Science to Practice, Ltd., Ljubljana, Slovenia and 3Science and Research Centre, Institutefor Kinesiology Research, University of Primorska, Koper, Slovenia

(Accepted 20 November 2013)

AbstractThe most common bike fitting method to set the seat height is based on the knee angle when the pedal is in its lowestposition, i.e. bottom dead centre (BDC). However, there is no consensus on what method should be used to measure theknee angle. Therefore, the first aim of this study was to compare three dynamic methods to each other and against a staticmethod. The second aim was to test the intra-session reliability of the knee angle at BDC measured by dynamic methods.Eleven cyclists performed five 3-min cycling trials; three at different seat heights (25°, 30° and 35° knee angle at BDCaccording to static measure) and two at preferred seat height. Thirteen infrared cameras (3D), a high-speed camera (2D),and an electrogoniometer were used to measure the knee angle during pedalling, when the pedal was at the BDC.Compared to 3D kinematics, all other methods statistically significantly underestimated the knee angle (P = 0.00;η2 = 0.73). All three dynamic methods have been found to be substantially different compared to the static measure (effectsizes between 0.4 and 0.6). All dynamic methods achieved good intra-session reliability. 2D kinematics is a valid tool forknee angle assessment during bike fitting. However, for higher precision, one should use correction factor by adding 2.2° tothe measured value.

Keywords: 2D kinematics, 3D kinematics, goniometer, electrogoniometer, cycling

Introduction

Bike fitting is an important process to adjust thegeometry of the bike to the needs of the cyclist.Optimal bicycle rider position may be consideredas a position in which force application and comfortare maximised, whilst resistive forces and risk ofinjury are minimised, in order to maximise bicyclevelocity (Iriberri, Muriel, & Larrazabal, 2008). Thefirst scientific papers on bike fitting were publishedin the mid-sixties (Hamley & Thomas, 1967), inwhich authors proposed an anthropometric-basedmethod to set the seat height. Subsequently altera-tions in body position and their effect on the vari-ables mentioned above were investigated. Based onthese studies, numerous methodologies have beenproposed to perform bike fitting (Burke, 2003;Holmes, Pruitt, & Whalen, 1994; Iriberri et al.,2008; Nordeen-Snyder, 1977).

Seat height is probably the most important para-meter set in the procedure of bike fitting. Improperseat height can result in over-compression of the knee

(Ericson & Nisell, 1987) and/or increased oxygenconsumption (Nordeen-Snyder, 1977; Price &Donne, 1997). On the other hand, recent researchsuggests that changes in seat height within 4% oftrochanter height do not affect cycling economy(Connick & Li, 2013). To avoid detrimental effects,various methods to set the seat height have beenproposed. The Hamely and Thomas method(Hamley & Thomas, 1967) defines the optimal seatheight (seat height defined as the distance betweenthe pedal axle and top of the seat) at 109% of inseamor, as recently revised, at 108.6–110.4% of inseam(Ferrer-Roca, Roig, Galilea, & García-López, 2012).LeMond and Gordis (1990) suggested to set the seatheight (seat height defined as the distance betweenthe centre of the bottom bracket and top of the seat)at 88.3% of inseam. The heel method determines theseat height by placing the heel of the foot on the pedaland incompletely extending the knee. The pedal mustbe at the bottom of the stroke (crank angle 180°) andthe cyclist must be sitting on the seat (Burke, 1994).

Correspondence: François-Xavier Li, School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom.E-mail: [email protected]

Journal of Sports Sciences, 2014Vol. 32, No. 10, 940–946, http://dx.doi.org/10.1080/02640414.2013.868919

© 2014 Taylor & Francis

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This method is not precise as the different cleat ped-als have different heights and it is difficult to deter-mine exactly where the heel is placed. To minimisethe risk of injury, Holmes et al. (1994) proposed to setthe seat height to the level where knee angle, when thepedal is in its lowest position, i.e. bottom dead centre(BDC), is between 25° and 35° (Figure 1). It hasbeen shown that the aforementioned static methodsto set the seat height vary significantly among eachother and do not always yield the same results(Peveler, Bishop, Smith, Richardson, & Whitehorn,2005). It is generally agreed that dynamic methodsprovide more realistic results than static methods(Ferrer-Roca et al., 2012; Peveler, Shew, Johnson, &Palmer, 2012). A study by Ferrer-Roca et al. (2012)compared a static (anthropometric measurements)versus a dynamic method (2D kinematics) to adjustthe seat height and they concluded that seat heightadjusted with static method (106–109% of inseamlength) was outside of the recommended range in56.5% of the participants. Therefore, in order to setthe seat height according to knee angle, direct mea-surements of knee angles should be adopted insteadof equations based on anthropometric data (Ferrer-Roca et al., 2012).

Based on the bike fitting recommendations,numerous studies on biomechanics of cycling at dif-ferent seat heights used knee angle as standardisationof the seat height (Bini, 2012; Bini, Hume, & Crofta,2011; Bini, Hume, & Kilding, 2012; Peveler,

Pounders, & Bishop, 2007). Knee angle in staticconditions is usually measured with a manual goni-ometer, where the axis is centred to lateral femoralcondyle, one arm pointing upwards to the greatertrochanter and the other arm pointing downwardsto the lateral malleolus of the ankle. Recently, severalcommercially available bike fitting systems are usingknee angle as a parameter based on which the seatheight is set.

Different kinematics systems do not necessaryprovide the same results as each system has differentdrawbacks and advantages (Vlasic et al., 2007).Umberger and Martin (2001) examined thedifferences in joint angles during cycling when ana-lysed in 3D and 2D, and reported that no significantdifferences exist between the two techniques. Theyused the same set-up and cameras for both analyses,but recruited only four participants and found nostatistically significant differences. Further researchshould be carried out in this direction to examine ifthere are any differences between 2D and 3D method.

Many kinematic systems have been tested for theirvalidity and/or within- and between-session reliabilityin other situations involving cyclic movements ratherthan during cycling. Studies on 3D kinematics duringgait reported moderate to good within-sessionreliability of joint angles in the sagittal plane(McGinley, Baker, Wolfe, & Morris, 2009). Cyclicmovements, such as cycling or gait, are assumed tohave good within-session reliability in kinematics para-meters. For gait, studies showed that an average of fivetrials achieved the level of confidence above 90%(Diss, 2001) for all kinematic patterns. To the authors’best knowledge, there is no published data on within-session reliability of kinematics data during cycling.

Knee angle has been well defined in relation to seatheight. However, there is a lack of systematic com-parison of the methods used to assess knee angle.Therefore, the first aim of this study was to comparethree dynamic methods for knee angle measurementto each other and against the static measure. Thesecond aim was to test the intra-session reliability ofthese methods. Based on our pilot studies andpractical experiences, our hypotheses were: (1) 2Dkinematics will significantly underestimate the kneeangle compared to 3D kinematics, (2) manual andelectronic goniometers will not provide valid kneeangle measurements in BDC compared to 3D kine-matics and (3) all three dynamic methods willachieve excellent intra-session reliability.

Methods

Participants

According to a priori sample size calculations basedon our pilot data, eleven participants ([mean ± SD]

Figure 1. Knee angle measurement when pedal is in the bottomdead centre (BDC).

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age 23.3 ± 2.8 years, body mass 71.6 ± 6.9 kg, bodyheight 179.8 ± 6.1 cm) were recruited from theUniversity’s cycling club. Six elite and five recrea-tional cyclists reached ([mean ± SD; min–max]peak oxygen consumption of 60.0 ± 7.7; 48–68 ml · kg–1min–1 and maximal aerobic power of342.6 ± 37.3; 300–420 W). Cyclists were trainingbetween 5 and 20 h/week. Before the experiment,each participant signed an informed consent form,which was approved by the local ethical committee.

Protocol

Participants were required to visit the laboratory ontwo occasions. The first involved an incremental testto exhaustion on an electromagnetically braked cycleergometer (Lode Excalibur Sport, Lode, Groningen,the Netherlands) in order to determine maximalaerobic power. Gas analysis was constantly moni-tored with a breath-by-breath gas analyser (JaegerOxycon Pro, Erich Jaeger GmbH, Hoechberg,Germany). Participants started pedalling at 100 Wat a self-selected cadence higher than 60 rpm.Resistance was increased by 25 W every 1 min,until the participant reached volitional exhaustionor cadence dropped below 60 rpm. Maximal aerobicpower was noted as the highest power output atwhich pedalling was maintained for at least 30 s.This test was performed to standardise the intensityfor the second session.

The second session was performed at least 48 hafter the incremental test. After a warm up (5 min at150 W, 80–90 rpm), each participant completedthree trials at different seat heights, and two at theirpreferred seat height on an electromagneticallybraked cycle ergometer (Lode Excalibur Sport,Lode, Groningen, NL). All trials lasted 5 min andwere carried out at 65% of maximal aerobic power.Seat height was adjusted according to the knee anglewhen the pedal was at the BDC. Saddle heightscorresponded to knee angle values of 25° (HIGH),30° (MID) and 35° (LOW), measured with a stan-dard manual goniometer in static conditions (Figure1). Preferred seat height was adjusted according tothe participant’s bicycle set-up. Participants wereinstructed to maintain a constant cadence (80 rpm)and to adapt a body position compared to real-lifeconditions simulating long duration training. Kneeangle measurements were performed three times foreach seat height with 10 s of pedalling between themeasurements. The handlebar position was adjustedaccording to the participant’s individual bicycle set-up, while the fore/aft position of the saddle was setaccording to bike fitting guidelines, where the patellais directly above the pedal spindle when the pedal isat a 90° position (Burke, 1994).

Set-up

In the second test, three systems were used torecord simultaneously kinematics. 3D kinematicsdata were captured using a Vicon MX motion ana-lysis system (Oxford Metrics Ltd., Oxford, UK)consisting of thirteen cameras recording with asampling rate of 250 Hz and which was calibratedwith a residual error less than 1 mm. Retro-reflec-tive markers were attached with double-sided adhe-sive tape over the greater trochanter, the lateralfemoral condyle and the lateral malleolus of thecyclist by the same tester to exclude inter-testervariability. Furthermore, reflective markers wereplaced on the pedal spindle and crank centre ofthe bicycle ergometer to identify crank position.One high-speed camera (Casio Exilim Pro EX-F1,Dover, NJ, USA) recording with a sampling rate of300 Hz, image resolution of 512 × 384 pixels andcalibration resolution of 8 mm was positioned per-pendicular to the participant at a distance of 4 m.An electrogoniometer (Biometrics Ltd., Newport,UK) operating at a sampling rate of 1000 Hz wasattached with double-sided adhesive tape to theright leg with one side attached to the middle ofthe shank (line between lateral malleolus and headof fibula) and the other to the middle of the thigh(line between lateral femoral condyle and greatertrochanter). Synchronisation between the Viconsystem and electrogoniometer was establishedthrough an A/D card (National Instruments,Austin, TX, USA). The high-speed camera wasnot synchronised with the Vicon system and theelectrogoniometer, but did record the same timeperiod (±3 s) as Vicon.

Data analysis

The first 15 cycles from the last 30 s of each trialwere used for analysis. Analysis of the 3D kine-matic data was performed using Matlab(MATLAB, MathWorks, Natick, MA, USA). 3Dkinematic data were low-pass filtered using afourth-order Butterworth filter with a cut-off fre-quency of 12 Hz. Knee angles for each trial wereobtained by dividing data into individual crankcycles using the BDC pedal position determinedas the point at which the pedal reflective markerreached its minimal vertical position, i.e., 180°.The knee angle from the 2D kinematic data wasextracted (Kinovea 0.8.15) with the software’sfunction”angle” at a visually determined BDCcrank position (with precision of 1.6°). Data fromthe goniometer was acquired and analysed withLabview (Labview, National Instruments, Austin,TX, USA). Crank angle from 3D kinematic datahas been interpolated to 1000 Hz and merged with

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the knee angle data from the goniometer in orderto calculate the knee angle when the pedal was inthe BDC position.

Statistical analysis

For each trial and method, the average of fifteenconsecutive cycles was taken for further analysisfrom each kinematic method. All data is presentedas a mean ± SD. A two-way repeated measureANOVA was used to test for method (3) × seatheight (3) interaction and differences among meth-ods at different seat heights. When sphericity wasviolated, Greenhouse–Geisser correction wasapplied. When the main effect was significant, posthoc comparisons with Bonferroni corrections werecarried out. The two trials with preferred seatheight were used to assess reliability. Differencesbetween two trials were assessed using two-tailedpaired t-test and 95% limits of agreement (Atkinson& Nevill, 1998; Nevill, 1996; Nevill & Atkinson,1997). Statistical analysis was performed usingSPSS V.20 (IBM Corporation, Somers, NY,USA) with levels of significance set to P < 0.05.Size effects (η2) are reported as partial eta squared.

Results

Reliability is presented in Table I. All dynamic meth-ods showed good reliability with not statistically sig-nificant t-tests (P = 0.37–0.51) and absolute limits ofagreement between 5.0 and 8.4. However inter-sub-ject variability measured by SD is substantially largerof the electrogoniometer (13.45) than of the 2D(7.45) and 3D (7.8) kinematics.

There was a significant main effect of methods atdifferent seat heights (F(1.039, 10.389) = 26.113;P = 0.000; η2 = 0.731) and a statistically significantinteraction between methods and seat height (F(4,40) = 4.449; P = 0.005; η2 = 0.681). Post hoccomparisons revealed that electrogoniometer sub-stantially underestimated the knee angle and showedsignificantly smaller angles (ES = 0.7–0.8) comparedto 2D and 3D kinematic methods at all seat heights(Figure 2). 2D significantly underestimated kneeangle at the HIGH seat height (P = 0.019;ES = 0.2) compared to the 3D methods.

All knee angle data measured with dynamic meth-ods were different compared to static measures(between 15.2% and 38.2% and effect sizes between0.4 and 0.6) which were used to set the seat height.The static measure underestimated the knee anglecompared to 3D and 2D kinematics, while overesti-mating the knee angle compared to the electro-goniometer.

Discussion

The aim of this study was to compare three dynamicmethods to assess knee angle during cycling at dif-ferent seat height among each other and against astatic method using a manual goniometer. We haveconfirmed our hypotheses by showing that: (1) anelectrogoniometer and 2D kinematics significantlyunderestimated the knee angle when compared to3D kinematics, (2) a manual goniometer underesti-mated the knee angle at the BDC compared to 3Dand 2D kinematics and (3) all three dynamic meth-ods achieved high intra-session reliability.

Various motion capturing systems do not necessa-rily provide the same results as they work on differ-ent basis (Vlasic et al., 2007), which has beenpartially confirmed in the present study. We haveshown that knee angles at specific points assessedwith two different passive kinematics systems andan electrogoniometer are substantially different.This is of practical importance when adjusting body

Table I. Intra-session reliability results for each method reported as mean ± SD for two trials (Trial 1 and Trial 2).

Method Trial 1 (°) Trial 2 (°) Difference ± SD Absolute limits P

2D camera 42.1 ± 7.4 43.8 ± 7.5 −1.7 ± 4.3 −1.7 ± 8.4 0.213D Vicon 42.9 ± 8.5 43.9 ± 6.7 −1.1 ± 4.1 −1.1 ± 8.0 0.41Electrogoniometer 32.3 ± 22.3 32.0 ± 20.3 0.3 ± 2.5 0.3 ± 5.0 0.73

Note: P, P-value of the difference between trials assessed with two-tailed paired t-test.

Kne

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in B

DC

(°)

50

60

40

30

20

10

0HIGH

2D Cam 3D Vicon Electrogoniometer

MID

Seat height

LOW

Figure 2. Knee angle values when the pedal is in the bottom deadcentre (BDC) measured with a high-speed camera (blackcolumns), Vicon system (dashed column) and electric goniometer(white columns). Dashed horizontal lines represent the anglebased on which the seat height has been set with the static goni-ometer (i.e. HIGH, MID and LOW at 25°, 30° and 35°,respectively)

Note: *Bonferroni post hoc (P < 0.05).

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position/technique. Overall for human motion ana-lyses, kinematic systems that monitor movement in3D space are more precise in assessing movementkinematics compared to 2D systems (Couto et al.,2008) and are therefore considered as gold standard.Moreover, a 2D method cannot be used to deter-mine the external or internal rotations of thesegments because this movement occurs in thetransverse plane. Given the fact that pedalling ispredominantly performed in the sagittal plane, 2Dkinematics should provide the same results as 3D.Using a small sample size (n = 4), a change of ~3° inknee angle, analysed with 2D and 3D kinematicsusing the same camera set-up, confirms that 2Dkinematics is valid for knee angle measurementsduring cycling (Umberger & Martin, 2001). In ourstudy, two independent systems were used for 2Dand 3D kinematics and were analysed using differentsoftware. Even though it resulted in significantlydifferent knee angles at BDC, the difference wasvery similar (2.2°) to the one found by Umbergerand Martin (2001). However, to achieve a higherlevel of precision of knee angle measurement with2D kinematics, one should use a correction factor of2.2° when assessing at higher seat heights.

Electrogoniometers are being regularly used inclinical practice and have been found to provideaccurate and reliable measure of knee angles whena standardised protocol is used (Piriyaprasarth,Morris, Winter, & Bialocerkowski, 2008; Rowe,Myles, Hillmann, & Hazlewood, 2001). The disad-vantage of the electrogoniometer is its susceptibilityto skin movement artefacts as it is attached to alarger skin area compared to retro-reflective markers,which were to a certain extent susceptible for skinmovement artefacts as well (Benoit et al., 2006).Another potential source of error is misalignmentof the electrogoniometer to the anatomical axis ofthe knee joint, leading to difficulties in determiningthe zero position (Kettelkamp, Johnson, Smidt,Chao, & Walker, 1970). On the other hand, electro-goniometers provide immediate feedback on themeasured knee angle, which is advantageous forbike fitting experts. It has been suggested (Petusheket al., 2012) that the electrogoniometer is a cost-effective and time-efficient alternative to videoanalysis for the assessment of knee flexion angle ifthe error is accounted for and the sensor is preciselyattached. To the authors’ knowledge, studies usingelectrogoniometers to measure knee angle duringcycling have not been previously published. Wehave observed that electrogoniometers significantlyunderestimated the knee angle when the pedal wasin the BDC, compared to 3D and 2D kinematics.Underestimation of the knee angle measured withelectrogoniometer and large between-subject varia-tion could be due to the difficulties of joint axis

alignment and establishing zero offset, in line withthe study of Kettelkamp et al., (1970). Futureresearch should focus on standardising the protocolfor determining zero position in order to use electro-goniometers for cycling analysis.

2D analysis with high-speed cameras has beenfrequently used in human movement analysis evenafter the introduction of 3D systems. In fact, the vastmajority of cycling studies on bike fitting have beenusing 2D kinematics with a camera (Ericson, 1986;Nordeen-Snyder, 1977). The disadvantage of the2D kinematics is the phenomenon of parallax error,which occurs when objects are viewed away from theoptical axis of the camera. Our results indicate thatusing a high-speed camera and open-source softwarefor post hoc analysis without a correction factor donot provide the same results on knee angle measure-ment compared to Vicon 3D kinematic systems. Wecan assume that commercial software would notmake any difference. Our results are similar to thefinding by Umberger and Martin (2001) who founda difference of 3° between 2D and 3D method forknee joint angle measurement during cycling. In thepresent study two independent systems have beencompared, whereas Umberger and Martin (2001)used the same cameras and set-up. Another note-worthy observation is that in our study the differ-ences between 2D and 3D results were not constantat all three seat heights. This could be the conse-quence of fixed camera position for all three trials,which is normally the case in bike fitting set-up. Ourresults indicate that 3D kinematics systems shouldbe used for exact knee angle assessment (e.g.research), but 2D kinematics could be used forbike fittings. It is worth noting that despite observingsignificant differences between 2D and 3D analyses,recent research suggests that changes of 10° in aknee flexion angle do not substantially affect physio-logical and biomechanical markers (Bini et al., 2012;Connick & Li, 2013).

Bike fitting based on knee angle in static conditionsis the most popular method among experts as it is thecheapest and the easiest method to use (de VeyMestdagh, 1998). For these purposes, bike fittersnormally use manual goniometer. This method hasseveral disadvantages, such as misalignment of thegoniometer, dropping or raising the heel and absenceof inertial momentum at the ipsilateral limb and forceaction in the contralateral limb. All these issues couldpotentially affect the knee angle. In our study, weobserved substantial underestimation of the kneeangle measured with a manual goniometer comparedto 3D kinematics and also other methods used in thisstudy (2D and electrogoniometer). Furthermore, thelarge inter-subject variability observed for electrogo-niometer suggests that it should be discouraged forbike fitting.

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The reliability and validity of knee angle measure-ments should be known in order to be usedappropriately. Intra-session reliability of knee anglemeasurement of all three methods in our study hasbeen found to be excellent (ICC > 0.94). Thisconcurs with the data on gait, where the highestreliability indices occurred in the hip and knee inthe sagittal plane (McGinley et al., 2009). Futurestudies should be focused on inter-session reliabilityas it is critical for any intervention using bike fitting.

Conclusions

Each method has certain advantages and disadvan-tages, but all three methods tested in this study haveshown high reliability. Experts should use 3Dkinematics systems for knee angle assessment duringcycling for the purpose of research, as this is the mostvalid way of knee angle assessment. Bike fittingexperts using a high-speed camera for bike fittingshould make sure that the camera is positionedparallel to the captured motion of the cyclist.Additionally, to reach higher precision by using ahigh-speed camera, one should employ a correctionfactor by adding 2.2° to the measured value for kneeangle measurement. In spite of the fact that electro-goniometers provide immediate feedback, due to itslarge inter-subject variability they are not suitable forbike fitting. Static measure of knee angle with manualgoniometers should be discouraged in bike fitting.

Acknowledgements

The authors would like to acknowledge the supportof Slovene Human Resources Development andScholarship Fund.

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