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The relationship between legpreference and knee mechanics duringsidestepping in collegiate femalefootballersScott R. Brownab, Henry Wanga, D. Clark Dickina & Kaitlyn J.Weissab

a Biomechanics Laboratory, Ball State University, Muncie, IN, USAb Sports Performance Research Institute New Zealand (SPRINZ),Auckland University of Technology, Mairangi Bay, Auckland, NewZealandPublished online: 10 Sep 2014.

To cite this article: Scott R. Brown, Henry Wang, D. Clark Dickin & Kaitlyn J. Weiss (2014): Therelationship between leg preference and knee mechanics during sidestepping in collegiate femalefootballers, Sports Biomechanics

To link to this article: http://dx.doi.org/10.1080/14763141.2014.955047

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The relationship between leg preference and kneemechanics during sidestepping in collegiate femalefootballers

SCOTT R. BROWN1,2, HENRY WANG1, D. CLARK DICKIN1,

& KAITLYN J. WEISS1,2

1Biomechanics Laboratory, Ball State University, Muncie, IN, USA, and 2Sports Performance Research

Institute New Zealand (SPRINZ), Auckland University of Technology, Mairangi Bay, Auckland, New

Zealand

(Received 2 January 2014; accepted 13 July 2014)

AbstractThis study examined the relationship between leg preference and knee mechanics in females duringsidestepping. Three-dimensional data were recorded on 16 female collegiate footballers during aplanned 458 sidestep manoeuvre with their preferred and non-preferred kicking leg. Knee kinematicsand kinetics during initial contact, weight acceptance, peak push-off, and final push-off phases ofsidestepping were analysed in both legs. The preferred leg showed trivial to small increases(ES ¼ 0.19–0.36) in knee flexion angle at initial contact, weight acceptance, and peak push-off, andsmall increases (ES ¼ 0.21–0.34) in peak power production and peak knee extension velocity. Thenon-preferred leg showed a trivial increase (ES ¼ 0.10) in knee abduction angle during weightacceptance; small to moderate increases (ES ¼ 0.22–0.64) in knee internal rotation angle at weightacceptance, peak push-off, and final push-off; a small increase (ES ¼ 0.22) in knee abductor moment;and trivial increases (ES ¼ 0.09–0.14) in peak power absorption and peak knee flexion velocity. Theresults of this study show that differences do exist between the preferred and non-preferred leg infemales. The findings of this study will increase the knowledge base of anterior cruciate ligament injuryin females and can aid in the design of more appropriate neuromuscular, plyometric, and strengthtraining protocols for injury prevention.

Keywords: Soccer, knee joint, anterior cruciate ligament, injury prevention

Introduction

Football (soccer) is the most popular and most played sport in the world involving over 265

million active footballers (FIFA 2007). In the USA alone, there are an estimated 24.5

million active footballers, roughly 7 million of which are female (FIFA 2007). Coinciding

with this large number of female participation is an equally large number of sport-related

injuries. Females who participate in sports that involve jumping or cutting manoeuvres are

at a higher risk (three to nine times) of injury when compared to males playing the same

q 2014 Taylor & Francis

Correspondence: Scott R. Brown, Sports Performance Research Institute New Zealand (SPRINZ) at AUT Millennium, Auckland

University of Technology, Level 2, 17 Antares Place, Mairangi Bay, Auckland 0632, New Zealand, E-mail: scott.brown@aut.ac.nz

Sports Biomechanics, 2014

http://dx.doi.org/10.1080/14763141.2014.955047

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sport (Arendt, Agel, & Dick, 1999; Gwinn, Wilckens, McDevitt, Ross, & Kao, 2000). The

incidence of football-related injuries for individual female players is estimated as much as

40 per 1,000 match hours and has been seen to increase in younger and less experienced

players (Agel, Arendt, & Bershadsky, 2005; Arendt et al., 1999). In addition, approximately

60–80% of all football injuries occur to the lower-extremities, specifically the anterior

cruciate ligament (ACL) (Agel, Evans, Dick, Putukian, & Marshall, 2007; Dick, Putukian,

Agel, Evans, & Marshall, 2007). Nearly 70% of all ACL injuries have occurred in a non-

contact situation (Brophy, Silvers, Gonzales, & Mandelbaum, 2010), reported to be

caused by no apparent contact with a stationary object, the ball, or with another player

(Agel et al., 2005).

The mechanism breakdown for non-contact ACL injury has been shown to involve a step-

stop action, cutting task, sudden change of direction, landing from a jump with unstable

lower-extremity mechanics, or a lapse in concentration (Boden, Dean, Feagin, & Garrett,

2000; Feagin & Lambert, 1985; Ferretti, Papandrea, Conteduca, & Mariani, 1992).

Expanding on these findings, studies (Besier, Lloyd, Cochrane, & Ackland, 2001; McLean,

Neal, Myers, &Walters, 1999) have reported that the greatest loads in the knee occur during

a deceleration manoeuvre combined with a change of direction (i.e. sidestepping). During

sidestepping, extreme external loads induced by kinematic changes have been seen in the

lower-extremities, introducing several different theories as to the exact mechanism of ACL

injury. Such external loads occur during knee hyperextension or excessive flexion, excessive

knee abduction or adduction, internal or external tibial rotation, and anterior tibial

translation (Besier, Lloyd, Cochrane, et al., 2001; Wascher, Markolf, Shapiro, & Finerman,

1993; Willems et al., 2005).

While biomechanical evaluations of gender, cutting speed, and cutting angle have been

examined to great length, the influence of leg preference on ACL injury has not. Several

retrospective studies have attempted to look into the role of leg preference on ACL injury but

have failed to standardise sport background and injury mechanism (Faude, Junge,

Kindermann, & Dvorak, 2006; Matava, Freehill, Grutzner, & Shannon, 2002; Negrete,

Schick, & Cooper, 2007). A recent study by Brophy, Silvers, et al. (2010) took these

limitations into account when examining footballers all with non-contact ACL injuries.

Containing an identical participant population and injury mechanism, results showed that

74% of male participants sustained an injury to the preferred kicking leg and 68% of female

participants sustained an injury to the non-preferred kicking leg, suggesting that leg

preference plays a gender-based role in non-contact ACL injury specifically in footballers

(Brophy, Silvers, et al., 2010). Authors further recommend prospective studies to look into

the relationship between leg preference and knee mechanics in healthy footballers to confirm

these findings.

Since football requires lower-extremity strength and endurance, wherein athletes are

required to run, cut, and kick over a 90-minute match, it appears pertinent to include

the effects of leg preference to the aetiology for ACL injury. As female athletes suffer

three to nine times higher risk of ACL injury than their male counterparts (Arendt

et al., 1999; Gwinn et al., 2000), it is important to examine the effects of leg

preference on knee joint mechanics during sidestepping in females. To authors’

knowledge, no such biomechanical examination has been performed. Therefore, the

primary purpose of this research was to examine the mechanical differences between

the preferred and non-preferred legs during sidestepping. The secondary purpose of

this research was to assess the magnitude of the mechanical differences between legs

during sidestepping.

2 S.R. Brown et al.

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Methods

Participants

Sixteen healthy females (age ¼ 20 ^ 1 years, body height ¼ 1.68 ^ 0.04 m, body

mass ¼ 62 ^ 6 kg, BMI ¼ 22 ^ 2 kg/m2) volunteered as participants for this study. An a

priori power analysis using previously collected and unpublished pilot data (e.g. knee

abductor moment) from females indicated that 14 participants were needed to detect

differences between legs with a power level of 80% and an a value of 0.05. All participants

were National Collegiate Athletic Association Division I varsity footballers, had a minimum

of 12 years of football experience (14 ^ 1 years), had at least 1 year of collegiate experience

(2 ^ 1 years), and preferred to kick a ball with their right leg. Recruitment and testing took

place following all in-season competition at which time all participants were engaged in field

training schedules 2–3 days per week and a strength training protocol 3 days per week under

the supervision of the singular head strength and conditioning coach following an identical

Olympic lifting programme. Participants were excluded from the study if they reported any

of the following: (1) any history of ACL injury to either leg, (2) any history of injury to the

knee joint of either leg, or (3) any physical or neurological condition that would impair their

ability to perform the required sidestep manoeuvre. Prior to participation, all aspects of the

research study were verbally explained to each participant, written informed consent was

obtained, and a coded number was assigned to each participant to ensure that the data

remain anonymous. All procedures used in this study were reviewed by the Institutional

Review Board and received full ethical approval in accordance with Human Subject

Research at Ball State University (ethics approval #280767-1).

Data collection

All participants were fitted with identical, size appropriate compression tops, bottoms, and

socks (Nike Pro Compression, Nike, Inc., Beaverton, OR, USA) and indoor football shoes

(EsitoFinale IT,Puma,Boston,MA,USA).The legwhich a player prefers to kick the ballwith

or which she can kick the ball the furthest with was noted as the preferred kicking leg. This

criterion was solely based on the definition previously used while looking at the effects of leg

preference and how it relates to ACL injury among footballers (Brophy, Silvers, et al., 2010).

A 12-camera (MX40) motion capture system (240 Hz) (Vicon Motion System Ltd.,

Oxford, UK) was used to track the three-dimensional (3D) trajectories of retro-reflective

markers placed on the body during the sidestep manoeuvres (see below for details). Two

embedded force platforms (Model OR6-7-2000, Advanced Mechanical Technologies, Inc.,

Watertown, MA, USA) were used to collect synchronised ground reaction forces at 2,400

Hz. The two force platforms were arranged next to each other along the runway to increase

the chances of obtaining complete foot contact during the sidestepping task. Vicon Nexus

(Version 1.8.5, Vicon Motion System Ltd., Oxford, UK) was used to reconstruct and

process the raw 3D trajectory data and ground reaction force data.

To create a static model, spherical retro-reflective markers (14-mm width) were placed

bilaterally on the following anatomical locations: superior border of the acromion, superior

border of the manubrium, cervical 7 vertebrae, thoracic 10 vertebrae, posterior superior iliac

spine, anterior superior iliac spine, iliac crest, medial and lateral femoral condyles, medial

and lateral malleoli, posterior calcaneus, base of second metatarsal, and the base of fifth

metatarsal. Ridged cluster marker sets consisting of four markers were placed on the lateral

thigh and shank. Knee and ankle medial markers were removed after a static calibration trial

was completed to allow for the dynamic movement of the testing protocol.

Leg preference and knee mechanics in female footballers 3

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Participants performed a general warm-up by jogging at a self-selected pace on a treadmill

for five minutes followed by a general self-selected lower-extremity dynamic warm-up as this

protocol was identical to the team’s weight training, practice, and game warm-

up procedures. Next, participants were instructed to perform a sidestepping manoeuvre

(online supplementary only) as previously described by McLean et al. (1999). The planned

sidestep manoeuvre consisted of participants sprinting 5 m at a speed of 4.5 ^ 0.5 m/s and

then performing an evasive manoeuvre (planting on one leg and pushing off to the other leg

in a new direction) at a 45 ^ 108 angle with their preferred leg and non-preferred leg. Tape

lines of the same width as the force platforms (508 mm) were provided to direct participants

through the initial runway and the 458 cutting paths. Participants were allowed adequate

familiarisation of the protocol, and testing began when they felt comfortable sidestepping in

both directions at the required speed. Approach speed was calculated by tracking the centre-

of-mass velocity at initial contact of the plant foot on the force platform. The frontal and

sagittal two-dimensional video of each trial was instantly reviewed to assess the quality of the

sidestep manoeuvre to ensure that a distinct change of direction occurred and not a

“rounding” manoeuvre. Specifically, if the participants ran outside of the taped lines but still

came in contact with the force platform, the trial was not considered a sidestep and was not

included in the analysis. Participants were required to perform five successful sidestep cuts

on each side given in a random order. A successful trial consisted of the participant staying

within the tape lines, having the planted foot land completely on the force platform, and

entering and exiting the cut within the required speed to simulate a match situation (Zebis,

Andersen, Bencke, Kjær, & Aagaard, 2009).

Data processing

Visual 3D (Version 4.91.0, C-Motion, Inc., Germantown, MD, USA) was used to perform

raw data smoothing and link model-based computations on lower-extremity joints.

Trajectory data were smoothed with a fourth-order Butterworth low-pass filter at 8 Hz.

Ground reaction force data were smoothed with a fourth-order Butterworth low-pass filter at

50 Hz. Knee joint angles and moments were calculated using standard inverse dynamics

equations (Winter, 2009) in the coronal, sagittal, and transverse planes. In addition, knee

joint power and knee flexion/extension velocities were quantified. Knee moment data were

normalised to body mass and body height and were expressed as Nm/kgm. Time data for all

variables were normalised to stance phase time (identified as the period from initial contact

of the foot to toe-off, as determined by the force platforms’ 10 N threshold readings) to

facilitate comparison between participants. Knee angles and moments were divided into

multiple phases within stance using previously established definitions (Besier, Lloyd,

Ackland, & Cochrane, 2001) and were identified using a custom Matlab (The MathWorks,

Inc., Natick, MA, USA) programme and are detailed as follows: initial contact, the time

where the vertical ground reaction force is higher than 10 N; weight acceptance, the average

between initial and the first trough in the vertical ground reaction force trace (the first 20% of

stance); peak push-off, the average of 10% either side of the peak vertical ground reaction

force; final push-off, the average of the last 15% of stance in the vertical ground reaction

force. Knee power and knee flexion/extension velocity were only divided into two phases

(braking and propulsive), as the two peaks of interest (power absorption/knee flexion velocity

and power production/knee extension velocity) are seen to occur in these phases and are

defined as follows: braking phase, initial contact of the foot to zero knee flexion velocity;

propulsive phase, zero knee flexion velocity to toe-off. Although it has been speculated

(Boden et al., 2000) that the majority of non-contact ACL injuries tend to occur during

4 S.R. Brown et al.

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weight acceptance, the purpose of this study was to illustrate a more complete picture of the

kinematics and kinetics of sidestepping; therefore, multiple phases of stance were examined.

Statistical analysis

To describe the results in detail, the following statistical analyses were performed: two-tailed,

paired Student’s t-test comparing the preferred and non-preferred legs at a significance level

of 0.05 were conducted in SPSS (Version 19.0 for Windows, SPSS Inc., Chicago, IL, USA);

the standard error of the measurement, the difference between the means (the mean of the

non-preferred leg minus the mean of the preferred leg) and 90% confidence limits were

calculated using Excel spreadsheets ( post-only crossover) found at Sportsci.org. The

magnitude of the difference was then assessed by standardisation; i.e. the difference between

the means was divided by the standard deviation of the preferred leg. The preferred leg was

chosen as the reference leg in this study as it is commonly chosen for analysis purposes

(Brophy, Backus, et al., 2010). For assessing the magnitude of standardised effects,

threshold values of 0.2, 0.6, 1.2, 2.0, and 4.0 represent small, moderate, large, very large,

and extremely large differences, respectively (Hopkins, Marshall, Batterham, & Hanin,

2009). Uncertainty in the estimates of effects on leg preference was expressed at 90%

confidence limits and as probabilities that the true value of the effect was substantially

positive and negative. Qualitative probabilistic inferences regarding the true effect were then

made, as described in detail elsewhere (Hopkins et al., 2009). In summary, if the

probabilities of the true effect being substantially positive and negative were both .5%, the

effect was expressed as unclear; otherwise, the effect was clear and expressed as the

magnitude of its observed value. The scale for interpreting the probabilities was as follows:

25–74%, possibly (*); 75–94%, likely (**); 95–99.5%, very likely (***); and.99.5%, most

(or extremely) likely (****).

Results

Approach speeds while sidestepping off the preferred and non-preferred kicking leg did not

differ substantially between legs at initial contact (4.33 ^ 0.36 m/s and 4.23 ^ 0.29 m/s,

respectively) and held similar stance times in both limbs (0.22 ^ 0.02 m/s).

Knee flexion angles (Table I) in the non-preferred leg were slightly smaller than those in

the preferred leg during initial contact (ES ¼ 0.19, -5.3%), weight acceptance (ES ¼ 0.28, -

4.5%), and peak push-off (ES ¼ 0.36, -3.5%). Knee abduction angles in the non-preferred

leg were slightly greater than those in the preferred leg during weight acceptance

(ES ¼ 0.10, þ 9.8%) but this effect was only trivial. Knee internal rotation angles in the

non-preferred leg weremoderately greater than in the preferred leg during weight acceptance

(ES ¼ 0.64, þ 32%) and slightly greater at peak push-off (ES ¼ 0.58, þ 25%) and final

push-off (ES ¼ 0.22, þ 20%). All other kinematic differences between legs were unclear.

Knee extensor moments (Table II) in the non-preferred leg were slightly lower than those

those in the preferred leg during peak push-off (ES ¼ 0.31, -5.8%) and final push-off

(ES ¼ 0.30, -22%). Knee abductor moments in the non-preferred leg were slightly greater

than in the preferred leg during weight acceptance (ES ¼ 0.22, þ 19%). Knee internal

rotator moments in the non-preferred leg were slightly lower than those in the preferred leg

during peak push-off (ES ¼ 0.42, -19%) and final push-off (ES ¼ 0.18, -11%). All other

kinetic differences between legs were unclear.

Figure 1 shows a depiction of preferred and non-preferred knee power and knee flexion/

extension velocity during the sidestep manoeuvre. Peak power absorption and peak knee

Leg preference and knee mechanics in female footballers 5

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flexion velocity in the non-preferred leg were slightly greater than in the preferred leg

(ES ¼ 0.14, þ 5.1% and ES ¼ 0.090, þ 2.7%, respectively). Peak power production and

peak knee extension velocity in the non-preferred leg were slightly lower than those in the

preferred leg (ES ¼ 0.34, -8.3% and ES ¼ 0.21, -5.2%, respectively).

Discussion and implications

Nearly 70% of female ACL injuries occur to the non-preferred leg even after accounting for

sport and injury mechanism (Brophy, Silvers, et al., 2010). However, the mechanisms of

ACL injury in females are far from being understood. The purposes of this study were to

examine the mechanical differences at the knee between preferred and non-preferred legs

and to estimate the magnitude of the effects of leg preference on knee mechanics during the

sidestep manoeuvre. This study assessed the probability of the risk of ACL strain due to the

effect of leg preference in female sidestepping.

During weight acceptance of sidestepping, the body is decelerating, relying on gross motor

skills to absorb the body’s force and stabilise the surrounding structures. The quadriceps

crossing the knee are lengthening, facilitating energy absorption around the knee joint

(Negrete et al., 2007). Knee joint power absorption indicates that kinetic energy is being

absorbed by the knee musculature and connective tissues such as the ACL. The ACL

experiences high tension when the knee joint angle nears the end points between 08 and 408of flexion during sidestepping-related activities (Draganich & Vahey, 1990; Yu & Garrett,

2007). In this study, during initial contact of sidestepping, the average knee flexion angle was

approximately 268 and increased to as much as 608 through peak push-off, suggesting that

the ACL is in a position of undergoing high tension. When compared to the preferred leg, the

Table I. Summary of knee joint angles during sidestepping and inferences for change of the means.

Non-preferred–preferred

Preferred

(8)

Non-preferred

(8)

p SEM Mean change;

^90% CL

Qualitative

inference

Flexion (þ)/extension (-)

Initial contact 27^ 7 25^ 6 0.258 3.3 -1.4;^ 2.1 Trivial*

Weight acceptance 35^ 5 33^ 5 0.117 2.6 -1.6;^ 1.6 Small* negative

Peak push-off 49^ 4 47^ 6 0.221 3.8 -1.7;^ 2.3 Small* negative

Final push-off 19^ 4 19^ 5 0.505 3.5 -0.85;^ 2.17 Unclear

Abduction (-)/adduction (þ)Initial contact -4.4^ 5.3 -4.3^ 3.2 0.986 2.8 0.017;^ 1.730 Unclear

Weight acceptance -7.0^ 6.3 -7.7^ 4.1 0.549 3.2 -0.69;^ 1.97 Trivial*

Peak push-off -9.0^ 6.6 -10^ 4 0.634 4.5 -0.78;^ 2.82 Unclear

Final push-off -4.6^ 4.2 -4.6^ 2.7 0.933 2.5 0.076;^ 1.557 Unclear

Internal (þ)/external rotation (-)

Initial contact 9.3^ 7.1 11^ 5 0.457 4.6 1.2;^ 2.8 Unclear

Weight acceptance 10^ 5 13^ 4 0.029 3.8 3.2;^ 2.3 Moderate** positive

Peak push-off 15^6 18^ 6 0.012 3.7 3.7;^ 2.3 Small*** positive

Final push-off 7.4^6.3 8.8^ 5.5 0.261 3.5 1.4;^ 2.2 Small* positive

Note: Values areM^SD, standard error of measurement (SEM) and mean change; ^confidence limits (CL) (90%).

(þ) indicates that a positive kinematic value is associated with the corresponding knee joint angle; (-) indicates that a

negative kinematic value is associated with the corresponding knee joint angle. Trivial, small, moderate and large

inference: *possibly, 25–74%; **likely, 75–94%; ***very likely, 95–99.5%. Positive, negative¼ substantial positive

and negative changes with non-preferred relative to preferred kicking leg.

6 S.R. Brown et al.

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Table

II.Summary

ofknee

jointmomen

tsduringsidesteppingandinferencesforch

angeofthemeans.

Non-preferred

–preferred

Preferred

(Nm/kg/m

)Non-preferred

(Nm/kg/m

)p

SEM

Meanch

ange;

^90%

CL

Qualitativeinference

Flexor(-)/extensor(þ

)

Initialco

ntact

-0.73^

0.27

-0.75^

0.31

0.672

0.14

-0.022;^

0.089

Unclear

Weightaccep

tance

0.62^

0.45

0.58^

0.43

0.663

0.28

-0.044;^

0.175

Unclear

Peakpush-off

2.6

^0.5

2.5

^0.4

0.196

0.32

-0.15;^

0.20

Small*

negative

Finalpush-off

0.28^

0.19

0.21^

0.12

0.178

0.12

-0.061;^

0.076

Small*

negative

Abductor( þ

)/adductor(-)

Initialco

ntact

0.16^

0.12

0.17^

0.09

0.568

0.065

0.013;^

0.040

Unclear

Weightaccep

tance

0.18^

0.15

0.22^

0.13

0.189

0.071

0.035;^

0.044

Small*

positive

Peakpush-off

-0.010^

0.292

0.0072^

0.1917

0.753

0.15

-0.017;^

0.091

Unclear

Finalpush-off

0.027^

0.098

0.015^

0.033

0.611

0.064

-0.012;^

0.040

Unclear

Internal(-)/externalrotator( þ

)

Initialco

ntact

-0.18^

0.22

-0.17^

0.21

0.828

0.11

0.0084;^

0.0668

Unclear

Weightaccep

tance

-0.71^

0.33

-0.69^

0.29

0.809

0.23

0.020;^

0.143

Unclear

Peakpush-off

-1.2

^0.5

-0.94^

0.45

0.088

0.35

0.23;^

0.22

Small**positive

Finalpush-off

-0.51^

0.29

-0.46^

0.20

0.314

0.15

0.055;^

0.092

Trivial*

Note:Values

are

M^

SD,standard

errorofmeasuremen

t(SEM

)andmeanch

ange;

^co

nfiden

celimits(C

L)(90%).(þ

)indicatesthatapositive

kineticvalueisassociated

withtheco

rrespondingknee

jointmomen

t;(-)indicatesthatanegativekineticvalueisassociatedwiththeco

rrespondingknee

jointmomen

t.Trivial,sm

all,moderate

and

largeinference:*possibly,25–74%;**likely,

75–94%.Positive,negative¼

substantialpositive

andnegativech

anges

withnon-preferred

relative

topreferred

kickingleg.

Knee

jointmomen

tswerenorm

alisedto

bodymass

andbodyheight.

Leg preference and knee mechanics in female footballers 7

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knee joint in the non-preferred leg exhibited greater peak power absorption, indicating

greater energy absorption in the knee structure. Furthermore, the peak knee flexion velocity

during the braking phase (,17% of the sidestep, similar to weight acceptance) was shown to

be greater in the non-preferred leg compared to the preferred leg. An increased rate of knee

flexion could result in increased rate of ACL lengthening. As tension developed in

viscoelastic tissue is dependent on mechanical loading and loading rate (Nordin & Frankel,

2012), the ACL of the non-preferred leg may experience greater tensile loading during

weight acceptance than that of the preferred leg. In addition, knee internal rotation angles

and knee abductor moments increased substantially in the non-preferred leg during weight

acceptance, potentially placing the ACL under dangerously high tension (Wascher et al.,

1993). It has been noted that during weight acceptance in sidestepping, individuals exhibit

increased knee abductor moments when pivoting on the plant leg to initiate the change-of-

direction (Besier, Lloyd, Cochrane, et al., 2001; McLean, Su, & van den Bogert, 2003).

Thus, increased power absorption and higher knee flexion velocity coupled with a greater

knee internal rotation angle and a greater knee abductor moment may place the ACL in the

non-preferred leg in higher tensile strain during weight acceptance. The possible higher

tensile strain may result in increased risk of ACL injury in non-preferred legs. Findings from

this study support the previous epidemiology report (Brophy, Silvers, et al., 2010) that 70%

Figure 1. Comparison of knee joint: power in watts per kilgroam of body weight (A) and flexion/extension velocity in

degrees per second (B) between the preferred and non-preferred kicking leg in female footballers during

sidestepping. Data are normalised to sidestep manoeuvre time; error bars are equivelant to one standard deviation;

and the vertical line indicates the division of the braking phase (initial contact of the foot to zero knee flexion velocity)

and propulsive phase (zero knee flexion velocity to toe-off).

8 S.R. Brown et al.

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of female ACL injury occurs in the non-preferred limb during sports requiring a sidestepping

component.

During peak push-off and final push-off of the sidestep movement, the body is accelerating

towards the direction of the cut, again relying on gross motor skills to transfer the absorbed

force in the new direction. The quadriceps are shortening and the hamstrings are

lengthening as the knee joint is extending. There is power production around the knee joint,

which reflects energy flowing through the knee joint to increase the kinetic energy of the

moving body. Interestingly, the preferred leg demonstrated a greater increase in power

production than the non-preferred leg. In addition, an increase in peak knee extension

velocity was seen to occur to the preferred knee. It seems that peak push-off is executed faster

by the preferred leg than the non-preferred leg, confirming that skill level may hold an

inference on cutting mechanics (Sigward & Powers, 2006). If the preferred leg is more skilful

in performing a sidestep compared to the non-preferred leg, then an increase in power

production in the preferred leg would be expected in the sidestep manoeuvre. Since the

preferred leg is determined by the leg with which the participant would kick a football the

furthest, it is expected that the preferred leg would be more skilled in hip flexion and knee

extension, which are common components in a football kick (Brophy, Backus, et al., 2010;

Brophy, Backus, Pansy, Lyman, & Williams, 2007). However, as the knee extension velocity

increases, the ACL lengthening also increases at a fast rate. Increasing lengthening velocity in

soft tissue results in pronounced tension development (Nordin & Frankel, 2012). Thus, the

ACL may experience an increase in tension. Furthermore, the preferred limb exhibited a

greater knee internal rotator moment compared to the non-preferred limb during peak push-

off and final push-off, potentially further placing tension on the ACL. Therefore, increasing

power production with increased knee extension velocity coupled with increased knee

internal rotator moment may expose the ACL of the preferred leg to high tension.

An increased risk of ACL injury associated with the preferred leg may be possible during

peak push-off of the sidestep manoeuvre.

In summary, mechanical differences between the preferred and non-preferred legs during

sidestepping were found. The knee of the non-preferred leg displayed small increases in

abduction angle and abductor moment, a moderate increase in internal rotation angle and

small increases in power absorption and knee flexion velocity during weight acceptance. The

preferred leg, on the other hand, demonstrated small increases in flexion angle, flexor

moment, and internal rotator moment during peak push-off and small increases in power

production and knee extension velocity compared to the non-preferred leg during

sidestepping. It appears that the non-preferred leg could be prompted to increased risk of

ACL injury during weight acceptance, while the preferred leg may face increased risk of ACL

injury during peak push-off.

It is necessary to address a few limitations in this study. First, we did not measure the speed

of the centre-of-mass at toe-off. Although participants elicited their 100% effort to perform

sidestepping in both directions, it is unclear whether the preferred limb changes direction

faster than the non-preferred limb as a result of better motor skills. Second, we did not

measure muscle volumes of the knee extensor and flexors; however, we evaluated the

isometric and isokinetic strength of the knee extensors and flexors. Both limbs possessed

comparable extensor and flexor strength; thus, the differences in knee mechanics between

the limbs did not likely originate from a difference in limb strength. Third, we expressed the

probability levels of the risk of ACL strain based on scaled differences in knee mechanics

between the two limbs. During which, the knee mechanics of the preferred limb served as the

reference leg. Thus, results from this study may not be compared to other ACL studies at this

time.

Leg preference and knee mechanics in female footballers 9

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Conclusion

During sidestepping, a female’s non-preferred limb experiences increased knee power

absorption, knee flexion velocity, abduction angle, and internal rotation angle during weight

acceptance, which may expose the ACL to an increased risk of high strain. The preferred

limb, however, experiences increased knee power production, knee extension velocity, and

internal rotator moment during peak push-off, which may result in an increased risk of ACL

strain.

Practical implications:

. Although small, differences in knee mechanics during sidestepping do exist between

legs, that is both knees react differently to sidestepping in each direction.

. Strength and conditioning staff should understand the above implication and

incorporate more single-leg deceleration/acceleration movements into their athletes’

strength training routines.

. An emphasis should be placed on proper landing techniques, joint stability, and

awareness and the appropriate utilisation of lower-extremity power.

Acknowledgements

The authors thank all the players, coaches, and strength and conditioning staff on the Ball

State University woman’s football team for their participation; Erin R. Feldman and Keith

D. Suttle for their contribution to the structure, organisation, and support of the study;

Conrad D. Schubert, Brandi R. Mixell, and Ryan P. Hubble for their assistance with data

collection; and Will Hopkins for his involvement in the analysis of this study.

Conflict of interest

None of the authors had a conflict of interest to declare.

Funding

This study was funded, in part, by a Ball State University ASPiRE grant (I513-12) and Scott

R. Brown was funded by the AUT Vice Chancellors PhD scholarship. These sponsors had

no such involvement with the study design, data collection, analysis, and interpretation of

data, in the writing of this manuscript, or in the decision to submit for publication.

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