Effect of turf on the cutting movement of female football players

Gerda Strutzenberger,Hue-Man Cao,Janina Koussev,Wolfgang Potthast, Gareth Irw in

aSchool of Sport,Cardiff Metropolitan University,Cardiff CF236XD,UK

bDepartment of Sport and Sport Science and Kinesiology,University of Salzburg,Hallein-Rif 5400,Austria

cInstitute of Biomechanics and Orthopaedics,German Sport University Cologne,Cologne 50933,Germany

Original article

Effect of turf on the cutting movement of female football players

Gerda Strutzenbergera,b,*,Hue-Man Caoa,c,Janina Kousseva,c,Wolfgang Potthastc, Gareth Irw ina,c

aSchool of Sport,Cardiff Metropolitan University,Cardiff CF236XD,UK

bDepartment of Sport and Sport Science and Kinesiology,University of Salzburg,Hallein-Rif 5400,Austria

cInstitute of Biomechanics and Orthopaedics,German Sport University Cologne,Cologne 50933,Germany

Purpose:The globalisation of artificial turf and the increase in player participation has driven the need to examine injury risk in the sport of football.The purpose of this study was to investigate the surface—player interaction in female footballplayers between naturaland artificial turf. Methods:Eightuniversity level female footballplayers performed an unanticipated cutting manoeuvre atan angle of30°and 60°,on a regulation naturalgrass pitch(NT)and a 3G artificial turf pitch(AT).An automated active maker system(CodaSport CXS System,200 Hz)quantified 3D jointangles at the ankle and knee during the early deceleration phase of the cutting,defined from footstrike to weightacceptance at20%of the stance phase.Differences were statistically examined using a two-way(cutting angle,surface)ANOVA,w ith anαlevelof p＜0.05 and Cohen’s d effect size reported.

Artificial turf;Football-specific movement;Gender;Injury risk;Kinematics

1.Introduction

Over the last decade artificial turf(AT)has been promoted as a viable alternative to natural turf(NT)by the major sporting international governing bodies,which utilise these playing surfaces(e.g.,Fe´de´ration Internationale de Football Association(FIFA),International Rugby Board(IRB),Rugby League(RL),National Football League(NFL),International Hockey Federation(FIH)).The rationale behind this promotion is based on,fi rstly economic reasons:AT reduces the cost of maintaining a grass-based surface,which is particularly challenging across diverse environmental and climatic conditions.Secondly,consistency of playing surface w ill provide a more congruent playing surface globally.Finally,providing longer playing hours,as well as a multi-purpose application support the global health agenda.

These surfaces have been particularly promoted and installed in professional football communities,w ith the 3rd generation(3G)AT being the most common system.1A 3GAT system is typically installed on a rigid base layer and consists of an elastic layer,an artificial grass carpet and infi ll material between the grass fibres.2Against the benefi ts stands the generally negative perception of male players on playing on AT w ith a subjective feeling of poorer ball control and greater physical effort3and greater difficulties in cutting.4Female football players in this Swedish study demonstrated a different response pattern.Both regular AT and NT players reported,no general influence of AT on the game but felt that running with the ball and passing was easier on AT.

Independent of gender,the players psychological perceptions identified a perceived higher injury risk when playing on AT.5These psychological observations were partly supported by epidem iological research6exploring football injuries on 3rd and 4th generation AT,which suggested an increased risk of ankle injury on AT.However,a recentepidemiological metaanalysis of football injuries,summarised the risk of injury by playing on different surfaces(AT—NT)from eight published studies7draw ing the conclusion that competing or training on AT generally reduces the risk of injury compared to NT.Another recentstudy identified generally no differences in acute injury rates when playing on AT compared with NT, butdemonstrated,that clubs w ith AT at their home venue had higher rates of acute training injury and overuse injury compared with clubs that play home matches on NT.8

Additionally the role of gender and the surface effects are inconsistently reported in the literature.Generally,knee and ankle injuries are the most common injuries for female football players.9,10Additionally they sustain a 2—3 times higher risk of ACL-rupture than their male counterparts.10,11While Fuller et al.10and Meyers12identified a lower injury risk for women on AT,Steffen et al.13found a trend towards higher risk of ankle sprains for female football players below the age of 17.Additionally,young female football players were very likely to sustain severe injuries on AT.6During training Fuller etal.9reported a higher risk ofankle sprains in men on AT,but no differences for women.Over a 5-year period,Soligard et al.14reported no difference in overall injury risk between AT and NT for male and female players.

These epidem iological studies provide useful information about the frequency and trends in injury occurrence.However, there is stilla gap between these descriptions and the aetiology of injury risk,w ith considerations forgender,age,and turf still under represented.Some evidence exists that surface changes lead to alterations in football-specific movementpatterns2,15,16of male football players,but to date no research was found by the authors,which investigates surface-induced effects on the movementof female footballplayers.Playing on AT includes, for example,increased peak torque and different rotational stiffness properties of shoe—surface interaction,decreased impact attenuation properties of surfaces and differing foot loading patterns.6While the approach velocity remained constant,the laststep to a kick was decreased on a rubberand sand fi lled artificial surface leading to a“more cautious braking behavior”.16Since female football players respond differently to football injury and perception of the AT than their male counterparts,investigating the female specific movements on different surfaces could enhance the understanding of injury risk and improve the quality of these surfaces.As approximately 50%of all season ending injuries during match play in female footballare ACL-tears,10itseems worthwhile investigating a movement task that is commonly representative for this injury.Female athletes tend to demonstrate less knee flexion,more knee valgus angles,greater quadriceps activation,and lower hamstring activation in cutting and running tasks than male athletes.17In non-contact situations an extended knee position(up to 30°)18—21as well as an anterior tibial draw combined w ith valgus and internal rotation moments22—24could induce excessive loads on the ACL causing it to rupture.Thirty-seven percent of the noncontact ACL injuries occur during cutting manoeuvres,followed by 32%in landings,16%land and steps,10%stopping/ slowing,and 5%crossover-cut manoeuvres.18Further,unanticipated cuttings are more likely to represent the movements during a game situation and are described w ith an increased risk of injury compared to anticipated cuttings.25

Therefore,the purpose of this study was to investigate the lower limb kinematics on differentsurfaces in female football players during an unanticipated cutting manoeuvre.This could lead to a more comprehensive know ledge of player—surface interaction and provide further understanding of the mechanism of injury risk and enhancement of artificial surfaces in football.Itwas hypothesised that AT would lead to increased contact times,no alterations in knee positions buthigherankle dorsiflexion,inversion,and rotational angles.

2.M aterials and methods

2.1.Participants and surfaces

Eight female university level football players(age: 21.5 ± 2.1 years;height:162.8 ± 7.1 cm;weight: 66.0±8.5 kg;football experience:13.3±4.1 years)participated in the study.The institutional ethical review board approved the study and additionally a w ritten consent form prior to participating was signed by allathletes.Athletes were free from injury over a 6-month period prior to testing.Leg dom inance was determ ined by the leg instantaneously used for a single-legged forward jump and only right-leg dom inant players were included in the study.Participants used their individual football shoes,which they would use on both AT and NT.

The data collection was performed on two neighbouring pitches:the natural surface pitch(NT)was a natural grass pitch approved fornational competition,and the AT pitch was a 2-star FIFA approved 3G AT pitch.As this was an outdoor testing,each participant underwent data collection for both surfaces in one session to keep the influence of weather and temperature change at a m inimum.

2.2.Data collection

A testing session consisted of an individual warm-up, habituation phase and data collection on surface A followedby data collection on surface B,whereas the order of the surfaces(NT,AT)was random ized.The habituation phase consisted of 5—10 cutting trials to fam iliarise the participants w ith the movementand the predetermined approaching speed of 4—5 m/s.17The movement contained an acceleration phase ofmaximum 8 m before cutting w ith a change of direction in a 30°or 60°angle,followed by a 5-m acceleration phase before decelerating and finishing the manoeuvre.The angle of the cut was predetermined and visually displayed by cones,butas the cutting direction(to the right or left side)was desired to be unanticipated,the participants received the direction of the cut in the acceleration phase by light signals in a random ised order.The data collection consisted of eightunanticipated cuts at 30°and 60°angle on each surface,leading to four cuts to the left and right side for each cutting angle and surface.A trial was declared successful when the predeterm ined speed and cutting point was hit.

Kinematic data were collected by an outdoor 3D motion capture analysis system(CodaSport CXS System;Charnwood Dynam ics Ltd.,Rothley,Leicestershire,UK)which collected data of active markers by two scanners w ith a sampling frequency of 200 Hz.Thirty active markers were placed on anatomical landmarks of the left lower limb and pelvis according to the Cleveland Clinic Lower Body Markerset(Motion Analysis Corp,Santa Rosa,CA,USA)to calculate knee and ankle joint angles in the sagittal,frontal,and transverse planes.Scanners were positioned to detect each marker by at least one scanner throughout the entire contact phase of the cutting movement.Approach velocity was determ ined via two pairs of infrared velocity tim ing gates(SMARTSPEED, Fusion Sport International,Coopers Plains,Australia),placed at the fi fth meter before the cutting point(Fig.1).

Fig.1.Measurement set-up.

2.3.Data analysis

Processed(labelled and gap fi lled)trajectory data were inserted in Visual 3D software(V3D,C-motion,Rockville, MD,USA)for further analysis.The trajectory data were fi ltered using a 4th order Butterworth fi lter implemented in the V3D software w ith 20 Hz.Stance phase was defined as the period from initial contactof the foot to toe off.These events were determ ined using acceleration data of active marker placed on the 5th metatarsophalangeal joint and 2nd interphalangeal joint follow ing a procedure described by Ast et al.26Early deceleration of the cutting movement was defined as beginning w ith foot strike(FS)at initial contact until weight acceptance(WA)at 20%of the stance phase.27The 3D ankle and knee angles were calculated via the 6°of freedom model inserted in the V3D software.28Data were time normalised during the early deceleration phase,as the majority of non-contact ACL injuries are reported to occur during this phase.23,27As parameters the sagittal,frontal,and transversal ankle and knee angles at FS and WA were determ ined.

Statistical analysis was calculated via a two-way(cutting angle,surface)ANOVA with repeated measurements,using SPSS 20 statistical software(IBM SPSS Statistics,20.0, Chicago,IL,USA).Significance levels were set atp＜0.05. Effect sizes were calculated using the partial eta squared (small:0:01＜η2p＜0:05;medium:0:06＜η2p＜0:13;large: η2p＞ 0:14)for main effects and Cohen’sdvalue(small: 0.20＜d＜0.49;medium:0.50＜d＜0.79;large:≥0.80)for interaction effects.Due to the low sample number medium and large effect sizes w ill also be discussed as indicator for movement changes.

3.Results

3.1.Globaleffect turf and interaction with cutting angle

The ground contact times did not reveala significant effect of the surface(p=0.465)and were on average for the 30°cut 0.180±0.020 s and 0.180±0.015 s on NT and AT,and for the 60°cut 0.185±0.015 s and 0.190±0.015 s on NT and AT,respectively.

At the ankle(Table 1)no significant effects were found for the main surface effect at FS and WA.However,large effect sizes appeared atFS for the ankle dorsiflexion angle(increased (mean over both cutting angles=factor surface)on AT compared to NT by 2.8°,η2p=0:15,p=0.303)and ankle inversion angle(increased on AT compared to NT by=0:19,p=0.243).At the point of weight acceptance large effect sizes remained for the ankle inversion angle(increased on AT compared to NT by 2.7°,η2p=0:16,p=0.284)and occurred for the ankle external rotation angle(decreased on AT compared to NT by 1.3°,η2p=0:35,p=0.091).Additionally,the interaction effect of the surface w ith the cutting angle demonstrated that the ankle inversion position at the 60°cutwas decreased for both FS(by 5.4°)and WA(by 5.0°),on the AT compared to the NT,while at the 30°cut no effect became evident.This reached significance level w ith higheffect size at FS(d=0.54,p=0.004)and a medium effect size at WA(d=0.52,p=0.115).

Table 1 Ankle and knee angles(°)(mean±SD)in 3D at foot strike and weightacceptance for two surface conditions(artificial(AT)and natural(NT))and two cutting angles(30°and 60°).

At the knee(Table 1)comparison of the surface showed a significant effectof surface type on the internal knee rotation angle of the knee(decreased on AT compared to NT by 5.4°, η2p=0:44,p=0.050)at FS.Atweightacceptance a large but insignificant effect remained(by 2.3°,η2p=0:19,p=0.092). Additionally large effect sizes w ithout reaching significant difference occurred for the knee valgus position at FS (decreased on AT compared to NT by 1.6°,η2p=0:21,p=0.217)and WA(decreased on AT compared to NT by 3.2°, η2p=0:35,p=0.084).The interaction effect of the surface w ith the cutting angle revealed medium and large but insignificant effect sizes for the knee valgus angle at FS by 3.1°(d=0.77,medium effect,p=0.094)and at WA by 5.1°(d=0.97,high effect,p=0.114),indicating an increased valgus positions at the 30°cuton NT compared to AT.The 30°cut on NT additionally seemed based on a medium effect to lead to a higher knee internal rotation by 5.6°(d=0.51, medium effect,p=0.235)at FS.

3.2.Global effect of the cutting angle

The ground contact times for the cut were w ith 0.190 s significantly higher for the 60°cut than for the 30°cut (0.180 s)（η2p=0:51;p=0:03）.The kinematic comparison of the effect of the cutting angle revealed for the 30°cut a significantly increased ankle dorsiflexion angle at FS by 2.8°（η2p=0:53;p=0:027）and WA by 2.1°（η2p=0:45; p=0:048）.The 30°cut indicates with large effect sizes an increased ankle inversion at FS by 1.4°（η2p=0:20;p=0:222）and WA by 1.6°（η2p=0:27;p=0:149）,as wellas a decreased external ankle rotation at FS by 0.8°（η2p=0:20;p=0:135）(Table 1).

Similarly to the ankle dorsal flexion angle the knee was significantly more flexed for the 30°cutting angle at FS by 4.4°（η2p=0:69;p=0:005）regardless of surface.

4.Discussion

The globalisation of AT across many football codes,w ith the combined increase in participation,has driven the need to exam ine the influence of surface on the injury risk.The purpose of this study was to investigate the surface—player interaction in female football players for an unanticipated cutting manoeuvre.Due to the low population number,medium and large effect sizes are discussed as a tendency towards a difference.Female athletes displayed a tendency to alterations mainly in the frontal and rotational plane of the knee and ankle w ith increased ankle inversion and external rotation angles and increased knee valgus angles as well as knee internal rotation angles for the AT in comparison to the NT.The only effectshow ing in sagittalplane was an increased ankle dorsiflexion at initial contact on AT.The ankle and knee joint angle strategies demonstrated by the female participants of this study revealed a movement strategy,which might be beneficial towards a lower risk of ACL injury on AT.

Ground contact times for the cutdid notdiffer between the two surfaces.As the participants approached the cut w ith the same velocity,this could give some indication of sim ilar grip properties.29

Non-contact ACL injuries are often described to occur in a position atwhich the knee is in a low flexion angle in combination w ith an increased knee valgus and internal rotation angle.19—22,24,30An increased ankle eversion and pronation may furtherpreload the ACL.31However,the cause and effectof the kinematics and ligament rupture are not yet fully understood. Currentevidence attained by investigatingin vivoloading patterns of the ACL suggests that the low flexion angle in combination w ith a posterior orientated ground reaction force is the primary cause of overloading the ACL.19,20The additional valgus and internal rotation position have the potential to slightly increase this load,20,30,32butplay a m inor role in terms of ACL-rupture.19,20Amore recentstudy even suggests thatthe addition of a valgus collapse pattern to a knee flexion angle of 30°does reduce the length of the ACL compared to the 30°flexed position only,indicating a lower strain on the ACL.21

Difference between female and male valgus angle during cutting on NT has been reported to be approximate 11°.17Additionally,Hewett et al.33identified an 8°difference in valgus angle during a jump-landing task between participants w ith an ACL-injured knee and participants w ith a healthy knee.The current study showed that there was a tendency towards a lower knee valgus angle by 1.6°—3.2°between differentsurfaces.Keeping in mind that the knee flexion angle did not significantly change between AT and NT,theimplications of these results are that cutting on AT does not appear to yield an increased ACL-injury risk for the female knee.The decrease in varus angle in combination w ith the observed decreased knee internal rotation angle and tendency toward an increased ankle inversion could further indicate that cutting on AT mighteven beara slightly lower injury potential than cutting on NT.The findings of this study support the literature demonstrating equal7,9,13,14and lower knee injury occurrence10,12for female footballplayers on AT and provide, on a basic kinematic level,a possible mechanistic explanation.

Frontal and transversal knee kinematics are challenging to capture and are susceptible to soft tissue artefacts.To keep this error to a m inimum,the Cleveland Clinical Markerset was applied,which uses anatom ical landmarks for static calibration in combination w ith cluster markers for targeting the movement.The 6°of freedom modelimplemented in V3D usesa least error approach to take movement artefacts into account. A lthough,the valgus angles in this study are higher than reported in previous research,27,31,34this may be partly due to the higher BM Iof the currentparticipants.However,the change in the range ofmotion of the valgus angle during the early deceleration phase corresponds well with these previous studies. Additionally,the reported intra-individual changes on knee valgus motion between AT and NT are consistent for each participant,which strengthens the confidence,that the demonstrated surface effectsoccur.The effectof the differentsurfaces on the ankle is less evident.Even though ankle sprains are among the most common ankle injuries,the mechanisms leading to the injury are unclear.The primary risk factorseems to be having sustained a previousankle sprain35,36and the majority of ankle sprains present an increased inversion or supination mechanism.37An increased plantar flexion at touchdown might also bear an increased risk.36However,as reported by Arnason etal.,38itwasnotpossible to identify football-specific screening tests to identify an increased risk of ankle sprain pre-injury, apart from having sustained a previous ankle strain.This study revealed on AT a tendency towards an increased dorsiflexion angle at touchdown,a trend towards higher external rotation atweightacceptance and for the 30°cut an increased inversion at the beginning and end of the early acceleration phase.Hence,no clear strategy to support or refute increased ankle injury risk derived outof thisstudy,and further research is needed to fully understand the surface—player effect on the ankle joint.

The current study has shown surface-induced alterations occurred in the kinematics of female football players,a more indepth analysis including ground reaction forces,joint kinetics,and EMG could reveal additional information and increase ourunderstanding of the interaction between the female player and the different surface systems in football-specific situations.It has to be noted that a variety of 3G AT systems exists and the differences in movement between ATs could become greater than between AT and NT.2Therefore the results of this study can only be applied to the differences between the specific AT and NT used.

Athletes wore the same football boot,which they would wear on both surfaces,which m ight not be the football boot used in match play.However,boot type(studdedvs.bladed) did not seem to impart differences in knee loading when used on AT,39and this approach allowed an investigation on surface-induced rather than shoe-induced effects.As the movement changes induced by AT are not well understood, and gender related responses mightbe affected by a variety of different aspects,such as climatic exposure,boot type,or playing experience,a number of key research questions remain unanswered,and our understanding of the influence of artificial surfaces needs to be further developed.These investigations should address more factorial approaches as including males and differentsoccer relevantmovements(e.g., straight runningvs.cutting w ith different angles).Finally,the present study investigated only a small sample size,as such, the findings should be interpreted w ith care and only can point out tendencies towards the discussed kinematic changes. Using a higher sample size could possibly lead to not only sim ilar or decreased effect sizes,but also current nonsignificant differences could become significant.

5.Conclusion

The overallpurpose of thisstudy was to investigate the lower limb kinematics on differentsurfaces in female footballplayers during an unanticipated cutting manoeuvre.The major finding of thisstudy wasthatthere wasno evidence to suggestthatthere isan increased risk of injury when performing w ith the same movement speed on an AT.The knee kinematics suggested that the ACL risk factorswerereduced in some cases.The ankle response was less clearand further investigation into this specific joint is needed.Significant changes in environmental conditions,as in this case through the playing surface,mustoccur in parallel to detailed biomechanicsanalyses,which can provide a mechanism of quantifying changes in performance and identifying whether there is a concurrentchange in injury risk.

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Received 4 June 2014;revised 12 June 2014;accepted 3 July 2014 Available online 7 August2014

*Corresponding author.School of Sport,Cardiff Metropolitan University, Cardiff CF236XD,UK.

E-mail address:Gerda.Strutzenberger@sbg.ac.at(G.Strutzenberger)

Peer review under responsibility of Shanghai University of Sport.

2095-2546/$-see front matter CopyrightⒸ2014,Shanghai University of Sport.Production and hosting by Elsevier B.V.A ll rights reserved. http://dx.doi.org/10.1016/j.jshs.2014.07.004

Results:A trend was observed on the AT,w ith a reduction in knee valgus and internal rotation,suggesting a reduced risk of knee injury.This findings highlight that AT is no worse than NT and may have the potential to reduce the risk of knee injury.The ankle joint during foot strike showed large effects for an increase dorsiflexion and inversion on AT.A large effect for an increase during weightacceptance was observed for ankle inversion and external rotation on AT.

Conclusion:These findings provide some support for the use of AT in female football,w ith no evidence to suggests that there is an increased risk of injury when perform ing on an artificial turf.The ankle response was less clear and further research is warranted.This initial study provides a platform for more detailed analysis,and highlights the importance of exploring the biomechanical changes in performance and injury risk w ith the introduction of AT.

CopyrightⒸ2014,Shanghai University of Sport.Production and hosting by Elsevier B.V.A ll rights reserved.