Lower extrem ity injury in female basketball players is related to a large difference in peak eversion torque between barefoot and shod conditions

Jennifer M.Yentes,Mx J.Kurz,Nichols Stergiou,c,*

aNebraska Biomechanics Core Facility,Department of Health,Physical Education and Recreation,University of Nebraska at Omaha,Omaha,NE 68182,USA

bMunroe-Meyer Institute,Department of Physical Therapy,University of Nebraska Medical Center,Omaha,NE 68198,USA

cCollege of Public Health,Department of Environmental Health,Occupational Health and Toxicology,University of Nebraska Medical Center, Omaha,NE 68198,USA

Lower extrem ity injury in female basketball players is related to a large difference in peak eversion torque between barefoot and shod conditions

Jennifer M.Yentesa,Max J.Kurzb,Nicholas Stergioua,c,*

aNebraska Biomechanics Core Facility,Department of Health,Physical Education and Recreation,University of Nebraska at Omaha,Omaha,NE 68182,USA

bMunroe-Meyer Institute,Department of Physical Therapy,University of Nebraska Medical Center,Omaha,NE 68198,USA

cCollege of Public Health,Department of Environmental Health,Occupational Health and Toxicology,University of Nebraska Medical Center, Omaha,NE 68198,USA

Background:The majority of injuries reported in female basketball players are ankle sprains and mechanisms leading to injury have been debated.Investigations into muscular imbalances in barefoot versus shod conditions and their relationship w ith injury severity have not been performed.The purpose of this study was to investigate the effects of wearing athletic shoes on muscular strength and its relationship to lower extrem ity injuries,specifically female basketball players due to the high incidence of ankle injuries in this population.

Methods:During pre-season,11 female collegiate basketball players underwent inversion and eversion muscle strength testing using an isokinetic dynamometer in both a barefootand shod conditions.The difference between conditions was calculated for inversion and eversion peak torque,time to peak torque as wellas eversion-to-inversion peak torque percentstrength ratio for both conditions.Lower extrem ity injuries were documented and ranked in severity.The ranked difference between barefootand shod conditions for peak torque and time to peak torque as well as percent strength ratio was correlated w ith injury ranking using a Spearman rho correlation(ρ)w ith anαlevel of 0.05.

Results:The ranked differences in barefootand shod for peak eversion and inversion torque at120°/s were correlated w ith their injury ranking. Ranking of the athletes based on the severity of injuries that were sustained during the season was found to have a strong,positive relationship w ith the difference in peak eversion torque between barefoot and shod(ρ=0.78;p=0.02).

Conclusion:It is possible that a large discrepancy between strength in barefoot and shod conditions can predispose an athlete to injury.Narrow ing the difference in peak eversion torque between barefootand shod could decrease propensity to injury.Future work should investigate the effect of restoration of muscular strength during barefoot and shod exercise on injury rates.

CopyrightⒸ2012,Shanghai University of Sport.Production and hosting by Elsevier B.V.All rights reserved.

Ankle sprain;Isokinetic dynamometer;Muscular imbalance;Strength

1.Introduction

More than 60%of all college women’s basketball injuries occur in the lower extremities.1Over a 16-year period,24.6% of these injuries were due to ankle ligament sprains during games and practices.Ankle ligament sprains were the second ranked injury leading to 10 or more days of activity loss,w ith knee internal derangement being the fi rst leading cause.1Furthermore,a history of ankle sprains would leave a player five times more likely to sustain another ankle injury.1The incidence of injury in female high school basketball playersdemonstrates a sim ilar pattern,w ith ankle sprains as the leading injury sustained.2

Several investigations into the primary etiology of ankle sprains have been conducted to probe the biomechanical mechanisms thatmay be responsible for the high incidence of ankle ligament sprains in female basketball players.Baumhaueretal.3concluded thateversion-to-inversion strength ratio was a predictive measure of ankle injury.This finding has not been consistently supported,as these results have not been clearly replicated.4,5Hence,severalother measures have been evaluated including ankle strength,4-7postural sway,6,7proprioception,5shoe height,8,9and peroneal reaction time.10,11Fong et al.12recently listed the two main causes to ankle sprains as improper foot positioning during heel strike and delayed reaction time of the peroneal musculature.Even still,the etiology of ankle sprains has yet to be clearly defined.

There is strong evidence that shoes can control the motion and position of the foot and provide cushion.13—16However, despite the advances in shoe construction,lower extrem ity injuries are still being reported in large numbers.1Prevention of injury may be dependent on intrinsic muscular strength of the ankle complex.In terms of footand ankle musculature,the tibialis anterior(invertor)and triceps surae could be considered as larger muscles,that are most responsive to movement in the sagittal plane and notas responsive to movement in the frontal plane.17Smaller musculature about the ankle and foot provide stability quickly and easily to the ankle joint complex by reacting faster to joint movement changes.17Nigg18has demonstrated that increased strength in these smaller,intrinsic muscles may lead to improved performance and protection, while the opposite can also be true.Therefore the strength of these smaller,intrinsic muscles may have an important relationship w ith susceptibility to lower extrem ity injury.

Avoidance of excessive movement about the ankle is provided by the ankle musculature,butonly if the musculature is properly activated.This is especially true for smaller intrinsic muscles which provide stability to the ankle joint complex by reacting faster to joint movement changes.17,18Essentially,deconditioned musculature may not only cause a decrease in the force production to controlexcessive subtalar motion,but also may delay neuromuscular responses.11Support for this notion has been demonstrated when soccer and cross-country runners w ith and w ithout ankle instability were tested for central and peripheral reaction times.It was found that players w ith severe ankle instability demonstrated peripheral latency of peroneal muscles.11When activated,the ankle and foot musculature take considerable m illiseconds (i.e.,92—133 ms)after the latency period before maximal muscular strength can be developed.8It is possible that deconditioning or atrophy of the muscular structure of the foot and ankle would cause a delay in peripheral reaction,leading to increased latency response of muscle activation and eventually a decrease in the ability to quickly generate force.19,20It has also been suggested thatdecreased sensations provided by wearing shoes may promote the skeletal musculature of the foot and ankle to become deconditioned.21This is not to say that if a shoe provides artificial strength,that barefoot play is recommended,rather the goal is to identify a testing method that w ill allow for identification of athletes predisposed for injury.

Therefore,the purpose of this study was to investigate the effects of wearing athletic shoes on muscular strength and its relationship to lower extremity injuries,specifically female basketball players due to the high incidence of ankle injuries in this population.It was hypothesized that individuals that demonstrated similarankle eversion strength between barefoot and shod conditions would be less susceptible to injury.Ankle evertor musculature provides support and functions as a dynamic stabilizer of the ankle against inversion;thus playing an important role in preventing inversion ankle sprains and/or lower extrem ity injury.In order to test this hypothesis, ankle inversion and eversion peak torque in both barefoot and shod conditions was measured prior to a college basketball season.Injuries were then measured prospectively and were recorded throughout the season.At the end of the season, athletic trainers ranked the athletes in terms of injury severity. Ranked differences in peak torque of the athletes were then correlated w ith ranked injury severity.Thus,a unique feature of this study is its prospective nature and such studies are scarce in the literature.

2.M ethods

Eleven female basketball players(age:20.4±3.2 years; height:172.0± 7.6 cm;mass:73.5± 15.9 kg)from the University of Nebraska at Omaha were consented and participated in the study.The participants were healthy and free from any present musculoskeletal injury.A ll testing was conducted during the basketball pre-season.A ll procedures were approved by the University’s InstitutionalReview Board.

Prior to testing,subjects warmed up on a Monarch stationary bicycle at a self-selected pace and resistance for a m inimum of 10 m in.Eversion and inversion muscular strength when barefoot(barefootcondition)and while wearing their own high-top,basketball shoes(shod condition)were recorded using an isokinetic dynamometer(Biodex System 2.0;Biodex Medical Systems,Shirley,New York,USA).The subjects wore their own shoes to m inimize any shoe-type effect by introducing discom fort or lack of adaptability due to the usage of a new shoe.Each subject was seated,w ith the trunk,thigh,and shank secured.Standard positioning for the ankle inversion and eversion testing was used according to the manufacturer’s guidelines.Subjects were seated and their right leg was raised so that the shank was perpendicular to the footplate attachment.W ith the shank supported,the right foot was secured into the footplate in neutral position and zero degrees plantarflexion.Isokinetic testing of the rightankle was administered at 120°/s w ithin a com fortable range of motion (mean±SD)for barefoot condition(76.8°±12.1°)and shod condition(71.1°± 16.7°).Three maximal repetitions were performed.A m inimum of24 h of restwas required before the subject returned to undergo testing under the second condition. Presentation of barefootand shod conditions was randomized between subjects.Prior to each recorded performance,thesubject was allowed to perform submaximal and maximal repetitions to prepare for each tested velocity.Verbal encouragementand visual feedback of the results were given in order to obtain maximaleffort.Afterall testing was completed three subjects(subjects 2,6,and 10)were eliminated from the analysis due to errors in data collection.

Inversion and eversion peak torque and time to peak torque was recorded for barefoot and shod conditions and the difference between conditions was calculated.A positive(+) difference indicated that the barefoot condition demonstrated greater torque and a negative(-)difference indicated that the shod condition demonstrated greater torque.A difference near zero would indicate sim ilar torque values in both the barefoot and shod conditions.For purposes of this study,either a large-+or large— difference in peak torque between conditions was considered detrimental.This is because,whether or not+ or-,the shoes had an affect on performance.In one case, a large+difference,in the shoe condition the athlete was weaker and for a large—difference the shoe has made the athlete artificially stronger.Therefore,the absolute values of the differences were then ranked.The largest absolute difference between barefootand shod conditions was ranked asa 1 and the smallest absolute difference was ranked as an 8.For time to peak torque+difference indicated that the barefoot condition demonstrated a greater amount of time to reach peak and a—difference indicated that the shod condition demonstrated a greateramountof time to reach peak torque.

In addition,eversion-to-inversion peak torque percent strength ratios were also calculated forboth barefootand shod conditions.The eversion peak torque is divided by the inversion peak torque and multiplied by 100.These were calculated in order to compare our results to previous studies that have investigated the relationship between percent strength ratios and injury occurrences.Percentstrength ratios were ranked for both the barefoot and shod conditions from largest percentage to smallest.The largest percentage was ranked as a 1 w ith the smallest percentage ranked as an 8.

During the basketballseason,injuries were recorded(Table 1). An injury was defined as a lower extrem ity impairment that caused a functional limitation of play or caused the athlete to m iss practice(s)or game(s).The University athletic trainers provided diagnosis and reporting of injuries.The university athletic trainers completed ranking of the athletes based on the severity of injuries that were sustained during the season.To maxim ize objectivity,injuries were fi rst divided into ankle/ foot complex injuries and all other lower extrem ity injuries. Ankle/footcomplex injuries were ranked fi rstand severity was based on the number of practices and games m issed.After the ranking of all ankle/foot complex injuries all other lower extrem ity injuries were ranked.Severity of lower extrem ity injuries was based on the totalnumber of practices and games m issed,as w ith the ankle/footcomplex.An injury ranking of 1 would indicate the mostsevere injury and a ranking of 8 would indicate no injuries.

Peak torque,time to peak torque,and percentstrength ratio were checked for normality using Shapiro—Wilk’sWtest. Thus,mean difference between conditions was investigated byemploying dependentttests.The ranked difference between barefoot and shod conditions for inversion and eversion,time to peak torque as well as the ranked percent strength ratio for both conditions was correlated with injury ranking using a Spearman rho correlation(ρ).Based on the hypothesis, a positive relationship would be present.An individualw ith an injury ranking of 1 would have a large difference in torque or large percent strength ratio;whereas,an individual w ith an injury ranking of8 would have a difference in torque nearzero ora smallpercentstrength ratio.Strengths of correlations were defined as follows:±1.00 to 0.80=very strong;±0.79 to 0.60= strong;±0.59 to 0.40= moderate;±0.39 to 0.20=weak;±0.19 to 0=no relationship.22All statistical analysis was done using SPSS 16.0(IBM Corporation, Armonk,NY,USA).Theαlevel was set at 0.05.

Table 1 Athletes’injuries documented and ranking based upon severity.

3.Results

There were no significant differences in peak torque between barefoot and shod conditions for either inversion or eversion(Table 2).There was no significantdifference in time to peak torque or eversion-to-inversion percent strength ratio between barefootand shod conditions.Ranking of the athletes based on the severity of injuries thatwere sustained during the season was found to have a moderate to strong positive relationship w ith the difference in peak eversion torque between barefootand shod conditions(ρ=0.78,p=0.02).There wasno significant correlation w ith the difference in inversion peak torque in barefoot and shod conditions(Table 3).Ranking of the athletes based on the severity of their injuries sustained during the basketball season did not demonstrate significant correlations w ith time to peak torque or eversion-to-inversion percent strength ratio while barefootor shod(Table 3).

Table 2 Mean±SD differences condition for each dependent variable.

4.Discussion

The current study investigated the relationship of the rank of lower extrem ity injuries sustained during a collegiate basketball season and the ranked difference in peak eversion and inversion torque between barefootand shod conditions in female basketball players.In agreement w ith the proposed hypothesis,the ranked difference between barefoot and shod conditions for peak eversion torque at 120°/s demonstrated strong correlations w ith ranked lower extrem ity injuries. Collegiate female basketballplayers thatdemonstrated a large difference in peak eversion torque between barefootand shod conditions demonstrated a greater tendency for lower extrem ity injuries during a collegiate basketball season.These findings indicate that the difference in evertor musculature performance between barefoot and shod conditions may play an important role in preventing lower extremity injuries.

In addition to acting as a dynam ic stabilizer of the ankle, the peroneal musculature provides support to the lateral ligaments of the ankle and functions as a static stabilizer of the ankle against inversion.To prevent ankle inversion injury,it has been hypothesized that preactivated evertor musculature can be employed as a strategy to stiffen the structures about the subtalar joint.23Ashton-M iller et al.23provided evidence that if the evertor musculature was fully activated,w ithout the use of high-top shoes,an orthosis or athletic tape,that this muscle group could enhance passive resistance atan inversion angle of 15°.In some cases,the evertormusculature alone was able to generate three times the amount of torque w ithout the use of high-top shoes,orthoses and/or athletic tape.23Ottaviani et al.9have further extended this notion by hypothesizing that for any given body size,increased muscular strength of the evertor muscle group would allow for greater resistance to inversion about the subtalar joint.On the other hand,extreme peak eversion torque has been related w ith complications in the Achilles tendon,by forcing the Achilles tendon laterally and distributing stress unevenly across the tendon.24It is apparent that the evertor musculature play an important role in preventing ankle injury;however,there is also evidence that too much ofa contribution from the evertors may also lead to injury.

Table 3 Results of the Spearman rho correlation.

Previous studies have found no significant differences in peak eversion torque between subjects w ith and w ithout ankle instability3,4,6and between dom inant and non-dom inant limbs.7However,we were unable to find a study that investigated the difference in peak eversion torque between barefoot and shod conditions correlated w ith ranked injuries sustained w ithin a competitive season.It is possible that a large difference between barefootand shod conditions would predispose an athlete to lower extremity injury.When an athlete is stronger in the barefoot condition than the shod condition,the shoe is making them weaker.Wearing shoes can lead to deconditioning in intrinsic ankle musculature through underutilization.17,25Habitually barefoot runners demonstrate altered mechanics26,27and possibly lower injury rates21yet, there is no clear evidence.17Based on previous findings,it is believed that persons thatwear shoes more often lose sensory feedback that is needed to produce protective adaptations to movement,such as dim inishing impact through the medial arch or alteration of mechanics.21Further,decreased proprioception due to previous ankle injury in addition to weakness exhibited in the peroneal longus and brevis muscles(ankle evertors)is also related w ith a history of ankle injury.5Prolonged peroneal reaction times have been targeted as a main cause of ankle instability,10,11leading to delayed generation of peak torque.8Neuromuscular deficits would then lead to a comprom ise in the protective effect of the evertor musculature on ankle joint stability.4Although,barefoot play is not feasible,it is possible that training of intrinsic musculature under barefoot conditions would be advantageous to the athlete during shod play.

Conversely,when the athlete is stronger in the shod condition than in the barefoot condition,the shoe provides artificial strength.Rehabilitation of foot musculature is possible,21allow ing for the skeletal muscle to adapt to barefoot conditions.Indirect evidence is supportive by suggesting that using a wobble board-based balance training program in healthy adolescents led to a reduction in sports-related injuries through increased strength of muscles crossing the ankle joint complex.28A sim ilar study investigating the effect of proprioceptive balance board training in adult athletes on ankle sprain re-injury is currently being conducted.29Future work should investigate the ability of rehabilitation of intrinsic foot musculature and its association w ith lower extremity injury in female basketball players.It is possible thatby increasing the strength of intrinsic musculature while barefoot,the ankle would better react to movement.

Previous studies have attempted to relate occurrences of injury to muscular imbalances,specifically eversion-to-inversion strength ratio.3,4Many of these studies have demonstrated that no differences in eversion-to-inversion strength ratios existbetween persons w ith and w ithoutankle instability.4-6To our know ledge,our study is the fi rst study that has related muscular imbalances between barefootand shod conditions to lower extrem ity injuries.In agreement w ith these previous studies,we did not find a significant association between injury and the difference between barefoot and shod conditions for eversion-to-inversion strength ratio(Table 3).

Lim itations of this study include the investigation of concentric torques only.Future work should investigate the difference in eccentric peak torque during barefoot and shod conditions as well.Previous work has demonstrated that subjects w ith and w ithout a history of injury demonstrate a lack of difference in eccentric peak in-eversion torque.4,6It is possible that a difference would exist in these individuals if tested w ith and w ithout shoes.In addition,the injuries reported in this current study were constrained to the lower extrem ity.The correlation between the difference in peak eversion torque in barefootand shod conditions may have been stronger if injury reporting was lim ited to only the ankle joint.In an attempt to overcome this lim itation, we ranked ankle/foot complex injuries fi rst,followed by all other lower extremity injuries.This would indicate that an injury ranking of 1 would be the most severe ankle/foot complex injury.Nevertheless,the strong correlation exists even w ith reporting all lower extrem ity injuries.Further, previous injury was not recorded.It is feasible that previous injury to the lower extrem ity predisposed individuals to current injury.

5.Conclusion

This study was the fi rst to investigate the ranked differences in ankle strength between barefoot and shod conditions and their relationship to ranking of the athletes based on the severity of lower extrem ity injuries that were sustained during a collegiate basketballseason.A unique feature of this study is its prospective nature and such studies are scarce in the literature.We found that the difference between barefootand shod peak eversion torque at 120°/s was significantly and strongly related w ith lower extrem ity injury severity.It is possible that a large discrepancy between strength in barefoot and shod conditions can predispose an athlete to injury.Future work should investigate the effect of restoration of muscular strength during barefoot and shod exercise on injury rates. Based on the findings of this current work,by narrow ing the difference in peak eversion torque between barefoot and shod conditions would decrease injury severity in female basketball players.

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Received 13 July 2012;revised 30 August 2012;accepted 12 November 2012

*Corresponding author.Nebraska Biomechanics Core Facility,University of Nebraska at Omaha,Omaha,NE 68182,USA.

E-mail address:nstergiou@unomaha.edu(N.Stergiou)

Peer review under responsibility of Shanghai University of Sport

2095-2546/$-see front matter CopyrightⒸ2012,Shanghai University of Sport.Production and hosting by Elsevier B.V.A ll rights reserved. http://dx.doi.org/10.1016/j.jshs.2012.11.004