Multiple factors contribute to the increased risk of noncontact

Transcription

1 Journl of Athletic Trining 2011:46(3): by the Ntionl Athletic Triners Assocition, Inc originl reserch A Preliminry Multifctoril Approch Describing the Reltionships Among Lower Extremity Alignment, Hip Muscle Activtion, nd Lower Extremity Joint Excursion Anh-Dung Nguyen, PhD, ATC*; Sndr J. Shultz, PhD, ATC, FNATA, FACSM ; Rndy J. Schmitz, PhD, ATC ; Richrd M. Luecht, PhD ; Dvid H. Perrin, PhD, ATC, FACSM ; *Deprtment of Helth nd Humn Performnce, College of Chrleston, SC; Applied Neuromechnics Reserch Lbortory, University of North Crolin t Greensboro Context: Multiple fctors hve been suggested to increse the risk of fulty dynmic lignments tht predict noncontct nterior crucite ligment injury. Few reserchers hve exmined this reltionship using n integrted, multifctoril pproch. Objective: To describe the reltionship mong sttic lower extremity lignment (LEA), hip muscle ctivtion, nd hip nd knee motion during single-leg squt. Design: Descriptive lbortory study. Setting: Reserch lbortory. Ptients or Other Prticipnts: Thirty men (ge = 23.9 ± 3.6 yers, height = ± 9.9 cm, mss = 82.0 ± 14.1 kg) nd 30 women (ge = 22.2 ± 2.6 yers, height = ± 6.3 cm, mss = 60.3 ± 8.1 kg). Min Outcome Mesure(s): Pelvic ngle, femorl nteversion, qudriceps ngle, tibiofemorl ngle, nd genu recurvtum were mesured to the nerest degree; nviculr drop ws mesured to the nerest millimeter. The verge root men squre mplitude of the gluteus medius nd mximus muscles ws ssessed during the single-leg squt nd normlized to the pek root men squre vlue during mximl contrctions for ech muscle. Kinemtic dt of hip nd knee were lso ssessed during the single-leg squt. Structurl eqution modeling ws used to describe the reltionships mong sttic LEA, hip muscle ctivtion, nd joint kinemtics, while lso ccounting for n individul s sex nd hip strength. Results: Smller pelvic ngle nd greter femorl nteversion, tibiofemorl ngle, nd nviculr drop predicted greter hip internl-rottion excursion nd knee externl-rottion excursion. Decresed gluteus mximus ctivtion predicted greter hip internl-rottion excursion but decresed knee vlgus excursion. No LEA chrcteristic predicted gluteus medius or gluteus mximus muscle ctivtion during the single-leg squt. Conclusions: Sttic LEA, chrcterized by more internlly rotted hip nd vlgus knee lignment nd less gluteus mximus ctivtion, ws relted to commonly observed components of functionl vlgus collpse during the single-leg squt. This explortory nlysis suggests tht LEA does not influence hip muscle ctivtion in controlling joint motion during singleleg squt. Key Words: knee injuries, nterior crucite ligment, risk fctors, posture, mllignment Key Points Sttic lower extremity lignment chrcteristics nd hip muscle ctivtion were directly relted to commonly observed components of functionl vlgus collpse during the single-leg squt. However, reltionships between sttic lower extremity lignment nd hip muscle ctivtion were not observed. Sttic lower extremity lignment my not influence hip muscle ctivtion in controlling joint motion during single-leg squt. Multiple fctors contribute to the incresed risk of noncontct nterior crucite ligment (ACL) injury. In fct, recent consensus sttement 1 hs highlighted the need for more integrted pproch cross risk-fctor ctegories (eg, ntomicl, neuromusculr, nd biomechnicl). A more comprehensive pproch to risk-fctor ssessment my llow clinicins to ccurtely identify nd understnd those relevnt risk fctors tht my contribute to t-risk knee positions during dynmic ctivity. Among the mny risk fctors suggested to contribute to ACL injury, neuromusculr function (strength nd ctivtion) of the hip musculture hs received incresed ttention becuse it is essentil to providing proximl stbility for lower extremity motion. 2,3 Neuromusculr deficits my compromise the stbility of the hip when it is loded during weight bering, resulting in fulty dynmic lignment of the lower extremity nd potentilly incresing the risk of injury. Authors 4 10 of retrospective studies hve reported decresed strength nd ctivtion of the 246 Volume 46 Number 3 June 2011

2 hip bductors in those with low bck pin nd lower extremity injuries; however, prospective investigtions of the reltionship between hip muscle function nd lower extremity injury re limited. Only one group 11 prospectively exmined the reltionship between hip strength nd lower extremity injury in collegite bsketbll nd trck thletes; those who were injured over the course of the seson hd less hip bduction nd hip extension strength thn the uninjured thletes. The uthors 11 suggested tht the decresed strength of the hip musculture reduced the bility to stbilize the hip, resulting in dduction nd rottion of the lower extremity nd, thus, fulty lignment, which contributed to lower extremity injury. This fulty dynmic lignment, commonly termed functionl vlgus collpse 12,13 nd chrcterized by dduction nd internl rottion of the hip nd knee vlgus, predicts ACL-injury risk. 14 Whether reltionship exists between decresed neuromusculr hip muscle function nd incresed functionl vlgus collpse is currently unknown. In ddition, sttic lower extremity lignment (LEA) hs been proposed s n independent, intrinsic risk fctor for ACL injury. 12,15 18 Authors of retrospective studies hve reported greter prontion, pelvic ngle, nd genu recurvtum in ACLinjured individuls. These nd other LEA chrcteristics tht increse sttic hip nd knee ngles my predispose individuls to incresed inwrd collpse of the knee during dynmic ctivities. The limittion of previous exmintions of the reltionship between ntomicl lignment nd neuromusculr function of the hip musculture is tht only one LEA chrcteristic or select LEA chrcteristics were exmined. No published studies hve ddressed the reltionship mong LEA, neuromusculr function of the hip, nd dynmic hip nd knee motion using collective set of ntomic lignment vribles tht re sufficiently descriptive of lower extremity posture. This reltionship my be importnt becuse one skeletl mllignment my cuse compenstory lignment chnges t other bony segments, resulting in bnorml stress ptterns or compenstory motions long the kinetic chin. Given the potentil link between decresed neuromusculr function of the hip musculture nd incresed functionl vlgus collpse, injury-prevention progrms hve been developed to trget the hip musculture. 23 However, the underlying cuses for this neuromusculr dysfunction of the hip musculture hve received little ttention. Differences in LEA my lter neuromusculr function of the hip muscles nd contribute to functionl vlgus collpse. This premise is bsed on reserch showing tht chnges in the length, tension, nd orienttion of the hip musculture directly influence the internl-moment rms of the muscle, resulting in chnges in hip muscle functions Few uthors hve exmined the direct influence of LEA on hip muscle function, but differences in LEA my be relted to chnges in the force nd ctivtion of the hip musculture. Using simulted hip model, n increse in gluteus medius (G med ) force ws necessry to mintin level pelvis when the femur ws positioned in more internlly rotted position ( position ssocited with femorl nteversion) compred with neutrl lignment. 27 Further, decresed ctivtion of the G med s mesured by surfce electromyogrphy (semg) mplitude ws demonstrted in those with incresed reltive femorl nteversion during isometric strength testing. 28 Collectively, these findings indicte tht individuls with incresed femorl nteversion require incresed force production to control the hip nd pelvis, yet they demonstrte decresed ctivtion; together, these fctors my severely reduce frontl-plne nd trnsverseplne hip control during functionl ctivities. Whether other lignment fctors t the pelvis, knee, lower leg, nd foot tht promote more inwrdly rotted or dducted hip posture further compromise hip muscle function is unknown. Although it is tenble tht differences in LEA chrcteristics my chnge the position of the femur reltive to the pelvis, thus potentilly ltering the length, tension, nd orienttion of the muscles nd their ultimte torque-producing cpbilities bout joint, these ssumptions re bsed primrily on findings from sttic model. Whether these reltionships would hold in dynmic nd constntly chnging joint during functionl ctivities is uncler. Therefore, we exmined whether sttic LEA chrcteristics nd hip muscle ctivtion were relted to hip nd knee kinemtics during single-leg squt, while ccounting for sex nd hip strength. Bsed on retrospective evidence tht ACL-injured individuls hd greter mgnitudes of sttic LEA nd the potentil for lignment to influence the neuromusculr function of the lower extremity muscles, 27,28 we wnted to explore both the direct reltionships of LEA nd hip muscle ctivtion on lower extremity kinemtics nd the potentil for indirect reltionships between LEA nd lower extremity kinemtics bsed on the ssocition of LEA with hip muscle ctivtion. Specificlly, we hypothesized tht (1) greter mgnitudes of sttic lignment of the lower extremity nd decresed hip muscle ctivtion would directly predict greter functionl vlgus collpse (incresed hip dduction nd internl rottion, knee externl rottion, nd vlgus excursion) during single-leg squt nd (2) indirect reltionships would lso occur such tht greter mgnitudes of sttic LEA would predict decresed G med nd gluteus mximus (G mx ) ctivtion (bduction nd extension) nd collectively predict greter functionl vlgus collpse. METHODS Thirty men (ge = 23.9 ± 3.6 yers, height = ± 9.9 cm, mss = 82.0 ± 14.1 kg) nd 30 women (ge = 22.2 ± 2.6 yers, height = ± 6.3 cm, mss = 60.3 ± 8.1 kg) were recruited from the university nd the surrounding community to prticipte in the study. Ech volunteer provided informed consent s pproved by the university s institutionl review bord. Prticipnts hd no history of surgery to either lower extremity nd no previous hip joint or knee joint injury within the lst 6 months. All mesurements were tken on the dominnt-stnce limb (ie, the stnce extremity when kicking bll). Alignment Mesurements We mesured 6 lignment chrcteristics on the pelvis nd lower extremity. These lignment chrcteristics were bsed on commonly identified vribles suggested to influence dynmic motion nd the risk of lower extremity injuries. All mesurement procedures were performed by single exminer who hd previously estblished good to excellent test-retest relibility on ll mesures (intrclss correltion coefficient [ICC][2,3] 0.87), 23,29 using techniques tht hve been previously described in detil All stnding mesures were tken in stndrdized stnce, with the left nd right feet spced equl to the width between the left nd right cromil processes nd toes fcing forwrd. The stnce ws chieved by instructing prticipnts to mrch in plce nd then tke step forwrd. They were dvised to look stright hed during ll stnding mesures, with weight Journl of Athletic Trining 247

3 evenly distributed over both feet. Pelvic ngle ws mesured in stnding position using n inclinometer nd represented the ngle formed by line from the nterior-superior ilic spine to the posterior-superior ilic spine reltive to the horizontl plne. 33 Femorl nteversion ws mesured in prone position using the Crig test. 34 Qudriceps ngle ws mesured in stnding position nd represented the ngle formed by line from the nterior-superior ilic spine to the ptell center nd line from the ptell center to the tibil tuberosity. Tibiofemorl ngle ws mesured in stnding position nd represented the ngle formed by the ntomicl xis of the femur nd tibi in the frontl plne. 29 Genu recurvtum ws mesured in supine position with bolster positioned under the distl tibi nd represented the sgittl-plne lignment of the femur nd tibi. 29 Nviculr drop ws mesured in stnding position nd represented the difference between the height of the nviculr in subtlr joint neutrl nd relxed stnce. 29 Ech mesure ws repeted 3 times. Electromyogrphy Procedures Surfce electromyogrphy signls of the G med nd G mx were obtined using 16-chnnel Myopc telemetric system (Run Technologies Compny, Mission Viejo, CA) with n mplifiction of 1 mv/v, frequency bndwidth of 10 to 1000 Hz, common mode rejection rtio of 90 db minimum t 60 Hz, input resistnce of 1 MΩ, nd n internl smpling rte of 8 KHz. The semg signls were detected with 10-mm bipolr Ag-AgCl surfce electrodes (Blue Sensor N-00-S; Ambu Products, Ølstykke, Denmrk; dimeter = mm; skin contct size = mm) with center-to-center distnce of 20 mm nd the electrodes were positioned ccording to procedures described by Crm nd Ksmn. 35 Electrodes were plced on the G med t position one-third the distnce from the greter trochnter to the ilic crest. Electrode plcement on the G mx ws midwy between the greter trochnter nd the first scrl vertebre. The semg electrodes were oriented perpendiculr to the length of the muscle fibers nd plced over the midbelly. The reference electrode ws secured to the medil spect of the tibi. Before the electrodes were ttched, we thoroughly clened ll skin res with isopropyl lcohol. Myoelectric dt were cquired, stored, nd nlyzed using DtPc 2K2 lbortory ppliction softwre (version 3.13; Run Technologies Compny) during the mximl voluntry isometric contrctions (MVICs) nd the single-leg squt. Strength Assessment A dynmometer (model 3; Biodex Medicl Systems, Inc, Shirley, NY) ws used to record hip bduction nd hip extension MVICs. Prticipnts performed 3 trils of 3-second MVIC for ech muscle, with 30-second rest period seprting trils. We modified technique described by Crci et l 36 to mesure hip bduction torque in weight bering. Volunteers stood djcent to the dynmometer, looking stright hed, with the trunk erect, feet fcing forwrd, nd rms crossed over the chest. The dynmometer xis ws ligned with the hed of the femur, determined by the intersection of medilly directed horizontl line from the greter trochnter nd distlly directed verticl line from the nterior-superior ilic spine. 37 The resistnce rm of the dynmometer ws positioned on the lterl side of the nonstnce leg, with the distl edge of the pd pproximtely 5 cm proximl to the lterl joint line nd the hip positioned in pproximtely 5 of bduction. Ech prticipnt performed the MVIC by bducting the hip while supporting his or her body weight on the dominnt-stnce limb nd mintining n erect posture. For ssessment of hip extension torque, ech individul performed hip extension in the supine position, with the hip flexed to 90 nd the dynmometer xis ligned with the greter trochnter. The resistnce rm ws positioned on the posterior thigh just proximl to the knee joint line. Previous work in our lbortory using these identicl MVIC mesurement protocols demonstrted good to excellent dy-to-dy relibility of torque production for stnding hip bduction (ICC[2,k] = 0.91, SEM = 0.03 N m/kg) nd hip extension (ICC[2,k] = 0.80, SEM = 0.46 N m/kg). Kinemtic Anlysis Kinemtic dt for the pelvis, thigh, shnk, nd foot were smpled t 100 Hz using 6 degrees-of-freedom electromgnetic sensors (Ascension Technology Corportion, Burlington, VT) nd Motion Monitor Softwre (Innovtive Sports Trining, Inc, Chicgo, IL) during the single-leg squt. Electromgnetic position sensors were ttched with double-sided tpe nd elstic wrp over the nterior midshft of the third mettrsl, the midshft of the medil tibi, nd the lterl spect of the midshft of the femur of the dominnt-stnce limb. An dditionl sensor ws secured on the scrum. Digitiztion procedures were performed using the defult selection with segmentl reference system defining body segments: the positive x-xis ws defined s the posterior-to-nterior xis, the positive y-xis ws defined s the distl-to-proximl longitudinl xis, nd the positive z- xis ws defined s the medil-to-lterl xis. An initil neutrl position ws estblished in stndrdized stnce with the left nd right feet spced equl to the width between the left nd right cromion processes nd the toes fcing forwrd. The nkle nd knee joint centers were estimted using the centroid method, whereby the nkle joint center ws clculted s the midpoint between the digitized medil nd lterl mlleoli, nd the knee joint center ws clculted by the midpoint between the digitized medil nd lterl femorl epicondyles. The hip joint center ws determined by the Lerdini et l 38 method. The strting position for prticipnts ws feet shoulderwidth prt, hips nd knees extended, toes fcing forwrd, equl weight on both feet, nd thumbs lightly touching the ilic crests (Figure 1). A plywood bord ws positioned t distnce nterior to the knee while volunteers performed double-leg squt to 60 of knee flexion bsed on rel-time goniometer vlues. The plywood bord ws positioned to provide individuls with feedbck indicting tht they hd reched 60 of knee flexion during ech tril nd while performing double-leg squt to ensure proper plcement of the bord. They then performed single-leg squt with instructions to squt stright down until they touched the bord with the knee while looking stright hed. A string ws positioned perpendiculr to the first toe t the level of the chest to monitor forwrd flexion of the trunk (Figure 2). Prticipnts were instructed to mintin n upright position without flexing the trunk forwrd or to the side in order to limit the influence of trunk motion on the hip musculture. Although we recognize tht this is constrined tsk, the rtionle for this stndrdized positioning ws to ccount for potentil confounding fctor tht my hve contributed to conflicting results in previous studies of hip muscle ctivtion dur- 248 Volume 46 Number 3 June 2011

4 Figure 1. Strting position for the kinemtic dt collection with feet shoulder-width prt, hips nd knees extended, toes fcing forwrd, equl weight on both feet, nd thumbs lightly touching the ilic crests. Figure 2. The single-leg squt ws performed to 60 of knee flexion. A string ws positioned perpendiculr to the first toe t the level of the chest to monitor forwrd flexion of the trunk during the single-leg squt. ing dynmic tsks. 39,40 Compred with men, women hd greter G mx ctivtion during single-leg squt 39 but less ctivtion during single-leg lndings. 40 Smll smple sizes nd methodologic considertions in performing the tsks my explin these contrsting findings. Specificlly, trunk motion, which hs direct influence on ctivtion of the hip musculture, did not pper to be controlled in these studies. 41 Ech single-leg squt tril ws initited by verbl commnd from the exminer nd performed t speed of 5 seconds from the strting position to 60 of knee flexion. The rte of the tsk ws controlled by metronome set t cdence of 60 bets per minute. Prticipnts trnsitioned from bilterl stnce to single-leg stnce during the first 2 bets with the nonstnce knee nd hip flexed pproximtely 45 nd 0, respectively. The squt then begn on the third bet nd ended t 60 of knee flexion on the fifth bet (totl squt time = 2 seconds). A force plte mrked the trnsition from double-leg stnce to single-leg stnce, nd 60 of knee flexion mrked the end of the tril. Volunteers were llowed sufficient prctice to ensure tht the tsk ws performed properly, nd dt were then collected during 5 cceptble trils. A tril ws deemed uncceptble if the individul (1) touched the string (indicting incresed forwrd flexion of the trunk), (2) touched the nonstnce leg to the ground or the stnce leg, (3) lifted either hnd off the ilic crest, or (4) filed to rech 60 of knee flexion s confirmed by rel-time goniometry. Dt Reduction nd Anlyses The verge of 3 mesurements for ech LEA chrcteristic ws used for nlyses. Dynmometer torque dt were recorded s the mximum pek torque obtined from 3 MVIC trils ech for hip bduction nd hip extension. Pek torque ws then normlized to the prticipnt s body mss nd reported in newton-meters per kilogrm of body mss. Kinemtic signls from the position sensors were low-pss filtered t 12 Hz using fourth-order, zero-lg Butterworth filter. Hip nd knee ngles were clculted using Euler ngle definitions with rottionl sequence of Z X Y. 42 Initil joint ngles were clculted s the verge joint positions during the first second fter trnsition from double-leg to single-leg stnce. Finl joint ngles were determined s the vlue when prticipnts chieved 60 of knee flexion. Single-leg squt joint excursions were clculted s the difference (finl minus initil) for ech tril, nd the verge cross 5 trils ws used for sttisticl nlysis. The semg of the G med nd G mx during the MVIC nd single-leg squt trils ws filtered from 10 Hz to 350 Hz using fourth-order, zero-lg Butterworth filter nd then processed us- Journl of Athletic Trining 249

5 ing centered root men squre (RMS) lgorithm with 100-millisecond time constnt. The pek RMS vlue obtined over 3 MVIC trils for ech muscle ws used to normlize the semg dt during the single-leg squt. The verge RMS mplitude of the 5 single-leg squt trils cross the entire tril (fter trnsition to single-leg weight bering to 60 ) ws then normlized to the individul s MVIC pek RMS vlue nd reported s percentge of the MVIC. Structurl eqution modeling ws used to evlute whether incresed LEA nd decresed hip muscle ctivtion (G med nd G mx, considered seprtely) predicted greter functionl vlgus collpse (chrcterized by incresed hip dduction nd internl rottion, knee externl rottion, nd vlgus excursion) during single-leg squt while ccounting for the individul s sex nd hip strength. Our rtionle in ccounting for these dditionl vribles ws tht LEA chrcteristics 31 nd hip strength 11,43 45 re known to differ by sex nd tht muscle-ctivtion mplitude of the primry hip bductor (G med ) nd hip extensor (G mx ) muscles my, in prt, depend on their bsolute force-producing cpbilities. 46 Hip bduction nd hip extension strength were included only in the specific pth models tht exmined the reltionships of G med nd G mx ctivtion, respectively, s they re the primry muscles tht perform hip bduction nd hip extension. The pth digrm exmining these reltionships is illustrted in Figure 3. Pth nlysis is n extension of multiple liner regressions with the purpose of modeling explntory chined reltionships between observed vribles. It provides estimtes of the mgnitude nd significnce of hypothesized cusl connections mong sets of vribles. Pth nlysis provides sttisticl pproch to understnding comprtive strengths of direct nd indirect reltionships mong set of vribles. 47 Becuse the totl number of vribles being estimted ws greter thn the totl smple size (resulting in the vrible estimtes being highly unrelible), ech full model ws reduced to more stble model by first removing the dependent mesures tht hd no sttisticlly significnt pths (ie, vribles tht hd no significnt predictors), followed by removing the predictor vribles tht did not pproch significnce or were nonsignificnt in explining ny of the remining outcome mesures (dependent vribles). Sttisticl significnce ws determined by the t-vlue sttistic, which reflects the rtio of the vrible estimte to its stndrd error. A t vlue greter thn +2 or less thn 2 is considered sttisticlly significnt. 47 All pth nlyses were performed using LISREL (version 8.72; Scientific Softwre Interntionl, Inc, Lincolnwood, IL). RESULTS Mesures of LEA, hip muscle ctivtion, joint excursion during the single-leg squt, nd hip torque re summrized in Tble 1. The men sttic lignment vlues re within the rnge of norml vlues reported in helthy dults using identicl mesurement methods Sex ws relted to LEA chrcteristics nd hip muscle ctivtion (ll P <.05): women hd greter pelvic ngle (t = 2.23), femorl nteversion (t = 4.60), qudriceps Figure 3. Full pth model for the dependent vribles glutel muscle ctivtion nd functionl vlgus collpse. 250 Volume 46 Number 3 June 2011

6 Tble 1. Descriptive Sttistics for Dependent nd Predictor Vribles Mesure Men ± SD Medin Rnge Lower extremity lignment Pelvic ngle, 11.1 ± to 21.0 Hip nteversion, 10.7 ± to 27.7 Qudriceps ngle, 12.9 ± to 29.0 Tibiofemorl ngle, 10.7 ± to 15.3 Genu recurvtum, 3.8 ± to 14.3 Nviculr drop, mm 6.6 ± to 25.7 Muscle ctivtion, % mximum voluntry isometric contrction Gluteus medius 0.27 ± to 0.72 Gluteus mximus 0.20 ± to 1.04 Joint excursion, Hip dduction 11.4 ± to 35.5 Hip internl rottion 2.3 ± to 12.8 Knee vlgus 0.1 ± to 17.0 Knee externl rottion 2.7 ± to 20.2 Hip strength, N m/kg Hip-bduction torque 0.69 ± to 1.33 Hip-extension torque 3.46 ± to 5.80 ngle (t = 2.58), tibiofemorl ngle (t = 3.09), genu recurvtum (t = 3.84), nd G mx ctivtion (t = 2.44) thn men. The inferentil goodness-of-fit index indicted tht both full models were perfect fit (χ 2 = 0.00, P = 1.00, RMS error of pproximtion = ) becuse the model ws sturted with 0 degrees of freedom. Reltionship Among LEA, G med Activtion, nd Joint Excursion The full model used to exmine the extent to which LEA chrcteristics predicted G med ctivtion nd the vribles collective influence on dynmic lignment during single-leg squt while ccounting for sex nd hip bduction torque ws reduced to more stble model (Figure 4). The vribles tht remined in the model were the dependent vribles of hip internl-rottion nd knee externl-rottion excursions nd the predictor vribles of pelvic ngle, femorl nteversion, tibiofemorl ngle, nd nviculr drop. The coefficients, stndrd errors of the coefficients, nd t sttistics for pths P 1 P 15 tht represent the reltionships mong the remining vribles re shown in Tble 2. The model explined 17% of the vrince in hip internlrottion excursion nd 24% of the vrince in knee externlrottion excursion during the single-leg squt. Smller pelvic ngle (P 6 ) nd greter nviculr drop (P 5 ) predicted greter hip internl-rottion excursion, wheres smller pelvic ngle (P 10 ) nd greter femorl nteversion (P 11 ) nd tibiofemorl ngle (P 12 ) predicted greter knee externl-rottion excursion during the single-leg squt. The model did not identify ny indirect (ie, sequentil or chined ) reltionships between LEA nd G med ctivtion in predicting joint excursion during the singleleg squt. Gluteus medius ctivtion Nviculr drop P 1 Sex Pelvic ngle P 2 P 5 P 13 Hip bduction torque P 3 P 6 P 14 Femorl nteversion P 7 Hip internl rottion Tibiofemorl ngle P 4 P 8 P 9 P 10 P 11 P 12 P 15 Knee externl rottion Figure 4. Finl model for the dependent vribles gluteus medius ctivtion nd dynmic vlgus lignment. Indictes significnt pth coefficient. See Tble 2 for pth coefficient vlues. Journl of Athletic Trining 251

7 Reltionship Among LEA, G mx Activtion, nd Joint Excursion The full model used to exmine the extent to which sttic LEA predicted G mx ctivtion nd the vribles collective influence on dynmic lignment during single-leg squt while ccounting for sex nd hip extension torque ws lso reduced to more stble model (Figure 5). The vribles tht remined in the model were the dependent vribles of hip internl-rottion, knee vlgus, nd knee externl-rottion excursion nd the predictor vribles of pelvic ngle, femorl nteversion, tibiofemorl ngle, nd nviculr drop. The coefficients, stndrd errors of the coefficients, nd t sttistics for pths P 1 P 20 tht represent the reltionships mong the remining vribles re shown in Tble 3. The model explined 27% of the vrince in hip internlrottion excursion, 17% of the vrince in knee vlgus excursion, nd 20% of the vrince in knee externl-rottion excursion during the single-leg squt. Smller pelvic ngle (P 6 ) nd greter femorl nteversion (P 7 ) nd nviculr drop (P 5 ) predicted greter hip internl-rottion excursion, wheres smller pelvic ngle (P 14 ) nd greter femorl nteversion (P 15 ) nd tibiofemorl ngle (P 16 ) predicted greter knee externlrottion excursion during the single-leg squt. Decresed G mx ctivtion predicted greter hip internl-rottion (P 18 ) nd decresed knee vlgus (P 19 ) excursion. Similr to the previous model, we did not identify ny indirect reltionships between LEA nd G mx ctivtion in predicting joint excursion during the single-leg squt. DISCUSSION The primry findings were tht LEA chrcteristics were directly relted to dynmic lignment during single-leg squt, with greter femorl nteversion, tibiofemorl ngle, nd nviculr drop predicting greter hip internl-rottion excursion nd knee externl-rottion excursion. Interestingly, greter pelvic ngle predicted decresed hip nd knee rottion. Direct reltionships were lso noted between glutel ctivtion nd dynmic lignment, with decresed G mx ctivtion predicting greter hip internl-rottion excursion but decresed knee vlgus excursion. These results provide empiricl support for previous theories tht differences in sttic LEA nd glutel muscle ctivtion contribute to greter hip joint nd knee joint excursions during functionl ctivities. However, no indirect (ie, sequentil or chined) reltionships were noted between LEA nd glutel ctivtion in predicting dynmic motion: no LEA chrcteristic predicted G med or G mx muscle ctivtion during the single-leg squt once n individul s sex nd muscle strength were ccounted for. Effects of LEA nd Hip Muscle Activtion on Lower Extremity Joint Excursion Bsed on previling theories, greter sttic hip nd knee lignment nd decresed hip ctivtion were hypothesized to predict greter frontl- nd trnsverse-plne joint excursion during the single-leg squt. Specificlly, individuls with more femorl nteversion nd nviculr drop went into more Figure 5. Finl model for the dependent vribles gluteus mximus ctivtion nd dynmic vlgus lignment. Indictes significnt pth coefficient. See Tble 3 for pth coefficient vlues. 252 Volume 46 Number 3 June 2011

8 Tble 2. Pth Coefficients of Lower Extremity Alignment Predicting Gluteus Medius Activtion, Hip Internl Rottion, nd Knee Externl Rottion Gluteus Medius Activtion Hip Internl Rottion Knee Externl Rottion Lower Extremity Pth Stndrd Pth Stndrd Pth Stndrd Alignment Pth Coefficient Error t Vlue Pth Coefficient Error t Vlue Pth Coefficient Error t Vlue Nviculr drop P P P Pelvic ngle P P P Femorl nteversion P P P Tibiofemorl ngle P P P Gluteus medius ctivtion P P Significnt pth coefficient (P <.05). Tble 3. Pth Coefficients of Lower Extremity Alignment Predicting Gluteus Mximus Activtion, Hip Internl Rottion, Knee Vlgus, nd Knee Externl Rottion Gluteus Medius Activtion Hip Internl Rottion Knee Vlgus Knee Externl Rottion Lower Extremity Pth Stndrd Pth Stndrd Pth Stndrd Pth Stndrd Alignment Pth Coefficient Error t Vlue Pth Coefficient Error t Vlue Pth Coefficient Error t Vlue Pth Coefficient Error t Vlue Nviculr drop P P P P Pelvic ngle P P P P Femorl nteversion P P P P Tibiofemorl ngle P P P P Gluteus mximus ctivtion P P P Significnt pth coefficient (P <.05). Journl of Athletic Trining 253

9 hip internl-rottion excursion nd individuls with greter tibiofemorl ngle nd femorl nteversion went into greter knee externl-rottion excursion, with both motions considered importnt components of functionl vlgus collpse. 12 The direct reltionship between greter femorl nteversion nd greter rottion of both the hip nd knee during dynmic motion seems logicl given tht more femorl nteversion hs previously been ssocited with hip internl rottion nd contributes to compenstory increse in knee externl rottion. 48 These observed reltionships suggest tht sttic LEA chrcteristics my directly influence dynmic hip nd knee ngles during functionl ctivities nd my offer potentil mechnism by which greter nviculr drop nd sttic knee vlgus ngles were ssocited with ACL injury An explntion for greter pelvic ngle predicting decresed hip internl-rottion nd knee externl-rottion excursion is uncler. Bsed on retrospective evidence 21 tht suggests reltionship between greter nterior pelvic ngle nd ACL injury, our expecttion ws tht more nterior pelvic tilt would be relted to more dynmic joint excursion. Additionl work is needed to better understnd the interction between the pelvis nd the femur nd its influence on dynmic lignment nd ACL injury. The hypothesized reltionship between hip muscle ctivtion nd functionl vlgus collpse ws prtilly supported. Decresed G mx ctivtion predicted greter hip internlrottion excursion. Although we found no studies tht directly exmined the reltionship between hip muscle ctivtion nd joint motion in helthy individuls, this observed reltionship does support current theories tht decresed hip muscle ctivtion my ffect dynmic stbility of the hip, resulting in n inbility to mintin neutrl lignment during single-limb weight-bering ctivities. 39,49 51 However, the positive reltionship of greter G mx ctivtion predicting greter knee vlgus excursion is the opposite of wht we expected. An explntion of this positive reltionship is uncler, but it my be tht hip ctivtion strtegies re different when controlling motions t the hip compred with motions t the knee. Dynmic knee vlgus observed during functionl tsks my reflect combined motion of knee vlgus nd hip internl rottion, which would further suggest positive reltionship between G mx ctivtion nd these motions. However, further exmintion of our dt indicted tht hip internl rottion ws negtively correlted with knee vlgus excursion (r = 0.370, P =.004). This observed reltionship between hip joint nd knee joint motion my be specific to single-leg squt tsk, nd, therefore, further studies re needed to determine whether the observed reltionships between hip muscle ctivtion nd lower extremity kinemtics re consistent cross functionl tsks. Although we observed direct reltionships between LEA nd joint excursion, it is uncler from these dt lone if sttic LEA directly predisposes individuls to the rottionl hip nd knee components of functionl vlgus collpse or whether these posturl effects ct through resulting biomechnicl chnges (ie, decresed hip muscle ctivtion) to increse dynmic hip nd knee mllignments. The use of pth nlysis model ws novel pproch towrd exmining multiple risk fctors, which llowed us to exmine the indirect reltionships between LEA nd functionl vlgus collpse by wy of their effects on hip muscle ctivtion. We hypothesized tht sttic mllignments would directly predict decresed hip muscle ctivtion, which would further predict incresed joint excursion. However, this sequentil or chined reltionship ws not observed: no sttic LEA chrcteristic ws relted to dynmic hip muscle ctivtion. Reltionships between LEA nd hip muscle function hve been observed using sttic models, but our results do not support this reltionship during dynmic ctivities when joint position is constntly chnging. These findings suggest tht sttic LEA lone my predispose individuls to greter hip nd knee rottions during dynmic ctivity, independent of G mx or G med ctivtion during dynmic tsks. Accounting for Sex nd Hip Muscle Strength We chose to ccount for sex in the pth-nlysis models becuse mny of the LEA chrcteristics 31 nd hip musclectivtion mesures 39,40 we exmined re known to differ by sex. By ccounting for sex in the model, we confirmed tht sex ws relted to LEA chrcteristics nd hip muscle ctivtion such tht women hd greter pelvic ngle, femorl nteversion, qudriceps ngle, tibiofemorl ngle, genu recurvtum, nd G mx ctivtion thn men. These sex differences in LEA chrcteristics nd hip muscle ctivtion my in prt explin why femles demonstrte greter dynmic knee ngles nd n incresed risk of ACL injury. Future uthors should exmine mles nd femles seprtely becuse the reltionships between mny of the postulted risk fctors nd ACL injury my not be the sme for ech sex. The purpose of ccounting for hip bduction nd hip extension strength in the pth nlyses ws to better clrify the reltionship between hip muscle ctivtion nd functionl vlgus collpse by tking into considertion vritions in the levels of hip strength mong prticipnts, which my itself explin differences in functionl vlgus collpse. Although uthors hve exmined ctivtion of the hip musculture during functionl ctivities such s single-leg lndings nd single-leg squts, either kinemtic dt were not collected 40 or hip strength ws not reported. 39,40 Bsed on these studies, the reltionship between posterior-lterl hip muscle function nd dynmic joint motion remins uncler. In theory, greter hip muscle ctivtion would be necessry to successfully perform desired motion in the presence of reduced hip muscle strength. The negtive reltionships we observed between hip bduction torque nd G med ctivtion (r = 0.275, P =.034) nd between hip extension torque nd G mx ctivtion (r = 0.612, P <.001) confirm tht greter posterior-lterl hip muscle ctivtion ws required in those individuls with decresed hip strength to successfully perform the single-leg squt. This inverse reltionship between hip muscle strength nd ctivtion suggests tht reltive increses in glutel muscle ctivtion my or my not, by themselves, indicte better hip control, depending on the ctul torque-producing cpbilities of the muscles. Limittions We cknowledge tht mesurement of femorl nteversion using clinicl methods hs the potentil for inconsistencies, with rnge of relibilities nd vlidities of this mesure reported in the literture. The mesurement technique we used ws bsed on originl work by Ruwe et l, 34 who reported good relibility between testers nd high correltions with intropertive mesurements. Consistent with other uthors who hve reported high intrtester 29,52 nd intertester relibility, 52 the tester in this study hd more thn 10 yers of clinicl experience nd hd estblished high level of relibility on this mesure. 254 Volume 46 Number 3 June 2011

10 However, recent group 53 hs since reported tht clinicl mesurements of femorl nteversion were underestimtes compred with vlues obtined vi mgnetic resonnce imging, questioning the vlidity of the Crig test in ssessing femorl nteversion. Our observed reltionships between femorl nteversion nd dynmic lignment, which were consistent with our hypotheses, indicte tht the clinicl mesurement of femorl nteversion represents some ntomicl spect of hip rottion nd remins n importnt fctor to consider when exmining risk of ACL injury. Further work is needed to identify the underlying chrcteristics being ssessed using the clinicl mesurement method. Aside from femorl nteversion, ll primry vribles were ssessed while the prticipnts were weight bering in n effort to better represent functionl position. However, hip extension torque ws mesured nonweight bering, nd more work is required to confirm if reltionships between strength nd G mx ctivtion would remin consistent if both were ssessed in more functionl position. Our findings re limited to the dominntstnce limb of helthy, college-ged dults nd should not be generlized to other popultions. Further, these findings re limited to controlled, functionl single-leg squt tsk performed in n upright position. Although we felt it ws importnt to control the influence of vrious trunk positions on hip muscle ctivtion 41 tht might hve contributed to inconsistent findings from previous studies, 39,40 we cknowledge tht this upright position my not be fully representtive of more unconstrined dynmic tsks potentilly ssocited with ACL injury. CONCLUSIONS A more integrted pproch to risk-fctor ssessment is needed to ccurtely identify nd understnd those relevnt risk fctors tht my contribute to t-risk knee positions during dynmic ctivity. The overll findings of this study reveled tht LEA chrcteristics cliniclly ssocited with sttic mllignment nd hip muscle ctivtion were directly relted to commonly observed components of functionl vlgus collpse during the single-leg squt. However, this explortory nlysis did not identify ny indirect reltionships between LEA nd G mx ctivtion in predicting joint excursion nd suggests tht LEA does not influence hip muscle ctivtion in controlling joint motion during single-leg squt. Future reserchers should continue to exmine the other fctors tht influence hip muscle ctivtion nd the mechnisms tht explin the reltionships between sttic nd dynmic mllignments. Although the identified reltionships were sttisticlly significnt, the ssocited pth coefficients were somewht low, which indictes tht other fctors could combine with LEA nd hip muscle ctivtion to further ffect dynmic motion. Future investigtors should confirm whether the reltionship mong LEA, hip muscle ctivtion, nd dynmic mllignment is consistent cross vriety of functionl tsks. In ddition, continued exmintion of differences in LEA chrcteristics mong both older nd younger individuls is needed to determine whether these postures chnge with mturity. This reserch will id clinicins in determining the most pproprite time to initite posterior-lterl hip strengthening progrms with the gol of reducing injury. Continued work in these res will help clinicins more effectively identify those t greter risk for injury nd, therefore, help us to develop intervention strtegies to reduce the risk of noncontct ACL injury. ACKNOWLEDGMENTS This study ws funded in prt by the NATA Reserch & Eduction Foundtion. REFERENCES 1. Shultz SJ, Schmitz RJ, Nguyen AD. Reserch Retret IV: ACL injuries the gender bis: April 3 5, 2008, Greensboro, NC. J Athl Trin. 2008;43(5): Kumgi M, Shib N, Higuchi F, Nishimur H, Inoue A. 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