The kinetics and stiffness characteristics of the lower extremity in older adults during vertical jumping

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1 Journl of Sports Siene nd Mediine (2008) 7, Reserh rtile The kinetis nd stiffness hrteristis of the lower extremity in older dults during vertil jumping Li-I Wng Deprtment of Physil Edution nd Ademi Pper Editing Clini, Ntionl Hulien University of Edution, Tiwn Astrt The purpose of this study ws to exmine the modulting effets of ge on lower lim stiffness nd net musle joint tivity degenertion when performing funtionl tivity involving SSC. Seven young mles nd seven older mles were reruited s sujets for this study. A high-speed mer nd fore plte were synhronized to ollet the iomehnil prmeters. The kineti prmeters were lulted with the inverse dynmis proess. The stiffness of lower lims ws lulted with the spring-mss model. The Student's t-test ws used to test the differenes of two ge groups. Sttistil signifine ws set t α = The present reserh showed tht the older group produed smller pek net musle joint moment t hip nd knee. There were no differenes in leg stiffness, hip stiffness, nd nkle stiffness etween the two ge groups. Knee stiffness ws smller in the older group. In elderly dults, redued musle strength in the lower lims, espeilly in the hip nd knee, nd redued stiffness of the knee, influene the si funtions of humn life nd inrese the risk of injury. Differenes in lower extremity kinetis nd stiffness in elderly dults during SSC movement my hve implitions for new preventive mesures. Key words: Aging, iomehnis, inverse dynmi, springmss model. Introdution Musle strength nd musle power re fundmentl to the performne of tsks of dily living, suh s wlking, liming up nd down stirs, nd prevention of flls et. (Bssey et l., 1992; Izquirdo et l., 1999). Previous studies using isokineti nd isometri tests hve shown tht the elderly hve signifintly deresed musle strength (Borst, 2004; Cress et l., 2004). However, sine dily tivities often involve omplex, multi-joint motions, nd isotoni nd isometri tests n only detet hnges of musle strength of individul joints during isolted joint motion, the deteted musle wekness of one joint nnot e used to dedue the hnges of the musle fores nd the ssoited ompenstory hnges t other joints in the elderly during multi-joint tivities. Therefore, in the study of motion of the elderly, etter understnding of the influene of musle wekness of eh individul joint on the performne of dynmi funtionl tivities should e hieved through dynmi multi-joint motion tests. Jumping hs often een used for musle strength testing of the lower extremities in the motion nlysis studies of different ge popultions (Elvir et l., 2001). In omprtive study on the performne during ountermovement jumping (CMJ) nd squt jumping (SJ), Psuke et l. (2003) indited tht older femles did not show the streth-shortening yle (SSC) during CMJ. From the temporl hrteristis of the iomehnil prmeters of the lower extremities in the elderly during SJ, Hguenuer et l. (2005) lso showed tht the proximl-distl sequene ws not present in the elderly during jumping. However, the internl dynmi vriles, suh s net musle joint moment nd power t eh joint of the lower extremities, were not inluded in the previous studies on the elderly during jumping. Therefore, musle dynmis nlysis of the joints of the lower extremities n e very useful in determining the net musle joint moment t eh joint during dynmi musle strength tests nd thus led to etter understnding of dynmi musle strength. In orgnism, the overll musle stiffness is determined y the properties of the musuloskeletl system suh s the stiffness of ligments, tendons, rtilge, musle, one nd joints (Ltsh nd Ztsiorsky, 1993; Owen et l., 2005). Menwhile, musle stiffness is determined y the spring-like hrteristis of the musle nd the neurl feedk system nd n e modulted y ontrolling musle tivity nd y neurl system feedk (Cvgn et l., 1977; Cnnon nd Zhlk, 1982; Derrik, 2004; Frley nd Gonzlez, 1996; Frley et l., 1993; 1998; Frley nd Morgenroth, 1999; Full nd Frley, 2000; Liu et l., 2006). Therefore, studying musulr stiffness eluidtes the mehnisms of dpttion nd modultion in the humn ody. In reent yers, the spring-mss model hs een pplied in the study of humn performne, injury nd regultion (Frley nd Gonzlez, 1996; Frley nd Morgenroth, 1999). This model ws developed sed on the elsti hrteristi of the humn musuloskeletl system. It integrtes the musle fiers, tendons, ligments nd the tivity of ll the musles of the lower extremities into simple liner spring. The resultnt fore trnsmitted in the spring is equl to the produt of the spring onstnt (k leg ) nd liner displement (F = k leg y), nd the spring onstnt is lled leg stiffness. Applying similr onept to ngulr joint dynmis, the joint stiffness n e defined euse the resultnt moment t joint is equl to the produt of the torsionl spring onstnt nd ngulr displement of the joint (M = k joint θ) (Brughelli nd Cronin, 2008; Frley et l., 1998; Frley nd Morgenroth, 1999; Full nd Frley, 2000). Furthermore, pproprited musulr stiffness Reeived: 21 Ferury 2008 / Aepted: 07 July 2008 / Pulished (online): 01 Septemer 2008

2 380 Kinetis nd stiffness in elderly during jump provides mehnism of prevention of injury during tivity, nd the inrese of musle nd joint stiffness inreses joint stility, whih helps to sustin rpidly nd effetively the sudden hnges of the joint. Hene, insuffiient stiffness my destilize joint motion nd inrese the inidene of soft tissue injury (Butler et l., 2003; Flngn nd Hrrison, 2007; Grnt et l., 2002; Kuitunen et l., 2007; Yoon et l., 2007). For this reson, study of the hrteristis of the elderly during dynmi multi-joint tivities in terms of the kineti prmeters of the lower lims nd the stiffness of lower lim will e helpful for etter understnding of this geing phenomenon. From the ove literture review, it is unler how the lower lim stiffness is hnged to ommodte for the redued musle strength of the elderly when performing dynmi multi-joint tivities. Therefore, this study exmined the modulting effets of ge on lower lim stiffness nd net musle joint tivity degenertion when performing funtionl tivity involving SSC. Methods Sujets Seven young mle (18.0 ± 0.3 yrs, 1.75 ± 0.06 m, 70.8 ± 9.92 kg) nd 7 older mle dults (67.1 ± 2.5 yrs, 1.63 ± 0.08 m, 66.7 ± 8.87 kg) prtiipted in the urrent study. None of the sujets undertook ny speifi exerise trining or hd ny sports injury in their lower extremities. They ll ould perform dily tivities normlly. Before the experiment, ll sujets red the experimentl notie, gve written informed onsent, nd ompleted helth survey. Prtiipnts were required to stnd on the fore plte with their hnds on their hips, nd the instrution given to the prtiipnts ws jump s high s you n. All prtiipnts wrm up with stndrd proedure efore the forml experiment. Eh prtiipnt hd to omplete three suessful trils, nd the highest jump ws seleted for nlysis. A MegSpeed high-speed mer (125 Hz) ws synhronized with n AMTI fore plte (1250 Hz) to ollet the iomehnis prmeters. Segment dt for foot, leg, thigh, nd hed nd trunk (HAT) were lulted sed on Dempster (1955) segment prmeters. Five ody lndmrks were fixed on the prtiipnts romion, greter trohnter, lterl epiondylus, lterl mlleous, nd tip of the foot t the height of the mettrsls on the left side; nd one referene point ws fixed on the fore plte. All these points were digitized nd frmed y Kwon3D system. A Butterworth Digitl Reursive Filter ws used to filter the rndom noise in the proess of digitiztion (fourth-order lower-pss with ut-off frequeny of 8 Hz). The ground retion fores (GRF) nd enter of pressure were ollet y KwonGRF system. The net musle joint moment (M), net musle joint power (P), nd net musle joint work (W) were lulted with the inverse dynmis proess (Winter, 2005). The leg stiffness (k leg ) ws lulted y the formul: (Fy hip-lowest ) / ( Y hip ). The Fy hip-lowest indited the vertil GRF while the hip ws t the lowest position. The Y hip ws the vertil displement of the hip from the strt to the point t whih the hip ws t its lowest. The joint stiffness (k joint ) ws lulted y the formul: (M jointminimlly) / ( θ joint ). The M joint-minimlly ws the net musle joint moment while the joint ws mximlly flexed. The θ joint ws the ngulr displement of the joint from the strt to the point t whih the joint ws t its miniml flexion. In this study, the jump height ws normlized on the sis of the height of prtiipnt, nd Height ws used s the unit. The normlized GRF ws the mesured GRF divided y the sujet s ody weight, nd BW ws used s the unit. The normlized net musle joint moment ws the net musle joint moment divided y the sujet s ody weight, nd Nm kg -1 ws used s the unit. Aording to the onept of inverse dynmis (Winter, 2005), positive net musle joint moment ws defined s extensors tivity, while negtive net musle joint moment indites the tivity of the flexors. The normlized net musle joint power ws the net musle joint power divided y the sujet s ody weight, nd the unit used ws W kg -1. The net musle joint power ws positive when musles ontrted onentrilly nd negtive when musles ontrted eentrilly. The normlized displement of the hip joint ws defined s the vertil displement of the hip joint divided y the sujet s leg length, nd the unit ws Length. The normlized leg stiffness ws defined s the GRF divided y the normlized displement of the hip joint, nd the unit ws BW/length. The normlized ngulr stiffness ws defined s the net musle joint moment divided y the joint ngulr displement, nd the unit ws Nm kg -1 /rd. The movement times were the solute time lulted s the rtio of the totl movement time. The totl movement time ws estimted y ounting the seond from initition of movement to the moment of tke off. All the lulted vriles were tested using n independent t-test for the differenes etween young nd older sujets during vertil jumping. A signifine level of α = 0.05 ws set for ll sttistil tests. Results The normlized jump height in the older group (0.17 ± 0.03 Height) ws signifintly lower thn tht of the young group (0.29 ± 0.03 Height, p < 0.05). The pek GRF ws signifintly smller in the older group (p < 0.05, Tle 1). Compred to the young group, the older group not only hd smller pek hip nd knee joint moments (p < 0.05) ut lso signifintly deresed pek net musle joint powers t the hip, knee nd nkle (p < 0.05). Tle 2 showed the men reltive times t four musle movement stges (i.e., FCC: flexors' onentri ontrtion, EEC: extensors' eentri ontrtion, ECC: extensors' onentri ontrtion, nd FEC: flexors' eentri ontrtion) t three joints for oth groups. In the movement times, no signifint differenes were found etween two groups t nkle (p > 0.05). There ws signifint differenes were found in the movement times of hip nd knee etween two groups (p < 0.05). No FCC of hip ws found in older group. The movement times of hip EEC nd knee FCC in older group were signifint longer (p < 0.05). In ddition, the older group ssigned shorter time of hip ECC, knee EEC, nd knee ECC (p < 0.05).

3 Wng 381 Tle 1. Kineti prmeters of lower lims during vertil jump. Dt re mens (±SD. Young Group Older Group Mximum normlized Fy (BW) 1.20 (.06) 1.02 (.13) * Mximum normlized M (Nm kg -1 ) Hip 2.45 (.36) 1.79 (.32) * Knee 1.52 (.16).81 (.26) * Ankle 1.86 (.18) 1.61 (.23) * Mximum normlized P (W kg -1 ) Hip (1.27) 5.39 (1.79) * Knee 6.19 (1.18) 2.58 (.99) * Ankle (3.21) (2.47) * Normlized totl W (J kg -1 ) Hip 2.73 (.73) 1.54 (.66) * Knee 1.62 (.27).83 (.39) * Ankle 1.80 (.17) 1.19 (.39) * signifint differene etween groups, p < 0.05 Tle 3 shows the prmeters of the normlized leg musulr stiffness during stnding vertil jumping. Figure 1 shows the reltionship etween the GRF nd the vertil displement of the hip for two typil sujets from the two ge groups. The experimentl results (Tle 3) showed tht the vertil displement of the hip nd the vertil GRF of the older group were ll signifintly smller thn those of the young group (p < 0.05) when the hip joint ws t the lowest position. There ws no signifint differene etween these two groups in leg stiffness (p > 0.05). The figure 1 shows tht the vertil GRF nd the vertil displement of the hip re greter in the young sujets thn in the older sujets in the rouh phse. Therefore, lthough the line - of the fore-deformtion reltion urve is shorter in the older sujets thn in the young sujets, the slopes re similr. Tle 3. Prmeters of leg spring-mss model during vertil jumping. Dt re mens ( ±SD). Young Group Older Group Normlized Fy hip-lowest (BW) 1.21 (.09).97 (.14) * Normlized Y hip (Length).41 (.07).31 (.08) * Normlized k leg (BW/Length) 3.03 (.56) 3.25 (.86) signifint differene etween groups, p<0.05 Tle 4 shows the normlized ngulr stiffness of eh joint during stnding vertil jumping. The reltionship etween the net musle joint moment nd the ngulr displement for the hip, knee, nd nkle of two typil sujets from the two ge groups re shown in Figures 2, 3 nd 4. The results (Tle 4) show tht the young group hd greter ngulr displement of the hip nd musulr stiffness of the hip nd knee (p < 0.05). The ngulr stiffness in knee joint of the older group ws signifintly redued (p < 0.05). There were no differenes in joint stiffness for hip nd nkle etween groups (p > 0.05). Figure 2 shows tht the net musle joint moment nd the ngulr displement of the hip re greter in the young sujets thn in the older sujets in the rouh phse. Although the line - of the moment deformtion reltion urve for the hip ws shorter in the older sujets thn in the young sujets, the slopes were similr. Figure 3 shows tht the net musle joint moment of the knee is greter in the young sujets thn in the older sujets in the rouh phse. However, the ngulr displement of the knee flexion is similr. Therefore, the slope of the line - of the moment deformtion reltion urve for the knee is fltter in the older sujets thn in the young sujets. Disussion The sequene of eentri musle tion immeditely followed onentri tion is defined s SSC, whih is nturl omponent of musle funtion in mny dily tivities. Mximl voluntry ontrtion nd SSC is involved during CMJ, thus reveling the musulr strength nd the unique tivtion strtegies of the neuromusulr system of the lower extremities (Enok, 1996; Kuo et l., 1999; Liu et l., 2006). Hene, pek GRF nd pek net musle joint moment during vertil jumping re importnt indies of musulr strength in the lower extremities. This study found tht, ompred to younger sujets, elderly sujets hve mximl voluntry CMJ with lower Tle 2. The men reltive times (%) t four musle movement stges. Dt re mens ( ±SD). Young Group Older Group Hip Flexors' Conentri Contrtion (6.88).00 (.00) * Extensors' Eentri Contrtion (9.23) (5.12) * Extensors' Conentri Contrtion (2.13) (3.66) * Flexors' Eentri Contrtion 1.28 (1.94) 1.88 (1.70) Knee Flexors' Conentri Contrtion 6.85 (4.44) (13.72) * Extensors' Eentri Contrtion (5.43) (14.45) * Extensors' Conentri Contrtion (3.12) (6.63) * Flexors' Eentri Contrtion 4.50 (2.13) 6.91 (6.10) Ankle Flexors' Conentri Contrtion.00 (.00).00.00) Extensors' Eentri Contrtion (2.49) (4.58) Extensors' Conentri Contrtion (2.44) (4.71) Flexors' Eentri Contrtion.18 (.48).30 (.53) * signifint differene etween groups, p<0.05

4 382 Kinetis nd stiffness in elderly during jump 3 young older Vertil Ground Retion Fore (BW) Vertil Displement of Hip (length) Figure 1. The fore-deformtion reltion for the leg during vertil jumping. Point indites tht the sujet strted downwrd motion. Point indites tht the hip rehed the lowest position. Point indites the moment of tke-off. The slope of the line - reflets the leg stiffness in the rouh phse. pek GRF s well s smller pek net musle joint moment t the hip nd knee joint. Elderly sujets therefore exhiit lower net musle joint power nd lower totl work of the lower extremities during onentri phse of the CMJ. This degrded dynmi musle strength impirs si funtionl tivity in elderly. In elderly, this musle wekness n lso severely impir multion, prtiulrly lne nd stility (Connelly, 2000; Lord et l., 1994). Further, lower lim stiffness my e ffeted neuromusulr system tivtion. During humn loomotion, the neuromusulr system dopts dequte musle tivity in different onditions to hieve suess in the intertion etween orgnism nd GRF, djusting the stiffness of the lower lims while in motion ould keep the movement in stte of stility (Cvgn et l., 1977; Cnnon nd Zhlk, 1982; Frley et l., 1998; Frley nd Morgenroth, 1999; Full nd Frley, 2000). Frely et l. (1998) suggested tht the ontrol of joint ngulr stiffness nd segment ngles ws very importnt for the modultion of leg stiffness, nd tht ws why one would hieve the sme movement pttern. The mjor findings of this study re tht, ompred to young sujets, elderly sujets perform mximl voluntry CMJ with less ngulr stiffness of the knee during CMJ. Leg stiffness nd ngulr stiffness in the hip nd nkle did not sttistilly differ. However, leg stiffness ws determined y vertil GRF nd hip displement, nd joint stiffness ws determined y net musle joint moment nd ngulr displement t the trnsition from the downwrd to upwrd phse during the CMJ. From ssessment of the leg stiffness nd ssoited prmeters in the elderly during CMJ, the GRF ws found to e redued signifintly when the hip joint ws t the lowest position, nd the displement of the leg length ws lso redued. Hene, lthough the line - of the fore-deformtion reltion urve ws shorter in the older group thn in the young group, the slopes were similr. The leg stiffness of the elderly ws not different from tht of the young. The similr phenomenon ws in the ngulr stiffness of the hip, the urrent study Tle 4. Prmeters of joint spring-mss model during vertil jumping. Dt re mens (±SD). Young Group Older Group Hip Normlized M hip-minimlly (Nm kg -1 ) 2.40 (.39) 1.71 (.39) * θ hip (degree) (12.81) (15.08) * Normlized k hip (Nm kg -1 degree -1 ).03 (.00).02 (.00) Knee Normlized M knee-minimlly (Nm kg -1 ) 1.48 (.15).78 (.28) * θ knee (degree) (8.34) (13.57) Normlized k knee (Nm kg -1 degree -1 ).02 (.00).01 (.00) * Ankle Normlized M nkle-minimlly (Nm kg -1 ) 1.71 (.22) 1.46 (.20) θ nkle (degree) (1.35) (7.51) Normlized k nkle (Nm kg -1 degree -1 ).05 (.01).05 (.02) signifint differene etween groups, p < 0.05

5 Wng young older Net Musle Joint Moment of Hip (Nm/kg) Angulr Displement of Hip (rd) Figure 2. The moment deformtion reltion for the hip during vertil jump. Point indites tht the sujet strted downwrd motion. Point indites tht the hip rehed the miniml joint ngle of flexion. Point indites the moment of tke-off. The slope of the line - reflets the joint stiffness of the hip in the rouh phse. found tht oth the net musle joint moment t the hip joint nd the ngulr displement of the hip were signifintly redued when the hip ws minimlly flexed. Hene, lthough the line - of the moment deformtion reltion urve ws shorter in the older group thn in the young group, the slopes were similr. The ngulr stiffness of the hip in the older group ws not signifintly different from tht of the young. This result suggests tht the elderly tended to derese the leg displement to modulte leg stiffness in suessfully performing the dynmi tivity. Otherwise, the older group deresed ngulr displement of the hip to modulte the ngulr stiffness of the hip. Restted, in the elderly, the hip joint stiffens during downwrd motion. A similr phenomenon ours during the prepring posture in SJ nd downwrd stepping in the elderly (Hguenuer et l., 2005; 3 young older Net Musle Joint Moment of Knee (Nm/kg) Angulr Displement of Knee (rd) Figure 3. The moment deformtion reltion for the knee during vertil jumping. Point indites tht the sujet strted downwrd motion. Point indites tht the knee hs rehed the miniml joint ngle of flexion. Point indites the moment of tke-off. The slope of the line - reflets the joint stiffness of the knee in the rouh phse.

6 384 Kinetis nd stiffness in elderly during jump 3 young older Net Musle Joint Moment of Ankle (Nm/kg) Angulr Displement of Ankle (rd) Figure 4. The moment deformtion reltion for the nkle during vertil jump. Point indites tht the sujet strted downwrd motion. Point indites tht the nkle hs rehed the miniml joint ngle of dorsiflexion. Point indites the moment of tke-off. The slope of the line - reflets the joint stiffness of the nkle in the rouh phse. Hortogyi nd DeVit, 2000). The uthors suggest tht the elderly tend to stiffen the trunk or legs in ompenstion for impired neuromotor funtions nd deresed musle strength to inrese stility. In elderly, this phenomenon elevtes ntgonist musles tivity, whih inreses musle otivity (Izquirdo et l., 1999). Elevted musle otivity inreses joint stiffness: hene, joint stility is lso inresed in the lower extremities (Brtt et l., 1988). In this study, nlysis of net musle joint moment nd power reveled tht the elderly do not exhiit onentri flexor ontrtion in the hip joint t the eginning of the rouh phse. The elderly therefore perform the downwrd motion using only eentri ontrtion of the extensors. Hene, elderly lerly hve longer eentri ontrtion period of extensors t the hip joint thn young dults. A likely explntion is tht the elderly inrese ntgonist musles tivity to derese hip flexion; hene, joint stility is inresed during the downwrd motion of CMJ. However, the elderly showed redued net musle joint moment t the knee joint when the knee rehed minimum flexion, ut the ngulr displement of the knee joint ws not redued. This phenomenon used the slope of line - of the moment deformtion reltion urve to e fltter in the older group thn in the young group. The ngulr stiffness ws redued signifintly. Greter leg stiffness during ountermovement motion might e dvntgeous euse it llows greter storge nd relese of elsti energy to inrese the fore of motion (Gollhofer et l., 1992; Komi, 1992). Hene, the urrent study shows tht the derese of knee stiffness ws ovious in the older group during the CMJ. This result my e relted to the finding of previous study, tht the enefit from SSC during ountermovement motion ws not ovious in the elderly (Psuke et l., 2003). Otherwise, joint stiffness is onduive to ngulr resistne to torque (Willims et l., 2007). Hene, enhned joint ngulr stiffness ould resist sudden ngulr displement, whih is enefiil to joint stility (Flngn nd Hrrison, 2007). Butler et l. (2003) suggested tht redued joint ngulr stiffness my inrese the dmge to rtilge nd ligments. Therefore, n pproprite mount of stiffness is relevnt to the prevention of injury (Kuitunen et l., 2007). The results in the urrent study suggest tht, with deresed musle strength, the elderly deresed the hip ngle to mintin its ngulr stiffness, strtegy tht n help protet the hip joint. Therefore, the elderly redued the ngulr displement of the hip during downwrd motion to djust the hip stiffness, ut the strighter motion of the trunk ould inrese the demnd t the ACL during downwrd movement (Deker et l., 2003). Conversely, irtiulr musles ross the hip nd knee (e.g., m. semitendinosus nd m. ieps femoris). Hene, in the initil downwrd motion, the longer eentri ontrtion of hip joint extensors in elderly my inrese onentri ontrtion of flexors t the knee joint, whih would not signifintly derese ngulr displement of the knee joint. However, the net musle joint moment t the knee in the elderly ws deresed without deresing the ngulr displement of the knee joint, leding to signifintly deresed ngulr stiffness, whih my e hrmful to the knee joint. In omprtive study on leg stiffness in mles nd femles during ontinuous hopping, Grnt et l. (2002) showed tht the ngulr stiffness of the knee joint in femles ws smller then tht in the mles during hopping nd suggested tht this ws the reson why femles hve higher risk of knee ligment injuries. In the urrent study, the redued ngulr stiffness of the knee joint in the older group during dynmi tivity my e

7 Wng 385 used s the referene for the study of the degenertion of the knee joint in the elderly. Furthermore, the net musle joint moment nd ngulr stiffness t the nkle joint in the older group were not signifintly different from those of the young group, suggesting tht no ovious degenertion t the nkle joint funtion ws present in the elderly during the dynmi, mulit-joint motion of the CMJ. Conlusion This study of hnges in net musle joint moment nd leg stiffness in the elderly during dynmi multi-joint tivity reveled signifintly deresed lower extremity musle strength in the elderly, prtiulrly in the hip nd knee musles. This hnge ffets their ility to perform funtionl dily tivities. Older sujets mintin hip ngulr stiffness y deresing joint ngles in order to protet the joint nd to inrese stility during movement. However, this mehnism ws not oserved in the knee joint. 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8 386 Kinetis nd stiffness in elderly during jump Key points The present reserh showed tht the older group redued musle strength in the lower lims, espeilly in the hip nd knee, nd redued stiffness of the knee, influene the si funtions of humn life nd inrese the risk of injury. There were no differenes in leg stiffness, hip stiffness, nd nkle stiffness etween the two ge groups. Older sujets mintin hip ngulr stiffness y deresing joint ngles in order to protet the joint nd to inrese stility during movement. In elderly dults, insuffiient ngulr stiffness of the knee joint my inrese the risk of knee injury during funtionl tivity involving SSC. AUTHORS BIOGRAPHY Li-I WANG Employment Assistnt Professor, Deprtment of Physil Edution, Ntionl Hulien University of Edution. Degree PhD Reserh interest Biomehnis of lower lims. E-mil: tennis01@mil.nhlue.edu.tw Dr. Li-I Wng Deprtment of Physil Edution, Ntionl Hulien University of Edution, No. 123, Hu-Hsi Rd., Hulien 970, Tiwn (R.O.C.).