步行至移動表面時預期性及反應性動作調控之生物力學分析

Abstract

步行至移動平面而失去平衡是跌倒的主要原因之一，且極可能造成嚴重的後果。建立步行至移動平面的完整下肢運動學與力動學分析將有助於設計預測或預防跌倒之方法，因此本研究旨在建立年輕人步行至移動表面時之下肢力動學資料，以瞭解行走至移動平面時之預期性及反應性動作的調控機制。本研究利用一配有七台紅外線攝影機之動作分析系統量測十五位健康年輕人分別步行至移動平面與靜止平面時之全身運動學資料，並利用兩塊測力板量測其運動過程中之地面反作用力，接著以逆向動力學分析獲得終端點參數、下肢關節角度、力矩、功率、以及全身質量中心與骨盆的運動，並以相依樣本t檢定比較兩種平面狀況的差異。結果顯示，以行走至靜止平面為基準，在行走至移動平面前的一個跨步期間，前腳跨距短於後腳跨距，前腳跨速也慢於後腳跨速，且前腳擺盪費時亦較長。行走至著地於移動平面過程之預期性動作調控早在後腳前一步著地時便發生，後腳踏地後之地面反作用力鋒值增加，致使垂直方向及前進方向的身體質量中心加速度提早結束，且其影響持續至前腳擺盪前期之身體質量中心速度及位置的驟降；於前腳擺盪後期，地面反作用力鋒值提前由阻力轉為推進力，因而促使質量中心速度提前重新快速向上、向前攀升，進而影響質量中心位置之改變，並導致前腳著地瞬間身體質量中心與足底壓力中心之間的角度及角速度皆大於行走至靜止平面之相對應瞬間的數值。預期性之動作調控於終端點控制的關鍵之一也發生在前腳擺盪後期，此時期健康年輕人藉由增加前腳之膝關節彎曲及減少後腳髖關節之伸直，來減少前腳趾與地面之距離，同時預備降低足部水平落地角度。此外，足跟之水平前進速度，後腳踝關節之屈曲角速度及屈曲功率也較大。由於在此期間，後腳踝關節蹠屈及髖關節外展力矩皆比行走至靜止平面小，顯示健康年輕人為了達到安全且平順的邁向移動平面，其所採用之預期性動作控制需要較精確的時間空間控制能力而遠勝於對肌力的要求。基於安全考量，為了減少前腳踩踏至移動平面之側向剪力及增加支持底面積，在踏上移動平面的前後瞬間，健康年輕人藉由增加骨盆前傾、前腳髖關節與膝關節彎曲以及前、後兩腳踝關節蹠屈，使之達到較小之足部-地面角度。由於在腳落地向前及腳端向前速度較大，加上跑步機之向前拖引力，使得身體質量中心於此雙腳著地反應期之總體活動度在前後與左右方向都較大，這也會導致身體質量中心位置有較趨於向前與偏向前腳的位置。為了維持在此一雙腳著地反應期之動態平衡穩定性，落地時之身體質量中心初始加速度必須迅速驟減，由此可知年輕健康人能夠藉由短暫著地反應期前期之質量中心加速而得以快速轉移，之後又驟然減少加速度，此一反應性動作調控可由加速度的微分值增加而顯示出其有極不穩定的特質。由本研究結果可助於瞭解健康年輕人在行走至移動平面時之預期性其反應性之動作調控，若無法達成此一動作調控策略或無法發展其他代償控制策略，則極可能增加跌倒發生的機會。因此建議未來可以本研究為基礎，進一步探討老年人或有平衡能力缺失的族群於步行至移動平面時下肢之生物力學，藉以鑑定出其跌倒之危險因子。

Causes of fall are complex and involve both human and environmental factors. Adapting our gait to cope with a predictably moving surface is a common fact of life in modern cities. Loss of balance or riders who were stuck by other passengers especially when steeping onto a moving walkway or escalators can lead to serious injuries. Awareness of the risks and the circumstances leading to those injuries allows for better direction of intervention strategies for injury prevention. However, the biomechanical strategies when walking onto a moving surface remained unexplored. Fifteen young subjects were recruited to participate in the study. Test conditions included walk along the walkway from the ground onto a moving surface (MS) or onto a static surface (SS). Subjects were instructed to initiate their gait from one end of the walkway through the movable walkway to the other end while kinematic and kinetic data were measured with a Vicon system and two forceplates. End point variables, joint angles, moments and power of the lower limbs as well as motion of the centere of mass (COM) and pelvis were calculated. The differences between surface conditions were analyzed using paired t-test (α=0.05). Compared to the baseline data in the SS condition, anticipatory locomotor adjustments (ALAs) to walk onto a moving surface started as early as the trailing heel-strike (T3) and mainly occurred during swing phase of the leading limb (T4-T5). With increased leading step speed and length but decreased trailing step speed and length during the anticipatory stride, decreased leading toe clearance and foot angle were found during the late leading swing phase which was longer. Compared to those in the SS condition, smaller ankle plantar flexor and hip abductor moments during the late stance suggested that not the muscular strength but proper temporal-spatial controls of locomotor system were essential to achieve a safe and smooth landing for the young subjects. Greater heel horizontal velocities, efficiency of work revealed by greater trailing ankle power and angular velocities were also observed during late anticipatory phase. ALA of COM acceleration occurred in the vertical and anterioposterior (A/P) directions at the end of leading SLS and trailing SLS (T5) when walking onto a moving surface to have position and velocity of COM decreased until the mid-swing and then increased at terminal swing in vertical and A/P directions. However, changes in COM accelerations might result in the more jerky motion of the body COM and thus lead to greater challenge in the control of the motion of COM. After T5, reactive adjustments revealed by altered heel contact dynamics were achieved through increased leading hip and knee flexion together with more anterior-flexed pelvis and plantar-flexed ankle. With greater initial A/P and M/L COM velocities and the forward moving surface, overall stability of the body in terms of the ROM of the COM was greater in the A/P and M/L direction during the reactive DLS, but inconsequence position of COM were overshot anteriorly and laterally to the leading side. To maintain dynamic stability, initial accelerations became smaller shortly after T5 and remained small during the reactive DLS. Faster weight-transferring but earlier break of accelerations were both essential for a safe walking landing with a. jerky motion during DLS. Knowledge of the ALA and reactive adjustments in compensating for moving surface in young subjects was established and that could be served as a baseline data for the management of other populations with balance deficits in the future. It was also suggested that training programs for patients who is at risk of fall should considered not only task-oriented management but also speed specific modulation.