維持人體站立平衡需要前饋與反饋機制相互合作才能達成。站立需要整合多種感覺與認知處理，發展出應對環境與生物力學限制的最佳動作策略。姿勢平衡需整合關節自由活動度，維持身體質量中心落在穩定的底面積內。雖然人類姿勢控制系統可簡化成以踝關節為軸心的單倒單擺模組，但在較具挑戰性的站立平衡，仍不能輕忽具協同重要性的多關節耦合動作。臨床上關節活動度限制(常見於肢體障礙的病患或穿戴義肢的病人)會破壞站立協調策略。因此本研究探討站立在不平穩平面時，踝或膝關節活動度的限制對姿勢穩定性、代償性動作策略與控制機制的改變。
本論文包含兩個獨立研究。第一個研究著眼在雙邊踝關節限制，站立在平衡板上所衍生的下肢姿勢系統重組。年輕健康受試者穿著一雙可鎖定與解鎖，調整雙側踝關節活動的踝足支架，維持在平衡板上的站立平衡(分別是踝限制(AC)與無限制(NC)狀況)；第二個研究著重在研究雙側膝關節限制對於站立在平衡板上所衍生的姿勢策略重組。受試者在膝限制(KC)與無限制(NC)情況下，穿著可鎖定與解鎖的雙膝支架，站立在平衡板上維持穩定站姿。主成分分析(principal component analysis, PCA)(簡化分析變數並可解釋充足變異性的統計學分析)用以分析踝與膝限制實驗所使用的運動策略。踝關節限制的實驗中，分析下肢關節角度變化；為了強調快頻關節活動對姿勢控制的影響，在膝關節限制的實驗，分析下肢關節的角速度變化。擴散穩定性圖分析(Stabilogram diffusion analysis (SDA))，分析在時間軸上一定時間間隔的姿勢位移，用來分析在踝限制與膝限制情況下姿勢變化的前饋(feedforward)與反饋(feedback)機制的偏移。
在第一個實驗中，雖然踝限制並無顯著影響平衡板的運動動態，在踝限制情況下，第一主成分，可以解釋超過80%的下肢動作變異性，其大小與規律性因踝限制而增加。同時，踝限制提升膝與髖關節活動在第一主成分的貢獻程度，顯示雙邊踝限制改變了直立站立的協調機制，增加依賴代償性膝關節動作維持姿勢平衡。相較於無限制狀況，踝限制增加第一主成分的振幅與規律性。為了深入分析踝限制後，不同表現受試者採用適應策略的差異，受試者分成踝限制後平衡板晃動振幅變小的D組與踝限制後平衡板晃動振幅變大的I組。對於踝限制後平衡板搖晃角度變小的受試者(D組)，其下肢動作的第一主成分振幅較小，複雜度變小較少，仰賴代償性的髖關節活動程度也較踝限制後搖晃角度變大的受試者(I組)低。擴散穩定性圖分析(SDA)結果顯示：踝限制狀況導致在平衡板站立前饋與反饋機制重新調整增益。在踝限制的狀況短時區斜率變大與姿勢動作變僵硬有關，長時區斜率在踝限制下減緩，可能與減慢搖晃速度增加感覺訊息處理有關。
第二個實驗中，膝關節限制顯著地增加平衡板角速度變化的規律性。第一與第二主成分可以解釋超過95%下肢關節角速度變化的變異性。膝關節限制強化了第一主成分(踝關節策略)的規律性，近乎達顯著的減少第二主成分(複合膝髖關節策略)的振幅，同時強化平衡板運動與第一主成分(踝關節策略)的耦合度，減弱平衡板運動與第二主成分(複合膝髖關節策略)的耦合度。在膝限制狀況，平衡板角速度的振幅大小與第一主成分的規律性與振幅呈正相關。但與平衡板與第一主成分的耦合度呈負相關。與踝限制狀況不同，膝限制(KC)並不顯著地影響擴散穩定性圖分析(SDA)中的相關參數。
總言之，站立於不平穩平面，踝膝限制會引發姿勢協調控制的重組，強調踝膝關節功能性連結在姿勢協同策略上的重要性。當踝關節受限時，雖不致影響整體平衡板運動動態，但膝關節在平衡板站立任務上扮演較重要的代償角色。從平衡板搖晃增加與需投入較多注意力於姿勢動作鏈的控制來推論，贅餘的髖關節動作在踝限制後的平衡板站立任務是不利的。踝關節限制增加短時區前饋機制的正增益作快速姿勢反應與長時區控制負反饋增益以因應較大程度的姿勢晃動。平衡板站立任務在膝關節受限時，其協同動作也進行重組，藉由增加仰賴踝策略來維持姿勢平衡。在膝限制時，當踝關節角速度較小與較規律，與平衡板耦合度越佳，其平衡板站立的姿勢穩定度越佳。但膝限制對前饋與反饋模式轉變影響不顯著。

Both feedforward and feedback mechanisms cooperate with each other to maintain human upright equilibrium. Upright stance requires multi-sensory integration and cognitive processing to optimize movement strategies in accordance with biomechanical constraints and environmental contexts. One of the key issues of postural balance is to regulate available degree of freedom in joint space, maintaining the motion of the center of mass within the stability region of the base of support. Although the human postural system can be simplified as a single inverted pendulum model pivoting at the ankle joint, yet synergistic joint couplings cannot be overlooked especially during challenging upright stance. Constraints of joint motion, commonly seen in patients with physical disabilities or wearing prosthesis, should violate coordination solutions for stance control. Therefore, this dissertation investigated variations in posture instability, kinematic compensatory strategies, and control mechanisms on an unsteady stance surface after ankle or knee constraint.
This dissertation contained two individual studies. The first study focused on reorganizations of the postural system in the lower limb due to bilateral ankle bracing during stabilometer stance. Young healthy subjects maintained their balance on a stabilometer plate, wearing a pair of unlocked or locked ankle-foot orthoses (AFO) to regulate bilateral ankle movements, known as the ankle constraint (AC) and non-constraint (NC) conditions, respectively. The second study focused on the reorganizations of the postural system in the lower limb due to bilateral knee bracing during stabilometer stance. Subjects were instructed to stand on a stabilometer plate wearing a pair of knee orthoses, unlocked or locked to modulate knee movement, in the knee constraint (KC), and non-constraint (NC) conditions. With principle component analysis (PCA), a statistical approach that accounts for sufficient variance property into fewer variables, different forms of kinematical synergies in response to ankle or knee constraints were characterized. Joint angular fluctuations of the lower limb were used in the ankle constraint study. Joint velocity fluctuations of the lower limb were utilized in the knee constraint study to highlight the roles of fast joint oscillations in posture control. With stabilogram diffusion analysis (SDA), the postural displacements between identified time intervals in the temporal domain were analyzed to determine shift in feedforward and feedback mechanisms for posture stability due to ankle constraint and knee constraint.
In the first study, despite an insignificant constraint-related impact on the dynamics of the stabilometer movements, the size and regularity of the first principal component (PC1) which explained more than 80% of the variance of joint movements in the lower limb was significantly increased with ankle constraint. PC1 exhibited higher communalities with angular movements of the knee and hip joints in the AC condition than in the NC condition. These scenarios indicated that bilateral ankle constraints altered the coordination solution for stabilometer stance, with increasing reliance on compensatory knee movements to maintain a balanced posture. In comparison to NC condition, ankle constraint led to increasing PC1 size and regularity. To analyze performance-related differences in adaptive strategy after ankle constraint, the participants were dichotomized into the D and I groups whose posture sway was decreased and increased with ankle constraint, respectively. For subjects in the D group, PC1 of these participants was smaller in size with less decline in complexity as compared to their counterparts (the I group). The D group relied less on the hip joint during ankle constraint on stabilometer stance than the I group did. The SDA results showed that ankle constraint led to rescale loop gains for feedforward and feedback mechanisms during stabilometer stance. The larger slope of the short-term regime in ankle constraint condition might be associated with a stiffening posture kinematic chain. Reduced the slope of the long-term regime in the ankle constraint condition indicated increasing reliance on sensory cues for posture control with negative feedback process.
In the second study, knee constraint led to a significant increase in regularity of the stabilometer angular velocity. More than 95% of the variance properties of joint angular velocities in the lower limb were explained by the first and second principal components (PC1 and PC2). Knee constraint enhanced the regularity of PC1 (the ankle strategy) but marginally reduced the size of PC2 (the combined knee and hip strategy), along with an increase in the kinematic coupling between stabilometer and PC1 and decreased in the kinematic coupling between stabilometer and PC2. In the KC condition, the size of stabilometer angular velocity correlated positively to the size and regularity of PC1, but negatively to the kinematic coupling between stabilometer and PC1. Contrary to ankle constraint, knee constraint did not alter any SDA variables without significant rebalance of feedback and feedforward mechanisms.
In summary, the coordination control after the physical constraint of the ankle or knee joints on an uneven stance was reorganized, which highlighted a supporting cooperative role of the knee-ankle linkage on posture synergy. Although ankle constraint did not affect the dynamics of stabilometer movement, the knee joint played a more important role in posture control during stabilometer stance. However, accessory hip movement on stabilometer stance during ankle constraint was disadvantageous, underlying increases in postural sway and attentive control of the degree of freedom in the kinematic chain. Ankle constraint enhanced enhancement feedforward gain of short-term for quick posture stabilization and negative feedback gain of long-term in case of greater posture way. Stabilometer stance synergy was also reorganized during knee constraint, with increasing reliance on the ankle strategy to maintain a balanced posture. Superior stabilometer stance stability during knee constraint correlated with coherent ankle-plate interaction and smaller more regular ankle angular velocity. However, knee constraint did not result in a significant paradigm shift in feedback and feedforward processes.

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