Functional reorganizations and motor strategies for postural control during standing.

坐下和下蹲是日常生活中的常规活动，通过弯曲躯干和腿部来降低体重，但显然每个目标姿势需要不同的运动策略。前者必须将支撑表面转移到座位上，而后者必须将重心保持在双脚的同一基座内。通过对两种动作的性能比较，研究了两种动作在重复过程中启动下向动作的机制和优化动作性能的过程。坐下运动是通过刻板的运动策略来实现的，其特征是腓肠肌破裂，并伴有竖脊肌的失活。前者在向前方向产生瞬时压力中心（COP）位移，同时解锁后备箱防止向后摔倒。相比之下，由于不需要在给定的方向上产生动力，各种各样的运动策略……更多的人可以开始下蹲。在重复坐下的过程中，在第二次试验后，预备阶段初始站立姿势的平均COP位置立即向前移动，导致受试者在转移到座位上时向后的动量减少。然而，在下蹲任务中，这些变化不能被识别出来。这些发现表明，在将身体质量转移到另一个支撑基座的情况下，中枢神经系统立即调整初始投掷的大小以优化性能。然后研究单侧腿神经分化后维持站立姿势的运动策略。具体来说，我们研究了正常成人受试者在长时间站立期间站立稳定性和参考位置的时间变化，直到神经分化发生。9名健康受试者被要求安静地站在相邻的两个受力平台上，膝盖伸展。两个独立的阶段包括长时间站立超过20分钟（控制任务），直到右膝上方充气充气袖带使h反射消失（实验任务）。第一期60 s记录双侧臀大肌（GM）、股内侧肌（VM）、胫前肌（TA）、比目鱼肌（Sol）的双侧髋关节、踝关节位置和肌电图，随后每3 min记录同一时间段的肌电图。根据力板数据得出的COP测量，即平均速度（MV）和均方根距离（RMSD），估计每个时期安静站立时的身体摆动。同时，计算每条腿的平均COP位置（身体摇摆中心；CBS），以确定参考位置的变化。在控制任务中，在长时间站立期间，所有测量参数均未发现任何与时间相关的变化。在实验任务中，虽然在前-后（a -p）和中-外侧（M-L）方向的MV在去神经分化后大于前几个时段，但RMSD仅在a -p方向的初始和最终时段之间存在显著差异（p<0.01）。结果表明，单侧下肢和足部感觉信息的丧失需要额外的调节活动来稳定A-P和M-L方向的站立姿势，但A-P方向的姿势稳定性不能发展到初始水平。脱神经后，右侧比目鱼肌活动明显减弱（P<0.01）， COP在前后、中外侧方向的平均速度增大，右脚下COP的平均位置较前期后移（P<0.01）。同时，左侧胫骨前肌和比目鱼肌以及双侧臀中肌也被激活。单侧腿部和足部感觉信息的丧失需要额外的调节活动来稳定站立姿势。新组织的姿势似乎部分模拟了单侧腿感觉障碍患者的站立姿势，如截肢者或偏瘫患者。少

Sitting down and squatting are routine activities in daily living that lower the body mass by flexing the trunk and legs, but they obviously require different motor strategies for each goal posture. The former must transfer the supporting surface onto a seat, whereas the latter must maintain the center of mass within the same base of both feet. By comparing the performance of both maneuvers, the mechanisms involved in initiating the downward-oriented movements and the process of optimizing the performance during their repetitions were studied. The sitting-down movement was achieved by a stereotyped motor strategy characterized by a gastrocnemius muscle burst coupled with deactivation of the erector spinae muscle. The former produced a transient center of pressure(COP) displacement in the forward direction, and simultaneous unlocking of the trunk prevented falling backward. By contrast, because of the absence of any need to produce momentum in a given direction, a variety of motor strat … More egies were available to initiate squatting. During repetition of sitting down, the average COP position of the initial standing posture in the preparatory period was immediately shifted forward after the second trials, resulted in a decreased momentum in the backward direction while the subjects were transferring themselves onto the seat. In the squatting task, however, these changes could not be identified. These findings suggest that in the case of transferring the body-mass to another supporting base the central nervous system immediately adjusts the size of the initial throw to optimize the performance.Then the motor strategies responsible for maintaining standing posture after deafferentation of the unilateral leg were studied. Specifically, we addressed the temporal changes in standing steadiness and reference position during prolonged standing until deafferentation would come on normal adult human subjects.Ninn healthy subjects were asked to stand quietly on adjoining two force platforms with their knees extended. Two separate sessions consisted of prolonged standing over 20 min(control task) and until the H-reflex would be disappeared by the inflated pneumatic cuff above the right knee (experimental task). Bilateral hip and ankle joint positions and EMGs of bilateral gluteus maximus(GM), vastus mediali (VM), tibialis anterior(TA), and soleus (Sol) were recorded in the first period for 60 s and the following periods for the same duration every 3 min0. Body sway during quiet standing in each period was estimated on the basis of the COP measures derived from force plate data, i.e. mean velocity(MV) and root mean square distance (RMSD). Simultaneously, the average COP positions(center of body sway ; CBS) in each legs were calculated for determining the change of reference position.In the control task, any time-related change during prolonged standing was not found in all of measured parameters. In the experimental task, although the MV in the anterior-posterior(A-P) and medial-lateral(M-L) directions were greater after deafferentation than in the previous periods, a significant difference in the RMSD was found only between the initial and final periods in the A-P direction(p<0.01). The results indicated that the unilateral loss of leg and foot sensory information necessitated additional regulatory activities to stabilize standing posture in the A-P and M-L direction, but the postural stability in the A-P direction could not be developed to the level in the initial period. After deafferentation, the right soleus activity was significantly decreased(P<0.01), so the mean velocities of COP in the anterior-posterior and medial-lateral directions were greater and the average COP position under the right foot shifted backwards compared to the ones in the previous periods(P<0.01). Also, the left tibialis anterior and soleus were activated as well as the bilateral gluteus medius.The unilateral loss of leg and foot sensory information necessitated additional regulatory activities to stabilize the standing posture. The newly organized posture appeared to partly simulate the standing posture in the patients with sensory disturbance of a unilateral leg, such as amputees or hemiparetic patients. Less