Buchanan J J, Horak F B
Texas A&M University, Department of Health and Kinesiology, College Station 77843-4243, USA.
Exp Brain Res. 2001 Aug;139(4):482-94. doi: 10.1007/s002210100798.
In this study, we examined flexibility in postural coordination by inducing transitions between postural patterns. Previous work demonstrated that the postural control system produces two task-specific postural patterns as a function of the frequency of support surface translation. For slow translation frequencies (<0.5 Hz), subjects ride on the platform reminiscent of upright stance (ride pattern), and for fast frequencies (> or =0.75 Hz) subjects actively fixed the head and trunk in space (head fixed pattern) during anterior-posterior platform motion. To study the adaptation of the postural control system, we had subjects stand on a support surface undergoing increases (from 0.2 to 1.0 Hz in 0.1-Hz steps) and decreases (from 1.0 to 0.2 Hz in 0.1-Hz steps) in translation frequency with the eyes open and closed. Kinematic measures of sagittal plane body motion revealed a gradual transition between these two postural patterns as a function of frequency scaling. In both the increasing and decreasing frequency conditions with visual input, center of mass displacements gradually decreased and increased, respectively, whereas upper-trunk (and head) displacement decreased gradually within the ride pattern until a head fixed pattern was observed without any significant changes in displacement for translation frequencies at and above 0.6 Hz. Without visual input, the scaling of the ride pattern was similar except the transition to the head fixed pattern never emerged with increasing frequency; instead, a less stable pattern exhibiting slow drift in head-trunk anterior-posterior motion (drift pattern) was observed at and above 0.5 Hz oscillations. The stability of the head fixed pattern at fast frequencies was clearly dependent on visual input suggesting that vision was more critical for trunk and head control in space at high than low translation frequencies. Head velocity was kept constant, and lower with vision, as translation frequency (and velocity) changed suggesting a head velocity threshold constraint across postural patterns. The gradual transition from the ride to the head fixed pattern was made possible by the recruitment of available degrees of freedom in the form of ankle, then knee, and then hip joint motion. In turn, the transition from the head fixed or drift pattern was made possible by the gradual suppression of available degrees of freedom in the form of reducing hip, then knee, and then ankle motion. The gradual change in postural kinematics without instabilities and hysteresis suggests that the ability to recruit and suppress biomechanical degrees of freedom allows the postural control system to gradually change postural strategies without suffering a loss of stability. The results are discussed in light of possible self-organizing mechanisms in the multisensory control of posture.
在本研究中，我们通过诱导姿势模式之间的转换来研究姿势协调的灵活性。先前的研究表明，姿势控制系统会根据支撑面平移的频率产生两种特定任务的姿势模式。对于缓慢的平移频率（<0.5Hz），受试者会像直立站立一样骑在平台上（骑行模式）；而对于快速频率（≥0.75Hz），在平台前后运动期间，受试者会在空间中主动固定头部和躯干（头部固定模式）。为了研究姿势控制系统的适应性，我们让受试者站在一个支撑面上，该支撑面的平移频率在睁眼和闭眼的情况下进行增加（从0.2Hz以0.1Hz的步长增加到1.0Hz）和减少（从1.0Hz以0.1Hz的步长减少到0.2Hz）。矢状面身体运动的运动学测量结果显示，随着频率缩放，这两种姿势模式之间会逐渐过渡。在有视觉输入的频率增加和减少条件下，质心位移分别逐渐减小和增加，而在骑行模式中，上躯干（和头部）位移逐渐减小，直到观察到头部固定模式，对于0.6Hz及以上的平移频率，位移没有任何显著变化。在没有视觉输入的情况下，骑行模式的缩放情况相似，只是随着频率增加，从未出现向头部固定模式的过渡；相反，在0.5Hz及以上的振荡频率下，观察到一种不太稳定的模式，在头部 - 躯干前后运动中表现出缓慢漂移（漂移模式）。快速频率下头部固定模式的稳定性明显依赖于视觉输入，这表明在高平移频率下，视觉对于空间中躯干和头部的控制比低平移频率下更为关键。随着平移频率（和速度）的变化，头部速度保持恒定，并且在有视觉输入时更低，这表明在各种姿势模式中存在头部速度阈值约束。从骑行模式到头部固定模式的逐渐过渡是通过以脚踝、然后膝盖、然后髋关节运动的形式募集可用自由度来实现的。反过来，从头部固定或漂移模式的过渡是通过以减少髋关节、然后膝盖、然后脚踝运动的形式逐渐抑制可用自由度来实现的。姿势运动学的逐渐变化没有不稳定性和滞后现象，这表明募集和抑制生物力学自由度的能力使姿势控制系统能够逐渐改变姿势策略而不会失去稳定性。我们将根据姿势多感官控制中可能的自组织机制来讨论这些结果。
