Lee W A, Patton J L
Program in Physical Therapy, Northwestern University Medical School, Chicago, IL 60611-2814, USA.
Biol Cybern. 1997 Sep;77(3):197-206. doi: 10.1007/s004220050380.
This paper tests the hypothesis that the central nervous system (CNS) learns to organize multijoint movements during a multijoint 'bouncing pull' task such that, after practice, motion of the anterior-posterior center of mass (CMAP) more closely resembles that of a conservative, one degree of freedom (DF), inverted pendulum model. The task requires standing human subjects to produce precise peak pulling forces on a handle while maintaining balance-goals that can be easily accomplished if movement is organized as in the model. Ten freely standing subjects practiced making brief, bouncing pulls in the horizontal direction to target forces (20-80% of maximum) for 5 days. Pulling force, body kinematic and force plate data were recorded. An eight-segment analysis determined sagittal-plane CM motion. We compared the effects of practice on the regression-based fit between actual and model-simulated CMAP trajectories, and on measures of CMAP phase plane symmetry and parameter constancy that the model predicts. If the CNS learns to organize movements like the inverted pendulum model, then model fit should improve and all other measures should approach zero after practice. The fit between modeled and actual CMAP motion did not improve significantly with practice, except for moderate force pulls. Nor did practice increase phase plane symmetry or parameter constancy. Specifically, practice did not decrease the differences between the pre-impact and rebound positions or speeds of the CMAP, although speed difference increased with pulling force. CMAP at the end of the movement was anterior to its initial position; the anterior shift increased after practice. Differences between the pre-pull and balance-recovery ankle torque (TA) impulses were greater on day 5 and correlated with the anterior shift in CMAP. These results suggest that practice separately influenced the force production and balance recovery phases. A modified model with damping could not explain the observed behaviors. A modified model using the actual time-varying TA profiles improved fit at lower force levels, but did not explain the increased postural shift after practice. We conclude that the CNS does not learn to organize movements like the conservative, inverted pendulum model, but rather learned a more complex form of organization that capitalized on more time-varying controls and more intersegmental dynamics. We hypothesize that at least one additional DF and at least one time-varying parameter will be needed to explain fully how the CNS learns to organize multijoint, bouncing pulls made while standing.
中枢神经系统（CNS）在多关节“弹跳式拉伸”任务中学会组织多关节运动，从而在练习之后，前后重心（CMAP）的运动更接近于保守的单自由度（DF）倒立摆模型。该任务要求站立的人体受试者在保持平衡目标的同时，在手柄上产生精确的峰值拉力，如果运动按照模型那样组织，这些平衡目标很容易实现。10名自由站立的受试者练习在水平方向进行短暂的弹跳式拉伸，以达到目标力（最大力的20%-80%），持续5天。记录拉力、身体运动学和测力板数据。八段分析确定矢状面CM运动。我们比较了练习对实际与模型模拟的CMAP轨迹之间基于回归的拟合效果的影响，以及对模型预测的CMAP相平面对称性和参数恒定性测量的影响。如果中枢神经系统学会像倒立摆模型那样组织运动，那么练习后模型拟合应得到改善，所有其他测量值应趋近于零。除了中等力度的拉伸外，练习并没有使模拟和实际的CMAP运动之间的拟合得到显著改善。练习也没有增加相平面对称性或参数恒定性。具体而言，练习并没有减小CMAP撞击前和反弹位置或速度之间的差异，尽管速度差异随着拉力增加。运动结束时的CMAP位于其初始位置前方；练习后向前的位移增加。第5天，拉伸前和平衡恢复时的踝关节扭矩（TA）冲量之间的差异更大，且与CMAP的向前位移相关。这些结果表明，练习分别影响了力的产生和平衡恢复阶段。带有阻尼的修正模型无法解释观察到的行为。使用实际时变TA曲线的修正模型在较低力水平下改善了拟合，但无法解释练习后姿势位移增加的现象。我们得出结论，中枢神经系统并没有学会像保守的倒立摆模型那样组织运动，而是学会了一种更复杂的组织形式，这种形式利用了更多的时变控制和更多的节段间动力学。我们假设，至少还需要一个额外的自由度和至少一个时变参数，才能全面解释中枢神经系统如何学会组织站立时进行的多关节弹跳式拉伸。
