Feldman A G, Adamovich S V, Levin M F
Research Centre, Rehabilitation Institute of Montreal, Quebec, Canada.
Exp Brain Res. 1995;103(3):440-50. doi: 10.1007/BF00241503.
Two versions of the hypothesis that discrete movements are produced by shifts in the system's equilibrium point are considered. The first suggests that shifts are monotonic and end near the peak velocity of movement, and the second presumes that they are nonmonotonic ("N-shaped") and proceed until the end of movement. The first version, in contrast to the second, predicts that movement time may be significantly reduced by opposing loads without changes in the control pattern. The purpose of the present study was to test the two hypotheses about the duration and shape of the shift in the equilibrium point based on their respective predictions concerning the effects of perturbations on kinematic and EMG patterns in fast elbow flexor movements. Subjects performed unopposed flexions of about 55-70 degrees (control trials) and, in random test trials, movements were opposed by spring-like loads generated by a torque motor. Subjects had no visual feedback and were instructed not to correct arm deflections in case of perturbations. After the end of the movement, the load was removed leading to a secondary movement to the same final position as that in control trials (equifinality). When the load was varied, the static arm positions before unloading and associated joint torques (ranging from 0 to 80-90% of maximum voluntary contraction) had a monotonic relationship. Test movements opposed by a high load (80-90% of maximal voluntary contraction) ended near the peak velocity of control movements. Phasic and tonic electromyographic patterns were load-dependent. In movements opposed by high loads, the first agonist burst was significantly prolonged and displayed a high level of tonic activity for as long as the load was maintained. In the same load conditions, the antagonist burst was suppressed during the dynamic and static phases of movement. The findings of suppression of the antagonist burst does not support the hypothesis of an N-shaped control signal. Equally, the substantial reduction in movement time by the introduction of an opposing load cannot be reconciled in this model. Instead, our data indicate that the shifts in the equilibrium point underlying fast flexor movements are of short duration, ending near the peak velocity of unopposed movement. This suggests that kinematic and electromyographic patterns represent a long-lasting oscillatory response of the system to the short-duration monotonic control pattern, external forces and proprioceptive feedback.
本文考虑了关于离散运动由系统平衡点移动产生的两种假说。第一种假说认为，平衡点移动是单调的，且在运动峰值速度附近结束；第二种假说则假定，平衡点移动是非单调的（“N形”），并持续到运动结束。与第二种假说不同，第一种假说预测，在控制模式不变的情况下，对抗负荷可显著缩短运动时间。本研究旨在基于两种假说各自对快速屈肘运动中扰动对运动学和肌电图模式影响的预测，来检验关于平衡点移动持续时间和形状的这两种假说。受试者进行约55 - 70度的无对抗屈曲（对照试验），在随机测试试验中，运动受到扭矩电机产生的类似弹簧负荷的对抗。受试者没有视觉反馈，并被指示在受到扰动时不要纠正手臂偏斜。运动结束后，移除负荷，导致手臂进行二次运动，最终回到与对照试验相同的最终位置（终点相同）。当改变负荷时，卸载前的静态手臂位置和相关关节扭矩（范围为最大自主收缩的0至80 - 90%）呈单调关系。由高负荷（最大自主收缩的80 - 90%）对抗的测试运动在对照运动的峰值速度附近结束。相位和紧张性肌电图模式取决于负荷。在由高负荷对抗的运动中，只要负荷持续存在，第一个主动肌爆发就会显著延长，并表现出高水平的紧张性活动。在相同负荷条件下，对抗肌爆发在运动的动态和静态阶段受到抑制。对抗肌爆发受到抑制的结果不支持N形控制信号假说。同样，在该模型中，引入对抗负荷导致运动时间大幅缩短也无法得到解释。相反，我们的数据表明，快速屈肌运动背后的平衡点移动持续时间较短，并在无对抗运动的峰值速度附近结束。这表明运动学和肌电图模式代表了系统对短持续时间单调控制模式、外力和本体感觉反馈的长期振荡反应。
