Hasan Z, Enoka R M
Exp Brain Res. 1985;59(3):441-50.
Since the moment arms for the elbow-flexor muscles are longest at intermediate positions of the elbow and shorter at the extremes of the range of motion, it was expected that the elbow torque would also show a peak at an intermediate angle provided the activity of the flexor muscles remained constant. We measured the isometric elbow torque at different elbow angles while the subject attempted to keep constant the electromyographic activity (EMG) of the brachioradialis muscle. The torque-angle relationship thus obtained exhibited a peak, as expected, but the shape of the relationship varied widely among subjects. This was due in part to differences in the variation of the biceps brachii EMG with elbow angle among the different subjects. The implications of these observations for the equilibrium-point hypothesis of movement were investigated as follows. The subject performed elbow movements in the presence of an external torque (which tended to extend the elbow joint) provided by a weight-and-pulley arrangement. We found in the case of flexion movements that invariably there was a transient increase in flexor EMG, as would seem necessary for initiating the movement. However, the steady-state EMG after the movement could be greater or less than the pre-movement EMG. Specifically, the least flexor EMG was required for equilibrium in the intermediate range of elbow angles, compared to the extremes of the range of motion. The EMG-angle relationship, however, varied with the muscle and the subject. The observation that the directions of change in the transient and the steady-state EMG are independent of each other militates against the generality of the equilibrium-point hypothesis. However, a form of the hypothesis which includes the effects of the stretch reflex is not contradicted by this observation.
由于屈肘肌的力臂在肘关节的中间位置最长，而在运动范围的极限位置较短，因此可以预期，如果屈肌的活动保持恒定，肘关节扭矩也会在中间角度出现峰值。我们在受试者试图保持桡侧腕长伸肌的肌电图（EMG）活动恒定时，测量了不同肘关节角度下的等长肘关节扭矩。由此获得的扭矩-角度关系正如预期的那样呈现出一个峰值，但这种关系的形状在不同受试者之间差异很大。这部分是由于不同受试者之间肱二头肌EMG随肘关节角度变化的差异。以下是对这些观察结果对运动平衡点假说的影响进行的研究。受试者在由重物和滑轮装置提供的外部扭矩（倾向于伸展肘关节）存在的情况下进行肘关节运动。我们发现在屈曲运动的情况下，屈肌EMG总是会有短暂增加，这似乎是启动运动所必需的。然而，运动后的稳态EMG可能大于或小于运动前的EMG。具体而言，与运动范围的极限相比，在肘关节角度的中间范围内达到平衡所需的屈肌EMG最少。然而，EMG-角度关系因肌肉和受试者而异。瞬态和稳态EMG的变化方向相互独立这一观察结果不利于平衡点假说的普遍性。然而，包含牵张反射效应的假说形式并不与这一观察结果相矛盾。
