It has been demonstrated recently that, when suitably instructed, subjects could alter the stiffness at the elbow in response to a slowly and imperceptibly changing elastic load. Although evidence was provided in favour of this occurring via changes in gain of the reflex response to stretch, changes in the degree of co-contraction could not be entirely ruled out. The major objective of the present experiments was to determine if subjects could alter stiffness at the wrist in a similar task, and then to determine whether they retained this ability when co-contraction was made impossible by anaesthetizing the nerve to the wrist extensors. A second objective was to determine if changes in stiffness could be controlled independently at the wrist and elbow. 2. Subjects, with eyes closed, initially held position constant against a constant force that loaded the flexors. For the wrist, they were instructed: (i) to keep the hand as still as possible (keep position constant) or (ii) to let the hand be moved by the perturbation (keep force constant). The perturbation was an initially imperceptible elastic load whose direction (loading or unloading) could not be predicted. Subjects were also asked to indicate when the perturbation was first perceived. 3. When asked to hold position constant or force constant at the wrist, subjects demonstrated task-dependent changes in stiffness prior to perception of the perturbation. These changes in stiffness were still achieved when the nerve to the wrist extensors was anesthetized and thus co-contraction was prevented. 4. Five subjects demonstrated the ability to control stiffness independently at the wrist and the elbow although most subjects had difficulty with the task we employed to demonstrate this. 5. The results demonstrate: (i) that for the wrist, set-dependent changes in stiffness that occur prior to perception of a slowly developing perturbation can be mediated by changes in gain of reflex responses to those perturbations, and (ii) that stiffness can be controlled independently at the wrist and elbow, presumably in part by changes in gain of stretch reflexes.
摘要
最近有研究表明，在得到适当指示时，受试者能够根据缓慢且难以察觉的弹性负荷变化来改变肘部的刚度。尽管有证据表明这是通过对拉伸反射反应增益的改变而发生的，但不能完全排除共收缩程度的变化。本实验的主要目的是确定受试者在类似任务中是否能改变手腕的刚度，然后确定当通过麻醉手腕伸肌的神经使共收缩无法发生时，他们是否仍保留这种能力。第二个目的是确定手腕和肘部的刚度变化是否能独立控制。2. 受试者闭眼，最初要在加载屈肌的恒定力作用下保持位置不变。对于手腕，他们接到的指示是：(i) 尽可能让手保持静止（保持位置不变）或 (ii) 让手随扰动移动（保持力不变）。扰动是一个最初难以察觉的弹性负荷，其方向（加载或卸载）无法预测。还要求受试者指出何时首次察觉到扰动。3. 当被要求在手腕处保持位置不变或力不变时，受试者在察觉到扰动之前就表现出了与任务相关的刚度变化。当手腕伸肌的神经被麻醉从而防止共收缩时，这些刚度变化仍然能够实现。4. 五名受试者展示了在手腕和肘部独立控制刚度的能力，尽管大多数受试者在我们用来证明这一点的任务中存在困难。5. 结果表明：(i) 对于手腕，在察觉到缓慢发展的扰动之前发生的与设定相关的刚度变化可以通过对这些扰动的反射反应增益的变化来介导，以及 (ii) 手腕和肘部的刚度可以独立控制，大概部分是通过牵张反射增益的变化来实现的。
