Milner T E, Cloutier C, Leger A B, Franklin D W
Institut de réadaptation de Montréal, Canada.
Exp Brain Res. 1995;107(2):293-305. doi: 10.1007/BF00230049.
In order to determine the maximum joint stiffness that could be produced by cocontraction of wrist flexor and extensor muscles, experiments were conducted in which healthy human subjects stabilized a wrist manipulandum that was made mechanically unstable by using positive position feedback to create a load with the characteristics of a negative spring. To determine a subject's limit of stability, the negative stiffness of the manipulandum was increased by increments until the subject could no longer reliably stabilize the manipulandum in a 1 degree target window. Static wrist stiffness was measured by applying a 3 degree rampand-hold displacement of the manipulandum, which stretched the wrist flexor muscles. As the load stiffness was made more and more negative, subjects responded by increasing the level of cocontraction of flexor and extensor muscles to increase the stiffness of the wrist. The stiffness measured at a subject's limit of stability was taken as the maximum stiffness that the subject could achieve by cocontraction of wrist flexor and extensor muscles. In almost all cases, this value was as large or larger than that measured when the subject was asked to cocontract maximally to stiffen the wrist in the absence of any load. Static wrist stiffness was also measured when subjects reciprocally activated flexor or extensor muscles to hold the manipulandum in the target window against a load generated by a stretched spring. We found a strong linear correlation between wrist stiffness and flexor torque over the range of torques used in this study (20-80% maximal voluntary contraction). The maximum stiffness achieved by cocontraction of wrist flexor and extensor muscles was less than 50% of the maximum value predicted from the joint stiffness measured during matched reciprocal activation of flexor and extensor muscles. EMG recorded from either wrist flexor or extensor muscles during maximal cocontraction confirmed that this reduced stiffness was due to lower levels of activation during cocontraction of flexor and extensor muscles than during reciprocal contraction.
为了确定腕屈肌和伸肌共同收缩所能产生的最大关节刚度，进行了实验。在实验中，健康受试者稳定一个通过正位置反馈使其机械不稳定的腕部操作器，以产生具有负弹簧特性的负载。为了确定受试者的稳定性极限，逐渐增加操作器的负刚度，直到受试者无法再在1度的目标窗口内可靠地稳定操作器。通过对操作器施加3度的斜坡保持位移来测量静态腕关节刚度，该位移会拉伸腕屈肌。随着负载刚度变得越来越负，受试者通过增加屈肌和伸肌的共同收缩水平来增加腕关节的刚度。在受试者的稳定性极限处测得的刚度被视为受试者通过腕屈肌和伸肌共同收缩所能达到的最大刚度。在几乎所有情况下，该值与受试者在无任何负载时被要求最大程度地共同收缩以强化腕关节时测得的值一样大或更大。当受试者交替激活屈肌或伸肌以抵抗拉伸弹簧产生的负载将操作器保持在目标窗口内时，也测量了静态腕关节刚度。在本研究中使用的扭矩范围内（最大自主收缩的20 - 80%），我们发现腕关节刚度与屈肌扭矩之间存在很强的线性相关性。腕屈肌和伸肌共同收缩所达到的最大刚度小于在屈肌和伸肌匹配的交替激活过程中测得的关节刚度所预测的最大值的50%。在最大共同收缩期间从腕屈肌或伸肌记录的肌电图证实，这种刚度降低是由于屈肌和伸肌共同收缩期间的激活水平低于交替收缩期间的激活水平。
