Reflex and non-reflex torque responses to stretch of the human knee extensors

Reflex responses to unexpected stretches are well documented for a range of muscles in both animal and human. Moreover, investigations of their possible functional significance, has revealed that reflexes can contribute substantially to the overall stiffness of a joint. Unfortunately due to obvious technical limitations, only the muscles spanning the human ankle joint have been investigated in the lower extremity in the past. This study implements a unique hydraulic actuator to study the relative contributions of the knee extensor stretch reflex to the overall knee joint stiffness. The quadriceps muscles were stretched at various background torques, produced either voluntarily or electrically and thus the purely reflex-mediated torque could be calculated. The contribution of the reflex mediated stiffness initially low, increased with increasing background torques for the range of torques investigated.