Most investigators believe that the stretch reflex is an important mechanism in the regulation of posture and movement, but the precise nature of its actions and the importance of this reflex in intact animals and humans have remained unclear. The uncertainty has resulted largely from an inadequate interpretation of the function of the stretch reflex. It is assumed that the function is to compensate for changes in load; in other words, the actions of the stretch reflex are supposed to reduce the influence of load on muscle length, either during the maintenance of a posture or the execution of a movement. This chapter presents the evidence against this view. An alternative hypothesis is also presented stating that the main function of the stretch reflex is to compensate for variations in the mechanical properties of skeletal muscle. The chapter also discusses the compensatory actions when different amplitudes of stretch and release are applied. The remarkable finding is that the highly nonlinear dependence of the underlying mechanical response on amplitude and direction was compensated to a considerable extent by reflex action. It is believed that this may be important in the central nervous control of the musculature.
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