Improvement in linearity and regulation of stiffness that results from actions of stretch reflex

The authors studied stretch reflexes of soleus muscles of intercollicularly decerebrated cats using a new technique for estimating the component of a stretch reflex that results from the purely mechanical properties of the active muscle (mechanical response). The difference between a net stretch reflex and its underlying mechanical response provided a direct measure of reflex action. The relative contributions of reflex action and the mechanical response are different for stretch and release. With stretch, reflex action is generally large and the mechanical response small. The opposite is true with release. The property that remains relatively constant, when stretch and release are compared, is the net stiffness in opposition to length change. The authors concluded that reflex action compensates for variations in the inherent stiffness of the muscle. Compensation is effective over a range of intermediate values of initial force, but it fails whenever the reflex force approaches zero or the maximal force within 22 ms of the onset of length change. This indicates that even during a fast gallop, there is sufficient time for reflex action. The mechanical properties of the active soleus muscle are highly nonlinear; e.g., muscular stiffness becomes negative transiently during stretch. In stable decerebrate preparations they found that reflex action resulted in a considerable improvement in linearity. Their results support two complementary hypotheses: stiffness may be the regulated property of the stretch reflex, and the main function of autogenetic reflexes may be to compensate for variations in the properties of skeletal muscle rather than to oppose changes in load.