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When sinusoidally fluctuating tensions were applied to the soleus muscle of a decerebrate cat, plots of tension against length showed a hysteresis loop, implying damping. Various tensions and frequencies of oscillation were used, with and without simultaneous head‐tilting or inhibition of the stretch reflex. The damping was not frequency‐dependent ( i.e. not viscous or due to velocity feedback). The pattern of the loops for overlapping ranges of tension indicated a “distributed simple friction” mechanism. The way the loops shifted during change of gamma bias indicated that the damping was located in the series‐elastic elements. During imposed length‐changes the response curve shifted sporadically along the tension axis and sudden twitch‐like increases in tension occurred in an unpredictable fashion. In contrast, the responses to tension‐changes were more consistent. This difference was most marked when other reflex effects were simultaneously elicited, e.g. by head‐tilting. The effects are accounted for by defining the stretch reflex as “a reflex increase in the stiffness (δT/δL) of a muscle in response to increased tension on its spindles”. The sequence of changes in the "clasp‐knife" response can also be explained in this way.
T. D. M. Roberts (Thu,) studied this question.