Damping in reflexively active and areflexive lengthening muscle evaluated with inertial loads

Studies of active areflexive muscle have shown that during a constant veloc ity stretch the increment in force elicited by an incremental length change falls dramatically after a few hundred micrometers of stretch, a finding l abeled as ''muscle yield.'' The mechanical behavior after the yield was lik e a viscous damper, in that force varied only with velocity. In light of th ese observations, our aims were to determine whether viscous properties are also evident under more physiological conditions, specifically under inert ial loading, and to evaluate the damping action of reflexively intact compa red with that of deafferented muscle. The active soleus muscle in a decereb rate cat was forcibly stretched by a simulated inertia with a specified ini tial velocity. We compared muscle length changes when afferent pathways wer e intact with those recorded after cutting the dorsal roots. Our findings w ere that areflexive muscle showed highly damped responses, with large chang es in mean muscle length, indicative of high viscosity relative to stiffnes s. In contrast, reflexively active muscle produced lightly damped oscillati ons, with minimal changes in mean length, reflecting low viscosity and high stiffness. It appears that the stretch reflect modifies the relative contr ibutions of elastic and viscous-like forces, maintaining elasticity, which in turn sustains oscillations. These differences highlight tradeoffs betwee n positional and velocity regulation, in that elastic properties of reflexi vely active muscle promote oscillations with modest change in mean muscle l ength, whereas viscous-like properties of areflexive muscle produce damped responses, with poor positional regulation.