Damping actions of the neuromuscular system with inertial loads: Soleus muscle of the decerebrate cat

A transient perturbation applied to a limb held in a given posture can indu ce oscillations. To restore the initial posture, the neuromuscular system m ust provide damping, which is the dissipation of the mechanical energy impa rted by such a perturbation. Despite their importance, damping properties o f the neuromuscular system have been poorly characterized. Accordingly, thi s paper describes the damping characteristics of the neuromuscular system i nteracting with inertial loads. To quantitatively examine damping, we coupl ed simulated inertial loads to surgically isolated, reflexively active sole us muscles in decerebrate cats. A simulated force impulse was applied to th e load, causing a muscle stretch, which elicited a reflex response. The res ulting deviation from the initial position gave rise to oscillations, which decayed progressively. Damping provided by the neuromuscular system was th en calculated from the load kinetics. To help interpret our experimental re sults, we compared our kinetic measurements with those of an analogous line ar viscoelastic system and found that the experimental damping properties d iffered in two respects. First, the amount of damping was greater for large oscillation amplitudes than for small (damping is independent of amplitude in a linear system). Second, plots of force against length during the indu ced movements showed that damping was greater for shortening than lengtheni ng movements, reflecting greater effective viscosity during shortening. Thi s again is different from the behavior of a linear system, in which damping effects would be symmetrical. This asymmetric and nonlinear damping behavi or appears to be related to both the intrinsic nonlinear mechanical propert ies of the soleus muscle and to stretch refer properties. The muscle nonlin earities include a change in muscle force-generating capacity induced by fo rced lengthening, akin to muscle yield, and the nonlinear force-velocity pr operty of muscle, which is different for lengthening versus shortening. Str etch reflex responses are also known to be asymmetric and amplitude depende nt. The finding that damping is greater for larger amplitude motion represe nts a form of automatic gain adjustment to a larger perturbation. In contra st, because of reduced damping at small amplitudes, smaller oscillations wo uld tend to persist, perhaps contributing to normal or "physiological" trem or. This lack of damping for small amplitudes may represent an acceptable c ompromise for postural regulation in that there is substantial damping for larger movements, where energy dissipation is more critical. Finally, the d irectional asymmetry in energy dissipation provided by muscle and reflex pr operties must be reflected in the neural mechanisms for a stable posture.