Nonlinear damping properties and postural stability of the neuromuscular system

Damped oscillations caused by external disturbances are commonplace in the human neuromuscular system. Although mechanical and reflex properties have been studied extensively under length-controlled conditions, their interaction with inertial loads, which are the most commonly experienced natural loads, have been infrequently studied. These interactions could be the key to understanding joint stability. The goal of this study was to characterize the damping properties of the neuromuscular system when coupled to inertial loads and analyze system responses with respect to postural stability. We performed two series of parallel experiments using the soleus in a decerebrate cat preparation and the interphalageal joint of the thumb in humans. Damping properties were assessed by applying a force impulse to a simulated inertia and measuring the energy transfer between the muscle and load. In human and cat preparations, damping depended upon oscillation amplitude and movement history. In contrast to linear systems with fixed parameters, these nonlinearities provide the advantage of initially promoting preservation of position and subsequently damping oscillations. It is postulated that the beneficial nonlinear mechanical behaviors arise from the automatic movement-induced modulation of reflex sensitivity interacting with intrinsic muscular properties