Quantification of intrinsic and reflexive properties during multijoint arm posture.

This study estimates intrinsic and reflexive properties of single- and two-joint muscles acting around the human shoulder, elbow and wrist joint during posture maintenance. External force disturbances were applied to the hand while subjects (n = 5) were instructed to minimize their hand displacement amplitude in a horizontal plane. To examine how the nervous system modifies intrinsic and reflexive muscle properties, we varied external damping, disturbance power and arm configuration. A new identification method is introduced to quantify a large set of model parameters describing intrinsic, activation and reflexive properties, the latter representing afferent feedback gains from muscle spindles and Golgi tendon organs. Consistent estimates were found for the gains of the different types of reflex pathways. The results showed that intrinsic visco-elasticity of shoulder muscles was minimal, whereas reflexive feedback was largest compared to the elbow and wrist joint. Intrinsic and reflexive properties of the two-joint shoulder-elbow muscles were larger than the two-joint elbow-wrist muscles. Contrasting to previous single-joint studies, the reflex gains did not vary with the experimental conditions. It is concluded that during redundant multijoint posture maintenance, the mechanical properties are efficiently controlled on the joint level and not on the endpoint level.