Viscoelastic properties of the wrist motor servo in man.

Viscoelastic properties play an important role in posture and movement. Such properties arise from muscle mechanics and from stretch-reflex actions. We describe experiments designed to characterize both linear and nonlinear elastic and viscous properties of the wrist motor servo in human subjects. First, we describe a trial comparison method for the identification of reflex responses that are unmodified by triggered reaction-time movements. Elastic properties were studied by applying step changes in load force that stretched or released the wrist flexor and extensor muscles. The properties were basically spring-like, but there was a short-range enhancement of stiffness that gave rise to a prominent hysteresis. Viscous properties were studied by applying ramp stretches at different velocities. Both EMG and force responses showed a weak fractional-power dependence on velocity similar to that described recently for muscle spindle receptors. Consideration is given to the possible advantages of this type of nonlinear feedback in the damping of postural responses and movements.