Movement and the Mechanical Properties Of

Recent studies of motor coordination have emphasized the importance of the mechanical properties of muscle in both load compensation-and in the control of movement. Teleologically, different mechanical characteristics may be optimal for different tasks. For example, a skiier may wish to adjust the stiffness of his legs in accordance with the terrain and snow conditions. How, and to what extent can subjects control the mechanical properties of their joints? To help answer this question, we investigated short term changes in muscle properties by measuring the response of the intact human elbow joint to mechanical perturbations. The resistance of the intact joint to such displacements reflects contributions from the muscles and passive tissues spanning the elbow joint. In order to explore the effects of both muscular activation and movement on joint properties we studied three conditions: (1) voluntary isometric contraction, (2) passive movement, and (3) voluntary movement. In the isometric condition, the subject maintained a constant arm position against various loads. In the passive movement condition the subject's arm was moved at a constant velocity. Finally, the voluntary movement condition combined muscular activation with movement. During all three conditions a small amplitude, high frequency (15-30 Hz) sinusoidal perturbation was applied about the elbow joint. Joint impedance was derived from the resistance of the elbow to this superimposed displacement. We obtained continuous measures of joint impedance, joint angle, and the biceps and triceps EMG's. During isometric contraction, joint impedance increased monotonically with increasing muscular activation (estimated by EMG). In contrast, during passive arm movements joint impedance decreased markedly. This was true even for very slow passive movements when no change in muscular activation could be detected. Finally, voluntary movements resulted in changes in joint impedance which were consistent with the combined effects of muscular activation and movement. We attribute these changes in joint impedance to variations in the properties of the muscles acting about the joint. We conclude that length