Abstract
Muscles have elastic- and viscous-like properties in the contractile mechanism itself. These elastic- and viscous-like properties are controlled by muscle activity. It is important to maintain muscle activity constant when these properties are investigated. The purpose of this study was to investigate the elbow joint torque-angle and torque-velocity relation at constant muscle activity in voluntary contraction. Our targets of experiments were static position control and isovelocity extending of the elbow joint on a horizontal plane. A subject was instructed to maintain the posture of the upper extremity at a pre-determined position (static condition) or to extend the elbow joint at constant velocity (isovelocity condition) and not to cause coactivation of antagonist muscles. First, we measured IEMGs at various elbow joint angles, extending velocities (0, 30, 60, 90[deg/s]), and with various external loads (0, 2.8, 5.6, 8.4[%] of the maximum voluntary contraction). Then we constructed a model describing the torque-angle-IEMG relation and torque-velocity-IEMG relation with the aid of an artificial neural network technique. Finally, we estimated the torque at various elbow joint angles or various extending velocities while the IEMGs were kept constant. The elbow joint torque increased with increasing elbow joint angle in both the static condition and the isovelocity condition. The increase of elbow joint torque means that the flexor muscles of the elbow joint have a spring-like property, because the extensor muscles are stretched with the increase of elbow joint angle. This property agrees with that of a monkey's triceps. The elbow joint torque decreased with increase in the extending velocity. This means that viscous-like property is not negligible. In the case of extracted muscle fibres, the torque-velocity relation represents a hyperbolic curve. However, in this study, the relation seems to be a linear function rather than a hyperbolic curve. The main reason was that the extending velocity was much slower than the maximum velocity of elbow joint extension (1400-2200[deg/s]). In this study, the elastic-like properties of some subjects were not enhanced while the muscle activity increased. However the properties of other subjects were enhanced. It is known that extracted muscle fibres have high elasticity when they are strongly stimulated. If we assume that the contractile force and the viscosity of muscles are proportional to both the muscle activity and the muscle length (elbow joint angle), the elbow joint angle is determined by the muscle activity.
