Simultaneous Modulation of Force Generation and Mechanical Property of Muscle in Voluntary Contraction

The purpose of the present study was to make it clear how the mechanical parameters (viscous and elastic coefficients) of muscle change with the activation level (contractile force) and to examine whether the estimated parameters were valid or not, by comparing them with previously reported values. Experiments were made from flexor pollicis longus muscle in five healthy subjects. The left hand was placed on a horizontal plate, and the subject was asked to maintain constant isometric force by watching the developed force displayed on a CRT. When the muscle force was kept constant, the thumb was ramp-stretched or shortened by a small amount (approximately 4.5 degree) by means of a servo-controlled motor. Time courses of the force (torque) and the length (angle) observed during the period ranging from the onset of length perturbation to 50ms were used for estimating the mechanical parameters. It was showed from examining EMG activities that the contractile force during this period was almost the same as the constant isometric force maintained before the length perturbation. Individual parameters were estimated by searching the best fit of the model response with the measured force and by analyzing the sensitivity of estimated values. The obtained results are summarized below. 1) Both the series and the parallel elastic coefficients increased almost linearly with the contractile force (isometric force). 2) The viscous coefficient could be estimated only at the range of slow velocity, while dynamic constants of the force-velocity relation (a and b) could not be obtained. The viscous coefficient increased linearly with the contractile force. 3) Linear increases of these mechanical parameters with the contractile force were shown in both stretching and shortening of the muscle. 4) Obvious differences were found between estimated values of each mechanical parameter obtained from the five subjects; that is, the mechanical property of the muscle was subject-dependent. 5) Meanings of the simultaneous modulation of the muscle force and the muscular activity in regulating limb movement and mechanical impedance are discussed from the control point of view.