Modulation of viscous elastic property of muscle in phasic isometric contraction

Muscles not only have contractile function but also viscous elastic like property. The viscous elastic like property was observed as the force in response to stretching the contracting muscle. While there are many reports concerning stretch reflex or stiffness, the viscous elastic like property originated from the contractile mechanism itself has not been studied enough. The purpose of the present study is to investigate the relation between EMG (electromyogram), isometric force and stretch evoked force. Three normal male subjects were instructed to change isometric contractile force of their thumb tracking a visible target displayed on an oscilloscope. Sinusoidal oscillation of the target was presented to the subjects. While the subject was tracking the target, length perturbation of short duration was applied to the thumb at random time. The thumb was extended with constant velocity 0.15 m/s for 20 ms, maintained for 30 ms at the extended position and then returned to the previous position. The position and flexing force of the thumb and the EMG of flexor pollicis longus muscle were measured. By subtracting both isometric force and the force of passive components from the measured force response to stretch, the stretch evoked force generated by viscous elastic components was calculated. The stretch evoked force at 15 ms after the onset of length perturbation was examined; the force at this time did not contain the force increased by the stretch reflex. The stretch evoked force followed the IEMG with a short latency when the isometric force varied sinusoidally. It was shown that the viscous elastic property of muscles did not depend on the isometric force.