Viscoelasticity of human thumb muscle modulated by intensity of voluntary contraction

The purpose of the present study was to examine the relation between the viscoelasticity and contractile force in the flexor pollicis longus(FPL) muscle of five male subjects. A small ramp stretch was applied to the FPL muscle when the subject maintained constant isometric force. The amount of stretch or shortening was 4.8 degrees around the articulatio interphalangeae pollicis joint and the duration was 30ms ; time courses of the length-perturbation were almost identical in all the experiments. The force(Fv) evoked by the viscoelasticity was calculated by the equation F-Fc-Fp only over the interval of 35ms after the onset of length-perturbation, where F was the measured force, Fc was the isometric force before the onset of perturbation and Fp was the force at the resting state obtained from the same length perturbation. These experiments were repeated varying the level of isometric force. The obtained result was as follows. The viscoelastic force(Fv) evoked by the length-perturbation was proportional to the contractile force (activation level of muscle) in both direction of stretching and shortening ; that is, viscoelasticity varied linearly with the level of contractile activity. The functional meaning of modulation of muscle viscoelasticity in limb movements was discussed from the control point of view.