Relation between intrinsic viscoelasticity and activation level of the human finger muscle during voluntary isometric...

The purpose of the present study was to isolate the length perturbation-evoked force attributed to intrinsic muscle viscoelasticity, and to investigate the relation between muscle viscoelasticity and the level of muscle activation during isometric contraction in five healthy male subjects. A small length perturbation (stretching or contraction) was applied to the flexor pollicis longus muscle while the subject maintained constant isometric force; the time courses of the length perturbation was found to be almost identical in all the experiments. The force (Fv) induced by the muscle viscoelasticity was calculated using the equation Fv = F - Fc - Fp over an interval of 35 ms after the onset of perturbation, where F is the measured force, Fc is the tonic isometric force before the onset of perturbation and Fp is the force at rest obtained from the same length perturbation. The force response attributed to the stretch reflex is not included during this interval. These experiments were repeated at varying levels of isometric force. An almost linear relationship was obtained between the muscle viscoelasticity-induced force and the tonic isometric force during both the stretching and contraction of the muscle, i.e. the intrinsic muscle viscoelasticity varied almost linearly with the level of isometric contraction.