MECHANICAL-BEHAVIOR OF SKELETAL-MUSCLE DURING INTERMITTENT VOLUNTARY ISOMETRIC CONTRACTIONS IN HUMANS

Changes in contractile speed and force-fusion properties were examined during repetitive isometric contractions with the knee extensors at t hree different target force levels. Seven healthy subjects were studie d at target force levels of 30, 45, and 60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s re st were continued until exhaustion. Contractile speed was determined f or contractions elicited by electrical stimulation at 1-50 Hz given du ring exercise and a subsequent 27-min recovery period. Contraction tim e remained unchanged during exercise and recovery, except for an initi al rapid shift in the twitch properties. Half relaxation time (RT1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT1/2 values were not fully restored to preexerc ise levels. During exercise at 60% MVC, the RT1/2 decreased for twitch es and increased for the 50-Hz stimulation. In the recovery period aft er 60% MVC, RT1/2 values declined toward those seen after the 30 and 4 5% MVC exercise. The force oscillation amplitude in unfused tetani rel ative to the mean force increased during exercise at 30 and 45% MVC bu t remained unaltered during the 60% MVC exercise. This altered force-f usion was closely associated with the changes in RT1/2. The faster rel axation may at least partly explain the increased energy cost of contr action reported previously for the same type of exercise.