Relaxation time during intermittent isometric contraction in subjects with different capacity for oxidative work.

Force and relaxation time were measured during intermittent electrical stimulation of the quadriceps muscle on 14 volunteers. Each stimulation had a duration of 1.6 s and the work to rest ratio was 1:1. Relaxation times increased during the first 20-30 contractions and then reached a plateau. The rate of increase in relaxation time during the first contractions as well as the plateau levels were found to correlate to maximal oxygen uptake capacity, predicted from submaximal cycle ergometer tests. Since slow-twitch fibres are known to produce work at a lower energy cost it is suggested that studies of relaxation time changes could provide functional estimates of fibre-type proportions in the quadriceps muscle.