Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans

Vøllestad, N. K., I. Sejersted, and E. Saugen. Mechanical behavior of skeletal muscle duringintermittent voluntary isometric contractions in humans.J. Appl. Physiol. 83(5):1557-1565, 1997.Changes in contractile speed and force-fusionproperties were examined during repetitive isometric contractions withthe knee extensors at three different target force levels. Sevenhealthy subjects were studied at target force levels of 30, 45, and60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in thetwitch properties. Half relaxation time(RT_1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT_1/2 values werenot fully restored to preexercise levels. During exercise at 60% MVC,the RT_1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after60% MVC, RT_1/2 values declinedtoward those seen after the 30 and 45% MVC exercise. The forceoscillation amplitude in unfused tetani relative to the mean forceincreased during exercise at 30 and 45% MVC but remained unalteredduring the 60% MVC exercise. This altered force-fusion was closelyassociated with the changes inRT_1/2. The faster relaxation mayat least partly explain the increased energy cost of contractionreported previously for the same type of exercise.