Effects of fatigue on isometric force- and relaxation-time characteristics in human muscle.

Effects of intermittent one leg isometric fatigue on maximal isometric force, force production, relaxation and blood lactate were studied using 29 male students as subjects. The relative changes of variables during fatigue and recovery were intercorrelated together with muscle structure variables, which were determined using needle biopsy technique. Maximal force decreased, force production and relaxation became slower and muscle lactate increased during fatigue. Change of maximal force, force production and lactate during fatigue as well as recovery of maximal force and lactate after fatigue were correlated significantly to muscle fiber distribution. Fatiguability of the force-time characteristics was therefore influenced by the differences in the metabolic profiles of the individual muscle fibers. However, ability to relax the muscle quickly was not observed to be dependent on muscle structure. This suggest that different fatigue mechanisms might be present in relaxation than in force production.