Abstract

The relationship between changes in muscle metabolites and the contraction capacity was investigated in humans. Subjects (n = 13) contracted (knee extension) at a target force of 66 of the maximal voluntary contraction force (MCV) tofatique, and the recovery in MVC and endurance (time to fatique) were measured. Force recovered rapidly (half-time (t 1/2) < 15 s) and after 2 min of reconvery was not significantly different from the precontraction value. Endurance recovered more slowly (t 1/2 ca. 1.2 min) and was still significantly depressed after 2 and 4 min of recocery. In separate experiments (n = 10) muscle biopsy specimens were taken from the quadriceps femoris muscle before and after two successive contractions to fatique at 66 of MVC with a recovery period of 2 or 4 min in between. The muscle content of high-energy phosphates and lactate was similar at fatique after both contractions, whereas glucose 6-phosphate was lower after the second contraction. During recovery, muscle lactate decreased and was 74 and 43 of the value at fatique after an elapsed period of 2 and 4 min, respectively. The decline in H+ due to lactate disappearance is balanced, however, by a release of H+ due to resynthesis of phosphocreatine, and after 2 min of recovery calculated muscle pH was found to reain at a low level similar to that at fatique. It is concluded that 1) after a fatiguing isometric contraction, force is restored more rapidly than endurance, 2) force is not limited bya high H+ concentration, and 3) the similar chemical composition at fatique suggests that this limits endurance, possibly because of an impaired capacity to rephosphorylate ADP. The relationship between acidosis and the fatique process often observed during exercise may be explained by a H+-mediated inhibition of the ATP-generating processes. Verf.-Referat