The metabolic basis of recovery after fatiguing exercise of human muscle.

We investigated the metabolic basis of human muscular fatigue and recovery utilizing 31P magnetic resonance spectroscopy and measurements of maximum voluntary contraction (MVC). We produced fatigue by sustained MVC for 4 minutes in 2 different muscles (adductor pollicis, tibialis anterior) and obtained similar results in both muscles. During fatiguing exercise, there was a nonlinear relationship between MVC and both phosphocreatine and total inorganic phosphate. By contrast, there was a roughly linear relationship between the decline in MVC and the accumulation of both H+ and H2PO4-. However, during recovery after exercise, MVC rapidly returned to control levels while H+ recovered with a much slower time course. On the other hand, H2PO4- rapidly returned to control values with a time course similar to MVC. In addition, the relationship of H2PO4- to MVC was similar during both fatigue and recovery. Thus, during fatigue as well as during recovery, changes in MVC correlate best with H2PO4-, suggesting that this metabolite is an important factor in human muscle fatigue.