Effect of muscle action and metabolic strain on oxidative metabolic responses in human skeletal muscle

A recent report suggests that differences in aerobic capacity exist between concentric and eccentric muscle action in human muscle (T. W. Ryschon, M. D. Fowler, R. E. Wysong, A. R. Anthony, and R. S. Balaban. J. Appl. Physiol . 83: 867-874, 1997). This study compared oxidative response, in the form o f phosphocreatine (PCr) resynthesis rates, with matched levels of metabolic strain (i.e., changes in ADP concentration or the free energy of ATP hydro lysis) in tibialis anterior muscle exercised with either muscle action in v ivo (n = 7 subjects). Exercise was controlled and metabolic strain measured by a dynamometer and P-31-magnetic resonance spectroscopy, respectively. M etabolic strain was varied to bring cytosolic ADP concentration up to 55 mu M or decrease the free energy of ATP hydrolysis to -55 kJ/mol with no chan ge in cytoplasmic pH. PCr resynthesis rates after exercise ranged from 31.9 to 462.5 and from 21.4 to 405.4 mu mol PCr/s for concentric and eccentric action, respectively. PCr resynthesis rates as a function of metabolic stra in were not significantly different between muscle actions (P > 0.40), sugg esting that oxidative capacity is dependent on metabolic strain, not muscle action. Pooled data were found to more closely conform to previous biochem ical measurements when a term for increasing oxidative capacity with metabo lic strain was added to models of respiratory control.