The ability of human skeletal muscle to provide anaerobically derived ATP during short-term, intense activity is examined. The paper emphasizes the information obtained from direct measurements of substrates, intermediates, and products of the pathways in muscle that provide anaerobically derived ATP. The capacity of muscle to provide ATP via anaerobic pathways is approximately 370 mmol/kg dry muscle (dm) during dynamic exercise lasting approximately 3 min. Anaerobic glycolysis provided approximately 80%, phosphocreatine (PCr) degradation approximately 16%, and depletion of the ATP store approximately 4% of the total ATP provided. When the blood flow to the working muscles is reduced or occluded, the anaerobic capacity decreases to approximately 300 mmol/kg dm. This reduction is due to a lower glycolytic capacity associated with an inability to remove lactate from the muscles. Directly measured maximal rates of anaerobically derived ATP provision from PCr degradation and glycolysis during intense muscular activity are each approximately 9-10 mmol.kg-1 dm.s-1. Evidence suggests that both of these pathways are activated instantaneously at the onset of maximal activity. Spring training does little to the capacity or rates of the pathways, although a 10-20% increase in glycolytic ATP provision has been reported. The only study comparing direct and indirect estimates of the anaerobic capacity in humans suggests that O2 deficit measured at the mouth accurately predicts the anaerobic capacity of a single muscle group and that O2 debt does not. There are many unresolved issues regarding the capacity of the PCr and glycogenolytic--glycolytic systems to provide ATP during short-term intense muscular activity in humans.(ABSTRACT TRUNCATED AT 250 WORDS)