Fig. 1. Maximal power output of human legs as a function of time during cycling. The symbols represent the highest power output that can be sustained for a given time (adapted from) (Wilkie, 1985). The upper left diagram illustrates that high power outputs cannot be sustained because the source of ATP (phosphocreatine, PCr) generates by-products that inhibit contraction (inorganic phosphate, P_i and H⁺). The upper right diagram illustrates that the continuous supply of ATP allows lower power outputs to be sustained.