Summary The onset of fatigue within a muscle depends on several factors, including the duration or intensity of work and amount of training. In aerobic work over prolonged time it appears to be the level of glycogen stored within the muscle fibers that becomes limiting. In anaerobic work the limiting factor may be a combination of the amount of substrate available to the cell and the ability of the cell to continue working at low pH. Further work is needed to establish which is the most limiting of these two factors, levels of stored glycogen or accumulating levels of lactate and concomitant lowering of muscle pH.
