Lactic acid accumulates because of anaerobic activity, lowering the ph, and glycogen reserves are exhausted; in aerobic activity depletion of glycogen puts a limit on muscular activity. -during aerobic work (free oxygen is readily available), cells can derive energy from glucose and fatty acids and work for a long time, until glycogen stores are exhausted. in anaerobic exercise (free oxygen is not available in sufficient supply), the ability to work is much more limited, and fatigue sets in rapidly. fermentation must be used as a source of atp, producing lactic acid that builds up (lowering the ph), causing pain in the muscle. since the atp derived from glycogen is much less in anaerobic than aerobic work, glycogen is more quickly used up in anaerobic than aerobic activities. the combination of chemical changes and glycogen depletion soon causes the muscle to fatigue.