In healthy individuals there is no direct evidence that exercise, even prolonged endurance exercise, is limited by fatigue of the heart muscle. Because the heart in effect gets first choice at cardiac output, the heart is well oxygenated and nourished, even at maximal heart rate. In addition, because the heart is "omnivorous" in its appetite for fuels, it can be sustained by either lactic acid (which rises in short - term work) or fatty acids (which rise in long-term work). In individuals free of heart disease, the ECG does not reveal signs of ischemia (inadequate blood flow) during exercise. If ischemic symptoms are observed - then this is, in fact, evidence of heart disease.
During prolonged work that leads to severe dehydration and major fluid and electrolyte shifts, or other situations in which exercise is performed after thermal dehydration or diarrhea, changes in plasma, Na+ , K+ , or Ca2+ . can affect excitation - contraction coupling of the heart. In these cases, cardiac arrhythmias are possible, and exercise is not advised.
