High-intensity running is different. While a small amount of high-intensity running is more beneficial to the aerobic system than is a small amount of low-intensity running, larger amounts of faster running offer no additional benefit. In fact, large amounts of high-intensity training are so stressful to the body that they suppress the parasympathetic nervous system, resulting in chronic fatigue and loss of performance. So the sum of what we know about the physiological effects of low-intensity and high-intensity exercise tell us that the optimal combination is one that is weighted heavily
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