Intensity Limits Volume
High-intensity interval training is very taxing on the neuromuscular system. One can do only so much HIIT without creating a chronic recovery deficit that eventually causes results to diminish, then cease, then reverse. The human body’s tolerance for moderate- to moderately high-intensity aerobic training is significantly greater.
Thus, while the individual who does only HIIT gets more results per minute of exercise, the greater overall potential for fat burning and aerobic fitness development lies with steady-state aerobic training, because you can do so much more of it without becoming overtrained.
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It’s no accident that the leanest athletes in the world are elite marathoners, about 80 percent of whose training is of moderate intensity. Elite marathoners have body fat levels that professional bodybuilders would kill for, yet the runners often get there without having to exercise any restraint in their diet, while the bodybuilders, who rely on HIIT, have to practically starve themselves to get that lean.