David Nieman, Ph.D., a health and exercise science specialist in Boone, NC, has found that 90 minutes or more of continuous, moderate- to high-intensity, exercise weakens the immune system, making you more prone to illness. This effect can last up to 72 hours. If you don’t give yourself proper time to recover after working out and put too much stress on your body while training, you could be doing more harm than good.
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