Here's the simple explanation of how it works:
When you exercise, you stress and damage your muscles. Tiny tears and pulls are created.
When you rest and provide adequate protein, vitamins, and liquids, that damage is repaired and the muscles become stronger.
When you don't give that damage time to repair, then you are stacking damage on top of damage and actually causing the muscles to become weaker rather than stronger.
Ideally you should exercise to the point of creating that damage, but not to excess. Then you should take a day off (or sometimes 2, if necessary) to allow the muscles to repair.
Repeat the cycle.
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