If you suddenly do really intense stretching-type workouts that your muscles aren't used to, it can seriously damage them — so much that the muscles actually get smaller for a few weeks.
Evidence from Studies
Supporting (1)
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MR measurements of muscle damage and adaptation after eccentric exercise
The study found that after intense muscle-damaging exercise, the muscle swelled at first but then became smaller than before and stayed that way for weeks, which suggests some muscle fibers were destroyed.
Contradicting (1)
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Muscle damaging eccentric exercise attenuates disuse-induced declines in daily myofibrillar protein synthesis and transiently prevents muscle atrophy in healthy men
The study found that intense eccentric exercise actually helped protect muscles from shrinking during short periods of inactivity, instead of making them smaller as the claim suggests.
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