Your muscles will grow only when subjected to an overload. They will not respond to anything less. Muscles will not grow bigger and stronger unless you force them to. Making your muscles contract against a level of resistance that they are not used to will eventually cause them to adapt and grow stronger. But once they have adapted, this progress will stop. When this happens, the only way to make your muscles grow is by further increasing the amount of overload to which you subject them to.
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