Hypertrophy is what everyone trying to gain lean mass is trying to achieve. Hypertrophy occurs when physical activity breaks down your muscles forcing them to rebuild bigger, leaner, and stronger. You have to work hard to achieve hypertrophy. The best way to know you've gotten there is to be finishing your workout and thinking..."Man, I am going to be sore tomorrow." Then you will be sore for a few days. During these days it is essetial to get adaquate rest so that your muscles can rebuild. Afterward you will be slightly bigger stronger and more defined.
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P.S. Hypertrophy is not only for bodybuilders. This information is beneficial to anyone who lifts weights.