Specific Adaptation to Imposed Demand — the SAID principle.

When you starve yourself, your metabolism slows. When you overfeed yourself, your metabolism increases. When you're dehydrated, you hold more water. When you're over-hydrated, your body releases water. When you practice Chopin on the piano, your nervous system builds neural connections that enhance finger control. If you don't practice piano, your nervous system doesn't build those neural connections because there's no reason to.

So when you train muscles frequently, your body must adapt to the imposed demand. What we currently know is that a muscle group becomes more neurologically efficient and grows bigger with training. These processes occur because the nervous system enhances those motor pathways, and new proteins are inserted into the muscles.

What I can't do is reference a human-based, hypertrophy-focused study that incorporates the type of training that I'm talking about. But what I can do is tell you to look at the delts of boxers, the thighs of speed skaters, and the lats of swimmers.

Sure, I can't prove that training with a high frequency results in the faster muscle growth because the NIH doesn't prioritize such information. But I can tell you to observe the world around you. After all, that's what science is all about.

— CW