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<p>Most 16-year old boys have very high metabolisms that keep the body fat off, with or without explicit exercise. However, those who do little exercise tend to gain fat later. </p>
<p>If he is lucky enough to be a rare natural mesomorph (gains muscle with little effort, does not gain fat as easily), then that is great – but a sports coach or personal trainer will not have as much credibility if s/he does not do the exercises that s/he is coaching. For example, sports coaches may say “you should feel these muscles work when you do this exercise”, “these muscles will probably be sore the next day after you do these exercises”, and “make this adjustment to your technique to perform better”.</p>