For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.
And for the first block along the street he walked very stiff and straight and awkwardly, until he forgot himself in his thoughts, whereupon his rolling gait gracefully returned to him.
Also, a natural pose is better than a stiff one so try to make your model feel comfortable and not nervous. Let them sit on a chair instead of just standing up in the studio.