Ten weeks of high-intensity interval training performed three times per week increases plasma testosterone concentrations by approximately 61.5% and the testosterone-to-cortisol ratio by 93%, while decreasing cortisol levels by 10.7% in healthy male adolescents aged 14–16.

Intense short bursts of running three times a week for ten weeks trigger a stress response that, over time, tells the brain to produce more muscle-building hormone and less stress hormone. The testes make more testosterone because the brain sends stronger signals to them, and the adrenal glands make less cortisol because they become less reactive to stress. This shifts the body’s balance toward growth and repair.