Resistance training in collegiate women tennis players over 9 months increases resting serum concentrations of insulin-like growth factor-1, testosterone, and cortisol, and periodized resistance training results in significantly higher cortisol levels compared to nonperiodized resistance training.

When athletes lift weights with changing intensities, their muscles are forced to work harder, especially during heavy lifts. This triggers nerves to activate more muscle fibers, which sends signals to the brain and adrenal glands to release more stress and growth hormones. The body responds by making more hormones that build muscle and more hormones that manage stress, and the more varied the training, the stronger this hormonal response becomes.