The Joy of Movement: How Exercise Helps Us Find Happiness, Hope, Connection, and Courage

by Kelly McGonigal

Irisin has been dubbed the “exercise hormone,” and it is the best-known example of a myokine, a protein that is manufactured in your muscles and released into your bloodstream during physical activity. (Myo means muscle, and kine means “set into motion by.”) One of the greatest recent scientific breakthroughs in human biology is the realization that skeletal muscles act as an endocrine organ. Your muscles, like your adrenal and pituitary glands, secrete proteins that affect every system of your body. One of these proteins is irisin. Following a single treadmill workout, blood levels of irisin increase by 35 percent. The Yukon Arctic Ultra required up to fifteen hours a day of exercise. Muscle shivering—a form of muscle contraction—also triggers the release of irisin into the bloodstream.