John Halliwill is an academic expert on exercise and environmental physiology with an emphasis on responses of the cardiovascular system. He studies post-exercise hypotension, in which a person's blood pressure drops after workouts. Halliwill has sought to identify hormonal, neural or metabolic factors responsible for heart, vascular, and muscle-related changes during exposure to environmental and physical stresses. He also is seeking to understand the mechanisms that underlie beneficial effects of exercise. He has studied conditions faced by astronauts, helped Olympic athletes prepare to compete in environmental extremes, and tested equipment for the US Marines.