Physical exertion refers to any bodily movement that results in a change in your overall body temperature. Physical exertion can include anything from brisk walking to running to weightlifting. "Physical exertion has a thermogenic effect, meaning it results in an increase in body temperature. This increase in body temperature is the result of elevated levels of energy storage in muscles and the liver," says Kerry Lynch, MS, CSCS, founder of Trim Fit Foods. "This stored energy can be used to create heat during times of excess heat production, such as during a workout, or when the body is defending itself from a cold environment.