Vigorous exercise boosts your body's heat production and can increase your body temperature by several degrees. Your working muscles are responsible for the increase in heat production, but your body's ability to retain or dissipate heat and your external environment also play a role in how high your core temperature rises during your workout.
Muscle Heat Production
To work, your muscles need energy, which they obtain by burning fuels such as fats and carbohydrates in a series of chemical reactions that produce heat. As your muscles warm up during your workout, blood circulating through the muscles is also warmed, producing a rise in core temperature. The amount of heat your muscles produce is related to the amount of work they perform. The more strenuous your workout, the more heat they produce. During very vigorous workouts, muscle heat production can increase 15 to 20 times above resting levels.
How high your temperature rises during your workout depends not only on how much heat your muscles produce, but also on how fast your body loses heat. In cold conditions, your body loses heat rapidly. In hot, humid weather, your body is less able to dissipate excess heat, making overheating more of a risk. A rise in core temperature to above 104 degrees Fahrenheit can result in life-threatening heat stroke, so your body has a number of mechanisms to keep your core temperature within fairly narrow limits, even during a strenuous workout.
As your core temperature increases during your workout, blood is shunted away from your core to your skin, which allows your skin to radiate more heat, reducing your temperature. Sweating also helps to cool you. As sweat evaporates, it carries off excess heat. Because less sweat evaporates when the humidity is high, you're more likely to overheat in muggy weather than in dry conditions. As you become more fit with training, your body's ability to dissipate heat improves, a process called acclimatization. You begin sweating earlier in your workout, and at a lower temperature.
If your core temperature rises above 104 degrees Fahrenheit, you're at risk for heat stroke, a potentially fatal condition that damages multiple body systems. To prevent the risk of heat-related illness during exercise, the American College of Sports Medicine recommends avoiding strenuous exercise during hot, humid weather, wearing light clothing and avoiding dehydration by drinking sufficient fluids before and during your workout. Dehydration greatly adds to the risk of heat-related illness.