When exercising in extreme conditions, the body’s primary focus is maintaining a stable internal temperature. Physical activity generates significant heat that must be managed. The environmental temperature dictates the physiological mechanisms used to either dissipate this excess heat or conserve it. This trade-off between generating energy for muscle movement and dedicating resources to thermal regulation is the core challenge of working out in either the heat or the cold.
Exercising and Heat Stress
When heat stress combines with physical exertion, the cardiovascular system faces competing demands for blood flow. Working muscles require oxygenated blood, but the skin also needs increased circulation to transport internal heat to the surface for cooling. This necessary vasodilation—the widening of blood vessels near the skin—increases the heart rate and strains the entire system by reducing the volume of blood returned to the heart.
Sweating is the body’s primary defense, allowing heat to escape through the evaporation of water from the skin’s surface. However, this process can lead to significant fluid loss, sometimes exceeding one liter per hour in intense exercise. This rapid dehydration decreases the total blood volume and increases the concentration of solutes in the blood plasma, further compromising the body’s ability to cool itself. If heat dissipation is insufficient, the core temperature can rise rapidly, leading to hyperthermia, which negatively affects cellular and organ function.
Exercising and Cold Stress
In a cold environment, the body’s immediate priority is to conserve core heat rather than expel it. The initial response is peripheral vasoconstriction, where blood vessels in the extremities and skin narrow to reduce the flow of warm blood to the surface. This action creates an insulating layer, protecting the internal organs by limiting heat loss to the cold air.
If heat conservation is not enough to maintain the core temperature, the body initiates shivering, an involuntary mechanism of rapid muscle contractions. Shivering is a form of thermogenesis that increases the body’s metabolic heat production significantly, though it also impairs fine motor control and coordination. While the core temperature is well-defended, the extremities are particularly vulnerable due to reduced blood flow, increasing the risk of localized tissue damage like frostbite when skin temperatures drop below -0.5°C.
Impact on Performance and Energy Use
The physiological strain imposed by heat limits peak performance more severely than cold. In hot conditions, the high cardiovascular load and rising core temperature decrease maximum oxygen uptake (V̇o2max) and increase the perception of effort, causing earlier fatigue. Furthermore, elevated muscle temperature shifts metabolism toward greater reliance on anaerobic pathways, increasing muscle glycogen utilization and lactate accumulation while reducing fat oxidation.
In contrast, cold weather allows athletes to maintain a higher intensity and exercise for a longer duration, as the environment aids in heat dissipation. This extended duration leads to a greater total calorie expenditure. While shivering burns a substantial number of calories, exercise-induced thermogenesis is usually enough to suppress it. Following cold exposure, there is often an increase in appetite, promoting greater post-exercise energy intake to compensate for the higher expenditure.
Preparation and Safety Measures
Mitigating heat stress requires focusing on fluid replacement and timing, as dehydration is the central challenge. Athletes should consume between four and eight ounces of fluid every 15 to 20 minutes during exercise to match sweat loss. Wearing loose-fitting, light-colored clothing with moisture-wicking properties facilitates evaporative cooling. Scheduling workouts to avoid the hottest period between noon and 3 p.m. and allowing 4 to 14 days for heat acclimatization can significantly improve the physiological response.
For exercise in the cold, the strategy centers on layering to manage moisture and insulation. A three-layer system—a wicking base layer, an insulating middle layer, and a windproof, water-resistant outer shell—is highly effective. Covering the head, hands, and feet is paramount, as these extremities lose heat quickly due to peripheral vasoconstriction. Athletes should be mindful of wind chill, which accelerates heat loss. They should also remove layers before excessive sweating occurs to prevent clothing from becoming wet and compromising its insulating properties.