Exercise- and heat conditioning- cause the core temperature to increase. To cool the body, blood is sent to the skin to transfer the heat from the core to the skin. The process of perspiration causes evaporation from the skin to cool the blood before it is returned to the core.(29) This process is called “thermo-genesis” and results in increased heart rate, stroke volume and cardiac output at any given exercise intensity.(29) In sufficiently hot and/or humid environments, the process occurs even without exercise. If heat is not dissipated, the core temperature will increase and the subject will experience fatigue and exhaustion.(29) Exercise combined with heat exposure increases body temperature and activates beneficial physiological responses more significantly than either exercise or heat alone.(32)
Cardiovascular improvements which help to maintain a stable core temperature include the following:
- Increased stroke volume: The amount of blood pumped by the left ventricle of the heart in a single contraction. Increased stroke volume reduces cardiovascular strain and lowers the heart rate for the same given workout.(2) Exercising in heat causes acclimation and increases stroke volume more than just exercise or heat exposure alone.(5,32)
- Increased heart rate and cardiac output: ATE can increase heart rate up to 100 beats per minute with moderate heat exposure and/or exercise intensity and up to 150 beats per minute with high heat exposure and/or exercise intensity.(97) Cardiac output is also increased with ATE.(29) Exercising in a hot environment increases cardiac output more than just exercise or heat exposure alone.(32) Studies have shown that hyperthermic conditioning causes rises in cardiac output proportionally to increase in heart rate, and can increase cardiac output. by as much as 75%! (108)
- Increased sweating rate: The rate of sweating is increased with both exercise and heat exposure. A study with 12 fit subjects exercising to exhaustion at 95 degrees F. (and 87% relative humidity) showed a 26% increase in sweating rate.(91) Heat acclimation increases the size of the eccrine sweat glands — and larger glands produce more sweat.(41) Thermal exposure combined with exercise results in greater increases in sweating than passive heat exposure alone.(32) Exercising in heat can cause sweat loss of from 2 to 6 pounds (1 to 3 liters) per hour, and each vaporized liter of sweat extracts 580 calories from the body! (29)
- Increased sweat sensitivity: Sweat sensitivity determines the body’s potential for evaporative cooling.(6) Sweat sensitivity increases during both heat acclimation and exercise conditioning.(32)
- Increased core temperature: A rise in core temperature triggers the body’s temperature regulating center for heat dissipation. Numerous studies have shown that exercise in hot, dry environments is limited by the attainment of a critical level of core temperature and that high core temperature, not circulatory failure or metabolic depletion, is the critical factor in heat stress.(5, 121)
- Increased blood flow to muscles: ATE increases the flow of blood to the skeletal muscles which keeps them fueled with glucose, fatty acids, and oxygen. At the same time, metabolic by-products such as actic acid are more effectively removed. Improved delivery of nutrients reduces muscles dependence on glycogen stores, which helps endurance athletes perform for longer periods.
- Increased blood plasma volume and red blood cell count (RBC): ATE has been shown to increased blood plasma volume by as much as 7.1% (13% in another study(5)) and increase red blood cell count (RBC) by 3.5%.(1) Increases in RBC results in increased the delivery of oxygen to the muscles.
- ATE reduces muscle glycogen use: Studies have shown that ATE reduces muscle glycogen use by 40 to 50% before heat acclimation.(7,8) It is believe that reduced muscle glycogen use results from the increased flow of blood to the muscles.(7)
- Enhanced endurance: The cardiovascular improvements described above have been shown to enhance endurance in both trained and untrained test subjects.(2,3,4) Heat acclimatization allows the organism to tolerate a higher core temperature and therefore prolongs the ability to continue exercising before exhaustion. (24) A study using a climatic chamber to study exercise in dry heat found that acclimation increased average endurance before reaching exhaustion of the study subjects from 48 minutes to 80 minutes.(5)