Heat, the Brain and Fatigue

sun hot heat sweat 

Heat limits athletic performance. The way heat causes fatigue may be through an elevated brain temperature.

 

 

Our bodies do not perform as well in hot conditions. Hyperthermia is when the body gets overheated resulting in a decrease in maximal power output. Most of us have experienced the fatigue that occurs with heat while exercising or during labor. Personal experience tells us that heat creates fatigue, but what is the physiologically basis of this heat-induced fatigue?

Before looking at how heat affects performance it is important to understand fatigue. There is much debate among today’s exercise physiologists about what actually limits performance during maximal-effort exercise. It is a muscles versus brain dissension. Do the muscles tire, signaling fatigue to the brain or does the brain, in an effort to avoid injuring muscle tissue, signal fatigue to the muscles? The idea that the brain is actually responsible for limiting performance was first introduced in the late 19th century, but has not yet been proven.

A hot environment makes it harder for the body to regulate its body temperature during exercise for several reasons. Blood cools off muscle tissue by moving into the body’s core and out to the skin. A hot environment means that the skin is unable to dissipate heat easily due to a reduced temperature gradient. The elevated temperature of the skin also lowers the temperature gradient between the body’s core and skin. During exercise in a hot environment, blood is simultaneously needed to deliver oxygen to the muscles and to thermoregulate by going to the skin. To make matters worse, blood pressure is reduced because water is being lost as sweat. This strain on available blood is why dehydration is so detrimental to athletic performance in hot environments.

One study found that regardless of the temperature of the exterior environment fatigue always sets in at the same core body temperature [Walters, et al. (2000). Exercise in the heat is limited by a critical internal temperature.]. The authors reasoned that a warm environment simply means that the “critical” core body temperature is reached sooner. However, this critical core body temperature theory has been challenged recently by studies showing ways of altering the critical core body temperature. Dopamine and caffeine were both shown to increase the core body temperature that fatigue sets in. Dehydration, on the other hand, lowered the core body temperature at which fatigue set in. Therefore, not only does being dehydrated mean that your body has a harder time regulating its body temperature, but the body temperature at which fatigue comes on is lower.

Force exerted as a function of time and hyperthermia condition. Four electrical stimulations (EL) were done demonstrating that the muscle was capable of greater exertion from the EL than brain stimulation. Hyperthermia had less effect on muscle contraction during electrical stimulation than brain stimulation.

The brain’s ability to regulate its temperature is stressed during exercise. Cerebral metabolism increases during exercise. Yet, the brain must deal with reduced cerebral blood flow because blood is being diverted to active muscles and thermoregulating skin. It is hard to determine if the core body temperature or brain temperature is responsible for fatigue. Because the brain gets arterial blood from the body core their temperatures are closely linked. Thus, a hot body core results in a hot brain. The brain is typically a fifth of a degree Celsius above the core body temperature. One recent study used experimental techniques to selectively heat the brain during exercise (Lars Nybo, 2012. Brain temperature and exercise performance). The results suggest that the brain’s temperature has a significant impact on performance. However, other studies have concluded that the skin’s temperature sensors are responsible for hyperthermia fatigue [Sawka et al. (2012). High skin temperature and hypohydration impair aerobic performance].

In conclusion, it appears that hyperthermia causes fatigue by elevating the brain’s temperature. Much debate still remains regarding the source of fatigue and the role that hyperthermia plays on fatigue. Understanding the way that heat decreases performance will provide avenues for increasing athletic capability in warm environments.