Most of the country is experiencing stifling temperatures, some areas in excess of 100 degrees. The impact of heat on exercise performance is well documented. Heat causes a reduction in muscle force and sustained contraction. Logically, it would seem that the precipitating event in reducing muscle performance is an increase in core body temperature and for a long time this was the accepted wisdom. Increased core body temperature is detected by sensors in the brain that release a signal to reduce muscle function. We now know that, although the brain is the central player in heat impaired exercise performance, the mechanism is far more sophisticated.
To determine the role played by core body temperature researchers designed a novel time trial study. One group of cyclists exercised in cool (59° F) conditions, the second in hot conditions (95° F). The researchers measured core body temperature, heart rate and muscle performance. There was no difference in heart rate between the two groups. Surprisingly they found that core body temperature rose similarly in both groups, however muscle performance began to decrease sooner in the hot exerciser group. The researchers found that, when exercising in hot conditions, the body has an early warning system that signals the muscle to slow down and stop generating as much heat, even before there is an appreciable rise in core body temperature. The nature of this early warning system may be a sensor that detects minute changes in blood temperature before one sees a rise in core body temperature.
The early warning signal reduces muscle power output and, in theory, allows you to complete your exercise bout at a reduced performance level. These results show the body temp sensing mechanism during exercise is far more sophisticated than we previously thought.