1. The relationship between body fat content, isometric endurance and deep muscle temperature was assessed in eight male and female volunteers trained to static effort.
2. Muscle temperature, measured in the belly of the brachioradialis muscle, was found to be directly related to the subjects' body fat content.
3. Associated with a reduction in body fat content, there was a marked decrease in deep muscle temperature and an increase in isometric endurance of the hand-grip muscles; no change in strength was noted.
4. Conversely, an increase in body fat content resulted in an increase in deep muscle temperature and a decrease in isometric endurance.
5. When muscle temperature was stabilized before and after weight loss by immersing the forearm in water at 37°C, a reduction in body fat no longer influenced endurance.
6. It was concluded that the change in isometric endurance associated with either loss or gain of weight can be accounted for entirely by the changes in muscle temperature.