Scrotal temperature change during running in naked humans

Abstract Male reproductive organs are functionally affected by high body temperature. This study aimed to clarify the thermoregulation mechanisms of the human scrotum during running, and experiments were conducted to investigate temperature changes in the whole body and the scrotum. Eleven male long-distance runners performed treadmill running at 60%VO2max for 60 min in an indoor environment at 27°C and 40% relative humidity. Immediately after the start of the run, the skin temperatures in areas other than the scrotum decreased or showed a tendency to decrease. In contrast, the scrotal temperature increased immediately after the start of the run. After the end of the run, the scrotal temperature decreased, whereas the skin temperatures in other body parts increased. An increase in scrotal temperature immediately after the start of the run was observed even in the situation where the scrotum was not affected by heating due to contact with the thighs. A possible explanation for this phenomenon is that the scrotum may have been elevated with the contraction of the dartos muscle and the cremaster muscle as a result of increased sympathetic nerve activity. Additionally, it cannot be ruled out that a specific vasodilation mechanism could play a role in this occurrence.