Scrotal temperature change during running in naked humans.

Plain-language summary

This study measured what happens to scrotal temperature during a run. Eleven trained long-distance runners ran on a treadmill for 60 minutes at 60% of their maximum aerobic capacity, in a room held at 27 degrees Celsius and 40% humidity, while temperature sensors tracked the scrotum and skin elsewhere on the body. The pattern was the opposite of what happens everywhere else. Skin temperature across the rest of the body dropped as soon as the run began, as sweat began to evaporate. Scrotal temperature rose immediately instead. When the run stopped the two reversed again, with scrotal temperature falling while the rest of the skin warmed. The rise happened even when the scrotum was held clear of the thighs, so it is not simply heat conducted from the legs. The authors put it down to the cremaster and dartos muscles pulling the testicles up towards the body as sympathetic nerve activity rises during exercise, and note they cannot rule out a blood-flow mechanism specific to the scrotum. Eleven men in one condition is a small study, and it measured temperature only, not sperm.

Key findings

• Scrotal temperature rose immediately after running began.
• Skin temperature elsewhere on the body fell, or tended to fall, over the same period.
• After the run ended, scrotal temperature fell while skin temperature elsewhere rose.
• The rise happened even when the scrotum was kept from touching the thighs, ruling out conducted heat from the legs as the sole cause.