So they asked their volunteers to consume large amounts of carbohydrates in the two hours after their session of hard pedaling but not to otherwise coddle their muscles.

Then on subsequent visits to the lab, they had the young people repeat the pedaling workout twice more, and immediately afterward, slip long cuffs over their arms that could be heated or chilled with water coils. The cuffs were warmed during one session to about 100 degrees Fahrenheit and chilled during another to about 5 degrees. The volunteers wore the cuffs for two hours while also downing carbohydrates.

Finally, at the end of each session, the men and women repeated the interval portion of their original pedaling, since it was the most tiring.

And each of them could pedal hardest at that point if their arm muscles had been warmed beforehand. Their power output then was “markedly better” than after the other two sessions, the scientists write in their paper, suggesting that their muscles had better regained strength. Their power was worst after their muscles had been cooled.

But these results, while interesting, could not explain why heat might be goosing recovery, so the inquisitive scientists next turned to individual leg-muscle fibers obtained from mice. They attached the fibers to a mechanism that could record the strength of contractions and then zapped the fibers with electricity so that they contracted, over and over. The researchers noted when these contractions slowed, indicating the fibers had grown pooped.

They then tired other fibers before dousing some of them with glycogen and subsequently warming or cooling all of the fibers and restimulating them a final time.

They also examined whether warming or cooling had affected how much glycogen the muscle tissue absorbed.