For a study published in the Journal of Physiology, researchers at the Karolinska Institute subjected five fit young men and women to grueling interval workouts on arm-pedaling machines, followed by 20 minutes of moderate pedaling. The workouts were designed to completely exhaust their arm muscles by depleting their levels of glycogen, the carbohydrate fuel that muscles feed off of. Once your muscles burn through their glycogen, they're pretty much done. Endurance athletes call this "hitting the wall" or "bonking," and it's why you'll often see runners and cyclists fueling with sugary treats in the middle of races to stave this off.

Once the volunteers were done with their tortuous workout, they rested for two hours and refueled with a plentiful helping of carbohydrates. That was the control session the researchers used to compare the subsequent workouts.

The next times they visited the lab, the volunteers did the same pedaling workout twice more, then slipped their arms into long cuffs that were either heated to 100 degrees Fahrenheit (38 degrees Celsius) or cooled to 59 degrees Fahrenheit (15 degrees Celsius). With the cuffs on, the volunteers did the same thing they had on the first visit: rested for two hours and gorged on carbs. But this time, after the resting session was up, they let their muscles come back to a normal temperature and then tried the interval workout again. The volunteers performed much better after using warmth to soothe their muscles and performed worse than normal after using chilly temperatures.

To find out why, the researchers did something akin to the same experiment on individual muscle fibers harvested from mice, using electrical impulses to mimic the pedaling workout. Then they warmed or heated the fibers while they rested, doused them with glycogen, brought them back up to a baseline temperature, and did the workout again. Just as in the human volunteers, the muscle fibers performed best after a warm recovery. When they weren't doused with glycogen, there was no difference, strengthening the hypothesis that muscles fatigue when glycogen runs out. But because they were using individual muscle fibers, this time they could tell why warmth was so much better: the cooled fibers were slower to take in the glycogen fuel than the warmed fibers.

This is just one study, but it builds on previous research showing the ineffectiveness of ice baths. A February 2017 study in the Journal of Physiology found that ice baths are no more effective than "active recovery" — something like a light jog or easy cycling — for reducing inflammation in muscles after a hard workout.

Other recovery techniques don't seem to be much better; a May 2017 study in the Proceedings of the National Academy of Sciences demonstrated that NSAIDs like ibuprofen actually prevent your muscles from repairing themselves as well after exercise. And as for stretching, a 2007 study in the American Journal of Physical Medicine & Rehabilitation found that it was no better than "passive recovery" — something like sitting on the couch watching Netflix — for getting muscles back to peak performance after exercise.

The results are in. After that marathon, century, or CrossFit competition, skip the ice bath, don't shell out for the cryotherapy, throw out the ibuprofen, and go straight for the jacuzzi.

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For more about the science of exercise, check out "The Runner's Body: How the Latest Exercise Science Can Help You Run Stronger, Longer, and Faster" by Ross Tucker, Jonathan Dugas, and Matt Fitzgerald. Clicking affiliate links helps support Curiosity!