If you're lucky enough to go to a gym with a sauna, you'll probably see people relaxing in it after their workout. But the heat won't do anything to prevent sore muscles or help you recover. Instead, the best time to use the sauna is before your workout.
Physical therapist Patrick Walsh tells Outside Online:
Walsh wouldn't recommend using the sauna after a hard workout, either. "When you recover from exercise, your heart rate should come back down to normal," he says. "Sitting in a sauna for more than five minutes is going to keep your heart rate up - it's essentially a form of passive exercise - so it's really going to delay your body from starting its recovery process."
Spending a few minutes in the sauna before your workout is a better idea, he says, and may indeed help you feel warmed up and relieve some immediate muscle pain.
Warming up is, in part, about literally warming up your body, so a sauna can help get that process started (although it doesn't completely replace your exercise-based warmup). The experts that spoke to Outside also emphasised that saunas can be great for relaxation, and even if they aren't helpful, they probably won't do any real harm, even after a workout.
The exception: if you're pregnant or have a heart condition, talk to your doctor; they may recommend avoiding long sauna sessions. Also, if you'll be spending time in the summer heat directly after your workout, it's probably best to skip the sauna altogether.
Do Saunas Help or Hurt Sore Muscles? [Outside Online]