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Open Instagram and you would think post-workout recovery begins and ends with an ice bath. After you hit ‘save’ on your Apple Watch, your run zips off to the Strava cloud, plenty of social media influencers advise you go for a cold plunge (if you have access) or a cold shower (if you don’t).
But a new human study on muscle healing suggests something different: if you actually want your muscles to repair themselves, you are better off turning the hot tap up than jumping into a tub of freezing water or opting for a cold shower.
Dr Freya Bayne, a sport and exercise scientist at London South Bank University, has just co-authored research in The Journal of Physiology comparing cold and hot water immersion after a simulated muscle injury.
“Cryotherapy, or cold therapy, is used really widely in a lot of sports medicine for muscle injuries, but the evidence to support that in terms of muscle regeneration is really lacking,” she said. “Prior to this study, no human studies on muscle regeneration have been done, so we really wanted to fill that gap.”
How do you “injure” a muscle in a lab?
To properly study regeneration, the team needed real muscle damage, closer to a strain than exercise-induced soreness.
“The electrical stimulation that we use is where you put a probe into one of the nerves to cause damage that would be the equivalent of a strain,” explained Dr Bayne. A sample of 34 healthy men took part in the study which involved having their thighs put through 200 electrically stimulated eccentric contractions. This was not a gentle gym session; this process kills off a chunk of muscle fibres and triggers a full repair response, similar to a serious sports injury.
Over the following days, the researchers tracked strength, soreness, blood markers of muscle damage and, crucially, took muscle biopsies to see what was happening inside the tissue as it tried to heal. After the damage was done, participants were randomly assigned to one of three daily treatments for 10 days:
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Cold water immersion: 15 minutes at 12°C
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Thermoneutral water: 30 minutes at 32°C
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Hot water immersion: 60 minutes at 42°C
The results
Credit: CC
So what happened when they crunched the data?
The muscle’s maximum strength took a hit after the simulated injury and had still not fully recovered after 10 days in any of the groups. Cold immersion did not speed this up. Heat did not magically restore power either, but it did change other markers that reveal how well the muscle is regenerating. Bayne and colleagues identified four ways hot water immersion seemed to help muscles bounce back quicker than cold water treatment .
The first was better blood flow and muscle waste clearance through improved circulation. “Hot water boosts blood flow to [the damaged muscle], delivers more oxygen, more nutrients to the damaged tissue, and then helps clear the waste production at a faster rate,” said Dr Bayne.
In the study, people in the hot-water group reported less muscle soreness over the following days and had lower levels of muscle damage proteins (creatine kinase and myoglobin) in their blood than those using cold or neutral water.
The second restorative process was “heat shock” proteins, referred to as “the muscle’s internal repair toolkit”. “When you’re exposed to heat, it triggers these protective proteins which stabilise muscle fibres and then supports that cellular repair,” explained Dr Bayne. The biopsies showed that two of these proteins were ramped up in the hot-water group but stayed flat or even blunted in the cold-water group.
The third indicator was that hot water treatment appeared to stimulate a faster switch from inflammation to healing. After an injury, your body undergoes an initial inflammatory phase (characterised by that typical inflamed swelling you see as part of injuries), but then the tissue switches into a calmer, rebuilding mode. In this study, hot water seemed to nudge that switch along.
“Hot water seems to accelerate pro-inflammatory to anti-inflammatory much faster than cold water. And we want that anti-inflammatory because that’s the one that promotes healing, and often with cold, it gets stuck with the pro-inflammatory, so you have that inflammation for a longer period of time,” said Dr Bayne.
Finally, the team looked at pathways linked with rebuilding and creating muscle proteins. Bayne said:. “We think that heat may help maintain protein synthesis pathways, and these are kind of essential for rebuilding muscle, whereas cold blocks those signals.”
Summing all these findings up, Dr Bayne said: “Essentially, we found the warmer the temperature, the faster the regeneration.” Cold therapy, meanwhile, did not reduce pain or blood markers of damage compared with neutral-temperature or hot water, and it appeared to blunt some of the helpful responses seen with heat.
What this means for your post-workout routine
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For elite athletes and clinicians, Bayne thinks these results “challenge the ‘ice for injury’ approach that everybody uses”. In her view, heat therapy “shows a promising reduction in pain and regeneration. Athletes and clinicians could use this as part of their rehabilitation after muscle injury, specifically strain.”
But what about delayed onset muscle soreness after a tough gym session or long run? Here, the evidence is still emerging. Bayne also mentioned a second study, due to be published next year, using more typical exercise rather than a lab-induced strain.
“We’ve shown the same thing, so heat will also be beneficial.”
Where ice has a role
None of this means ice baths are useless. Cold can still temporarily numb pain and there is some evidence it has mental health benefits. And according to this study, healing will happen when using cold water — just not as fast as heat.
The bigger shift may come in hospitals, where patients are routinely handed ice packs after injury and surgery. “I think the biggest thing we’ll see is actually going to be in the hospital setting,” Dr Bayne said. “Now I think that’s going to completely change.”
But the science is still emerging. “We’re really at the start of it, and this is that proof of concept,” said Dr Bayne.
Future work needs to test heat therapy across different sports, injury types, and importantly in women. For now, though, if you are standing in the changing room wondering whether to brave the ice barrel or run a hot bath, the science suggests you can skip the shivers and turn up the heat instead.
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