Physiology Friday #226: Optimizing Post-exercise Recovery: Embrace Heat and Forget about Cold
The science behind "warming up" after your workout.
Greetings!
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My favorite recovery gadget? A hearty meal and a good night of sleep.
If you’re someone who exercises, then you’re likely aware of how big the “recovery industry” has become. We’ve transformed a passive process into something active. Everyone is convinced that they need to take the initiative when it comes to recovering from exercise — sitting around just won’t cut it!
Recovering has become a workout in and of itself.
I have of course fallen prey to the clever marketing of the recovery companies and their scientific-sounding jargon about how their product helps my muscles regenerate quicker so I can tackle my next workout with more vigor and intensity (and less soreness!)
But what’s actually happening when we’re “recovering” from exercise? Why do our muscles feel fatigued after a hard workout, and how does this prevent us from performing well during our next exercise session? Furthermore, are there ways to “speed up” recovery?
One correlate of muscle fatigue is exercise-induced muscle damage, also known as EIMD. Muscle damage during exercise is the primary reason you feel sore after a particularly intense workout, maybe not right away, but 24–48 hours after (hence the term delayed onset muscle soreness or DOMS). Muscle damage and muscle soreness are strongly associated with the inflammatory/immune response to exercise.
You’ve probably observed that when you’re experiencing DOMS or any other subjective marker of muscle fatigue, your performance suffers. The day after a hard workout, your force-generating capacity is lower, reaction time is slower, and running or cycling speed is reduced. This indicates that we’ve not fully recovered from our prior workout.
What can we do to recover faster? The litany of companies selling massage boots, supplements, and special recovery sandals would like you to think that they’ve got the solution, but the evidence on most recovery techniques is scant. Everyone has likely found something that they wear, eat, or do that “works for them” in terms of recovery, but most of the claims made by those selling a recovery product are unfounded. They sound good in theory but rarely work in practice (or when subjected to a randomized controlled trial).
One technique that does appear to work is heat and/or cold exposure.
Cold exposure, aka cryotherapy (e.g., an ice bath or cold plunge) has long been used by athletes and coaches to promote recovery after workouts. And it does work…sort of. Cold exposure seems to be good for reducing muscle soreness, lowering post-exercise inflammation, and improving subjective measures of recovery.
But this might come at a cost. The “dangers” of cold exposure for athletes have recently been highlighted — a meta-analysis found that post-exercise cold water immersion reduced gains in muscle strength and size when compared to a control condition. The benefits probably don’t outweigh the potential risks especially if cryotherapy is close to your workout.
What about heat exposure, also called thermotherapy?
This appears to be more promising. Post-exercise hot water immersion does appear to improve indices of neuromuscular recovery as long as it’s hot enough — somewhere around 41℃/105℉ appears to be the sweet spot.
In contrast to cold water immersion, heat exposure does not appear to come with the associated risks of reducing strength or hypertrophy. In fact, hot water immersion and sauna both improve cardiovascular health, and post-exercise heat exposure might enhance resistance training and endurance training adaptations! Better recovery and better gains? Sign me up.
Should you go cold or go hot after your workout if you aim to maximize recovery? Thanks to a new study published in Medicine and Science in Sports and Exercise, we have an answer.1
For the study, a total of 30 young men (average age of 22) were randomly assigned to one of three different conditions:
Cold water immersion: Participants completed full-body cold water immersion (head out) at a water temperature of 11℃/51.8℉ for 11 minutes.
Hot water immersion: Participants sat in hot water (41℃/105.8℉ ) after exercise until their core body temperature reached ~39℃/102.2℉ . They spent 25 minutes immersed waist-deep in hot water at this core body temperature. Fluid ingestion was encouraged throughout and after the recovery protocol.
Control group (warm bath): Participants replicated the protocol of the hot water immersion group except that the water temperature was 36℃/98.6℉, resulting in no change in core body temperature.
All of the participants completed an exercise session to test quadriceps strength and neuromuscular function before, 24 hours after, and 48 hours after their respective recovery protocol. This test consisted of maximal concentric knee extensions to assess peak torque (maximal strength), rate of force development (explosive strength), and the amount of neural drive (measured using electromyography) in the quadriceps muscle (neuromuscular function/fatigue). Pressure pain threshold — an indicator of DOMS — was also assessed at each post-exercise time point.
After the first exercise session, the participants completed an exercise-induced muscle damage protocol, during which they performed 7 sets of 10 maximal eccentric contractions of the quadriceps muscle. This protocol was designed to elicit an initial 15% to 25% loss in peak torque — indicative of significant exercise-induced muscle damage.
How does hot water compare to cold water, and are they better than a warm bath? Let’s find out.
Results
Peak torque: Compared to baseline values, peak torque decreased 24 hours after exercise-induced muscle damage in the control condition and the hot water immersion condition, but not in the cold water immersion condition. At the 48-hour time point, peak torque had returned to baseline values in the hot water immersion condition but remained lower than baseline values in the control condition.
RFD: Rate of force development was similar to baseline values at the 24-hour time point in all 3 conditions. However, 48 hours after exercise, the rate of force development was lower in the control condition and cold water immersion condition compared to baseline. The rate of force development was similar to baseline values in the hot water immersion condition when measured 48 hours after exercise.
Pressure-pain threshold: Pressure pain threshold was lower (muscle soreness was higher) in the control condition and the cold water immersion condition 24 hours after exercise compared to baseline but was unchanged in the hot water immersion condition. At the 48-hour time point, the pressure pain threshold was still lower in the control and cold water immersion conditions and unchanged in the hot water immersion condition.
Peak torque (left), rate of force development (middle), and pressure pain threshold (right) at each time point in the hot bath (purple), cold water immersion (teal), and hot water immersion (red) conditions.
Neural drive: The rate of the increase in neural drive during the exercise test was higher in the hot water immersion condition at all time points when compared to the control condition and the cold water immersion condition. Furthermore, the low-frequency power of the electromyography signal (where higher values indicate greater neuromuscular fatigue) was elevated at the 48-hour time point in the control condition and the cold water immersion condition compared to baseline, whereas no change was observed in the hot water immersion condition.
Low-frequency power of the electromyography signal (EMG) at each time point in the hot bath, cold water immersion, and hot water immersion conditions. Higher low-frequency power indicates greater neuromuscular fatigue.
The main takeaway: heat appears to be most effective for mitigating muscle soreness and the decline in explosive strength after muscle-damaging exercise. Soreness subsides within 24 hours and explosive strength returns to baseline in as little as 48 hours.
Furthermore, in contrast to some widely held beliefs, cold water immersion is not effective for reducing muscle soreness or mitigating muscle damage-related declines in explosive strength, although it appeared to have a small benefit for improving the recovery of maximal strength.
Finally, post-exercise hot water immersion would seem to benefit some indices of neuromuscular recovery, as indicated by a more favorable electromyography signal during exercise.
Thus, this study would highly suggest that the optimal post-exercise recovery protocol involves heat exposure — cold exposure might just be a waste of time.
I will note that this study did not reveal significant differences between conditions at the various time points. For example, the rate of force development and pressure pain threshold were not different between the cold water immersion and hot water immersion conditions 24 or 48 hours after exercise. Rather, these variables merely changed within these conditions compared to baseline. This is an important distinction, but it doesn’t invalidate the benefits of the hot water immersion protocol used.
Importantly, this study provides us with a protocol that can be used to initiate the recovery process after exercise. Indeed, immersing yourself in water with a temperature around 41℃ or 106℉ and staying in it for at least 30 minutes (to maintain an elevated body temperature) is probably necessary if you want to realize the benefits observed in this study.
That’s because heat shock proteins — stress-response proteins that are implicated in protein folding and repair, cellular protection, and adaptation and recovery — are upregulated at a core body temperature of around ~39℃/102℉. You need to meet this threshold to reap the benefits of the heat shock protein response. This is why sauna is particularly good for inducing exercise-like adaptations — it potently elevates heat shock proteins by elevating core body temperature, and I would speculate that replacing the hot water immersion protocol used in this study with an equal-length sauna session would provide similar results.
The authors do note that it took some of the participants 60 minutes or more to reach this core body temperature — potentially imposing a time limitation. But I’d speculate that you could achieve lesser (albeit still significant) benefits by sitting in hot water for less time.
Would a hot shower work just as well? Probably not, but I think taking a post-workout hot shower could have some benefits for improving skin blood flow and lessening some muscle soreness. If you don’t have enough time for a 30-minute hot bath, crank the shower heat up for a few minutes.
Temperature manipulation appears to be one of the simplest and most effective tools that we have to modulate our physiology. Cold exposure does have certain benefits that are likely related to mood and cognition rather than performance or recovery improvements. Heat exposure, on the other hand, is gaining notoriety as a performance and recovery enhancer. The proverbial icing on the cake is that, when paired with exercise, heat exposure augments strength, hypertrophy, and VO2 max improvements.
If you’re not currently a fan of “thermotherapy”, I suggest you start to warm up to it.
Thanks for reading. See you next Friday.
~Brady~
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Interesting study, Brady. Of note, another way I think that heat therapy (I’m thinking sauna, here) could aid in recovery is as a substitution for the latter portion of a long run during a de-load week. This allows you to cut back on mileage without sacrificing your progress, in that, as you mentioned at the end of this post, there’s some research suggesting that adding sauna to endurance training doesn’t just maintain endurance performance but can improve it.
https://pubmed.ncbi.nlm.nih.gov/16877041/
Also, though this is fairly indirect evidence, I’m aware of one study showing that localized therapy on the quads can mitigate muscle atrophy during immobilization—not quite the recovery you were talking about here, but potentially another meaningful benefit heat therapy has to offer for recovery. I’m also aware of a rodent study showing similar results using one whole-body hyperthermia session prior to 8 days of essentially simulated bed rest.
https://pubmed.ncbi.nlm.nih.gov/31046520/
https://journals.physiology.org/doi/full/10.1152/jappl.2000.88.1.359?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org
Anyways, thanks for sharing another reason for me to keep up my sauna attendance.
It's funny how things come full circle. When I was playing competitive sports, rugby and football, forty years ago, the conventional wisdom was to have a good soak in a hot bath. In fact many changing rooms used to have a big hot bath for players post game. Now I run and I'm older, I'm happy that heat is probably best, however I do concur, that I always feel great after a cold shower!