A hot tub for bigger muscles?

Should we heat our muscles after exercise to stimulate muscle growth? In a recent blog (see blog post) we discussed some of the research...

A hot tub for bigger muscles?

Should we heat our muscles after exercise to stimulate muscle growth? In a recent blog (see blog post) we discussed some of the research demonstrating that a cold bath after exercise may lowers muscle protein synthesis as well as gains in muscle mass and strength. So, if the main goal is to promote muscle growth, we suggested that it is not smart to use cold-water immersion as a recovery strategy following resistance exercise.

Hot water immersion after exercise does not help recovery despite increase in blood flow

The reasons by which cold-water immersion likely impairs muscle growth, is due to its effects on reducing skeletal muscle temperature and blood flow. A reduction in muscle temperature will lower (anabolic) enzyme activity and a reduction in blood flow. This reduction in blood flow will lower the provision of amino acids towards the muscle (which is an important process for stimulating muscle growth). One could speculate that hot water immersion could elevate muscle temperature and blood flow and have the opposite effects.

So, we were interested in finding out whether increasing muscle temperature and blood flow could actually improve muscle growth.

Applying hot-water immersion following exercise will increase muscle temperature and blood flow.Therefore, could this be an effective strategy to increase muscle protein synthesis and long-term gains in muscle mass and strength?

In order to answer this question, we decided to conduct an experiment to assess the effect of hot-water immersion following resistance exercise on muscle protein synthesis (1).

Muscle protein synthesis experiment

We recruited 12 healthy young male adults that were familiar with resistance-type exercise. We asked them to come to our laboratory and let them perform a resistance-type exercise session followed by one leg that was immersed (for 20 minutes) into hot water (46°C) and the other leg into thermoneutral water (30°C). Immediately after water immersion we also gave 20 grams of an intrinsically labeled (milk) protein shake. This “intrinsically labeled” means that we had some amino acids within that protein shake “specifically labeled” so we could see exactly where those amino acids would end up in the body and, particularly, in the muscles. Together with an infusion with tracers and taking blood and muscle samples for 5 hours after post-exercise water immersion, we could then exactly see what happened to the amino acids from the protein shake into our muscles and what happened to muscle protein synthesis.

NO difference in protein synthesis with hot water

When looking at what happened to the amino acids from the protein shake, we observed that there was no difference in their incorporation into either the leg muscle that was heated (46°C) or the leg that recovered at the normal (30°C) temperature. In addition, we also did not observe any difference in muscle protein synthesis over 5 hours following exercise between the heated or thermoneutral leg. Therefore, our acute experiment did not show any benefit of hot-water immersion for increasing muscle protein synthesis following exercise.

Would this also mean that there is no benefit of hot-water immersion in increasing muscle mass and strength gains following more prolonged resistance exercise training over several weeks or longer?

No effect of hot water immersion after exercise on protein synthesis
Long term effects of hot baths after exercise

There is only limited data available looking into the effects of hot-water immersion as a heating strategy following resistance exercise training. However, the available evidence so far does not show that hot-water immersion helps making more gains in the gym. Recently, a paper came out looking into the effects of 4 weeks of hot-water immersion (15 min at 39°C) following resistance exercise (2x per week) in highly trained rugby athletes during an in-season competition phase. The authors did not report any benefit of hot-water immersion on lean body mass after 4 weeks (2). Of course, this study was only 4 weeks and also in-season, so it was not necessarily expected that there would have been (substantial) gains in lean body mass. However, it does not support the suggestion that hot-water immersion is a potent strategy to further increase gains in muscle mass over weeks of resistance training.

No benefits of 4 week hot bath use for muscle mass and strength

There is also some unpublished work, available online (3). In this study the authors looked into the effects of 10 weeks of resistance exercise training and whether hot-water immersion (10 min at 45°C) following each resistance exercise session (2x per week) would lead to more gains in muscle mass and strength. In line with the acute muscle protein synthesis and the 4-week training study, they also did not observe any benefits of repeated post-exercise hot-water immersion on increasing gains in muscle mass and strength over a 10-week training period.

so far there is no reason to assume that going into a hot tub following resistance exercise will not result in greater gains in muscle mass and strength.
Conclusion

Despite limited evidence, there is so far no reason to assume that going into a hot tub following resistance exercise will not result in greater gains in muscle mass and strength. The good news is that if you really enjoy a hot tub after exercise, it will also not hinder your gains in muscle mass and strength. So, you can still apply it without worrying (which is not necessarily the case with cold-water immersion).

References
  1. Fuchs, C.J., et al., Hot-water immersion does not increase postprandial muscle protein synthesis rates during recovery from resistance-type exercise in healthy, young males. J Appl Physiol (1985), 2020. 128(4): p. 1012-1022.
  2. Horgan, B.G., et al., No effect of repeated post-resistance exercise cold or hot water immersion on in-season body composition and performance responses in academy rugby players: a randomised controlled cross-over design. Eur J Appl Physiol, 2022.
  3. McGorm, H., The effects of hot water immersion on recovery, performance and adaptation to resistance exercise. PhD Thesis, 2019