Two weeks ago, we looked at when and how cold-water immersion, or ice-bathing, can be useful in training. Through a combination of hydrostatic pressure and cooling from the low-temperature water, ice baths seem to aid recovery by “squeezing out” the waste products from a workout and clamp down on localized inflammation, reducing soreness and the related drop in performance.
This week, we’ll take a look at the other end of the thermal spectrum: when is the right time to use hot water immersion?
Hot water immersion, or at least heating with hot packs from a hydrocollator, is a common practice in athletic training rooms at high schools and colleges. Outside of that, though, it does not seem to permeate out much into the wider world of injury care. Surprisingly, not a whole lot of research has been done on large-scale questions on when and how to use hot water immersion; rather, the bulk of the scientific work has been on more specific aspects of how the body responds to aqueous heating. So, we are left to interpret from those smaller, specific studies.
The science of using heat for injuries and recovery
Many of the body’s reactions to hot water are the inverse of its response to cold water. As demonstrated in a 2005 study by Kimberly Fiscus and coworkers, blood flow to the lower legs increases when immersed in warm water. The vessels that deliver blood and retrieve waste products dilate, resulting in the increased flow.
However, other work (albeit using heat packs, not water immersion) appears to show that heating and increased blood flow only penetrate about 1-2 centimeters into the body. From a survival perspective, this is a good thing; the body’s ability to maintain a constant temperature allows us to survive in very cold and very warm temperatures. However, when it comes to modifying blood flow to a muscle, heating leaves a bit to be desired.
From some research in Denmark published in the early 1990s, it seems the blood flow to the skin and the tissue immediately beneath it actually comes at the expense of the blood flowing to the deep muscles. Fortunately, the magnitude of this effect is not particularly large, but room-temperature water may actually be better for loosening up muscles than hot water.
Indeed, muscle flexibility does not improve with heat alone, although when combined with stretching, it does seem to help.
When heat should not be used
It’s important to note when heat should not be used too. Heating not only increases blood flow near the skin, but it also increases the flow of lymph fluid, the body’s “general plumbing” system for the interstitial fluid that fills up the empty space in your body. Many of these lymph vessels happen to be close to the surface of the skin, and as such are susceptible to being opened up by external heat.
In one study of 30 brave volunteers with newly-sprained ankles, the ankle swelled by 25% in a group treated with hot water immersion of their foot. Contrast this with only a 3% increase in size in the group treated with cold water immersion.
So, heating any kind of injury, especially an acute one, seems like a bad idea if you aren’t going to cool it down later. It’s also plausible that this same effect can manifest with the micro-trauma to muscles from hard training.
Heating your legs after a hard run might increase soreness the next day because of the increased blood and lymph flow to the area
Final thoughts
In all, hot water immersion does not appear to have too many uses for a distance runner.
- It is best used before a workout to heat up an area that’s a bit sore or needs some extra blood flow, but it won’t be very effective if this area is more than an inch or so deep. So heating up your foot will work great if you’ve got a tendon that’s a bit stiff, but it won’t do much for your hamstring or your quads.
- If you do heat up something before a workout, you should make sure you ice it thoroughly afterwards, as you don’t want the blood and lymph flow to the area to go on unchecked.
- Definitely don’t heat up an area with an acute injury like an ankle sprain or a bruise unless you plan on icing it soon after.
New research is coming out on “contrast baths,” which involve alternating hot and cold water immersion multiple times in a single session. The research there is spotty, and it’s a topic for another day, but it may yet give hot water immersion a broader range of applications.
