In June of 2005 I finished my second Ironman in Coeur d’Alene, Idaho. After taking some time to recover in the athlete area, I was very stiff and sore as I hobbled with my wife back towards where we had parked our car. My quads were so tight they were screaming with every step and I began to wonder how I was going to make it. We had only gone about fifty feet when we came to an area that had been set up for athletes—and right there were two big hot tubs.

As physicians, both my wife and I had been taught pretty much forever that when you have acute pain or soreness due to any kind of injury you don’t use heat to treat it, because physiologically that isn’t a great idea. As I was standing there though, unable to walk, my wife and I both thought maybe this would help, it certainly didn’t seem like it would hurt. And so I gingerly went up the steps and settled in to the hot tub, where I sat for a good 15 minutes. When I emerged, I couldn’t believe it: my legs were totally rejuvenated.

I’ve thought back on that experience many times over the years and wondered if anyone had done good quality research to answer the question of whether or not the application of heat, rather than cold, might be beneficial after endurance events. As it turns out, many people have and the results, while not overwhelming, are interesting and not at all what you might think.

First, it’s worth revisiting the physiology of injury and especially of delayed onset muscle soreness (DOMS) and the evidence behind the use of local cold application (ie. icing), as well as the research around the use of cold water immersion (ie. ice baths).

At its most basic level, injury is essentially the destruction of tissues and cells. The more severe the injury, then the more significant the damage. For example, fractures or sprains result in the breakage of bones or ligaments, while strains merely involve damage to muscle-sometimes as tears in the tissue or sometimes just as cellular damage.

The consequences of those injuries are pretty uniform, with increased permeability in the blood vessels in the area resulting in fluid extravasation and an increase in inflammatory cells and chemicals. This starts a cascade of events that manifests as swelling, redness, warmth, and pain.

DOMS is an example of an injury that occurs at the cellular level, where breakdowns in the integrity of cellular structures result in swelling and inflammation that causes stiffness and soreness in the affected muscle and can impact performance and prolong recovery. You’re sore because, in essence, your muscle cells are breaking down and you need to let them rebuild.

The application of cold to an injured area, in the form of an ice pack for example, has a long history of use in medicine and a fair amount of research to support it. The cold causes vasoconstriction and reduces swelling and inflammation. Interestingly though, cold application does not impact the time to healing nor does it really affect recovery. It simply reduces pain and swelling, both of which have positive effects.

Cold water immersion, like ice baths, expand on this, but the research to date in fact suggests that this treatment strategy may actually impair recovery and worsen muscle performance. This is true despite the fact that ice baths do provide positive psychological benefits—in other words, it feels good.

Now with respect to heat, the thinking on this has historically been pretty consistent. Because the response to heat is to increase blood flow, it has always been thought that in the presence of injury heat is a bad thing, because it only exacerbates the swelling and inflammation. However, these long-held beliefs didn’t stop some from testing their theory.

Dr. Bruno Roseguini, PhD and an associate professor of exercise physiology at Purdue University, saw that previous research showed heat therapy helped restore exercise performance in patients with peripheral artery disease—so would it work for others as well?

“We did studies to evaluate heat treatments in young people to see whether or not this might help,” he said. The results were encouraging. “We found that heat caused a small increase in muscle blood flow. As a vascular biologist, it always made sense that increasing blood flow to a damaged muscle cell would improve recovery by improving the delivery of oxygen and nutrients and inflammatory cells that are needed for repair.”

Dr. Roseguini’s group has now done several studies to try and answer this question, as well as provide an answer for why heat might actually be a better way to treat DOMS than traditional ice methods. By taking muscle biopsies, they have demonstrated that the application of heat actually leads to a more rapid restoration of glycogen stores and correlates with improved muscle function when compared to the application of cold.

“There is a potential for heat to accelerate recovery and allow for athletes to train better more quickly. Heat increases the activity and rate of enzymatic processes both for repair and refueling,” he said. In addition, “it doesn’t end there, vascular dysfunction after hard resistance workouts are mitigated by heating. Mitochondrial abnormalities are also caused by extreme muscular efforts and are potentially reversed by heating.”

Perhaps unsurprisingly then, more and more evidence shows that the pain associated with delayed muscle soreness is impacted by heat more than by cold.

A recent meta analysis from researchers in China also looked at several studies that compared local cold application and cold water immersion to local heat application and heat immersion in the form of hot water or sauna use in athletes after intense bouts of exercise. The analysis showed that all modalities were effective in reducing or preventing the pain associated with DOMS, but the most effective modality was local heat application.

Dr. Roseguini concedes that to date most of the research in this area is on sedentary individuals or recreational athletes, as it is difficult to perform these kinds of studies on high-performing athletes. However, he feels that the results are very likely to be extrapolated to higher performing athletes, and that for the prevention of and recovery from delayed-onset muscle soreness heat—not cold—is really the best way to go.

Perhaps in the future more races will be offering hot tubs in the finish area, and athletes will be able easily walk (not hobble) away.

The MedTent answers your science and medical triathlon-related questions. It’s written by Jeff Sankoff, an ER physician, triathlete, and coach who runs TriDoc coaching and hosts the TriDoc podcast.