Train blocks ache, however just for believers

Exercise is a powerful analgesic that helps relieve any feeling of pain you may have. I believe that and that’s why it’s true to me. But would it stay true even if I didn’t believe it?

That is the question at the center of a recent study by researchers at the University of Southern Denmark published last month in Medicine & Science in Sports & Exercise. The study investigates a phenomenon known as exercise-induced analgesia, which simply represents the decrease in pain sensation commonly seen after exercise. There are several theories as to why this occurs, including the release of pain-blocking brain chemicals like endorphins. Or it may simply be that the discomfort of movement desensitizes us to subsequent discomfort.

The researchers recruited 83 volunteers, roughly half male and half female, and divided them into three groups. One group was told that exercise can reduce the sensation of pain; Another was told that exercise can increase the sensation of pain. and nothing was said to the third group. Then all subjects did a three-minute, unsupported wall squat with legs bent at almost 90 degrees. Before and after the squat, they performed a series of tests to measure pain perception.

The results were pretty much what you would predict. The main result was the pain threshold, which was measured with an algometer – basically a blunt stick that applies carefully calibrated pressure to your skin. If exercise-induced analgesia is effective, it can be expected that subjects will tolerate more pressure after squatting compared to their base test. Here is the change from before to after in the three groups:

(Illustration: Medicine & Science in Sport & Exercise)

The group that received positive information (left) saw a significant increase in pain threshold. The group with no information in the middle saw a similar but slightly smaller increase. And the group who were told that exercise would make their pain worse saw a slight decrease in pain threshold. These results were recorded on the quadriceps that were active while squatting; similar results were observed on the shoulder, which was not the case.

The authors discuss these results in the context of attempts to prescribe exercise to treat people with chronic pain. While exercise-induced analgesia is a fairly robust effect, it doesn’t seem to work as well in patients with chronic pain. Some of these patients may have “expectations shaped by previous unhelpful information or reports from healthcare professionals, non-evidence-based web sources, or negative treatment experiences,” they write.

The results remind me of a study I wrote a few years ago that correlated ultra-endurance athletes’ attitudes towards pain and the likelihood of making it to the end of a race. They are also reminiscent of one of the classic studies on the perception and thinking of pain published in 2013 by Fabrizio Benedetti of the University of Turin. Benedetti and his colleagues have inflicted pain on their volunteers by breaking the circuit to their arms. One group was told that the procedure would hurt (which it naturally did). The other group was told it would hurt, but that temporary blockage would be good for their muscles – and the longer they held out, the more benefit they would get. The results were spectacular: those who thought the experiment was good for their muscles endured the pain for about twice as long.

But here’s the twist that makes Benedetti’s study so interesting. Some of those who were told the pain was beneficial were given a drug called naltrexone, which blocks the effects of opioids. These subjects did not see such a large increase in pain tolerance. Others were given a drug called rimonabant, which blocks the effects of marijuana-like cannabinoids. These subjects also saw a smaller increase in pain tolerance. And a final group received both naltrexone and rimonabant – and those subjects saw no flare at all. In other words, the beliefs we form about why we endure pain have real biochemical effects on our brains. Telling people that their suffering was beneficial triggered the body’s own version of opioids and cannabinoids to be produced to relieve the pain. When these brain chemicals were inhibited by naltrexone and rimonabant, belief no longer mattered.

It is sometimes tempting to dismiss studies like the new Danish one as a mere trick or to dismiss the poor results of the negative information group as a moral failure. After all, they clearly gave up early, didn’t they? But the Benedetti study reminds us to avoid this trap – to remember that, according to the words of a classic study from the 1960s, “psychology is a special case of brain physiology”. And it doesn’t take much to change that physiology. The Danish researchers note that their briefing sessions only lasted two to three minutes and were still enough to completely eradicate the pain-blocking effects of exercise. Beliefs are fragile, so cultivate them carefully.

Hat tip to Chris Yates for additional research. If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.

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