Exercise is a powerful analgesic, dulling the sensation of whatever pain you may be experiencing. I believe that, and therefore it’s true for me. But would it remain true even if I didn’t believe it?
That’s the question at the heart of a recent study from researchers at the University of Southern Denmark, published last month in Medicine & Science in Sports & Exercise. The study explores a phenomenon called exercise-induced analgesia, which is simply the reduction in pain sensation commonly observed following exercise. There are various theories for why this happens, including the release of pain-blocking brain chemicals such as endorphins. Or it may simply be that the discomfort of exercise 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 experience of pain; another was told that exercise can increase the experience of pain; and the third group wasn’t told anything. Then all subjects did a three-minute unsupported wall-squat with legs bent to nearly 90 degrees. Before and after the squat, they did a series of tests to measure pain perception.
The results were pretty much exactly what you’d predict. The main outcome measure was pain threshold, which was assessed with an algometer—basically a blunt rod that applies a carefully calibrated amount of pressure to your skin. If exercise-induced analgesia is working, you’d expect the subjects to tolerate more pressure after the squat compared to their baseline test. Here’s the change from before to after in the three groups:
The group that got positive information (on the left) saw a significant increase in pain threshold. The no-information group, in the middle, saw a similar but slightly smaller increase. And the group that was told exercise would worsen their pain saw a slight decrease in pain threshold. These results were taken at the quadriceps, which were active during the squat; similar results were seen at the shoulder, which wasn’t.
The authors discuss these results in the context of attempts to prescribe exercise to treat people suffering from chronic pain. While exercise-induced analgesia is a pretty robust effect, it doesn’t seem to work as well in chronic pain patients. It may be that some of these patients “have expectations shaped by previous unhelpful information or narratives from healthcare professionals, non-evidence-based web sources or negative treatment experiences,” they write.
The results remind me of a study I wrote about a couple of years ago that drew a link between how ultra-endurance athletes think about pain and how likely they are to make it to the finish of a race. They also bring to mind one of the classic studies of pain perception and mindset, published in 2013 by Fabrizio Benedetti of the University of Turin. Benedetti and his colleagues inflicted pain on their volunteers by cutting off circulation to their arms. One group was told that the procedure would hurt (which, of course, it did). The other group was told that it would hurt, but that the temporary circulation blockage would be good for their muscles—and the longer they endured, 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 effect of opioids. These subjects didn’t see as big a boost in pain tolerance. Others were given a drug called rimonabant, which blocks the effects of marijuana-like cannabinoids. These subjects also saw a smaller boost in pain tolerance. And a final group received both naltrexone and rimonabant—and these subjects saw no boost at all. In other words, the beliefs we form about why we’re enduring pain have real biochemical effects in our brains. Telling people that their suffering would be useful triggered the production of the body’s own version of opioids and cannabinoids to dull the pain. When those brain chemicals were inhibited by naltrexone and rimonabant, belief no longer mattered.
It’s sometimes tempting to dismiss studies like the new Danish one as mere trickery, or to dismiss the poor results of the negative-information group as a moral failing. After all, they clearly gave up prematurely, right? But the Benedetti study reminds us to avoid that trap—to remember that, in 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 information sessions lasted only two to three minutes, and yet they were enough to completely wipe out the pain-blocking effects of exercise. Beliefs are fragile, so nurture them carefully.
Hat tip to Chris Yates for additional research. For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.