As much as running sometimes hurts, it can also relieve pain — so much so that exercise is sometimes suggested as a pain-management approach for chronic pain patients. But it's not clear exactly how exercise reduces pain, or what types of exercise are best. Here are a couple of interesting results that were presented at this year's American College of Sports Medicine conference:
1. Endocannabinoids or opioids?
Two sets of brain chemicals are typically invoked in discussions of runner's high: endocannabinoids, which are the brain's version of cannabis, and opioids such as endorphins, which are the brain's version of morphine. Both sets of chemicals may also play a role in analgesia (i.e. pain suppression). For example, a Brazilian study published last year presented evidence that endocannabinoids are responsible, at least in part, for exercise-induced analgesia (EIA) in mice.
At ACSM, researchers from the University of Wisconsin presented data from a study investigating the interactions between these two systems, since previous research has suggested that they influence each other. The basic idea was to block the opioid system and see if that changed the response of the endocannabinoid system to exercise.
The study had 58 volunteers; half of them were given a drug called naltrexone that blocks opioid receptors, while the other got a placebo. Their pain response (to heat and pressure) was tested before and after a three-minute exercise bout. In both cases, pain perception was reduced after exercise, and levels of endocannabinoids increased to the same degree, with no difference between groups. In other words, blocking the opioid receptors didn't have any apparent effect — it was the endocannabinoids that mattered.
There was one odd difference between the two groups. Levels of a molecule called oleoylethanolamine (OEA) increased after exercise, but this increase didn't happen when the opioid receptors were blocked. OEA is a marker of satiety, so this seems to indicate that opioid signalling plays some role in keeping appetite in check after a workout.
It's difficult (and risky) to read too much into the results of mechanistic studies like this, but it's interesting to get a sense of what's going on in the brain and body after a workout.
2. Hard or easy?
The other ACSM study, from researchers at Texas Tech, looked at the effect of different intensities of exercise on something called "conditioned pain modulation," which is a pretty interesting phenomenon.
The idea is basically that "pain inhibits pain." If I give you an electric shock, you might rate it as 7 out of 10 on a scale of how painful it is. Now if I jab a sharp stick into your arm and leave it there for a while (let's say the stick is a 3 on the pain scale), then give you another electric shock while the stick is still jabbing you, you'll rate the shock as less painful — maybe 5 of out 10. The reason is that, in order to deal with the jabbing stick, your central nervous system has "turned down" its pain reception. This is conditioned pain modulation (CPM), and how well it works is an important factor in conditions like chronic pain.
What the new study looked at was the link between CPM and patterns of physical activity in a group of 48 volunteers. Previous studies have shown that vigorous exercise improves CPM. This makes sense, because an important part of the training process is learning to tolerate discomfort. The new study was trying to assess whether moderate physical activity has similar effects. However, after levels of vigorous exercises were accounted for, doing more moderate exercise wasn't associated with any improvements in CPM. You have to suffer in order to reduce your suffering, apparently.