by Paul Govern
Six weeks of aerobic exercise can lead to sustained increases in endogenous opioid function and significant relief for chronic low back pain, according to a randomized controlled trial by Vanderbilt University Medical Center’s Stephen Bruehl, PhD, and colleagues, reported in Pain.
Low back pain is a worldwide cause of disability that has been examined most closely in Western industrialized societies, where it affects 60-80% of adults at some time in their lives (with causes in most cases remaining unknown).
While the benefits of exercise for chronic pain have been previously reported, the investigators designed this trial to shed light on underlying mechanisms that may be specifically to thank. The trial focused on the potential role of endogenous opioids, popularly known as endorphins, which are neuroactive peptides (short strings of amino acids) that can reduce pain and anxiety and enhance mood, acting on the same opioid receptors in the central nervous system as opioid pain medications.
Investigators at VUMC and Rush University in Chicago recruited low-activity participants who had experienced chronic low back pain on average for more than six years. In the trial, 38 participants were randomized to 30-minute sessions of supervised aerobic exercise in three sessions per week for six weeks, while a control group of 44 participants was instructed to stick with their normal activity.
Before and after this intervention, participants reported their level of chronic pain and the interference it had wrought in their daily lives. Also, before and after, participants underwent double-blinded laboratory sessions to assess responses to a controlled “heat pain task” that participants were exposed to after being given intravenous fluids containing either naloxone or saline placebo.
Naloxone, unlike saline, blocks endogenous opioid receptors and thereby temporarily eliminated any endogenous opioid analgesia. With participants and investigators blinded as to whether naloxone or placebo had been infused, participants repeated the evoked pain task under both conditions. Changes in pain responses between the saline and naloxone conditions provided an index of endogenous opioid function.
“Relative to controls, our exercise participants reported greater decreases in their chronic pain intensity and interference, and they showed reductions in responsiveness to evoked pain in the lab under placebo conditions that were not seen in the control group,” said Bruehl, professor of Anesthesiology.
In the exercise group, 44% of participants reported at least a 30% reduction in average pain from baseline to post-intervention, compared to only 31% of the control group. Among the exercise group, 32% reported being “much improved” or “very much improved,” whereas in the control group only 9% reported this level of positive overall change.
“Regarding why aerobic exercise might help with chronic pain management, it’s especially telling that we found a dose response linking exercise intensity to pain reduction and endogenous opioid function,” Bruehl said. Exercise group participants who exercised more vigorously (according to recorded heart rates) showed significantly larger increases in endogenous opioid function than those who exercised less intensely. Greater increases in endogenous opioids were, in turn, related to greater reductions in chronic pain.
According to the authors, the published evidence on aerobic exercise and chronic low back pain has been scant and somewhat in doubt, with most of the research having been conducted without randomization or adequate experimental controls.
“Results of this trial not only confirm a causal connection between aerobic exercise and reduced chronic pain, but they also suggest that increased endogenous opioids may be an important reason why regular exercise improves chronic pain. Our results indicate that exercise-related increases in endogenous opioids may be relatively prolonged rather than transitory, continuing a week or more after exercise,” Bruehl said.
