‘Pain sensitivity is reduced by exercise training’ - A Meta-Analysis & Systematic Review
Musculoskeletal pain (i.e., the tissue source of pain is muscle, ligament, tendon, joint, cartilage or discogenic) is driven by a number of factors, including (but not limited to) nociceptive pain drivers, nervous system dysfunction, comorbidity drivers and cognitive-emotional drivers. In chronic pain, alterations in the normal processing of pain occur, leading people to become more sensitive to pain, or even prone to feeling pain to stimuli that aren’t usually painful.
Exercise training is an important and effective treatment strategy for managing pain and disability for adults with chronic pain conditions. Whilst exercise training traditionally focused on improving strength and endurance, it is likely that the mechanism of action of exercise in improving chronic pain is not due to these musculoskeletal factors alone. Factors such as self-efficacy, and central nervous system adaptation likely play a role.
This paper aimed to systematically review and conduct meta-analysis of the literature to examine whether exercise training interventions used as a standalone conservative treatment were effective in reducing peripheral and/or central pain sensitisation compared to no exercise training or to other conservative, non-exercise training interventions.
Included studies in the search strategy were ones which compared an exercise training intervention to a no-intervention control, or treatments that involved passive treatment by a therapist (e.g. manual therapy, chiropractic, passive physiotherapy, osteopathic, massage or acupuncture) and treatments that involved practitioner interaction only (e.g. general practitioner management, education or psychological interventions).
Prescribed exercise training programs including one or more of the following exercise modes: resistance, stabilisation/motor control, Pilates, yoga, McKenzie, flexion, aerobic, water-based or stretching without the addition of other treatments (e.g., massage, electrotherapies, cognitive behavioural therapy, pain education).
Studies were required to include either an outcome measure of peripheral pain sensitivity and/or central pain sensitivity. Peripheral sensitisation is an increase in localised nociceptor response to tissue damage, with reduced afferent threshold for conduction at the sensory neuron peripherally. Central sensitisation is defined as a dysfunctional enhancement in nociceptor responsiveness to either normal or subthreshold afferent input within the central nervous system (CNS), causing pain sensitivity which may be disproportionate to noxious, or innocuous stimuli.
From the fifteen studies were eligible for quantitative they found that exercise training may be effective for reducing pain sensitivity when compared to non-exercise training comparators. This effect persisted when compared to treatments that involved practitioner interaction only without exercise. Further, exercise was also effective in fibromyalgia and in neck/shoulder pain. Notably, the evidence was low to very-low quality overall, as assessed by the GRADE criteria.
An acute bout of exercise (one session) is known to reduce pain sensitivity and increase pain thresholds in healthy people, yet in chronic pain populations, this effect is less consistent. The current study adds that exercise training (i.e. exercise performed over a number of sessions) can result in a reduction of pain sensitivity.
The authors comment that reasons for pain and disability are always very multi-factorial. Thus, they attribute the reductions in pain sensitivity from a single bout of exercise to increased cerebral perfusion and cortical inhibition, moderating inflammatory and immune response to perceived or actual harm. Potential reasons for pain sensitivity reductions in long term exercise included central neuroplastic changes, local musculoskeletal adaptations, reduced fear and avoidance of movement and improved mental health status.
Three particular studies in the review assessed pain sensitivity changes over time in response to exercise training local to the pain region and remote to it. Their analysis showed that the effect size of pain sensitivity locally to the pain region was larger in magnitude than remote regions, and only the effects locally were statistically significant.
The review also found that, in comparison to non-exercise treatments (i.e. where an intervention other than exercise was provided), the effect size favoured exercise. The authors commented that whilst this implies that exercise training may be better for reducing pain sensitivity than non-exercise treatments, the quality of the evidence was low.
Overall, the review concluded that there was low to very low quality evidence that exercise training alone may be an effective treatment for reducing pain sensitivity in adults, as well as those with fibromyalgia or neck/upper quadrant pain specifically. And that exercise training could be more efficacious as a therapeutic tool to reduce pain sensitivity than passive modalities like massage or dry needling. However, the optimal exercise prescription required to achieve reductions in pain sensitivity is unclear, as interventions in the included studies ranged between 4-16 weeks in duration.
References
1. Belavy, D. L., Van Oosterwijck, J., Clarkson, M., Dhondt, E., Mundell, N. L., Miller, C. T., & Owen, P. J. (2020). Pain sensitivity is reduced by exercise training: Evidence from a systematic review and meta-analysis. Neuroscience & Biobehavioral Reviews.
2. Naugle, K. M., Fillingim, R. B. & Riley, J. L. A Meta-Analytic Review of the Hypoalgesic Effects of Exercise. J. Pain 13, 1139–1150 (2012).
3. Rice, D. et al. Exercise-Induced Hypoalgesia in Pain-Free and Chronic Pain Populations: State of the Art and Future Directions. J. Pain 20, 1249–1266 (2019).