Author: Mary Exton
At first glance, it is easy to view pain as a symptom of injury in a specific body tissue. Pain driven by activation of nociceptors in the acute phase of tissue injury and healing more closely fits this model. However, the longer pain persists, adaptive changes occur in the functioning of the nervous and immune systems that increase tissue sensitivity and its response to movement. The International Association of Pain describes this as nociplastic pain.2 Nociplastic adaptations, as well as the negative mood changes frequently comorbid with chronic pain, can act to increase pain during exercise even when the exercise is not causing tissue harm. As science behind the complex relationship between chronic pain and exercise becomes clearer, practitioners will have a better understanding of how to harness the beneficial effects of exercise to reduce chronic pain so that pain is less of a barrier to becoming more active.
The effect of exercise and activity on body systems: Movement, whether general physical activity or specific exercises, influences the regulation of multiple body systems. Important functions that are influenced by movement and that play a role in the pain experience are the excitability of the nervous system, the immune system, and the regulation of psychological mood factors.
Nervous system effects: During an acute bout of exercise, typically there is a release of opioids, serotonin and cannabinoids from the brainstem. This produces the pleasant and pain reducing responses that can be achieved with exercise known as ‘exercise induced hypoalgesia’.4 Release of higher quantities of endogenous opioids with intense exercise may lead athletes to experience a ‘runners high’. The term ‘descending inhibition’ is used to describe the function of releasing opioids from the brainstem. This function serves to inhibit the tone of the pain pathways and provide an analgesic effect. Research has shown that more active individuals generally release higher quantities of opioids with activity,4 and are therefore more likely to report that exercise makes them feel good. Conversely, in the case of a more sedentary lifestyle, the functioning of the descending inhibition centres reduce and an individual will typically produce lower analgesic effects when exercising. In conditions such as fibromyalgia, there is a failure of the inhibitory system to release opioids with exercise. Typically, people with fibromyalgia will not experience much or any analgesic effects with exercise.
Immune system effects
The immune system releases cytokines in response to physical activity. Cytokines are signalling molecules that can produce pro-inflammatory or anti-inflammatory effects. Pro-inflammatory factors increase the excitability of pain pathways leading to increased pain levels. Anti-inflammatory cytokines have an inhibitory effect on pain pathways and the experience of pain. An individual’s general activity levels influences the balance of pro or anti-inflammatory cytokines released by the immune system. In general, more active individuals release more anti-inflammatory cytokines in response to exercise, and a sedentary lifestyle is associated with a greater release of pro-inflammatory cytokines with activity.4 The balance between the release of pro and anti-inflammatory factors is also altered in people with chronic pain compared to people without pain. Those with chronic pain have a sensitised system where the balance is tipped towards the release of pro-inflammatory factors during activity. This increase in pain with activity often leads to those same people becoming less active to avoid a flare-up. The unfortunate result of prolonged rest and inactivity in an attempt to reduce pain is that the immune and nervous systems adjust to lower activity levels and the balance is further tipped towards increased excitability of pain pathways with movement. If chronic pain is confined to a local body region, the person may experience an increase in pain in that local area due to the release of pro-inflammatory factors, but experience exercise induced hypoalgesia when non-painful body parts are exercised. To take advantage of the beneficial effects of exercise in this situation, a person may benefit from higher intensities of exercise of non-painful body regions, but will need to start more gently where it is locally painful. In the case of a widespread pain condition such as fibromyalgia, widespread sensitisation of tissues may result in a more widespread release of pro-inflammatory factors with exercise. A person with fibromyalgia is more likely to experience pain at lower levels of exercise intensity due to both an increased release of pro-inflammatory factors with exercise and a reduction in the ability of the descending inhibitory system to release opioids.
Psychological mood factors: Expecting that something will be painful increases the excitability of pain pathways and the severity of pain experienced with a noxious stimulus. This is known as the nocebo effect. Activating the nocebo effect via increasing expectations of pain can counteract the analgesic effect that is produced by the descending inhibitory centres on exercise. This has implications for someone with chronic pain who aims to become more active but is also anticipating an increase in pain. To reduce the impact of the nocebo effect, it is important to feel safe when exercising and to start gradually. Fear of increased pain may prime the pain systems to be more excitable. Emotional distress such as anxiety and catastrophic thoughts, anger, frustration and depression have been shown to increase the perception of pain. Being physically active may help improve negative mood states such as depression and may improve many of the psychological factors that enhance the pain experience. Negative mood factors may initially act as a barrier to exercise due to their impact on the pain experience. However, with gentle persistence, these factors may gradually improve with adherence to an appropriately graded activity program.
Effects on locally painful tissues: It is important to assess for local movement dysfunction and biomechanical factors that can overload joints and soft tissues. Optimal movement and loading of painful tissue may improve circulation, promote healing and reduce concentrations of local inflammatory factors. This in turn will result in less activation of nociceptors in the tissues, and an improvement of pain with movement. If there is suboptimal loading on joints and soft tissue, local release of inflammatory factors may lead to increasing pain with increased activity dosage. If biomechanical factors and movement dysfunction are not addressed, muscle tightening, spasm and loss of joint range may secondarily increase pain, making it even more difficult to remain active.
Exercise for treatment of chronic pain: Although chronic pain can present a barrier to activity, the beneficial effects of exercise can be harnessed to reduce chronic pain. Maximal efforts are not required. These beneficial effects can be seen on lighter exercise.5 Adhering to a regular program is more important. So, whilst a single bout of exercise can increase chronic pain, persisting with an appropriately designed and graded program can gradually help to reduce the neuroimmune sensitisation seen in chronic pain. Motivation and feeling safe to confidently persist for the longer term are key.