Pain, Perception, and Physical Activity

Summary of article

• Pain can be understood as “an opinion on the organism’s state of health rather than a mere reflexive response to an injury”.

• Nociceptors are nerves that detect mechanical, chemical, and thermal changes which may cause tissue injury.

• Nociceptive signals can be up and down regulated as they travel towards the brain.

• The brain interprets nociceptive information in the context of other inputs and decides what to pay attention to and what to discard.

• Being physically active can sometimes be painful, but this is not typically an indication the activity is causing tissue damage.

• Physical activity is generally protective against the development of pain and can also be used to treat multiple painful conditions.

Article

Pain can be weird. This weirdness is perfectly encapsulated in the famous case report of a construction worker who stepped onto a 15cm nail (1). The worker was rushed to hospital in excruciating pain, the nail travelling clean through his boot. As he was preparing to have the nail removed, the unfortunate man was provided with large doses of analgesic medication to control his symptoms. However, when the nail had been extracted and the boot removed, it was quickly determined that the nail had passed between his toes. Apart from perhaps a bruised ego, the man had sustained no injury at all (1).

Demonstrating the weirdness of pain in the opposing direction is the story of a man who was in a car accident in 1963 (2). Surviving a head on collision with truck, the man managed to recover and move on with his life. However, in 1999 he was alerted to the fact there may some remnants of medical equipment in his arm when he set off metal detectors in a courthouse. Fast forward to 2014 an x-ray investigation of a now painful and swollen arm revealed his car’s turn signal had been embedded in him for the past half-century. Seemingly the only reason the man was now experiencing issues was that the 51-year-old metal rod was now beginning to corrode (2).

These dramatic stories clearly demonstrate that pain does not behave simply as a 1:1 readout of tissue damage. Instead, pain can better be understood as “an opinion on the organism’s state of health rather than a mere reflexive response to an injury” (3). This is not to say that pain is “all in your head”, or that if you are experiencing chronic pain that you are somehow deficient. Instead, it is a recognition of the complexity of pain, so that we may better understand and manage it.

Pain Neurophysiology (Over)Simplified

While there is no such thing as a “pain receptor”, what we do have are danger receptors called nociceptors. Nociceptors detect mechanical, chemical, and thermal changes which may cause tissue injury (4). When changes in these variables exceed certain thresholds, the nociceptors may be stimulated and begin firing off signals. While there are different types of nociceptors, each with different thresholds for stimulation and different rates of signal conduction, this foundational principle is common across them (4). For example, touching a hot stove will stimulate peripheral thermal nociceptors in your fingers, which will then send signals to your spinal cord and brain.

The sensitivity of nociceptors appears to be modifiable. Specifically, in the context of inflammation some otherwise unresponsive nociceptors will become sensitive to mechanical force (5). For example: a non-inflamed shoulder tendon is exposed to tensile and compressive loads and tolerates this easily. The same tendon, after becoming inflamed, will now be painful when exposed to those exact same forces.

Nociceptors that are functioning correctly are stimulated when changes are detected in their target tissues. When a nociceptor is damaged, however, it can start generating signals spontaneously. This is seen in lumbar radicular pain (sciatica), where inflammation of a nerve root as it exits the spinal canal can result in painful symptoms being felt in the leg and foot (6).

At the spinal cord nociceptors pass-off their signals to other nerves, which then propagate the signal up towards the brain. This pass-off represents another location in which signals can be up-regulated, down-regulated, or otherwise scrambled. For example, where nociceptive signals from one area converge on nerves from another area, a person might experience referred pain (6). Classically, someone with neck pain due to an inflamed and sensitive cervical joint may also experience headache and/or shoulder pain.

Repeated stimulation of nerves in the spinal cord by nociceptive signals can cause these nerves to become hypersensitive (7). In this scenario the same degree of mechanical, thermal, or chemical change in the tissue can result in greater amounts of pain.

Also affecting signal propagation in the spinal cord are descending pathways from the brain. These can down-regulate signals in the spinal cord when they are stimulated by endorphins (like those released during exercise or sex) or medications (such as morphine) (7).

Arriving at the brain, nociceptive information is then forwarded to numerous different regions including the:

• Somatosensory cortex

• Insular cortex

• Prefrontal cortex

• Anterior cingulate cortex

• Thalamus

• Periaqueductal grey

• Cerebellum

Each of these regions play different roles and synthesise the nociceptive information in the context of other inputs (8). For example, the insular, anterior cingulate, and prefrontal cortices modulate pain in the context of mood, motivation, and cognition (8). At all times, your brain is trying to determine what information to pay attend to, and what to discard, based on these different factors. The result of this modulation is that the pain perception can be vastly different between people who are exposed to the exact same nociceptive input.

It is worth noting again that this does not mean “pain is just in your head”. What it does mean, however, is that to fully understand and address pain we must consider all the electrochemical differences in our brains which can turn our perception of pain up or down.

How Physical Activity Can Cause Pain

When functioning desirably, nociceptors are activated before a tissue injures (9). For example, a person completing a heavy barbell squat may feel a pang of pain in their knee, which causes them to shift weight back to their hips as they complete the repetition. If this avoids the tissues of the knee being injured, then nociception has done its job.

Where tissues are exposed to forces that they cannot withstand or are unable to recover from the loads that are applied, they can injure (10). As described in the previous section, the change in mechanical forces and inflammatory milieu of a tissue can cause nociception and pain. As tissue healing occurs (which can be assisted by appropriate loading, even in the context of some discomfort), the nociceptive input and pain reduces (11).

For some conditions the tissue changes resulting in nociception may not resolve. For example, a person with knee osteoarthritis is not expected to recover the articular cartilage and surface characteristics of a non-arthritic knee. A person with ankylosing spondylitis (an inflammatory condition of the spine) is not expected to cure themselves of this condition. In these conditions, being physically active can stimulate nociceptors and cause short-term pain, but this is not an indication that the activity is causing harm (12,13).

Finally, in people with nervous system sensitisation resulting in overactive nociception, or upregulation of signals at the spinal cord or brain, physical activity can be acutely painful. This pain is a reason that may people with this type of pain become less active. However, as with other conditions this pain is not an indication of tissue damage (14).

How Physical Activity Can Address Pain

Being physically active is generally protective against the development of pain and can help to address pain when it does occur.

People who are physically active are 11-16% less likely to develop chronic low back pain, and 10-38% less likely to develop any form of chronic pain (15,16). On average, people with knee osteoarthritis, chronic low back pain, and patellar tendinopathy can reduce their pain by 1.2/10, 1.5/10 and 3.5/10 respectively, when engaged with an exercise intervention (13,17,18). While these pain reductions may not sound large, they are equal to or better than pain reductions from strong opioid medications (19). Additionally, while the negative health effects of opioid misuse can include addiction, overdose, cognitive impairments, and death, being physically active is beneficial for overall health (19).

There are many proposed mechanisms by which physical activity can reduce pain including:

• Improved neuromuscular function (increased strength, control, or tissue length)

• Improved local tissue factors (increased circulation and reduced inflammation)

• Changing neuro-endocrine-immune factors (downregulation of nociceptive signals)

• More robust psychological status (greater self-efficacy and coping) (20)

However, research to determine which mechanisms, and to what extent, are responsible for mediating improvements in pain for different conditions. As this area of research matures, interventions for painful conditions may be better tailored to achieve the desired outcomes.

In Summary

Pain does not have a 1:1 relationship with tissue damage. Instead, pain may be better though of as “an opinion on the organism’s state of health” (3). While peripheral nociception does inform this opinion, it is modulated by multitudes of other factors (7,8).

While being physically active can sometimes cause increased reports of pain in the short term, over the long term being physically active is one of the best ways to manage pain.

References

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2. As It Happens. (2015, February 13). We talk to man who found 1963 Thunderbird turn signal embedded in arm 51 years after car crash. CBC Radio. https://www.cbc.ca/radio/asithappens/tuesday-keystone-veto-belgian-right-to-die-turn-signal-in-arm-and-more-1.2940953/we-talk-to-man-who-found-1963-thunderbird-turn-signal-embedded-in-arm-51-years-after-car-crash-1.2956523

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