Pain and fatigue models
What is pain? And what is fatigue? The short answer is that both pain and fatigue are experiences. While physiological phenomena like muscle damage or a wound can exist separate from the nervous system, pain and fatigue cannot. They arise from our nervous system and are an experience within our mind. Since pain and fatigue are important elements of trail and ultramarathon races, this section will look at the psychological models of pain and fatigue to motivate the later sections, which will detail strategies to manage both pain and fatigue.
Let’s start off with the biopsychosocial model of pain. Traditionally, people think of pain as simply alerting us to physiological damage. This can be true to an extent, as we all know that you usually experience pain when you suffer a physical injury. However, the context of that injury matters. The relationship between pain and physical damage isn’t perfectly linear (and we will discuss this more in the returning from injury section). An extreme example of this is the phantom pain experienced by amputees; they can experience excruciating pain in a limb that is no longer there.
Perhaps a more relatable example for ultramarathon runners is the personal story of the first time I lost a toenail in a 100km race. I was completely unaware that a hotspot was developing on that foot until I could see the finish line, at which point my toe was suddenly throbbing, and I could barely put any weight on it (even though the blister must have been forming for hours prior). As soon as I crossed the finish line, I had to rip my shoes and socks off!
Why had I not felt any pain up until that point? It’s the context of the race; the psychological and social expectations were that I would be in pain, and at that point, the neurological signals that the blister was forming, and the toenail was coming off, were insufficient to consciously register as pain.
Anyone who has experienced a chronic running injury will also be aware of how the pain can fluctuate day by day. Factors like daily stress are a big factor – a niggle tends to feel a lot worse after a stressful day at work! This is known as allostatic load, which takes into consideration all forms of stress. That might be work stress, stress coming from family, or it might be simply stressing out about the injury. All these factors contribute to the psychological state and the experience of pain. Thus, when looking at pain we should consider the “bio” (what physiologically is going on?), the “pyscho” (what is the athlete’s psychological state and expectations?), and the “social” (what is the social context?) factors.
Similar models exist for fatigue in endurance activities (i.e., those lasting more than a couple of minutes). Two of the most common models you may have heard of previously are Tim Noakes’s central governor model (Noakes, 2012) and Samuele Marcora’s psychobiological model (Marcora, Staiano, & Manning, 2009). There are some key distinctions between the central governor model and the psychobiological model.
In the central governor model, the physiological factors that we associate with fatigue are sub-consciously used to calculate what is the maximum sustainable output while avoiding catastrophic failure (by comparing resources required to the physiological resources available). It’s hypothesised that this is achieved by limiting the neurological recruitment of muscle fibres. Hence, during a maximum effort there will be a roughly linear increase in perception of effort over time due to these anticipatory calculations, with some potential adjustments along the way. Time to exhaustion and perception of effort will be based on earlier subconscious decisions made.
On the other hand, the psychobiological model has two key principles. The first core principle is that endurance performance depends on perception of effort and potential motivation – that is the psychological part. The second principle – the biological part – is that the mind emerges from lower-level neurobiological processes. Therefore, biological aspects are determining endurance performance at the psychological level through these underlying neurobiological processes.
In the psychobiological model, time to exhaustion will be limited by conscious decisions made at the end of exercise based on perception of effort. In this model, there is no subconscious central governor, as a conscious perception of effort is constantly being generated. The progressive increase in perception of effort is reflecting progressive increases in neural signals being processed by the brain and possible changes in the way that the brain is processing those signals. It is then motivation that caps performance, as an athlete will stop when their perception of effort reaches their level of motivation.
What is the evidence for these different models? There are many papers written on these topics, so we’re just going to look at some of the key evidence. One of the main pieces of evidence given for the central governor model is the common phenomena of a finishing kick in races. This seems impossible in a peripheral fatigue model (whereby fatigue is caused purely due to physiological shut down), but in the central governor model the finishing kick is possible due to the recruitment of extra muscle motor units as the end of exercise is approaching. The central governor is allowing that increase in output as it determines that there are sufficient physiological resources available. However there have also been findings that mental fatigue can impair physical performance, which makes most sense in the psychobiological model.
In a 2017 systematic review it was found that endurance performance was impaired by mental fatigue despite physiological variables traditionally associated with the endurance performance such as heart rate, blood lactate levels, oxygen uptake, cardiac output, and maximal aerobic capacity, all being unaffected. Importantly this was different to anaerobic work such as maximal strength and power, which were not shown to generally be affected by mental fatigue (Cutsem, et al., 2017), and thus fatigue in shorter duration efforts may be limited by different factors.
Further evidence that our experience of fatigue is of psychological origin comes from nerve block studies, in which an epidural blocks afferent feedback related to peripheral muscle fatigue. If our experience of fatigue was dependent on feedback of muscular fatigue, then endurance performance should be improved by blocking this information. However, in a 5km cycling time-trial, Amann et al. (2008) found the opposite.
In the central governor model we can train physiological capacities to improve and through doing this training we will also better train the central governor to regulate effort. We may be able to better train the central governor with sessions that involve pushing hard when an athlete is fatigued, e.g., a surprise extra rep at the end of session, or a fast-finish at the end of a long run. However, the psychobiological model offers the potential to use various mental strategies to improve performance, as any strategy that affects perception of effort or the level of motivation will be able to influence performance. Based on this, in later sections we’re going to look at various strategies that can help an athlete perform during training and races.
Key takeaways
Pain and fatigue are psychological experiences
The biopsychosocial model of pain suggests that pain is not just caused by physical damage, but also influenced by psychological factors and the social environment
The central governor model of fatigue suggests that fatigue arrives from neural circuitry monitoring physical feedback and this circuitry acts to prevent catastrophic physical failure
The psychobiological model suggests that physical performance depends on how perception effort compares to the athlete’s level of motivation in a given moment
Amann, M., Proctor, L., Sebranek, J., Eldridge, M., Pegelow, D., & Dempsey, J. (2008). Somatosensory feedback from the limbs exerts inhibitory influences on central neural drive during whole body endurance exercise.. Journal of applied physiology, 105 6, 1714-24 . https://doi.org/10.1152/japplphysiol.90456.2008.
Cutsem, J., Marcora, S., Pauw, K., Bailey, S., Meeusen, R., & Roelands, B. (2017). The Effects of Mental Fatigue on Physical Performance: A Systematic Review. Sports Medicine, 47, 1569-1588. https://doi.org/10.1007/s40279-016-0672-0.
Marcora, S., Staiano, W., & Manning, V. (2009). Mental fatigue impairs physical performance in humans.. Journal of applied physiology, 106 3, 857-64 . https://doi.org/10.1152/japplphysiol.91324.2008.
Noakes, T. (2012). Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis. Frontiers in Physiology, 3. https://doi.org/10.3389/fphys.2012.00082.