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For exercise professionals, fatigue is an intriguing phenomenon that remains poorly understood. Fatigue has been defined in so many ways that consistent explanations and interpretations are rare. For example, exhaustion (the inability to continue exercising at any intensity) is often used interchangeably with fatigue (a temporary inability to sustain a specific exercise intensity; Phillips, 2005). From training and performance perspectives, those are two different problems. Cairns et al (2005) argue that fatigue likely sits somewhere along a continuum. So, there is probably no single identifiable critical failure point, at least not one that remains constant within or across individuals. Bishop (2012) describes fatigue as a reversible, exercise-induced, reduction in power output or speed of movement speed. Even though an individual might be able to continue exercising when they’re fatigued, they’ll be incapable of producing their maximum speed or power – so their ‘performance’ is effectively impaired. Will that individual sense it? Will it be noticeable in a given performance? Given that fatigue is notoriously difficult to measure, those are intriguing questions. My guess is that many individuals are continuing to train and compete through imperceptible (to others) fatigue. I’m also guessing that the effect of fatigue may not be particularly noticeable for the individual, because they may not remember any different. It all becomes part of the accepted conditioning process, and likely their athletic peers – their athletic network – dwell in a similarly fatigued state!
Fatigue has both central and peripheral determinants. Peripheral determinants include the availability of fuels (metabolic substrates), ionic disturbances (e.g. ions such as Ca2 and K that are integral to muscle activation), and accumulation of metabolites (metabolic end products such as lactate). Essentially everything distal to the neuromuscular junction (where nerves that activate specific muscles connect to muscle) is classified as a peripheral factor capable of impairing muscle activity. Central factors include the brain and its various connections via the nervous system to the motor neurons. Regardless of the peripheral determinants, if there is inadequate motor command from the central nervous to stimulate muscle contractions, then performance will deteriorate. Similarly, in the face of peripheral fatigue, motor commands from the central nervous system would need to be ‘ramped up’ in order to continue – so more effort would be required to sustain that exercise. Clearly, there are multiple sites and mechanisms that can interact, and that can and do influence and induce exercise-related fatigue.
Fatigue may have a biophysical basis, but it is also (obviously) highly influenced by psychosocial elements. Everyone who has exercised knows that while there are physiological and biochemical rationales for fatigue, motivation and determination also affect how fatigue manifests itself and ultimately how it influences performance. When we exercise, we intuitively gauge our effort. We consciously sense the strain or intensity of a physical task. Fatigue or the motivation to persist when feeling fatigued is also influenced by social factors. These can diminish or strengthen motivation, through upward social comparison (trying to keep up with peers) or social loafing (coasting within a group setting). Psychobiological models of fatigue acknowledge that an individual’s perception of effort will interact with motivational factors to determine the exercise that an individual is willing to tolerate (Staiano et al, 2018). I can certainly relate to that – it may be a long time ago, but I can recall how much discomfort I was prepared to tolerate at rugby training (very little) versus what I was prepared to put up with in a game (bring it on)! Yet games were undoubtedly more physically demanding (in those days) than practices!
Task failure will occur if a participant decides to terminate their exercise, or if the compensatory adjustments made by the central nervous system still fail to match the power demands of the task. In the latter case even though a participant might want to continue exercising, they will face a basic power failure. The relative contribution of factors will depend on the individual, the type of exercise being undertaken, and the conditions under which that exercise is taking place. For example, different exercise intensities, durations, and contraction types will shape fatigue. Put simply, people fatigue at different rates, have different thresholds of acceptance, likely have different thresholds for when performance deteriorates, and will also recover at different rates (Gathercole et al, 2015). Individuals will have a certain pain tolerance and motivation to exercise and whether there is any ‘reserve’ may depend on this, along with factors such as health, age, and familiarity with ‘hard’ exercise. I know that’s not particularly helpful, but it’s important to recognise that level of variability when it comes time to consider the monitoring of fatigue. After all, as exercise professionals, some form of fatigue monitoring is a constant part of our roles.
Balancing the work that we want clients to complete, with their individual fatigue profiles is always a ‘trick’, and individual capacities and tolerances for high-intensity efforts may be something that we tend to overestimate. Burnley & Jones (2018) remind us that our capacity to sustain high power outputs is astonishingly small. They claim that almost 70% of the power-generating capacity of the neuromuscular system is unsustainable and that task failure at high power results within minutes. So, we really don’t have a lot to play with there! Exercise professionals are perhaps deceived by the fact that lower power outputs, masquerading as full training efforts, can be continued for hours. I’m wondering whether we have a bit of a blind spot when it comes to training and fatigue. There’s that grey area where an individual may be fatigued but can continue to train or exercise at a level. We don’t really know, but I think we prefer to assume that training under those conditions is not only effective but may also be necessary. As professionals we need to constantly think about exercise objectives and whether the intensity or the volume of an activity should be taking precedence. Fatigue over form may be a perilous pathway? Maybe less, done really well is a better exercise objective!
Part of the problem is likely the methods that we use to detect the presence of fatigue (not to be confused with workload monitoring – that’s our next topic). We tend to use strength measures or maximal efforts to identify fatigue, but as Cairns et al (2005) suggest, these are poor fatigue markers. Force measures tend to underestimate functional impairment. These authors recommend measuring performance in dynamic exercises and measuring power outputs. For example, most team sports involve repeated sprint efforts so a single sprint effort may not really mean very much. Instead of looking at an individual’s maximum squat, we could have them complete multiple vertical jumps or a 10 second Wingate type test on a cycle ergometer. They might be able to ‘get up’ for their maximum back squat or reproduce their best vertical jump, but in a fatigued state, they won’t be able to sustain a high work output! Their first rebound might be perfect, but that 15th rebound might be a different story!
While strength declines rapidly and progressively during sustained maximal efforts, rapid but partial recovery normally occurs pretty quickly. Full recovery of maximal strength is much slower but it usually does happen some hours after the cessation of exercise (Carroll et al, 2017). Low-frequency fatigue (LFF) is potentially more problematic as it results in longer lasting effects on low-frequency force-generating capacity, the type of efforts that are vital to many sports performances. LFF is multifactorial fatigue usually resulting from high-intensity exercise that involves moderate-to high-forces, and repetitive eccentric or stretch-shortening cycle activities (Fowles, 2006). Those types of exercise have been associated with structural damage and impaired muscle activation. LFF is challenging because it is only detectable through low-frequency muscle stimulation (something not readily available outside lab settings) and while it may not impair a single maximal effort, it will affect the ability to sustain high power outputs. LFF is insidious and I’m guessing a lot of highly trained athletes are continually in a LFF state. I wonder what that might mean in terms of their injury exposure. Those concerns extend to the fitness industry where extreme fatigue is often promoted as the pathway to success.
As part of being a competent exercise professional, I think we need to reflect more often on what we are trying to achieve with training or exercise and recognise that individual responses and situations will call for different exercise prescriptions and exercise adjustments. Some believe that fatigue is a failsafe protective mechanism that helps prevent someone from exercising to the point of metabolic catastrophe (Burnley & Jones, 2018). Others see fatigue as an inconvenience, something to be outsmarted and leveraged for greater training gains. I’m not convinced we are that smart!
- Bishop, D.J. (2012) Fatigue during intermittent-sprint exercise Proceedings of the Australian Physiological Society 43: 9–15
- Burnley, M., Jones, A.M. (2018) Power–duration relationship: Physiology, fatigue, and the limits of human performance, European Journal of Sport Science, 18:1, 1– 12.
- Cairns, S.P., Knicker, A.J. , Thompson, M.W., et al (2005) Evaluation of models used to study neuromuscular fatigue. Exerc. Sport Sci. Rev. 33 (1); 9–16.
- Carroll T.J., Taylor, J.L., Gandevia, S.C. (2017) Recovery of central and peripheral neuromuscular fatigue after exercise. J Appl Physiol 122: 1068 –1076.
- Fowles, J.R. (2006) Technical Issues in Quantifying Low-Frequency Fatigue in AthletesInternational Journal of Sports Physiology and Performance,;2:97–99.
- Gathercole, R., Sporer, B., Stellingwerff, B. et al (2015) Alternative Countermovement-Jump Analysis to Quantify Acute Neuromuscular Fatigue. International Journal of Sports Physiology and Performance 10; 84–92
- Hill, C.R. (2019) The Köhler Effect: A Motivational Strategy for Strength and Conditioning. Strength and Conditioning Journal PAP
- Phillips, S. (2005) Fatigue in Sport and Exercise, Routledge, London
- Staiano, W., Bosio, A., De Morree, H.M. et al (2018) The cardinal exercise stopper:Muscle fatigue, muscle pain or perception of effort? Progress in Brain Research 240;175 – 200.