Fatigue,  Health & wellbeing,  Strength & conditioning

Tired….

Dealing with training fatigue

Regardless of work setting, an exercise professional primarily focuses on stimulating physical or physiological adaptations through the cunning manipulation of exercise stressors.  Achieving positive adaptations involves the somewhat vague challenge of trying to balance the complex interaction between work and recovery. Central to that task is fatigue – we are seeking to produce fatigue in order to provoke adaptations, but we are also wanting to avoid fatigue that might interfere with subsequent training, or fatigue that might persist and threaten health or performance. Given that there are so many elements (e.g. age, gender, training status, training potential, and current fatigue status) that can influence both work and recovery, it’s hardly a surprise that our training adjustments are neither precise nor that well supported scientifically. It seems that most exercise professionals are resigned to accepting fairly generic exercise prescriptions when highly individualised strategies are really what’s required. I am intimating here that even when following an individual-specific programme, there will be a need for ongoing monitoring, and adjustments will be inevitable. 

Fatigue

Fatigue is the bane of the sportsperson (in fact most people!). It’s insidious, it’s vague, it’s poorly defined, it’s nearly impossible to measure or detect with any sort of accuracy (more to come on that), and athletes seemingly run blindly into its arms!  An underperforming athlete might be fatigued but they are just as likely to convince themselves that they simply haven’t done enough work and aren’t fit enough. Anyone who has spent time around athletes and sport knows that it is never that simple – ‘more’ is very rarely the correct answer! 

Cairns et al (2005) captured the essence of fatigue by describing it simply as reduced force production or power output from specified muscles over time.  In their terms, it’s a temporary inability to develop or maintain an expected force or power level, and in functional and sporting settings that is usually going to mean impaired performance. When we experience fatigue through engaging in everyday activities we have the choice of moderating or adapting the activity – using different muscle groups – or taking a rest before continuing. With those adjustments, we are able to continue an activity albeit with reduced power output and/or speed (Bishop, 2012). For competitive sport, those choices are less appealing and less acceptable.

Trying to understanding fatigue

Like many aspects of human performance, fatigue is confusing (to me anyway).  We suspect it has something to do with metabolism because energy use and recovery have to be important. We know it has something to do with the contractile apparatus because the ability to recruit and contract muscle fibres has a direct influence on our force producing capabilities. But fatigue also has a powerful psycho-social element. We know for example that it’s possible to feel fatigued at rest in the absence of any metabolic or muscular stress – it’s a very complex phenomenon. We routinely harness fatigue during workouts as we attempt to stimulate adaptations by progressively overloading an energy system or a muscle group through the use of acute fatigue. We know how to provoke fatigue through tweaking intensity, duration, and recovery, and we often measure and monitor those things with apparent precision –  yet when it comes to recovery we seem to trust that the ‘time off’ until the next workout will be sufficient (based on some science) to restore things to normal for that individual.  That’s not intended to be critical – it’s merely an observation on how we operate. 

Most of us are familiar with the long list of processes that are involved between thinking about an action and having a muscle exert force; processes that include motor control, physiological and biochemical responses. Experts have considered the origin of fatigue as being dependent on which side of the neuromuscular junction processes might be affected and causing the observed functional impairment. “Peripheral fatigue” is usually used to describe a drop in force production due to processes failing in and around the muscle, whereas “central fatigue” is held accountable when the central nervous system is unable to activate sufficient muscle (Caroll et al, 2017).  Peripherally the alleged culprits are factors related to diminishing energy availability, metabolite accumulation or failure of the contractile apparatus. In terms of central nervous system failure, a poorer motor command from the motor cortex can mean diminished recruitment of muscle fibres which in turn means less force, less accuracy, lowered stabilising activity from accessory muscles, and impaired proprioception (Clarke et al, 2015). Depending on the nature of the fatiguing activity, recovery time frames are variable and provide no clearer answers on fatigue mechanisms. And so the research continues.


Monitoring fatigue

One of the obstacles to understanding and dealing with fatigue seems to be how we measure or assess it. Research studies tend to gravitate toward measuring the decline in muscle force, usually with something like maximal voluntary contractions. Unfortunately, these measures don’t have a lot of relevance to how fatigue affects sporting performances. Most sport doesn’t involve a lot of maximal voluntary contractions of isolated muscle groups! A lot of the related research seeks to identify a valid and reliable biomarker of fatigue (more to come on that also), work that has been equally frustrating. Many of these methods make the assumption that fatigue is a phenomenon with a threshold. It seems more likely that fatigue is a continuous (and highly individualised) variable rather than a specific ‘failure point’ that can be crossed or avoided (Cairns et al, 2005).  In jumping back into this fatigue research literature there were a couple of things that piqued my interest, so I’ll share here…..


Crossover fatigue

We often think of fatigue as specific to the muscle group(s) exercised but that’s not strictly true. The concept of crossover fatigue has been described by several researchers (e.g. Halperin et al, 2015). Crossover fatigue or as the boffins call it ‘non-local muscle fatigue’ (NLMF) is when a temporary decline in performance is observed in a non-involved muscle group. So, the affected group could be the other limb, another muscle group on the same limb, or in a different region of the body altogether. In a review of this phenomena, Halperin et al (2015) noted that studies have demonstrated that isometric and cyclical contractions have a greater NLMF effect than the dynamic contractions (the ones we normally use in resistance training). Bilateral exercises had a larger NLMF effect than unilateral exercises, and one of the most interesting observations (for me) was that lower body muscle groups seemed to be affected more by NLMF than the upper body. What this means is that a high-intensity workout like arm cranking (upper body – cyclical) would then impair something like squatting ability – but cycling intervals would have little effect on arm curls. There is also a hint in the research that males may be more susceptible to NLMF. Without getting into the suggested mechanisms, NLMF could have implications for how we structure programmes and order our exercises, and how we condition for certain sports.  


Low-frequency fatigue

Low-frequency fatigue (LFF – sometimes called long lasting fatigue) is a state that has been associated with muscle damage following training.  This type of fatigue is linked to failure in muscle excitation-contraction coupling.  Recovery of force in LFF is reputed to be slow and the effects can persist in the absence of any other signs. According to Twist & Highton (2013), LFF is significant for athletes from sports that involve a lot of tissue damage and prolonged muscle soreness. So impact sports or sports that include heavy loads and an emphasis on stretch-shortening cycle (SSC) or eccentric dominant movements. They note that perceived fatigue and soreness might alter an athlete’s sense of effort and cause them to down-regulate their training effort (Twist & Highton, 2013). Unfortunately, LFF can only be assessed by stimulating a motor nerve or muscle and will be typified by a proportionately greater loss of force at low stimulation frequencies.  The absence of a handy test is a real pity as it would be interesting to monitor for LFF across a training group and training period.

The link between strength and conditioning and injury in sport always stimulates my curiosity. While the PR surrounding modern strength and conditioning promotes its injury prevention benefits, the rising injury rates tend to suggest otherwise. Given the high training loads carried by many athletes, I’ve often wondered whether some athletes carry a constant background level of fatigue (e.g. LFF) that exposes them to injury and affects performance. I don’t have any evidence for this – only the observation that athletes seem to train too hard and too much, and rest too little. Fatigue is known to modify proprioception (Clarke et al, 2015) and anything that represents an impairment of neuromuscular control has potential to increase injury susceptibility.I’m wondering if LFF reduces the ability to sense and react to loads in a timely fashion, and this is leading to many of the injuries that we are now seeing?   

Closing

In spite of the gallant efforts of researchers and exercise professionals biomarkers of training fatigue remain woefully inadequate (I will explore workload monitoring in a blog soon.). The measures tend to be dissociated from performance outcomes, they lack immediacy, and most importantly they lack the necessary specificity and sensitivity to enable decisions to be made about when to scale back training load.  In the meantime, we seem to continue training more and harder. Fowles et al (2006) suggested that there may be some promise in performance tasks that could assess the effect of LFF on individuals’ work capabilities using tests that require low to moderate muscle activation.  However, twelve years on and those tests haven’t eventuated. Perhaps by exercise professionals acknowledging that fatigue can undo a lot of good when they write and deliver training programmes, we will be one step towards better vigilance around fatigue. Better awareness should translate into healthier and better performing athletes.


Selected References

  1. Bishop, D.J. (2012) Fatigue during intermittent-sprint exercise Proceedings of the Australian Physiological Society 43: 9-15
  2. Cairns, S.P., et al (2005) Evaluation of models used to study neuromuscular fatigue. Exerc. Sport Sci. Rev. 33 (1); 9–16.
  3. Carroll T.J. et al (2017) Recovery of central and peripheral neuromuscular fatigue after exercise. J Appl Physiol 122: 1068 –1076.
  4. Clarke, N. et al (2015) Direct and indirect measurement of neuromuscular fatigue in Canadian football players. Appl. Physiol. Nutr. Metab. 40; 464–473
  5. Fowles, J.R. (2006) Technical Issues in Quantifying Low-Frequency Fatigue in Athletes International Journal of Sports Physiology and Performance 2:97-99.
  6. Halperin, I. et al  (2015) Non‐local muscle fatigue: effects and possible mechanisms Eur J Appl Physiol 115:2031–2048
  7. Twist, C. & Highton, J. (2013) Monitoring Fatigue and Recovery in Rugby League Players International Journal of Sports Physiology and Performance 8;467-474

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