What is Fatigue?

Dr Ollie Stoten, ultra-marathon runner, Polar explorer and genuinely all round top bloke takes a closer look at what fatigue actually is and highlights just how important being honest about our rest days is. If you aren't too tired...read on!

Fatigued? Photo courtesy of Freedom Racing

Fatigue is controversial. Fatigue is multi-faceted and difficult to define. Fatigue affects all of us at some point. Fatigue is a perception of tiredness, and should be separated from sleepiness. Feeling tired at night is part of your circadian rhythm, your sleep wake cycle, and can be alleviated by periods of sleep. You can be tired (sleepy) without being fatigued, and you can be very fatigued without being sleepy.

Fatigue is a normal part of every day life, a result of work, stress, overstimulating or under stimulation. Fatigue is also a non-specific symptom of disease or pathology in a medical context, which we won’t dwell on here.

Fatigue is a normal part of training, pushing the body to adapt, however there is a continuum. Normal training-induced fatigue takes 1-2 days to overcome; functional overreaching taking up to a couple of weeks to fully overcome, perhaps from an intense training camp, part of a planned program to improve performance longer term; then non-functional overreaching pushing into overtraining where excessive fatigue especially over long periods causes no long term advantage and can take months to overcome.

Fatigue is often split into physical or mental fatigue. Physically we can think about a lack of substrates to fuel muscles, whether it be ATP, glycogen or creatine. 

Mental fatigue is a cross over between a psychological and biological state caused by prolonged periods of demanding cognitive activity. It has a negative effect on endurance performance, with greater perceived exertion, often without effecting physical variables such as heart rate, and causes earlier cessation of activities. Think about a time when you’ve had a hard day at work, nothing too physical, but you’ve had to make lots of hard decisions under time pressure, concentrating all day and things just out of your control have been going wrong. It can all build up to make you feel knackered at the end of the day, even though you’ve done minimal activity. 

Although it seems simple enough to divide fatigue between physical and mental, they are inextricably linked. Locomotion is powered by the brain through nerves to muscles in one complicated and beautifully brilliant system. I find it much more useful to consider overall stress and resultant fatigue from everyday life, work and training. Must of us are spinning several plates and we can’t ignore our lives outside of sport. Even pros have life stresses outside of training. TrainingPeaks can give us very useful insights into our training but remember life stress and fatigue is very difficult to objectify and account for, but is crucial to consider.

Adrenaline and caffeine can reduce the perception of effort or fatigue in the short term, but it will catch up with you if you don’t take the time for proper restorative physical and mental rest. Don’t forget that stress prolongs recovery, so a proper rest day doesn’t mean no exercise substituted for a very stressful day at work.

Dr Ollie Stoten