How mental fatigue keeps you from running

Training after a work day… a scenario that requires to deal with the “mental fatigue” of the day. In short, the effort is hard while the intensity remains low-moderate. Moreover, we are often not even physically tired. Something going on… in the head?


Mental fatigue, concretely, what is it? It is that feeling of lack of energy after intense and prolonged mental activity, or after a short night’s sleep. The same feeling as in the pre-race when stress is important: you are emptied before you even start.

Mental fatigue is a daily companion. Every day our brains are put through severe challenges to manage the gaps between our spontaneous behaviours and our specific goals. These discrepancies remain insurmountable without the cognitive functions that allow us to adjust. Examples: inhibiting an impulse, anticipating the sequence of events, switching between different rules, reasoning on several pieces of information, remaining focused… These are crucial functions to adapt to vagaries, indeed, but they are also sources of fatigue because they are energy consuming!

Like a basin full of resources, mental fatigue develops when cognitive reserves deplete. The degradation of these resources varies according to (i) the intensity of the mental effort to accomplish the task, (ii) the duration of the effort, and (iii) the person’s mental endurance – which explains why atypical days are particularly arduous… Today, we know that in laboratory conditions with cognitive tests (eg. alertness, memorization), it takes only 30′ to 90′ to induce a state of mental fatigue.


At the behavioural level, mental fatigue leads to an impaired effectiveness in response to a signal: precision decreases, reaction time lengthens, forgetfulness recurs. There is also an increased level of impulsivity: choices turn towards an immediate gain, patience decreases. These observations gradually manifest as the task demands more mental resources than the subject has:

  • In the short term, this imbalance is associated with punctual discrepancies (eg. difficulty in interacting, driving, etc.);
  • In the medium term, it leads to excesses (eg. violent acts, excessive consumption, eating disorders, etc.).

In the field of exercise more precisely, it is established that many aspects of motor skills are automatic (eg. one can run without even thinking about it). Since they are automatic, mental fatigue would have little or no effect on these gestures. But this is not the case for technical tasks with high motor control (eg. scales work). It’s also different for endurance events. Indeed, for the latter, several performance factors depend on the athlete’s ability to use his or her cognitive resources during exercise (below).

Thus, while it has no impact on strength/anaerobic performance (exercise), mental fatigue is systematically harmful AND growing on endurance events.

Example in cycling: at 80% PMA the performance of trained cyclists in time-to-exhaustion tests drops by ~2′ (over a total effort of ~12′) when 90′ of cognitive task precedes the test. We thus estimate the decrease in power by 10-15W. In running, the impact on pacing is also unequivocal: the pace adopted is immediately lower (example here on 5km).


The question of the mechanisms by which mental fatigue operates has long remained a grey area. Today, we know that the drop in performance is not linked to:

  • A peripheral physiological change: heart rate, ventilation frequency, oxygen consumption, neuromuscular function… All remain unchanged!
  • A change in blood glucose levels – while glucose is the brain’s preferred source of energy!
  • Decreased motivation. We keep the same desire, but we don’t get the same result!

In this context, the main obstacle of mental fatigue on performance, it’s in fact the increased effort felt. Specifically, studies that included EEG (electroencephalography) measurements showed that the activity of the brain’s ß-waves was more important in a state of mental fatigue, while the task at hand was the same and performance was no better. These waves characterize intense cognitive activity (eg. concentration). When we now look at the data obtained under MRI: studies carried out on individuals fatigued by a mental effort of ~6h show a progressive deactivation of the medium frontal cortex: this is a region of the brain that is particularly crucial for cognitive control, which also intervenes in motor control!


Thus, the veil begins to rise on the mechanisms. However, this state is still difficult to see coming: we are still spectators of the onset of mental fatigue. For proof, a drop in endurance performance was observed in athletes who did not even declare themselves mentally tired (ie. after a cognitive effort of ~30′). This suggests that a decline in cognitive resources may be sufficient to be counterproductive to exercise, but not enough to qualify as “tired”. In this context, the sensation of vigour could be a useful guide because it seems to precede the impression of mental fatigue.


Last point: the strategic aspect. As we have seen, motivation is not affected by mental fatigue (you can be motivated but not very effective). Despite this, increasing motivation and/or improving mood are useful strategies to counteract the effects of mental fatigue:

  • Positive emotions have a positive effect on the impression of effort. Examples: a photo, a souvenir or just smiling (the brain likes these actions!);
  • The announcement of a rewards reactivates areas of the brain related to the control of effort. Basically, the game is now worth it when it was not before. It can even allow you to achieve higher performance than individuals who are not even mentally tired;
  • The intake of caffeine (~5 mg. kg-1 ~90min before exercise) also makes it possible to achieve higher performances than those of subjects who are not mentally tired (see attached with an exercise at 80% MAP until exhaustion). Be careful though, the systematic consumption of caffeine can reduce your body’s sensitivity and limit its positive effects;
  • Napping, physical exercise (a few abs, pumps, jogging 10′ s), mental imagery, self-talking, make it possible to get reactivated, at least temporarily.
  • Conversely, some factors accelerate the effects of mental fatigue on performance. This is the case with heat and the feeling of disappointment: you become less efficient faster.

All these points show that mental fatigue can also be used as a training strategy. For example, running voluntarily at the end of a stressful day’s work to make the session harder… Fatigue, a travel companion to choose rather than to endure.



Your training application GUTAÏ seeks to consider the arguments posed in this article. Two examples illustrate this. The first is obvious and concrete: your Wellness. Every morning, GUTAÏ invites you to indicate your level of stress and fatigue, your mood… These parameters depend on your mental resources at the time. If you are stressed, chances are it will wear you out during the day and make evening training more difficult. Your Wellness influences GUTAÏ’s calculations to suggest the training that suits you best.

The second is the measure of your RPE that corresponds to the feeling of effort of the session that you note between 1 and 10. It then seems obvious that the more mentally tired you are, the less easy the session will seem. GUTAÏ then discusses this RPE in its analyses.

From a more hidden point of view, know that GUTAÏ has fine analysis calculations based on variations in your heart rate and heart rate recovery data. These data are important because they reflect the proper functioning of your nervous system. However, this nervous activity varies according to your level of fatigue, even if it is “only” mental. A very simple example is the case of burn-out, where mentally exhausted people are diagnosed with developing cardiovascular diseases. Without getting there, the logic of GUTAÏ remains the same: to rely on the data of your head and your body to advise you the best (rest, low intensity, high intensity…).