Endurance performance: Paying the price of mental fatigue

More than 2,000 years ago, Siddhartha Gautama, the Indian philosopher and the founder of Buddhism proclaimed that “The mind is everything. What you think, you become.” Since then, the connection between mind and body has well understood across the ages, but its significance has only recently been appreciated in the context of sport performance. Yes, it’s true that sports psychology has long been a component of fine tuning an athlete’s expectations and mental approach for a race event. However, the impact of the brain on actual physical performance has only become a serious topic of research during the past couple of decades.

Brain and performance

In a number of previous SPB articles, we have explored evidence clearly demonstrating that the brain is the master controller of the fatigue your muscles experience when performing exhaustive exercise. In short, the fatigue you experience during exercise isn’t just a biochemical phenomenon occurring in muscles and blood – it’s also a neurological phenomenon dependent on the brain and your thoughts/perceptions. This explains why certain types of music can enhance performance and lower perceived effort levels^(1,2), and even how different expectations of an exercise session can impact how much effort can be sustained⁽³⁾.

Mental fatigue and performance

Given the powerful link connecting brain to exercise performance, it would be incredible if loading up the brain with mental and psychological demands before exercise didn’t affect performance, and this indeed is what we find. In a previous SPB article, Andrew Sheaff highlighted a recent review of all the research investigating whether performing resistance training while mentally fatigued would result in worse performance compared to performing exactly the same resistance training without prior mental fatigue⁽⁴⁾. As Andrew explains, the results were very clear: the inclusion of mentally fatiguing activities prior to intense resistance exercise significantly reduced the number of repetitions subjects were able to achieve during a set. Indeed, the presence of mental fatigue, even in the absence of physical fatigue, negatively impacted performance.

There’s also plenty of recent research the impact of mental demands during exercise itself – so-called ‘dual tasking’. The research on this topic is fairly unambiguous: adding extraneous cognitive loading (ie mental demands) during a motor task (ie exercise) will typically lead to worse performance, as compared to normal or single-task conditions⁽⁵⁾. Good examples of this include:

·        Worse performance in soccer players who try to solve arithmetic problems while juggling a ball⁽⁶⁾.

·        Poorer time trial performance in cyclists asked to monitor multiple forms of feedback during their effort⁽⁷⁾.

·        Reduced accuracy, balance and smoothness when performing exercise requiring good motor coordination^(8-10).

Prior mental fatigue and endurance

In the examples above, we cited research showing that ‘dual tasking’ during exercise can negatively affect endurance performance and also that prior mental fatigue has a detrimental effect on resistance training and carrying our strength tasks. But can prior mental fatigue negatively affect endurance performance? In other words, should you expect to turn in poorer times for a training session or evening event after for example a demanding day in the office or after a long and stressful commute?

Before we answer that question, let’s try and define what we mean by ‘mental fatigue’, which is a fairly broad term. Technically, mental fatigue has been defined as the psychobiological state (ie a state brought about by both psychological and biological processes) induced by a sustained cognitive effort⁽¹¹⁾. This mental fatigue manifests itself by subjective feelings of tiredness and lack of energy, alterations in brain function and impaired cognitive function (ie reduced ability to mentally process information)⁽¹²⁾.

Evidence from swimming

Despite plenty of anecdotal evidence from coaches and athletes about the negative effects of prior mental fatigue on their endurance performance, researchers have only recently started to investigate this phenomenon using carefully designed studies. However, the experimental evidence accumulated over the past 15 years most definitely suggests that prior mental fatigue negatively affects endurance performance.

For example, a study with young swimmers observed a reduction in 1500m freestyle performance following a 30-min Stroop task (a demanding mental task where participants have to continually distinguish between the actual color of the ink in which a word describing a color is written in and the meaning of the word) in 12 of 16 athletes⁽¹³⁾. The pacing analysis showed that the participants were slower when they had become mentally fatigue beforehand in every 300m split (ie at 300, 600, 900, 1,200, and 1,500m).

In another study, international-level swimmers used social media for 30 minutes to induce mental fatigue before performing 50m, 100m and 200m swimming time trials in the pool⁽¹⁴⁾. In the 50m time trials, prior mental fatigue induced no effect – perhaps to be expected for a race when concentration in only needed for less than 30 seconds. On the other hand, in the 100 and 200m time trials, inducing prior mental fatigue resulted in a significant performance drop.

More research on the effects of prior mental fatigue on swimming performance comes from amateur triathletes⁽¹⁵⁾. On three separate occasions and in a random order, seven recreational triathletes  watched a documentary, utilized a smartphone, or performed mental tasks where continual responses were required on a keyboard, then performed a 15-minute warm-up followed by 6 × 200m at constant pre-set speed plus one 200m at maximal effort. The results of this study showed no negative effect of prior mental fatigue, which is contrary to other studies. However, this study had limitations in that the sample size was small (just seven athletes), and there was no control condition (ie just doing nothing before the swim). Also, this study used recreational athletes, whereas most of the previous studies have used either professional/elite or very well trained amateurs.

New research

To try and clarify the impact of prior mental fatigue on endurance performance, we can turn to a new study by a team of Italian researchers that has been investigating the effects of mental fatigue on perception of effort and performance in national level swimmers, details of which have just been published in the prestigious journal ‘Frontiers in Psychology’⁽¹⁶⁾. In this study, the researchers set out to examine the effects of prior mental fatigue on endurance performance, which was measured with a 400-meter front-crawl test. In addition, they sought to discover if and how prior mental fatigue impacted on measures of blood lactate, heart rate and perceived exertion while the swimmers performed an extended steady-state swim consisting of 12 sets of 100m performed at the swimmers’ normal lactate threshold.

What they did

Ten swimmers (six men and four women) were recruited to participate in the study and all completed it. All the swimmers were ranked in the top ten nationally in their respective events, with six having competed internationally – ie of elite status. The first step was a familiarization trial where all the swimmers attended the lab and acquainted themselves with the ‘AX-CPT’ challenge, which was used to induce mental fatigue in the later experimental trials. In this test, the swimmers were asked to sit in front of a computer screen in a silent room where a sequence of variable random letters, one at a time, was presented them. Using the mouse, they had to respond but only when they observed a 2-letter sequence of the letter ‘A’ followed by the letter ‘X’. This ‘A-X’ sequence occurred in 7 out of 10 instances, with the other 30% being random combinations to throw the swimmers off course!

The second part of the familiarization session consisted of a set of 7 × 200m swimming repeats, each separated by five minutes. These repeats started off gently but gradually increased in intensity until they became maximal effort. By taking blood samples in between each 200m swim, the researchers were able to individually determine the speed of each swimmer’s lactate threshold (the swimming speed at which blood lactate begins to rapidly accumulate), which would be used in the next stage of the experiment.

The experiment

In the next stage of the study, all the swimmers attended on two separate occasions in a fully rested state and performed a 600m warm up in the pool followed by a set of 12 x 100m repeats at their pre-determined lactate threshold. This was then followed by a 400m front-crawl performance test, which the swimmers were asked to complete in the fastest time they could. In all respects, these two trials were identical except the period before the trial (the order of trials was chosen at random):

·        Mental fatigue condition – in this trial, the swimmers completed the AX-Continuous Performance Task for 90 minutes beforehand to induce mental fatigue.

·        Control condition - in this trial, the swimmers simply rested for 90 minutes beforehand.

Before and after the AX test, mood scores were taken (to determine mental fatigue). Heart rate, lactate and perceived exertion (RPE) were measured during the 12 x 100m segment and again after the 400m flat-out time trial, where times were also recorded. A graphic showing the overall protocol for the experiment is shown in figure 1.

Figure 1: Experimental protocol