Effect of mental fatigue on half-marathon performance
Participants reported that the mentally fatiguing task was mentally demanding and effortful, and it increased the feelings of mental fatigue significantly more than the control treatment (reading magazines). Therefore, the experimental treatment used in this study was successful in inducing a state of mental fatigue.
The traditional NHST analysis showed that mental fatigue did not have a significant effect on half-marathon performance. This null result is contrary to the results of most previous laboratory/indoor-based studies showing significant effects of mental fatigue on shorter-term endurance performance [
2,
4,
5,
7]. One possible explanation for this null result may be the difference in fitness level between our participants and those recruited in earlier studies. Indeed, the negative effect of mental fatigue on short-term endurance performance seems to be larger in less fit subjects [
12]. However, in the present study, participants’ fitness level was similar or even slightly lower compared to previous experiments, hence this factor is very unlikely to explain our null result. Another potential reason might be the duration of our test of endurance performance (a real half-marathon) which is considerably longer than the time trials and time to exhaustion tests used in previous studies. However, it is generally assumed that psychological factors like mental fatigue become more, not less, influential as the duration of the endurance event increases [
41,
42]. Therefore, differences in testing duration do not seem a plausible explanation for the discrepancy between our null result and the results of most previous studies. A further aspect that could be considered is the duration of the cognitive task (50 min) used as experimental treatment which perhaps was not long enough to induce a state of mental fatigue. However, this speculation is in contrast with the results of our manipulation checks and with the findings of a recent meta-analysis which revealed that cognitive tasks of shorter as well as longer duration can induce mental fatigue [
43]. A more plausible explanation for the discrepancy between our null result and the results of most previous studies is the difference in motivational climate. To the best of our knowledge, our study is the first experiment investigating the effects of mental fatigue during a real mass-start endurance competition in which participants competed against other participants. This head-to-head competition increases motivation which, in turn, may have reduced the negative effect of mental fatigue on endurance performance compared to previous studies in which participants were tested alone [
44,
45].
In our opinion, the most likely explanation for the null results of our between-subject, posttest-only experimental study is the lack of statistical power compared to previous experimental studies that had a more powerful within-subject design or a pretest [
43]. Participants in the mental fatigue group completed the race, on average, four minutes slower than the participants in the control group with an effect size of 0.333 (Cohen’s
d). This effect size is similar to the effect sizes found in previous within-subject, lab-based experiments in which the effect of mental fatigue on endurance performance was statistically significant [
5,
7]. Furthermore, the effect size in our pragmatic trial is larger than the significant pooled effect size (Hedge's
g = 0.26) found for the effect of prior cognitive exertion on aerobic performance in a recent meta-analysis of lab-based studies [
43]. However, because of the between-group, posttest-only experimental design of our pragmatic trial, a post hoc power analysis estimated that a sample size of 286 runners (143 per group) would be necessary to demonstrate statistical significance for a Cohen’s
d = 0.333, alpha = 5% and beta = 20%. It is clear that conducting experiments of such sample size during real endurance competitions is extremely difficult. Due to the limited funding available and the logistical barriers to recruitment and testing, the maximum number of participants we were able to recruit over three consecutive years was 48. After accounting for the two dropouts during the race in the mental fatigue group, post hoc power analysis gave us a statistical power of 19.7% which is very low. Therefore, it is not surprising that the effect of mental fatigue on half-marathon performance is not significant according to NHST. However, finding a
p value larger than 0.05 or a 95% confidence interval including zero does not necessarily mean that there is ‘no difference’, ‘no effect’ or ‘no association’ [
38].
To prevent the common misinterpretation of
p values larger than the alpha level as support for the absence of a true effect [
39,
40], we also implemented the TOST equivalence testing procedure [
39,
40]. This novel statistical approach has been proposed as a more valid alternative to the magnitude based inference approach, previously used in sport performance research and recently abandoned by statisticians and scientific journals [
36,
37]. The TOST equivalence testing procedure rejected the hypothesis that mental fatigue has a beneficial effect on half-marathon performance in amateur runners but failed to reject the hypothesis that it has either a trivial or a harmful effect (Fig.
4). In other words, the results of the present study demonstrate that mental fatigue can either have no meaningful effect or a detrimental effect on half-marathon performance in amateur runners.
Potential mechanisms
Notwithstanding the limitations with regards to NHST analysis and lack of adequate power in our pragmatic trial, it is worth briefly discussing the effects of mental fatigue on other variables as they may provide some hints to the mechanisms underlying the potential negative effect of mental fatigue on long-term endurance performance.
Very small effect sizes and no statistically significant differences were found for success motivation, intrinsic motivation, and performance expectations. Therefore, it is unlikely that the potential negative effect of mental fatigue on half-marathon performance in amateur runners was mediated by a large reduction in motivation or a nocebo effect caused by the experimental treatment [
46].
In agreement with previous studies [
47‐
49], the present experiment showed that running speed significantly decreased throughout the half-marathon in both groups (i.e. positive pacing pattern, which consists in a fast start followed by a progressive reduction in running speed). Even though no statistically significant differences were found between the two groups, running speed in the mental fatigue group was on average 3% slower than in the control group. Heart rate values were also 3% lower throughout the entire race in the mental fatigue group compared to the control group. Moreover, RPE and the ratio between RPE and running speed during the race were similar (at 7 km) or higher (at 14 and 21 km) in the mental fatigue group compared to the control group. The directions of these between-group differences, although not statistically significant, are consistent with the significant differences observed in more powerful within-subject studies showing that mental fatigue [
8] and the fatigue that accumulates during multi-day races [
50] are associated with an increase in RPE for a given speed/power or HR. According to the psychobiological model of self-paced endurance performance [
51,
52], such increase in perception of effort would lead to the conscious decision to reduce the running speed during the half-marathon race to avoid premature exhaustion with the consequent reduction in HR and the increase in half-marathon time. The directions of the changes in running speed, RPE, HR and half-marathon time observed in the present study are consistent with such theoretical prediction and the results of previous studies on the effects of mental fatigue on self-paced endurance performance tests in the laboratory or on an indoor track [
3‐
5].
Practical applications and directions for future research
In conclusion, because of low statistical power, the data collected do not provide reliable evidence that mental fatigue reduces long-term endurance performance. However, the effect size of the present study is in line with previous studies on shorter-term endurance performance [
8]. Furthermore, the TOST procedure suggests that mental fatigue can either have no meaningful effect or a detrimental effect on half-marathon performance in amateur runners. Given that avoiding mentally fatiguing tasks before a race is feasible and without negative side effects, it seems prudent to recommend that endurance athletes avoid or reduce engagement in tasks that may induce mental fatigue (e.g. dealing with transport and logistics, engaging with social media, ruminating, or inhibiting emotions) before any race, including those lasting more than 45 min. Because sleep deprivation exacerbates mental fatigue [
53], endurance athletes should also implement strategies to improve their sleep before an important race [
54].
Additional studies with much larger sample size or two races (baseline and follow-up) will be required to draw a firmer conclusion on the hypothesis that mental fatigue has a negative effect on endurance events longer than 45 min, and to provide more precise estimates of the effects of mental fatigue on endurance performance during outdoor mass-start competitions. Given some evidence that elite endurance athletes may be more resistant to mental fatigue than amateurs [
12], future studies should include participants of higher competitive level. Further research is also required to confirm our hypothesis that the main mediator of the negative effect of mental fatigue on long-term endurance performance is primarily the increase in perception of effort associated with mental fatigue.