The effect of mental fatigue on time to exhaustion during different intensities of cycling exercise

Purpose. It has been demonstrated that mental fatigue (MF) impairs exercise tolerance. Althoughsome authors have hypothesised that the higher the exercise intensity the lower the impact of MF, itis unclear whether physiological determinants of endurance performance at different intensitydomains are modulated by the ergolytic effects of MF. Thus, we evaluated the effect of MF on timeto exhaustion (TTE) during cycling at heavy- and severe-intensity exercise. Methods. Twenty-threehighly trained endurance subjects (26 ± 5 yr; 71 ± 6 kg) performed an incremental cycling test toexhaustion immediately followed by a 2-minute all-out test, enabling the detection of the CriticalPower (CP). Each subject underwent four testing sessions on different days, involving either 45-minof intervention followed by constant work rate cycling exercise to exhausthion at heavy (90% CP;CWRHVY) or severe (110% CP; CRWSEV) intensity. During interventions, documentary watching(Control) was used as a control task and a continuous choice reaction task (Simon) was used to induceMF. The time-course of reaction time (tr) and error-rate (ER) during SIMON was monitored.Subjective rating of MF, assessed by VAS scale (VAS-M), and mental workload (TLX), evaluatedby NASA TLX questionnaire, were collected before exercise. Cardio-respiratory and metabolicresponses to exercise were monitored. Results. Maximal oxygen uptake (V̇O2max) and maximal workratewere 53.5 ± 8.0 ml∙min-1∙kg-1 and 368 ± 65 W, respectively. CP was 279 ± 58 w. CWRHVY andCWRSEV time to exhaustion were significantly lower after Simon (CWRHVY: 1766±907s; CWRSEV:438±270s) compared to Control (CWRHVY: 2004±903s; CWRSEV 393±248s; both p<0.05), whereasphysiological responses to exercise were not different between conditions. No significant differencesin performance loss due to MF between intensity domains were observed (CWRHVY 10.7%; CWRSEV8.2%, p>0.05). VAS-M score and TLX were significantly higher after Simon than Control (VAS-M:Control 8.3±4.4 vs Simon 13.2±4.3, p<0.01; TLX: Control 23.3±22.9 vs Simon 74.5±15.4 p<0.01).During Simon, tr did not change over time (p>0.05) but ER increased significantly (p<0.01).Conclusions. Completion of 45 min cognitive task induced significantly higher ratings of MF andmental workload, decreasing exercise tolerance to a similar extent in both heavy and severe domains.Thus, performance impairments induced by mental fatigue do not seem to be exercise-intensitydependent. Our results suggest avoiding tasks that may induce MF before activities and competitionsirrespectively of their intensity.