Research reports often tout the beneficial effects of exercise on mental performance, but can the opposite be true when a cerebral task is performed before
exercise? Yes, says a study published in the March issue of the Journal of Applied Physiology (2009; 106, 857–64). In response to many studies that focused on
fatigue in drivers and airline pilots, the authors investigated the relationship
between mental fatigue and physical performance. The 16 participants rode
a stationary bicycle to fatigue (described as the inability to maintain a cadence
of at least 60 revolutions per minute for more than 5 seconds) after completing two tasks: a 90-minute computer task that required heightened mental concentration and 90 minutes of viewing train and automobile documentaries. The tests were performed on different days. During the exercise bouts, the
researchers measured oxygen consumption, heart rate, cardiac output, blood pressure, ventilation and blood lactate levels. The participants also completed
a survey to gauge perceived effort and level of motivation.

The researchers found that the subjects ceased exercising about 15% earlier when they were mentally fatigued. On each occasion, participants stopped exercising at the same level of perceived effort; however, when mentally fatigued they perceived greater levels of exertion at the start of the session and so halted exercise sooner. Nonfatigued individuals experienced higher heart rate and blood lactate levels, potentially due to the extended exercise session. “Our study provides experimental evidence that mental fatigue limits exercise tolerance in humans through higher perception of effort rather than cardiorespiratory and musculoenergetic mechanisms,” stated the authors. Athletes and exercisers engaging in high-intensity exercise should do so when mentally rested, they added.