Feeling mentally exhausted? Maybe you should put off that run

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The end-of-the-day workout isn’t the easiest thing in the world to do, which is why many personal trainers recommend exercising in the morning.

They may be on to something — a new study from researchers at Bangor University in Wales suggests that being mentally fatigued may affect endurance exercise performance.

In the randomized crossover study (participants served as both the intervention group and the control), 16 people (10 men and six women) prepared for both trials by drinking the same amount of water, sleeping for at least seven hours, abstaining from alcohol, not doing any vigorous exercise the day before the test, and eating the same breakfast before cycling. In the intervention round, participants were given a challenging 90-minute cognitive test to induce mental fatigue. During the control round, men and women watched two videos: ‘World Class Trains—The Venice Simplon Orient Express’ and ‘The History of the Ferrari — The Definitive Story,’ chosen because watching them, apparently, encourages neutral moods and stable heart rates (we’re thinking more along the lines of boredom, but that’s just us).

In both trials the participants then rode a stationary bike to the point of exhaustion, while researchers measured oxygen consumption, heart rate, cardiac output, blood lactate levels and blood pressure. Participants were asked to rate their levels of motivation and perceived exertion. As an added incentive the men and women were also offered money for the best performances on the exercise and cognitive tests.

All that brain work evidently took a toll. While mentally fatigued, cyclists stopped exercising an average 15% earlier. The average RPMs during cycling weren’t much different in the two trials, but the time to exhaustion was significantly lower after the mental challenge compared to the control. While there were physiological changes during the trials, mental fatigue didn’t greatly affect any of them. And while rates of perceived exertion increased in both trials, they were much higher in the post-cognitive test trial than in the control.

The authors argue that the study not only has implications for endurance athletes but also for the military and people with chronic fatigue syndrome. The study appears in the March issue of the Journal of Applied Physiology.

-- Jeannine Stein