When participants performed a mentally fatiguing task prior to a difficult exercise test, they reached exhaustion more quickly than when they did the same exercise when mentally rested, a new study finds.
The study also found that mental fatigue did not cause the heart or muscles to perform any differently. Instead, our “perceived effort” determines when we reach exhaustion. The researchers said the next step is to look at the brain to find out exactly why people with mental fatigue perceive exercise to be more difficult.
Samuele M. Marcora, Walter Staiano and Victoria Manning of Bangor University, Wales, the United Kingdom, did the study, which appears in the March print edition of the Journal of Applied Physiology.
The 16 study participants rode a stationary bicycle to exhaustion under two conditions: Once when they were mentally fatigued and once when they were mentally rested. The trials took place in the laboratory on different days.
The mental fatigue sessions began with a challenging 90-minute mental task that required close attention, memory, quick reaction, and an ability to inhibit a response. After undergoing this session, participants reported being tired and lacking energy. The control session consisted of watching neutral documentaries for 90 minutes and was not mentally fatiguing.
After each of the 90-minute sessions – mentally fatiguing or non-fatiguing – the participants did an intense bout of exercise on a stationary bicycle. They rode until exhaustion, defined as the point when they could not maintain a cadence of at least 60 revolutions per minute for more than five seconds.
The participants stopped exercising 15 percent earlier, on average, when they were mentally fatigued. The researchers speculate that the perception of effort occurs in the brain. Marcora, lead study author, says the study findings indicate mental fatigue may affect dopamine, a brain chemical that plays a role in motivation and effort.
This research could provide a way to study chronic fatigue syndrome, Dr. Marcora said. People with chronic fatigue report they lack energy and experience “brain fog,” just like the mentally fatigued participants in this study. In addition, as in this study, people with chronic fatigue perceive exercise to be more difficult despite physiological responses considered normal during exercise.
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