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Mental Fatigue Can Make Exercise Harder

Study Shows People Feel Exhausted More Quickly After a Taxing Mental Task
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By Caroline Wilbert
WebMD Health News
Reviewed by Louise Chang, MD

Feb. 27, 2009 -- If you have a hard time getting through your exercise routine after a stressful day at the office, you are not alone.

A new study shows that people feel exhausted sooner during exercise if they have performed a mentally taxing task beforehand. While the mental fatigue doesn't affect cardiovascular capacity, it has an impact on the exerciser's perceived level of exhaustion.

For the study, 16 participants rode stationary bicycles under two different conditions. In one scenario, they rode the bicycles after participating in a challenging 90-minute mental task presented on a computer screen.

In the other scenario, they rode the bicycles after watching a 90-minute documentary that was not mentally fatiguing. Both times, participants rode the bicycles until exhaustion, defined as the point when they could not maintain pedaling of at least 60 revolutions per minute for more than five seconds.

The group included 10 healthy men and six healthy women. All participants were between the ages of 18 and 44 and already doing regular aerobic training.

Throughout the exercise sessions, researchers tracked cardiovascular response to exercise by assessing oxygen consumption, heart rate, cardiac output, blood pressure, ventilation, and blood lactate levels. Participants answered questions about perceived effort and motivation.

Participants who performed the mental task reported greater fatigue afterward compared to those who had watched the documentary. The participants stopped exercising an average of 15% sooner after doing the mental task (640 seconds) compared to watching the documentary (754 seconds).

The study did not show an impact of mental fatigue on cardiovascular response to intense exercise except for a higher heart rate, which was seen in participants who had watched the documentary. The researchers write that this is "most likely because of the longer exercise duration" that was seen for this group.

Researchers from Bangor University in Wales completed the study, which is published in the March issue of the Journal of Applied Physiology.

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