FOR IMMEDIATE RELEASE
February 24, 2009
Contact: Donna Krupa
Office: (301) 634-7253
commoff@the-aps.org
Mental Fatigue Can Affect
Physical Endurance
Findings could become research model to help those with chronic fatigue
 BETHESDA, Md. (Feb. 24, 2009)
− 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, “Mental
fatigue impairs physical performance in humans.” The study will appear in
the March print edition of the Journal of Applied Physiology. The
American Physiological Society published the study.
The study
The 16 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 participants got the same amount of sleep, drank the
same amount and had the same meal before each of the sessions.
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.
Throughout both exercise sessions, the researchers
tracked a variety of physiological measures, such as oxygen consumption,
heart rate, cardiac output, blood pressure, ventilation, and blood lactate
levels. The participants completed surveys to measure their motivation and
perceived effort. The researchers offered monetary prizes for the best
performance on the exercise and mental tasks as a way to keep motivation
high.
Results
-
The participants stopped exercising 15% earlier, on
average, when they were mentally fatigued.
-
The participants stopped at the same perceived effort
level in both the fatigued and non-fatigued trial. However, mentally
fatigued participants started at a higher level of perceived effort and
reached the endpoint sooner.
-
The cardio-respiratory and musculo-energetic measurements
did not vary between the two trials when compared at specific points in
time. However, because the non-fatigued trials went longer, heart rate
and blood lactate levels were higher at the end of those trials.
-
Motivation was the same in both trials and was not a
factor.
The researchers speculate that the perception of effort
occurs in the brain. Dr. Marcora said his team is considering two
possibilities:
-
mental fatigue lowers the brain’s inhibition against
quitting, or
-
mentally fatigue affects dopamine, a brain chemical that
plays a role in motivation and effort
One interesting note is that demanding mental tasks
activate the anterior cingulate cortex of the brain. Previous research has
shown that rats with a lesion in the anterior cingulate cortex would not
work as hard for a reward compared to rats with no lesion. This area of the
brain may be where perception of effort originates, Dr. Marcora said.
Applying the results
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.
The research model may also be helpful for military
personnel. They do physically demanding tasks after long period of
vigilance. Vigilance produces mental fatigue.
Finally, the study suggests that people doing high
intensity training, such as competitive athletes, should do their training
while mentally rested. However, people who exercise after work should
continue doing so, even if mentally fatigued. Most people work out at a
moderate intensity, which still gives plenty of physiological and
psychological benefit, including relief from stress and improved mental
performance.
NOTE TO EDITORS: To interview Dr. Marcora,
please contact Donna Krupa at (301) 634-7253 or at
commoff@the-aps.org.
Funding: School of Sport, Health and Exercise
Science, Bangor University, Wales, U.K.
***
Physiology
is the study of how molecules, cells, tissues and organs function to create
health or disease. The American Physiological Society (www.The-APS.org/press)
has been an integral part of this discovery process since it was established
in 1887.
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