Scientists at Johns Hopkins Medicine using MRI scans and computer modeling say they have further pinpointed areas of the human brain that regulate efforts to deal with fatigue.
The findings, they say, could advance the development
of behavioral and other strategies that increase physical performance in
healthy people, and also illuminate the neural mechanisms that contribute to
fatigue in people with depression, multiple sclerosis and stroke.
Results of the research were published online Aug. 12
in Nature
Communications.
“We know the physiologic processes involved in
fatigue, such as lactic acid build-up in muscles, but we know far less about
how feelings of fatigue are processed in the brain and how our brain decides
how much and what kind of effort to make to overcome fatigue,” says Vikram
Chib, Ph.D., assistant professor of biomedical engineering at the Johns Hopkins
University School of Medicine and research scientist at the Kennedy Krieger
Institute.
Knowing the brain regions that control choices about fatigue-moderating
efforts can help scientists find therapies that precisely alter those choices,
says Chib. “It might not be ideal for your brain to simply power through
fatigue,” says Chib. “It might be more beneficial for the brain to be more
efficient about the signals it’s sending.”
For the study, Chib first developed a novel way to
objectively quantify how people “feel” fatigue, a difficult task because rating
systems can vary from person to person. Physicians often ask their patients to
rate their fatigue on a scale of 1 to 7, but like pain scales, such ratings are
subjective and varied.
To standardize the metric for fatigue, Chib asked 20
study participants to make risk-based decisions about exerting a specific
physical effort. The average age of participants was 24 and ranged from 18 to
34. Nine of the 20 were female.
The 20 participants were asked to grasp and squeeze a
sensor after training them to recognize a scale of effort. For example, zero
was equal to no effort and 50 units of effort were equal to half the
participant’s maximum force. The participants learned to associate units of
effort with how much to squeeze, which helped to standardize the effort level
among individuals.
The participants repeated the grip exercises for 17
blocks for 10 trials each, until they were fatigued, then were offered one of
two choices for making each effort. One was a random (“risky”) choice based on
a coin flip, offering the chance to exert no effort or a predetermined effort
level. The other choice was a predetermined set effort level. By introducing
uncertainty, the researchers were tapping in to how each subject valued their
effort — a way, in effect, of shedding light on how their brains and minds
decided how much effort to make.
Based on whether the participant chose a risky option
versus the predetermined one, the researchers used computerized programs to
measure how participants felt about the prospect of exerting particular amounts
of effort while they were fatigued.
“Unsurprisingly, we found that people tend to be more
risk averse — to avoid — effort,” says Chib. Most of the participants (19 of
20) opted for the risk-free choice of a predetermined effort level. This means
that, when fatigued, participants were less willing to take the chance of
having to exert large amounts of effort.
“The predetermined amount had to get pretty high in
relative effort for participants to choose the coin toss option,” says Chib.
Among a separate group of 10 people trained on the
gripping system but not given numerous, fatiguing trials, there was no
significant tendency toward picking either the risky coin toss or defined
effort.
Chib’s research team also evaluated participants’
brain activity during the gripping exercises using functional magnetic
resonance imaging (fMRI) scans, which track blood flow through the brain and
show which neurons are firing most often.
Chib’s team confirmed previous findings that brain
activity when participants chose between the two options seemed to increase in
all participants in an area of the brain’s known as the insula.
Also using fMRI scans, they took a closer look at the
motor cortex of the brain when the participants were fatigued. This region of
the brain is responsible for exerting the effort itself.
The researchers found that the motor cortex was
deactivated at the time participants “decided” between the two effort choices.
That finding is consistent, Chib says, with previous studies showing that when
people perform repeated fatiguing exertions, motor cortex activity is
decreased, associated with fewer signals being sent down to the muscles.
Participants whose motor cortex activity changed the
least, in response to fatiguing exertion, were the ones who were most risk
averse in their effort choices and were most fatigued. This suggests that fatigue
might arise from a miscalibration between what an individual thinks they are
able to achieve and the actual activity in motor cortex.
Essentially, the body attunes to the motor cortex when
fatigued, because if the brain kept sending more signals to muscles to act,
physiological constraints would begin to take over, for example, increased
lactic acid, contributing to even more fatigue.
These findings, says Chib, may advance the search for
therapies — physical or chemical — that target this pathway in healthy people
to advance performance and in people with conditions that are associated with
fatigue.
Journal article: https://www.nature.com/articles/s41467-020-17855-5
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