Why do some people find physical effort easier than others? Researchers found an answer for that – Dopamine! Dopamine, a brain chemical long associated with pleasure, motivation, and reward-seeking, also appears to play an important role in why exercise and other physical efforts feel ‘easy’ to some people and exhausting to others, according to results of a study of people with Parkinson’s disease. A progressive loss of dopamine-producing brain cells is an indicator of Parkinson’s disease.
The results, which were published in NPG Parkinson’s Disease, may eventually lead to new treatments for fatigue linked to depression and many other conditions, improved methods of assisting individuals in starting and maintaining exercise regimens, and an increased understanding of Parkinson’s disease.
The amount of dopamine available in the brain is a key factor in physical efforts
“Researchers have long been trying to understand why some people find physical effort easier than others,” says study leader Vikram Chib, Ph.D., associate professor in the Department of Biomedical Engineering at the Johns Hopkins University School of Medicine and research scientist at the Kennedy Krieger Institute. “This study’s results suggest that the amount of dopamine availability in the brain is a key factor.”
According to Chib, people’s perceptions, and self-reports of the effort they put in after engaging in physical exercise differ, which influences their choices for additional exertion. The present study focuses on dopamine’s function in people’s self-assessment of effort needed for a physical task, without the promise of a reward. Previous research has demonstrated that persons with greater dopamine are more likely to perform physical effort for rewards.
Chib and his colleagues from the Kennedy Krieger Institute and Johns Hopkins Medicine enrolled 19 adults with Parkinson’s disease for the study. Parkinson’s disease is characterized by a progressive loss of dopamine-producing neurons in the brain, which results in uncontrollable movements like tremors, exhaustion, stiffness, and problems with balance and coordination.
Assessing the association between dopamine and physical efforts
Squeezing a hand grip fitted with a sensor was the identical physical exercise that ten male and nine female participants, with an average age of 67, were instructed to complete at Chib’s lab on two separate days, four weeks apart. On one of the days, the patients were instructed to take their regular daily dose of synthetic dopamine as prescribed. On the other hand, they were instructed to wait at least 12 hours before doing the squeeze test after stopping their medication.
The patients were instructed to press a grip sensor at different degrees of defined effort on both days. After that, they were requested to squeeze and record the number of units of effort they exerted.
Due to low level of dopamine, the perceived task becomes physically harder
The participants’ judgments of their own units of effort expended were more accurate when they had taken their usual synthetic dopamine medication than when they hadn’t. Additionally, when the researchers cued them to squeeze at various effort levels, they demonstrated accurate squeezes with reduced variability in their efforts.
By contrast, the patients who had not taken the medicine repeatedly overreported their efforts, indicating that they thought the activity was physically tougher, and their grip variability increased dramatically after being cued.
Dopamine’s influence on risk-taking preferences is specific to physical effort-based decision-making
In a different experiment, the patients had to flip a coin and take a risk of having to exert either no effort at all or a very high degree of effort, or they could flip the coin and be guaranteed to be able to squeeze on the grip sensor with a reasonably modest amount of effort. When the participants took their medication, they were more open to the possibility of needing to exert more effort than when they did not.
A third experiment gave participants the option of receiving a fixed amount of money or receiving a larger or lower amount based on the toss of a coin. The respondents did not differ in the results between the days they took and did not take their medicine. According to the researchers, this finding implies that dopamine’s impact on risk-taking inclinations is unique to decision-making based on physical exertion.
Dopamine’s role in understanding the chemistry and biology of motivation
All these results, according to Chib, point to the importance of dopamine levels in assisting individuals inappropriately estimating the level of effort required for a physical task, an assessment that can have a big impact on the amount of effort an individual is willing to put in for subsequent tasks. For instance, someone may be less motivated to perform a physical task if they believe it would require a significant amount of effort.
More knowledge about the chemistry and biology of motivation may lead to improvements in the way that physical therapy and exercise regimes are motivated, according to Chib. Furthermore, widespread weariness during cancer therapies and in situations like depression and prolonged COVID could be explained by inadequate dopamine signaling. He is currently researching the role of dopamine in clinical weariness with his colleagues.
Materials provided by Johns Hopkins Medicine. Note: Content may be edited for style and length.
Purnima Padmanabhan, Agostina Casamento-Moran, Aram Kim, Anthony J. Gonzalez, Alexander Pantelyat, Ryan T. Roemmich, Vikram S. Chib. Dopamine facilitates the translation of physical exertion into assessments of effort. npj Parkinson’s Disease, 2023; 9 (1) DOI: 10.1038/s41531-023-00490-4
Johns Hopkins Medicine. “Whether physical exertion feels ‘easy’ or ‘hard’ may be due to dopamine levels, study suggests.” ScienceDaily. ScienceDaily, 12 April 2023. <www.sciencedaily.com/releases/2023/04/230412131051.htm>.
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