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Whether Exercise Feels Easy or Hard May Be Up to Your Dopamine Levels

By Jiri Kaloc

Dopamine, a brain chemical usually linked with joy, desire, and reward, also seems to be crucial in shaping how we perceive physical effort. This insight came from a study involving Parkinson’s disease patients, conducted at Johns Hopkins University. Parkinson’s is ideal for studying dopamine because it’s characterized by a gradual loss of dopamine-producing brain cells.

Perceived effort determines if you’ll exercise again

We know that people can perceive their effort during physical activity differently and this perceived effort makes has an impact on how likely are people to do an activity again. Previous studies have shown that people with increased dopamine are more willing to exert physical effort for rewards, but the current study focuses on dopamine’s role without the promise of a reward.

Testing grip strength with and without dopamine

For this new study, 19 adults (10 men, 9 women) with Parkinson’s disease and an average age of 67 were recruited. They were asked to perform a grip strength test on 2 different days.

  • On the control day, the patients took their daily synthetic dopamine medication before the grip test.
  • On the test day, they were asked not to take their medication for at least 12 hours prior to the grip test.

Both of these days consisted of two phases. In the first phase, participants were trained to associate between grip force exerted on the handheld dynamometer and effort levels from 0 to 100. The display would light up green when they hit the target force but remained red until they did. In the second phase, they were shown a target amount of force without knowing how much it was. They were asked to squeeze this much. Only the green light showed them they hit the force required. And then they were asked to estimated how much effort they exerted to hit this unknown amount of force.

Cycling training
Dopamine seems to be crucial in shaping how we perceive physical effort. © Profimedia

Lack of dopamine makes efforts feel harder

The results of the study showed that when the participants took their synthetic dopamine medication, their self-assessments of effort exerted were more accurate than when they didn’t take the drug. They also had less variability in their efforts, they were more accurate in their squeezes at different levels of effort. In contrast, when the patients didn’t take the dopamine medication, they consistently over-reported their efforts. They felt the task was physically harder than the same task with dopamine. They also had significantly more variability among grips.

“Researchers have long been trying to understand why some people find physical effort easier than others. This study’s results suggest that the amount of dopamine availability in the brain is a key factor,” said study leader Vikram Chib, Ph.D.

Dopamine makes you more willing to take a risk if physical activity is involved

The researchers also did two more experiments in this study to find out the impact of dopamine on risk-taking. In the first of these two, participants were given a choice between either squeezing at a low effort or flipping a coin to decide whether they won’t have to squeeze at all or squeeze at high intensity. When participants took their dopamine medication, they chose the risky coin flip more often. In the second of the two experiments, participants were given the choice between getting a small amount of guaranteed money or, with the flip of a coin, getting either nothing or a higher amount of money. Here, the dopamine didn’t make a difference in their willingness to take the risky option. These results suggest that dopamine’s influence on risk-taking is specific to physical effort-based decision-making.

Overall, the findings of the study tell us that dopamine is a critical factor in helping people accurately assess how much effort a physical task requires. This can influence how likely they are to exercise. It can also help explain the pervasive fatigue in conditions such as depression, long COVID, and during cancer treatments.