Why Exercise Feels Good for Some and Miserable for Others, According to CU Anschutz and FIU Researchers

Every February, the same thing happens at the gym: the crowds thin out as New Year's resolutions lose their shine. The numbers behind that familiar pattern are stark. Nationwide, only about 30% of people meet the federal guidelines for physical activity. In Colorado, a state with a reputation for being outdoorsy, the figure is just 35%.

That gap is puzzling because the evidence that exercise helps mental health is clear, even if studies disagree on how large the effect is. If movement gets the endorphins flowing and lifts mood, why do so many people struggle to keep at it? Researchers at the University of Colorado Anschutz Medical Campus and Florida International University offer several answers.

Katrina Oselinsky, a post-doctoral researcher at the Health and Wellness Center on the Anschutz campus, says the popular idea that exercise produces endorphins that raise mood is not wrong, but it oversimplifies how brain chemistry and mental states interact. The same person can experience the same workout differently from one day to the next, she said, depending on mood, what else is happening in their life and where they are exercising. "The same workout can feel energizing for one person and uncomfortable for another," she said.

Emily Hemendinger, an assistant professor of psychiatry at CU Anschutz, points to the runner's high. Some people feel it right away during intense exercise, but not everyone does, and those who don't can end up disappointed. Brains also differ in how sensitive they are to rewards, which primes some people to enjoy exercise more than others, though past experiences with exercise matter too. Exercise does release chemicals that give most people at least some pleasure, Hemendinger said, but not enough to drown out discomfort with one's body, fear of being judged, perfectionism, boredom, physical pain or exhaustion. "It's not like doing drugs, and what that lights up in our brain right away," she said.

Marcelo Bigliassi, an assistant professor at Florida International University, offers a historical view. For most of human history, exercise was not a separate activity. People moved because they needed food or because they were doing something rewarding, like dancing. Now people must find a reason to exercise, and it isn't always obvious. The brain releases dopamine, which creates the desire to repeat exercise, but it can also get "really creative" at finding ways to avoid discomfort. His advice is to find something challenging enough to bring a sense of accomplishment but not so hard that people quit. He does not want people to pick something they dislike, train at high intensity, injure themselves and feel like failures.

Bigliassi said the data show exercise improves mental health and cognitive abilities such as problem-solving over time. But many people quit long before those benefits arrive, because they set themselves up to fail or aren't prepared for bumps along the way. "Consistency is much more important than everything else," he said.

Oselinsky said people who believe they can accomplish their goal report more enjoyment than those who feel defeated at the outset. Support from people around you improves the experience, and new exercisers tend to feel better about it as their skill grows. For those who don't like exercise, her suggestion is to pick an activity they enjoy or pair exercise with something pleasant: time outside, a friend or pet, music or podcasts. "If you're trying to force yourself to stick with something you don't like, it's going to be harder," she said.

A gym isn't required, she added. Movement through the day can be as good as a dedicated session, whether that is physical labor at work, carrying toddlers who don't feel like walking, or tending a backyard garden in spring. Federal guidelines recommend 150 minutes of moderate activity or 75 minutes of vigorous activity weekly, plus muscle-strengthening exercise on two days. Hemendinger said any movement someone enjoys is a step in the right direction, from jumping on a trampoline to playing with a dog. "Joyful movement's hard to find, but it's important," she said.