Every new year, the resolutions fly. This is the year to get in shape, exercise daily, run a marathon. And then the motivation fades.

Why does it seem so consistently, and annoyingly, harder for some of us?

“Neurochemicals in your brain,” said Vikram Chib, assistant professor of biomedical engineering in the Johns Hopkins University School of Medicine. “They influence your willingness to exert.”

His new research suggests some people naturally have more of these chemicals, specifically dopamine, which is central to a person’s mood and sense of motivation and reward. Those with higher levels may remain more likely to stick with a fitness routine because the exertion feels like less of a burden.

Chib says understanding this may help people set more reasonable fitness goals for themselves.

For the research, Chib looked at MRI studies to map the area of the brain responsible for the feeling of exertion in people with Parkinson’s disease, which causes loss of dopamine-producing cells in the brain. His findings, published online April 1 in NPG Parkinson’s Disease, showed those with low levels of dopamine often overestimated their exertion during tasks such as squeezing a grip, compared to those who received dopamine supplements.

Healthy people have a range of dopamine levels, and Chib, who is also a research scientist at the Kennedy Krieger Institute, said he will continue to study how to overcome the motivational drop by “reframing the task.” That may mean committing to a two-day-a-week fitness routine only, or breaking workouts into parts that are less overwhelming.

He suggested “experimenting on yourself, and be mindful of how you feel during an activity.”

Other fitness experts agree that adopting routines that are doable, for someone’s fitness and schedule, is a good idea for consistency and safety.

Josh Levinson, founder and CEO of Charm City Run, said many people gravitated to walking and running during the coronavirus pandemic because there was a will to get out of the house and gyms were closed. Plus, he said people saw that those most sickened were those who were less healthy, and that made them want to improve their fitness.

But he knows many people, who flood his stores this time of year with ambitious resolutions, don’t stick with those routines. Those that did often have some things in common. They went on to invest in a gym membership or a training program and made themselves accountable to a group or coach.

“The motivation comes from that accountability,” he said. “You don’t want to let others down.”

Plus, it’s social, he said. You can meet or be with like-minded people with any fitness level.

Of course, there is a risk in doing too much too fast, said Dr. Matthew Sedgley, a MedStar Health sports medicine primary care physician. He sees plenty of overuse injuries stemming from people’s plans to jump into a big new fitness routine.

But after years of treating athletes, and being a runner himself, he agrees group activities suited to your level and made a part of your schedule help with motivation. That’s especially true if you find a sport or activity you really like.

Sedgley said, however, there is something else he finds is key.

“The big thing is why you pick your resolution, whether it’s diet or exercise or cutting alcohol,” he said. “It really helps to know why.”

His last New Year’s resolution was to lose 25 pounds, which he did over 12 months, because he wanted to stave off blood pressure medications. This year, after serving as the chief doctor at many big races, he’s considering a marathon because he wants to run with his son.

And, Sedgley adds, you aren’t locked into resolutions only on Jan. 1. He said, “I’m going to take another week or two to decide.”