SACRAMENTO, Calif. — Fitness trainer Melissa Tafoya coaches boxers whose traitorous bodies give them a dozen reasons not to show up for class. Some lose their balance just trying to cross a room. Others wake up with an overwhelming sense of exhaustion. A few have muscles so rigid that their bodies contort.

All struggle with symptoms of Parkinson’s disease, and just getting to class takes a certain amount of intestinal fortitude.

Tafoya runs the Sacramento region’s first Rock Steady Boxing affiliate, a fitness regime that uses noncontact boxing drills to help people with Parkinson’s improve their agility, hand-eye and foot coordination, muscular endurance, speed and overall strength. Although her Rock Steady classes now take up half of her time, Tafoya said, they provide only a third of her income.

“It helps to keep me afloat, and it’s filling my heart beyond measure,” Tafoya said. “It’s something I never want to stop. … Everybody tells me, ‘This is growing really fast. Why don’t you start grooming new coaches for Rock Steady?’ Well, I don’t want to stop training the fighters.”

Rock Steady “boxers” or “fighters” are swinging back at Parkinson’s, and every one of Tafoya’s clients who spoke for this story emphasized that, while the disease comes with a life sentence, it is not a death sentence. Indeed, researchers now believe that regular, vigorous exercise can dramatically slow the disease’s progression.

Kaiser physical therapist Erin Vestal explained: “From a neurologic perspective, what they’re gaining is dopamine. It’s a neurochemical that is deficient for people who have Parkinson’s, and so when they’re exercising, particularly when they are exercising at an intense level, we see that they are utilizing dopamine more efficiently and effectively. From a movement perspective, that’s what allows them to move more normally.”

Roseville resident Ed Abbott said that although he still gets tired during the workouts, he has gained more self-confidence and a little bit more stamina since joining Tafoya’s classes in January.

“I’m not afraid to tackle the things that I was afraid to do before,” Abbott, 83, said. “I’m ready to get up in the morning and do whatever I have to do to keep moving. It’s really easy to say, ‘Today’s not a good day. I’ll just stay in bed.’ But I don’t. I get out and do.”

Some people find irony in the idea that people with Parkinson’s are training in a sport that is commonly believed to have led boxing legend Muhammad Ali to develop the disease. Neurologists have said that, while multiple traumas to the head likely may have caused symptoms of Parkinson’s to develop earlier in Ali’s life, it is unknown whether the battering caused it.

The Rock Steady boxers take no hits, but they deliver plenty to punching bags. Their boxing drills largely focus on developing coordination, strength and endurance. Vestal said the medical community is trying to get the message out that this type of vigorous exercise is as close to a cure as they have. People aren’t experiencing disability or changes in their cognition or depression as quickly as they might have if they weren’t exercising, she said.

Long before she worked with people with Parkinson’s, Tafoya had begun training clients with physical disabilities — multiple sclerosis, aging bodies, competitive athletes with debilitating injuries. She has a degree in fine art design from Cal Poly San Luis Obispo, but she left behind a career in graphic design after a fitness gym manager saw her training her mother for surgery and complimented her technique. He told her he would give her a job if she got the certifications, she said, and her mom encouraged her to do it. She now has credentials in personal training, corrective exercise, Rock Steady and more.

Tafoya’s involvement with her Rock Steady clients began at the gym door, but it hasn’t ended there. She goes to potlucks and movie nights at their homes. They carpool together to conferences on aging and Parkinson’s disease. They meet up at River Cats games.

Many of Tafoya’s clients link their discovery of the Rock Steady method to “60 Minutes” correspondent Lesley Stahl’s segment on how the training helped her husband, Aaron Latham, but their coach heard about it from an NPR report. Tafoya had clients whose Parkinson’s hadn’t been diagnosed fully and she thought it would be a great service to add to her business, but she had a lot on her plate and didn’t immediately explore it.

Then, she said, she went to a continuing education class where the woman in front of her had a shirt on that read: “Rock Steady San Francisco.” Tafoya saw it as kismet, and she introduced herself to the woman during a break and ended up volunteering for several months at her gym to see the impact of the training. She was content, she said, with the volunteering, but the trainers at the gym kept pushing her to go to Indianapolis for training and open an affiliate in Sacramento. She registered for training in October 2015, so she could celebrate her birthday while she was there.

Neurologists from the University of Indiana and trainers from the Rock Steady home gym immersed Tafoya and other fitness coaches in classes. Then they dropped them into a training session with boxers who had the most severe symptoms of Parkinson’s disease, she said, and they began learning how to adjust their routines and drills to fit different patient levels.

“I thought, ‘Aren’t these boxers going to be a little ticked off that they’re working with strangers?’ ” Tafoya said. “‘Don’t they want to stay within their comfort zone and work with their (Rock Steady) family?’ What I didn’t realize was how proud they were, how safe they felt, how strong they felt and how far they had come. They wanted to teach us. … I was one of the only ones from the West Coast, and they were so enamored with that. They said, ‘You take it to Sacramento. You grow this up. We need more of you guys.’ ”