Like a lot of self-isolating people, Madisen Li is vacillating between nervous, bored, grateful and irritable. One of her main forms of relief comes from her weekly training sessions over Zoom with her personal trainer of nine years, Hector Guadalupe.

Earlier this month, amid a circuit of curls and crunches, Mrs. Li, 32, who is the director of footwear merchandising for Kenneth Cole, let off some emotional steam too. “We were working out, and I was complaining to him that my husband and I had been in a very small apartment for weeks now and we are getting on each others’ nerves,” she said.

“Hector said, ‘Madisen, I was in isolation for a long time and you are lucky that you guys have each other.’”

Mr. Guadalupe is not referring to being cooped up in a city apartment or a suburban home. He’s talking about being in a federal prison cell. Six by nine feet, unless he was in solitary confinement: “in the hole,” as he put it. Then his space was notably smaller.

“I’m very blessed,” said Mrs. Li, who lives in the Chelsea neighborhood of Manhattan. “And I’m very blessed to have Hector to remind me of that.”

Mr. Guadalupe spent 10 years in various federal prisons, after pleading guilty to interstate drug distribution, in 2003. “I was in the hole a total of about 31 months,” said Mr. Guadalupe, who is 41 and was released from a federal correctional institution in Elkton, Ohio, in 2012.

He is also the founder of a nonprofit foundation called A Second U that trains former inmates, most of whom were convicted of nonviolent offenses, for careers in the fitness industry. Since starting A Second U four years ago, he and a staff of 13 have helped 196 people (mostly, but not all, men) get certified as personal trainers. In addition to classes on anatomy, physiology and movement, the six-week course provides mentorship, a support network and basic business development skills.

“We are at the intersection of wellness and social justice,” Mr. Guadalupe said.

A Second U happens to be well positioned for a new era, when trainers need to be experienced in giving clients a workout without the trappings of a well-stocked gym. And who better to give encouragement and tips for staying physically and mentally fit during confinement?

“In isolation is where I really learned to reflect, to think about what I wanted my life to be and what I was going to change,” Mr. Guadalupe said. “It’s where I practiced yoga, it’s where a cellmate taught me to meditate.”

Second U trainers have been discussing how prison mentally fortified them with each other, and their clients. “There are huge parallels,” said Tommy Morris, one trainer. “I’ve had people say to me, ‘I don’t know how you did it in prison, I’ve been home two weeks and it’s driving me nuts.’ The most important thing is you have to have a regimen, you have to have a steady schedule of regular things you do. And if you can, take time out every morning for prayer, yoga, meditation, whatever you do to get in tune with yourself so you know fundamentally that you are OK.”

Part of the Zoom Boom

Kerry Faherty, a founder and the chief impact officer of Faherty Brand, a clothing company, met Mr. Guadalupe when she invited him to speak about social justice at a company retreat after hearing about his work from an employee. Since then, she has joined the organization’s board and is trying to help him scale his model nationally.

“These people come out of jail with nothing,” said Ms. Faherty, 36. “They don’t have money, they don’t have a support system, many are coming from traumatic childhoods, many have been out of society for 10 to 30 years, and you have a criminal record — how are you going to get a job? What Hector is doing makes a lot of sense.”

She and her husband, Alex Faherty, also work out with Mr. Guadalupe weekly, and in the last several weeks, Faherty Brand has offered group fitness classes (normally $35 per person, per hour) for free to its employees in Zoom sessions led by trainers from A Second U. So have Bombas, the sock company, and others. Individual sessions cost $45 per hour. The screen can offer a layer of comfort on both sides.

Mr. Guadalupe is working to make sure that the coronavirus pandemic doesn’t prevent the newest class of students from embarking on a path from halfway houses and uncertainty to a making a living, and possibly building a career. Distance classes of anatomy, physiology, CPR and computer skills will begin later this month.

This was not the life he pictured for himself when he was a child, growing up in public housing in the Williamsburg section of Brooklyn. Mr. Guadalupe said his mother died of cancer when he was 15, and his father was already dead.

“I was 13 when I started selling drugs,” he said. “It was a very chaotic time in New York. I saw a lot of death, a lot of police corruption, a lot of gun violence. It was a very hectic environment.”

By the time he was 18, Mr. Guadalupe said, he had 80 drug dealers reporting to him and was overseeing a cocaine distribution network from Cape Cod to Florida. “I learned a lot about sales and operations and the management of teams,” he said.

He wasn’t surprised, or even that upset, to end up in prison. “I didn’t have options,” he said. “It was jail or I was going to die. Weirdly, it was going to prison that led me to find my ‘second you,’” he said. “Fitness gave me my second chance at life.”

One summer, while in the Federal Correctional Institute in Fort Dix, N.J., Mr. Guadalupe, who was overweight, thought he was going to sweat to death. “During the days it was 120 degrees, with no A-C.”

Encouraged by a friend, he started going to the prison gym daily and learning how to lift weights and do calisthenics. He soon added an in-cell workout. He learned yoga, got serious with a vigorous vinyasa practice and added meditation. He lost 90 pounds.

Eventually, Mr. Guadalupe saw a business opportunity. He enrolled in a by-mail correspondence program — the original distance learning — to get certified as a personal trainer and started taking fellow inmates on as clients. A client’s wife, for instance, would wire money to a relative of Mr. Guadalupe, he said. “I saw a market in training guys from the Russian and Italian mob and white-collar guys. They needed help.”

Encouraging his fellow inmates to get in shape and get involved in this black-market personal training business, Mr. Guadalupe built a team that he managed. “Training brought in about 20 to 30 grand a month for me and my entire team,” he said. During his 10 years in prison, Mr. Guadalupe was transferred among five federal correctional facilities and he said he replicated his fitness operation in each location he moved to after Fort Dix.

After he was paroled, Mr. Guadalupe moved into a federal halfway house in the Bronx. He now lives in East Harlem.

‘Hardest-Working Guys’

Mr. Guadalupe was turned down for fitness jobs because of his criminal record, until he went with a friend to work out at a New York Sports Club in Union Square in Manhattan. He met the manager and begged her for an opportunity. She gave him a job, and he started amassing private training clients and teaching group fitness classes.

Having a solid job with health benefits and a living wage was revelatory to Mr. Guadalupe. As friends from prison were released and as he met other former inmates through relationships with halfway house administrators, he encouraged peers to embrace careers in fitness as a way to avoid returning to crime, drugs and fast food.

“We don’t have nonprofits in the city that can give jobs that pay $35 to $70 an hour,” he said. “The wellness industry can provide a livable wage and give these guys access to business men and women as clients who connect them to more clients. People are not going to go back to crime if they can feed their families.”

In 2016, he decided to formalize his program of mentoring released felons, beginning A Second U, alluding to the importance of second chances and the federal Second Chance Act. The organization gained 501(c)3 accreditation in 2018.

Rohan Hales, 37, is one of Mr. Guadalupe’s key aides. As A Second U’s program director, he helps to oversee the curriculum, gives weekly biomechanics and human anatomy classes, and arranges sessions on contemporary technology and professional correspondence.

Mr. Hales, who served seven years in federal prison after pleading guilty to conspiracy to distribute drugs, is also involved in the candidate vetting process, interviewing all of the applicants. “Mostly what I ask them is what did they do when they were in prison, because how they spent their time when they had very few options and choices is most important and tells you the most about a person,” he said.

A Second U is supported by donors, including those who train with Mr. Hales and Mr. Guadalupe. Bradley Tusk, 46, a venture capitalist and onetime adviser to Michael Bloomberg, has been working out with Mr. Hales about three times a week for the last eight years. When Mr. Hales told Mr. Tusk of his past, and about A Second U, he gave money, introductions and advice.

Ruthy Rodriguez, a former manager for New York Sports Club and personal trainer who works as A Second U coach, helping to train its recruits, says corporate gyms benefit from hiring these trainers.

“They are the hardest-working guys you will ever meet,” said Ms. Rodriguez, 45. “In a lot of cases, being at work in the gym is the only nice place they were allowed to be. They came from a mentality where they needed to be the best on the streets to survive and you adapt that ethos to the gym.”

One trainer Ms. Rodriguez helped mentor is Joel Ramsey, 34, who served more than 11 years in prison on a robbery conviction. “Fitness got me through my sentence, it kept me sane,” Mr. Ramsey said. After he was released, he heard about A Second U. “It worked wonders for me,” he said.

After graduating, he began to work at a New York Sports Club in Manhattan and took on private training clients like Samantha Majic, an associate professor of political science at the City University of New York’s John Jay College in Manhattan. Ms. Majic, 42, who had tried every boutique fitness trend on both coasts, now just works out with him twice a week. “Training with Joel, I’m in the best shape of my life,” she said.

Mr. Ramsey currently works as a recruiter for A Second U, finding candidates and helping to vet them. “I go to the same halfway house in Brooklyn that I came from and try to let them know they have support and that we’ve all been through what they’ve been through,” he said.

One former inmate under Mr. Guadalupe’s guidance is Mr. Morris, 45, who was released from prison nearly two years ago after serving almost 27 years for two concurrent homicide convictions. He committed the crimes when he was 16. “I’m proud of none of this,” he said.

Mr. Morris grew up in Brooklyn, and both of his parents were drug addicts. (His mother died of AIDS, he said. His father disappeared while on federal parole and his son presumes him dead.)

Mr. Morris was living on his own from the time he was 13, sleeping on subways and in abandoned cars, when he was recruited by a crack dealer. Selling drugs “felt like the greatest opportunity,” he said.

When he was 16, he said, he was shot by a rival dealer and he retaliated, killing another rival. Three weeks later he shot another person and was arrested. “I have come to understand the way poverty and drugs pit kid against kid, like crabs in a barrel,” he said. “I am not my past experience. I am trying to be a different man.”

When he was released, he moved into a transitional housing run by a nonprofit and joined a running club of other former inmates. Through that network he was introduced to Mr. Guadalupe.

Because of Mr. Morris’s violent crime conviction, he has struggled to get hired by a corporate gym. A Second U has connected him to private clients, and since the coronavirus moved the world to remote fitness, he has led group classes through his phone.

He leads classes from the spare and tidy kitchen of his girlfriend, with whom he is self-isolating in the East New York neighborhood of Brooklyn. Earlier this month, dressed in black sweatpants and a red T-shirt that showed his sinewy tattooed arms, he demonstrated a bracing circuit of squats, lunges, crunches and planks.

“Don’t forget to breathe,” Mr. Morris said, “because that’s what connects the body to the mind, and the heart to the soul.”