Along with the usual reasons for losing weight, like fitting into a bikini and improving health, fitness experts say raising money for a good cause is another incentive for people to get in shape.
Entering a charity run in memory of a loved one or a bicycle ride for a worthy cause has pushed many couch potatoes from their sedentary lifestyle on to the path of fitness.
Kelly Flynn, running coach for Team In Training, a charity sports endurance program from the Leukemia & Lymphoma Society, has been teaching novice runners to tackle the Boson Marathon since 2005.
"The people who've never run before and have a motivation, like a loved one, are the easiest people to coach," said Flynn, a 40-year-old Boston-based attorney. "They stick it out. And when they cross the finish line, they've become running junkies."
Flynn, a soccer and softball player in her teenage years, said it was the death of a high school friend from lymphoma that inspired her to become a running coach for charity.
"I saw a flyer (from Team in Training) and went, on a lark," she said. "His death was my catalyst."
Team in Training, which is 25 years old, has raised more than $1.4 billion, with more than 600,000 people from across the country taking part in different endurance events to raise money for Leukemia & Lymphoma Society.
Flynn, whose 200-strong team soon will start training for 2014, has helped to train more than 800 Boston marathoners and her teams have raised over $4 million.
For Sarah Jane Constantine, a self-described recreational runner, marathon running was a bucket-list dream until she started running for charity.
"I didn't think at 39 I was going to start marathoning," said Constantine, a 41-year-old manager for a pharmaceutical company in Boston who has raised money for cancer research.
"That's what I'm most proud of," said Constantine, who finds some way to exercise every day, from running to strength training to swimming with her daughter.
Constantine dedicated her first marathon to her stepfather, who died of melanoma at 44. Last year it was for her daughter's school friend's mother, who died of lung cancer.
This year she plans to run for the cancer-stricken daughter of her husband's co-worker.
"You feel helpless," she said. "I'll be running for money for research and treatment."
Charity events are a nationwide staple of Flywheel Sports, a chain of indoor cycling studios founded in 2010. New York-based co-founder and creative director Ruth Zukerman said the fundraising events her company holds throughout the year are beneficial for people new to the sport, for charities and for her business.
"A lot of people come to these charity rides who've never been to us before. Typically some 80 percent of the people are new," she said. "They come to us to give, they love it, and they become clients of ours."
Zukerman said her regulars often approach her with their own pet causes to raise money, which attracts even more newcomers to indoor cycling.
"Some of our celebrity riders, such as (actresses) Kyra Sedgwick and Jessica Alba, draw a lot of attention because of their names," she said.
Last spring Alba hosted a class in Los Angeles to raise funds for Baby2Baby, which redistributes baby gear and clothes to the needy. In December, Sedgwick will host a ride for the Food Bank for New York City.
Zukerman believes charity fitness is here to stay.
"So many typical charity events involve going to speeches with a tedious sit-down dinner," she said. "People see this as an opportunity to contribute in a newer, fresher, more fun way."