Here’s Christina Morris’s idea of fun:

Jump straight up in the air and land on a box about 2½ feet high, 20 times. Next, lift a barbell with 80 pounds from the floor to chin level, 15 times. Follow that by lying flat on your stomach and walking your feet up a wall behind you to a handstand position, five times. Finish by climbing a rope dangling from the ceiling.

Then repeat the whole sequence — four times.

Morris, 27, a former elementary school teacher, ditched her career about a year and a half ago to follow her passion: She became a trainer in South Boston for CrossFit, a high-intensity workout program that started in California and is leapfrogging cross-country.

While many desk-bound, cubicle-dwelling Americans struggle with weight and fitness problems, a distinct subset is diving into a brand of exercise often advertised as military-based and that might seem extreme to outsiders. CrossFit, Tough Mudder, boot camp-style programs, TRX, and other high-octane workouts are growing in popularity, even as health specialists caution that for some people, the risk of injury is high.

“You have to know your limits,” said Dr. Thomas J. Gill, the chief of sports medicine at Massachusetts General Hospital and an associate professor of orthopedic surgery at Harvard Medical School.

“Just because you can run a marathon doesn’t mean you can do a Tough Mudder,” Gill said.

Tough Mudder, for the uninitiated, is a grueling 12-to-15-mile trail run with more than 20 military-style obstacles — slogging through hip-deep mud, belly-crawling under barbed wire, scaling walls, dashing around flames — designed by former members of British Special Forces, according to the Tough Mudder company.

Gill, medical director of the New England Patriots and team physician for the Boston Bruins, is a recent CrossFit convert who likes the way the program can condition a wider array of muscles more quickly and efficiently than traditional workouts.