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.
CrossFit uses diverse actions — dreaded push-up-to-leaping movements known as Burpees, sprinting, rowing, jumping rope, weight-lifting, and carrying odd objects — in ever-changing combinations to form prescribed “Workouts of the Day,” or WODs.
Despite his expertise and fitness (he also runs and plays squash), Gill, 47, recently aggravated an old back injury by lifting too much weight in a CrossFit class. He said he has treated a number of knee and shoulder injuries, mostly strained and torn cartilage, among CrossFit and Tough Mudder participants.
Tough Mudder started as a 500-person competition in Allentown, Pa., in 2010 and has morphed into a global phenomenon with an estimated 500,000 participants in events across the United States, United Kingdom, Australia, and Canada this year, according to the company.
Among them is Sebastian Buhaley, 36, a Jamaica Plain CrossFit devotee who recently completed his second Tough Mudder.
“I wanted to raise the intensity level,” said Buhaley, a former competitive runner who said he has not injured himself in his CrossFit workouts because the variation of the exercises has helped him avoid overuse injuries and get significantly stronger.
CrossFit has grown from 13 affiliates (trainers or gyms specially certified in the method) in 2005 to over 4,000 now, mostly in the United States, according to the company.
Data about the health benefits versus the risk of injuries in the new breed of high-intensity workouts is hard to come by.
A 2005 article by Greg Glassman, founder of CrossFit, noted at least five cases of “rhabdo,” short for rhabdomyolysis, a potentially lethal breakdown of muscle cells that can lead to kidney failure, associated with CrossFit workouts. All five people were hospitalized.
And there have been scattered media reports and blog postings of broken bones, hypothermia, and hospitalizations during Tough Mudder events in several states.
Some small studies on the preceding generation of popular high-intensity exercises — typically very short bursts of intense sprinting — found measurable health benefits, most notably a significant drop in body fat, when compared with traditional cardiovascular training that involved 30 minutes or more of slower, sustained cycling or running.
“Fitness clients want maximum benefit for their time limits and their money, and non-traditional, high-intensity exercise gives this to people,” said Mike Bracko, an exercise physiologist in Calgary, Alberta.
“The fitness industry is cyclical and, in this case, people are bored with long slow distance, traditional cardio training,” Bracko said. “Any kind of ‘non-traditional’ exercise is popular now.”
Though not a classic high-intensity exercise, TRX, is certainly non-traditional and challenging. Also known as suspension training, the system was created by a former Navy Seal, and uses ropes suspended from a frame that allows users to work against their own body weight in dozens of different positions to build strength.
Since 2006, TRX has grown from a training course with 122 graduates, to one with over 45,000 trainers and coaches, according to the company.
Jim Thornton, president-elect of the National Athletic Trainers’ Association and head athletic trainer at Clarion University in Pennsylvania, said his concern about injuries is not so much with TRX as with the high-intensity workouts, especially for baby boomers who are anxious to get back in shape.
“It’s very attractive to people who were former high school and college athletes who performed at a high level. It brings back their glory days,” Thornton said.
For newbies dead set on trying high-intensity workouts, Thornton recommends finding an instructor who pays attention to each participant’s history of injuries and abilities.
Despite the warnings about risk of injury, many agree that the military-style workouts foster an esprit de corps that is missing in more traditional exercise classes.
For Linda Sheehan, 44, a Melrose mother of two who has tried and quit a number of exercise programs, the one that has endured is a boot camp program.
Sheehan gets up before dawn three mornings a week to scamper across bleachers, sprint, and do a battery of push-ups, lunges, and other punishing exercises on a Melrose football field with two dozen other hardy souls, rain or shine.
“The thing that made it stick for me is the sense of community,” Sheehan said.
She has struggled with lingering hip and leg pain from a childhood car accident, and that injury was one of her concerns when she first started the boot camp. But Sheehan has found the muscle conditioning has helped ease those pains.
“I remember thinking I can’t do this, when I first started,” Sheehan said. “My goal was to get through the [boot camp] 1-mile run without stopping to walk.”
Today, Sheehan can run 5 or 6 miles, and is entering road races.
“Now,” she said, “I am working on faster times.”
Kay Lazar can be reached at klazar@
globe.com. Follow her on Twitter @