As I run flat out along East 2nd Avenue, carrying a nine-kilo medicine ball, the words of CrossFit owner Craig Patterson ring in my ears: "CrossFit is not for everyone."

The run, which I perform as onlookers point and stare, is part of a fitness test that Patterson administers to anyone interested in joining CrossFit, a rigorous program that combines various functional exercises done in short, intense intervals. Patterson's gym is the first in Canada to use CrossFit, a fitness method that has become popular with policemen and the military across the U.S. The system also gained recognition from its association with the 2007 film 300, where many of the outrageously chiselled actors portraying Spartan warriors used a variation of CrossFit (plus some CGI help) to fit into their loincloths.

Although Patterson welcomes anyone who walks into his gym, his workout philosophy tends to separate the wheat from the chaff rather quickly. "I'd say 80 percent of people out there aren't meant to do CrossFit," he says.

During my hourlong workout, Patterson checks my progress by running me through a series of short, intense session of chin lifts, squats, and sit-ups. These old-school gym exercises are at the core of the CrossFit method: a variety of routines that combine Olympic weightlifting, gymnastics, sprints, and functional movements with little time to rest between sets.

Just to remind you how intensive these workouts get, there is a drawing of CrossFit's mascot, a vomiting clown known as Pukie, on a sidewall. It's not unheard-of for more dedicated gym members to work out until they're nauseous.

After my first workout, my muscles screech with pain, but I avoid a visit from Pukie. After trundling me through a set of basic exercises, Patterson says that my 30-something body has the core strength of a 60-year-old grandmother, making me part of the 80 percent not cut out for CrossFit.

While most people won't make the cut, there are plenty willing to try. Patterson's gym has more than 200 clients, including several Olympic athletes, and he is in the process of moving to bigger premises to accommodate more clients.

His business is growing despite doing no advertising. (He receives new clients almost entirely through word of mouth.) Potential members are given a fitness evaluation and, if successful, then book a series of private sessions with an instructor, often using intense sets of reduced weights and reps designed to promote 10 elements of fitness including cardiovascular endurance, strength, agility, coordination, and balance. Once they've made enough progress, members join group classes where they often compete to see who can complete their workouts the fastest. (Private sessions cost $80 per hour; group classes cost between $150 and $200 per month. Drop-ins run $20.)

Both private and group instruction include exercises in as many combinations and patterns as possible. For CrossFit members, routine is the enemy. "There is a certain element of play," says Tony Leyland, a senior lecturer at SFU's School of Kinesiology and a CrossFit devotee. "One day you're out there flipping huge tires; the next day you're doing something completely different. Their argument is that they're trying to get the client ready for whatever life throws at them."

CrossFit's trainers find that only a relatively small percentage of the workout program's potential clients are able to keep up with its intense demands.

The CrossFit method was developed by American Greg Glassman, a former gymnast who started developing CrossFit more than 20 years ago, then began teaching classes in the mid '90s. The program started to take off in 2001 when he posted workout regimens on his Web site, www.crossfit.com. Today, there are more than 200 affiliates in the U.S. and five other countries–including Iraq, Afghanistan, and Qatar.

The intensity of the experience can make CrossFit clients so enthusiastic that the gym sells T-shirts that say CrossFit: It's Not a Cult, I Swear. One of the reasons for this fervour is that members are encouraged to create exercises of their own. Patterson, who used to work as a mechanical engineer, compares CrossFit to open-source software like Linux, which allow average users to make incremental changes to the programs. CrossFit users often create their own exercises–some may involve lifting tires or sacks of sugar, or trying to dead-lift the bumper of a car–and post them to the CrossFit Web site for others to try out. "It's the attitude of 'Whatever works,'" Patterson says. "Whatever is the most efficient and produces the best results is what we're interested in."

CrossFit also understands that what works for one person may not work for another. Coaches adapt the program to each client's fitness level. Although it has gained fame as a workout regimen for soldiers, police officers, and movie stars, scaled-down programs have been created for senior citizens. Such inclusiveness is encouraging for those of us with the core strength of a granny.

Patterson insists there is hope for me yet. During a second session, he runs me through a new set of exercises, and I feel slight improvements in strength and flexibility. My test scores take me from a 60-year-old woman to a 45-year-old chain smoker.

Patterson, who at first talked about how four-fifths of the population isn't cut out for CrossFit, softens his stance. "Everybody should be doing this," he says. "This is a natural thing. We wouldn't have evolved and gotten where we are if we couldn't do it. Everyone can do it; it's just that 20 percent of the people choose to do it."