See the rest of Slate’s Fitness Issue.

It’s late on a cold night, and I am holding two large Italian-style tomato cans and vigorously swinging my arms in forward and backward circles. One arm comes up forward—whoosh!—while the other heads backward—crack! Crack, as you might imagine, was not the sound I’d been hoping to hear. I am trying to stabilize my core, which is trying—valiantly, but falteringly—to stabilize me. I wobble. I am sweating. I am exhausted.

This is CrossFit, and in trying it out I am joining some uncounted yet increasingly muscled masses—millions, perhaps, but at least hundreds of thousands of Americans. But I am failing. Perhaps this is because, earlier in the evening, I had my first encounter with CrossFit’s sleeker cousin—P90X, a powerful, 90-day extreme workout system, hence the menacing acronym.

I am an intermittent runner, and I’m not in terrible shape in the scheme of American corpulence. But I am a lover of all things butter, and someone who has been castigated for her lack of “core and upper-body strength” by a Pilates teacher who mainly caters to elderly women. When I started asking around about how to get leaner and more buff, CrossFit and P90X were the two names that kept coming up. “You get so strong!” one friend cooed. “That shit should be illegal!” said another, apparently positively.

More to the journalistic point, CrossFit and P90X are the iconic fitness movements of the last half-decade or so. Looking at the workouts themselves and the businesses behind them, the relationship seems fraternal. The two systems share a lot of the same DNA: They both focus on intensive strength and agility training and employ video-based workouts. Both have enthusiastic—and sometimes, frankly, crazy—fan bases who love posting homemade videos of their morning routine or their insane feats of strength or their bodily transformation. And both have been touted not just as systems to whip even the laziest among us into shape, but as ways of life, cults, revolutions.

Of the two workout brothers, P90X is the slick, camera-ready pro athlete, while CrossFit is the sweatshirt-clad, hometown college favorite. The former is the best-selling brainchild of Beachbody, a company that makes and sells workout DVDs, mostly via infomercials (e.g., “P90X: The Proof“) and the Internet. Since 1998, the company has made $1.3 billion in sales to 10 million customers, with P90X the shiniest jewel in a crown that also includes titles like Brazil Butt Lift and Hip Hop Abs. (The most popular iteration of P90X was first released in 2003.) The videos are slick, expensive-looking, and decidedly aspirational. They are also, it must be said, incredibly campy. Workout sessions are led by the preternaturally tan Tony Horton and followed on camera by a series of fantastically chiseled co-eds; the setting is a kind of artfully distressed warehouse space.

CrossFit knows no camp—no distressed warehouse spaces, no Malibu beaches, no oiled, flexing washboard stomachs. The brand’s unofficial mascot is Pukie, a clown who encourages participants to, yes, work out so hard they vomit. The backbone of the company, founded by former gymnast Greg Glassman (known affectionately as Coach with a capital C) and his ex-wife Lauren Glassman, is a frills-free Web site. The workouts themselves are free and instantly accessible to anyone, and practitioners are encouraged to devise and post their own. (Coach calls the approach “Darwinian/free-market.”) While CrossFit first got its toehold in police forces and Army units, it is now the rage among everyone from suburban moms to hay-bale-lifting farm boys. It is apparently also quite popular among prison inmates.

The company makes most of its money training and certifying CrossFit instructors, bringing in an estimated $6.5 million a year (PDF). (Certification seminars currently cost $1,000.) It welcomes CrossFitters to open CrossFit-branded gyms (charging some licensing fees) and runs the annual CrossFit Games.

The similarities between CrossFit and P90X end when it comes time to look at the bottom line. Beachbody has sold $420 million worth of P90X DVDs since 2005, with those sales making up about half the company’s revenue. (It’s a private company, so it doesn’t disclose profit figures.) Though company executives fret about slowing sales growth, revenue for the line still increased a whopping 30 percent in 2009. CrossFit’s finances, however, seem something of a sore spot. Indeed, the company’s publishing arm, CrossFit Journal, features a kind of defensive advertorial that makes the case that “CrossFit is hugely profitable, and its growth has only accelerated during the worst economic downturn since the 1930s.”

“The world knows nothing of the virtual company,” Glassman says in the article. “Venture capitalists don’t get it, the MBAs don’t get it, and the media don’t get it. The very people who should understand it best are aghast at the concept.”

True, we dollars-and-cents types don’t get the give-away-the-exercises-for-free model, money-wise. But we certainly get it follower-wise. The videos, the fan base, and the social component are what keep anyone sticking with either of these decidedly painful, results-demanding workout systems. The Glassmans created a system that does not need their chiseled abs to keep growing: CrossFit is more of a community than a business.

For followers of both systems, much of the joy comes from gloating or whining about your crappy workout at CrossFit’s forums or P90X’s virtual community. CrossFitters are encouraged to post grainy homemade videos of themselves doing different bits of workouts—lifting bales of hay , then doing squats, for instance—to YouTube or the CrossFit site. They also head to the Internet to report on the growth of their biceps or to mythologize about rhabdomyolysis, a condition that develops when you work out so hard your muscles break down and release dangerous chemicals in your bloodstream. (The condition is rare, but not unknown (PDF) in the CrossFit community.)

These videos and chat boards help members who can’t share in a group workout or visit a CrossFit gym feel included. The same goes for its high-def relative, P90X. Enthusiasts have put up thousands of YouTube clips of themselves flexing and squatting and giving impromptu home testimonials—testimonials the company has incorporated into its infomercials—and have built Web communities to support each other through the first grueling days.

It’s those funny videos—where can I find a bale of hay to lift?—that I’m thinking about as I lay on the floor after a few hours of pain. I’m not sure which system I like better. They both hurt. But they both work. As I flex my slightly bruised muscles in the mirror, I’m thinking I just might post a video once I’ve got my six-pack.

Also in Slate’s Fitness Issue: Elizabeth Weingarten flexes her cheeks and winks creepily to see if face exercises really work, Torie Bosch searches for a fat-girl-friendly exercise DVD, and our handy flowchart helps you navigate awkward, naked gym situations.

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