I first saw the kettlebell at the gym. Amidst the mirrors, the gleaming weight machines, the whir of ellipticals and treadmills, it looked conspicuously medieval, primitive, even a bit sinister, as though it were part of an intricate and now obsolete system of weights and measures, or a weapon used by the barbarians when they stormed Rome. And yet, there was something comforting about the pleasingly round shape, like a potbellied stove or a witch’s cauldron.

My wife was pregnant with our second child. Surely this had something to do with my attraction to it—the feeling that I must prepare. Unlike the birth of our first child, which was a natural one, the second birth was a C-section. My wife’s hospital gown buttoned at the back. The lights were bright and harsh, as if in a stadium. Her hair was bunched beneath a shower cap. She shivered—the room was freezing. And then, after my son was allowed a brief communion with his mother, we were sent to a room to sit by ourselves, where I performed that curious father dance—the manic, loose-kneed bouncing, the shrugged shoulders, the little consoling noises, all to no avail—while a new life screamed in my arms.

Perhaps the mystery of my attraction to the kettlebell, and where it led me, is solved in this very digression. In the run up to such a vulnerable moment, you need to be strong.

The kettlebell was, it turns out, a gateway drug to the fitness cult to which I now belong: CrossFit. Joining CrossFit is similar to joining a gym, but the spaces, called “black boxes,” are barren, utilitarian, almost willfully ugly. There are no mirrors. There is a faintly post-apocalyptic vibe to the culture, as though it were training for survival. The workouts are done in groups, ideally to very loud music, and each one begins with everyone in a circle, saying their names and, in my experience, volunteering some outrageously harmless bit of personal information like their favorite cereal as a kid, or their plans for the coming weekend. In the supportiveness, the positivity, there may be touches of est and A.A., but when I say that CrossFit is something of a cult I don’t mean that it brainwashes you. I mean that, while CrossFit’s focus is the body, it addresses a need that is mostly mental—a need for personal transformation.

CrossFit, founded in 2000, is structured around a central accrediting organization and its affiliates, the number of which has increased by about fifty per cent each year for the past several years and is now above five thousand. The exercises that draw all this interest are a mixture of old-fashioned physical culture—chin-ups, push-ups, sit-ups, and sprints, all with tricky little variations—and the clean, press, and jerk movements of Olympic weight lifting that, as practiced by potbellied Russians in the seventies, circa “The Wide World of Sports,” had fascinated me. There is a smattering of gymnastics: headstands, rings, squats, box jumps, pistols (squats on one leg). There are a few rather random movements, like lifting gigantic truck tires end over end, or pulling around a weighted sleigh with chains. Many of these movements, one way or another, involve squatting. And then there are the kettlebells and their faintly Ottoman air. My favorite kettlebell workout is called “The Turkish Get-Up,” which involves standing up from a prone position while holding a kettlebell over your head in a state of adrenalized fear that you might drop the thing and crush your face.

CrossFit attracts control freaks and perfectionists, as well as people with soldierly dispositions, like cops, firefighters—one of whom memorably did a workout in full gear—and military veterans. There are as many middle-aged people as young ones.

It’s been nearly three years since I first joined. Some of this time has been spent in New York, most of it in New Orleans. When I am in New York, I go to CrossFit workouts sporadically, maybe once or twice a month. In New Orleans, the CrossFit need is much stronger.

What is behind the CrossFit need, which is a variation of compulsive intense training? And I mean training, not playing a sport—training as a form of preparation.

The answer moves along several parallel axes:

1. The Axis of Wanting to Be a Middle-Aged Man Who Can Dunk.

I have always been on the very cusp of being able to dunk a basketball, at times able to finish and at other times falling short, pathetically, the ball banging against the front rim or bouncing off the back. My relationship to a dunked basketball, in the course of an actual game, has mostly been that of the person being dunked on. This is a point of pride: it means I am good enough to be on the court with people who can dunk. But it is also depressing. Getting dunked on is itself not too bad, but the gleam in the eye of the dunker when he or she first lays eyes on you is. One of the most famous victims of a dunk is a guy named Frédéric Weis, a calamitously soft, inept Frenchman whose great moment in basketball history came when Vince Carter more or less jumped over him to dunk in the 2000 Olympics. C’est moi.

An idea has been percolating recently among basketball fanatics that being dunked on is a good thing, a sign of character and integrity. It means you have contested a shot and tried to make a defensive play. This is better and more useful than saying, “Oh boy, here comes a potentially ego-devastating experience. Let me turn my back and get out of the way.”

It would be dishonest to write that I joined CrossFit so I could be a middle-aged guy who can still dunk. No, I joined CrossFit not wanting to sustain my game but to improve it. I have played basketball for most of my life, starting at around ten. Could I have used something like CrossFit five or ten (or twenty or thirty) years ago? Most certainly. I wish there had been something like it. But the frantic culture of physical self-improvement in which we live did not exist thirty years ago, or even fifteen. Maybe I am more under the influence of the zeitgeist than I would like to admit, but my desire to stay fit enough to keep playing is earnest.

And yet, as powerful as this motivation is, I don’t think it is the primary explanation for the CrossFit need.

2. The Axis of Parenting. And Within That, the Axis of Having a Son.

I have a wildly physical daughter with whom I have run around and roughhoused from her earliest years. She is no slouch, my daughter, when it comes to wanting to be thrown in the air, or to play tag or swim. But it was when I found out that our second child would be a boy that I had my CrossFit freak-out.

While my daughter presented me with a demanding physical challenge, I was thinking about her now, the ongoing now of parenthood. This now is so forceful that seeing a photograph of your kid taken two or three years earlier is always a shock. My son, though, provoked a new way of regarding time. Even before he was born, I was looking into the future in a way I hadn’t done with my daughter. I wasn’t worried about playing with my son when he was a week or a year old; I was, in some way I can still only barely perceive, thinking about whether I could play with him when he was twelve and fourteen and sixteen.