I tend to have a short attention span when it comes to new workouts. As such, I've taken a lot of classes (Pilates, yoga) and lessons (snowboarding, ski jumping) and used a lot of trainers (boxing, cycling). They all begin much the same: The instructors are encouraging and — relentlessly — positive. So I expected a sort of tough-love version of the same as I stretched in the Combatives room at Manhattan's John Jay College of Criminal Justice, awaiting my introduction to Krav Maga ("contact combat" in Hebrew). The self-defense system was developed by Imi Lichtenfeld, who grew up learning to defend himself against anti-Semitic thugs in Czechoslovakia in the 1930s. Lichtenfeld later refined what he'd learned into a system of practical self-defense in Palestine, and in 1948 began teaching the Israeli Defense Forces. Krav Maga is now their official fighting technique; it's also been adopted by civilian police forces around the world and by such mere civilians as Hilary Swank, Matt Damon, and Angelina Jolie.

Soon enough, a barrel-bellied man in a white gi stalked to the front of the room and issued forth a brief guttural scream that had me and the 20 or so acolytes in the room scrambling to the mat for a rapid-fire series of 29 separate but equally brutal "warm-up" exercises, including jumping jacks, push-ups, planks, squats, and crunches. Then out marched our instructor, a compact and fierce man named Rhon Mizrachi, to deliver a brief verbal syllabus: "If you want to come in here and work hard and do what you're told, fine. If you don't? I. Don't. Care. We are not going to call you if you miss a class and ask how you're feeling, O.K.? I do not listen to excuses, and I do not give a shit if your dog died!" I was sold.

Krav Maga employs the familiar belt-ranking system. But unlike most martial arts and schools of hand-to-hand combat, there is no Eightfold Path beyond what Rhon — who has been studying the technique since he was seven under Lichtenfeld's protégé, Haim Zut — articulated in that first class: "We take people and we prepare them for war. Maybe you're walking down the street at night and somebody confronts you; maybe he says something about your mother. And before you even have time to think about reacting, you're looking down at this person crumpled on the ground, and you're wondering why you have his blood on your hands. Because if you are thinking about what you should do, you are dead. Any questions?"

This instinctual ass-kicking supremacy is incubated in me through the next few months of classes with Rhon. Each class (after the warm-up, a grueling speed workout that leaves everyone heaving and pouring sweat) features instruction in a handful of techniques: escape moves from strangleholds coming in all directions; how to throw cutting elbows to the side, from below and from above; kicks and defenses. Any defensive move (a block or escape) finishes with an offensive move (sharp fingers to the eye or a knee to the face).

These techniques are performed on — I mean with — a partner. My classmates range from the 82-year-old founder of International Creative Management and a middle-aged cardiologist at NewYork-Presbyterian Hospital to 20-something men (and a decent number of women) obsessed with the UFC. Since most of these moves are merely pantomimed, there's little worry about being KO'd. Even so, when I came home from my first class, my girlfriend asked me why I had scratch marks all over my neck. I muttered, "Stranglehold escapes" but left out the part about Rhon eviscerating me for not strangling someone hard enough. In my second class, an escape move finished with a (mock) elbow to the opponent's teeth, though when my partner tried it out I ended up tasting my own blood for the rest of the hour. Midway through my fourth class, I — inadvertently, of course — sent my partner wincing and screaming to the mat, and then to the E.R. (conveniently located directly across the street) with a dislocated shoulder.