Three nights a week I spend an hour rolling, kicking, jumping, dancing, punching and, from time to time, yelling "Kiai." I do it because my foot broke, and I realized last summer that I was getting older.

For years I ran and ran. First leisurely jogging; then running, 6- to 10-mile road races. These led to a marathon and a badly strained knee. After laying off for several months. I tried to get back in shape too quickly and pulled a ligament in my foot.

Four more idle months and I became irritable, couldn't sleep, lost interest in my job and began cursing my dog, child and wife.

I became convinced that I was getting fatter by the inactive day. My wife said that six feet, 155 pounds was not fat. But reason fled: I was getting fat; something had tobe done.

My foot was only one reason I needed some other exercise: Working out to make up for not running, I suffered a severely cramped neck muscle. The doctors discovered that I had bone spurs in my spinal column where the nerves from my spinal cord came through. I'd developed arthritis earlier, and this was a warning of more to come. My body had failed me; I'd better do something to help reduce the potential effects of the arthritis. Running would not help: I needed to keep my joints and muscles flexible and resistant to calcium deposits.

Tennis was out: I'd spent years teaching and playing tennis and hated the sociability of it. I wanted something I could do alone.

Like many who don't let on, I'd long had a curiosity about the martial arts, heightened by two secret, suppressed fantasies:

One, to be attacked by one or more -- preferably more -- muggers, only to surprise them with a series of well-orchestrated kicks, punches and throws and a final screaming "Kiai," having single-handedly rendered justice. Two, to learn the true mystic ways associated with the martial arts and become a traveling monk, dispatching villians and teaching universal awareness and harmony.

My wife, knowing that I tend to overindulge in things at times, had two horrors: One, of spending the rest of her life playing Peter Sellers to my Kato, as I leapt from behind doors and out of refrigerators attacking her. Two, of being left to explain to friends and family after I'd gone off to become a traveling Shaolin monk.

Despite my fantasies and curiosity, I'd always shied away from taking any martial-art lessons. I'd heard all the talk about how true experts were gentle and wise, more likely to avoid trouble than others; still, deep down, I felt that most people take up a martial art with one aim -- to inflict bodily harm upon another human being. The people I would find in martial-arts studio, I suspected, would be the same people I'd met in high-school football locker rooms.

Wiping these notions from my mind, I set out to learn something about the martial arts, and perhaps to find one for myself. There are as many such arts as running shoes, both being too numerous to name. The most common are: tae kwon do (karate), aikido, judo, hap ki do and kung fu. Tae kwon do , a Korean form of karate , the most popular, is considered by most teachers a "lower" martial art. Basically little more than defensive punching and kicking, it's considered a "lower" art because it doesn't require the same degree of mental training and discipline as most others.

Kung fu, whose reputation has suffered from the commercialization by television, is considered one of the highest martial arts, if not the highest.A Chinese art, today it most nearly resembles the first martial art taught by the Zen Buddhist master, Bodhidharma, to the monks of the Shaolin Temple. Without a doubt, it requires the most physical and mental discipline.

Most schools have a monthly membership with a three- to nine-month minimum requirement. Monthly fees, of $35 to $59, entitle you to two to four group lessons a week -- plus open time to work out in the school on your own. Private lessons are about $20 per hour. At some schools you can pay by the lesson, about $3 for a half-hour group session. A good school will require you to take at least two lessons a week, and will frown on your missing even one lesson. Good instructors want good, serious students.

Though not requiring that you purchase it from them, most schools will sell you a ghee (uniform) for about $30. Whether you already have one or buy one from the school when you start your lessons, you must have a ghee for your first lesson.

The schools are generally located in shopping centers, office buildings, above and in storefronts; they're usually one large room with a padded mat on the floor and two dressing rooms. There are seldom lockers or showers. One unusual karate school in Northern Virginia is actually a club, with a patent gym and outdoor swimming pool; you can join one or all of its aspects.

After visiting about a dozen schools and much deliberation, I decided to take up hap ki do at a school very near my house, basically on the advice of the teacher there, whom I liked very much. He listened to what I wanted out of the study of a martial art and recommended hap ki do , a Korean art with aspects resembling karate, judo and kung fu. Though there is kicking and punching in every martial art, hap ki do motions are designed to resemble running water. Students are trained to go with force, not against it.

In my class, we are taught to meditate as well, stopping periodically during exercise or sparring to meditate with closed eyes.

Each class begins with limbering and breathing exercises; the breathing exercises are practiced by breathing out and tightening the muscles of the lower stomach. Holding your breath, you then perform a series of body movements lasting from 30 to 60 seconds. Properly executed, this is a very difficult exercise.

Next, students are paired to practice the various throws and holds. The teacher makes his rounds, usually teaching one new technique and asking for a demonstration of one already learned but not perfected. There's a great deal of personal attention.

Practicing various kicks and kata , or set moves, or routines resembling dances, offer more exercise than one would think: Often, I've found that my heart rate equals what it did during running, though not for as long.

The first lesson was painful.A borrowed ghee was to save me money until I was certain that this was what I wanted. The first night, I put on the too-short uniform, dashed from my house so as not be observed by neighbors, and did the same upon arriving at the school. There I entered and gave the customary bow to the flag and teacher. A smiling Korean lady asked if I wanted to buy a ghee. Something was wrong. I already had one. It was, alas, the wrong kind. Not only wrong, but as I watched other students arrive, I realized that you do not wear the ghee outside, but change in a dressing room I didn't know existed. Standing there feeling silly, I waited as a new dark blue ghee was brought to me, paid for the first month's lesson and the ghee and made my way quickly to the dressing room, arriving to the stares of other students. The pain of the first lesson was not the kind I'd expected.

So far, I love my training and study. Three nights a week I leave my white-collar job and join about a dozen or so other people in a brightly lit studio in a small shopping center to kick, roll, jump, punch and finally "kiai" for an hour. Mornings I exercise for about an hour stretching joints and muscles that may be sore for the rest of my life, trying to pull then to the point that I can make the required height with my kicks. I do my exercises, practice my katas. To date I have not kicked my dog, child or wife, and have had no desire to leave home and wander the countryside righting wrongs and dispatching villains. I sleep better, my imaginary weight gain has subsided and I feel healthier.

I'm not sure if studying a martial art wil fulfill my fantasies; nor am I sure I'll stick with it. I am sure, though, that I won't live long enough to be a master, because of the devotion and time it takes. Maybe one can acquire wisdom from such study, but few people will learn more than to defend themselves when faced with inescapable danger. For most, that should and will have to be enough. Simply learning to yell properly should suffice in most situations.

For me, it's accomplishing what I want. But each person will have to find out for him or herself -- and that is, after all, the first lesson of the martial arts. In any case, there is one fewer runner in Washington, and one more closet kicker.

So far my wife shows no fear of the refrigerator, but there does seem to be some trepidation when I'm not in sight. "Kiai."