I have an exercise-loving friend who says all you really need to get fit is a couple of kettlebells, the things you may have seen in your gym that look like cannonballs with handles. He may be on to something. A small study published by the American Council on Exercise (scroll down for the link to a PDF) found a particular kettlebell workout burned as many calories per minute as running at a six-minute-mile pace. And, because you're lifting something heavy, you're getting both a strength and aerobic workout.
That doesn't mean that a beginner is going to immediately achieve the same kind of intensity. The 10 volunteers tested by researchers at the University of Wisconsin–La Crosse were healthy 29-to-46-year-olds who were already experienced in kettlebell training. And the workout was tough: Following a warm-up, they did 15 seconds of one-armed snatches with their dominant hand, rested for 15 seconds, and did 15 seconds of one-armed snatches with their nondominant hand. A snatch involves lifting the kettlebell by the handle, swinging it back through your legs, then using that momentum and your own strength to swing it back and up above your head. (It's easier seen than described; search for "kettlebell snatch" on YouTube and you'll have plenty of examples.) They repeated that work/rest sequence for 20 minutes, then cooled down. Each used a 26.4-, 35.2-, or 44-pound kettlebell, depending on such factors as gender, body weight, and experience level.
The number of snatches the volunteers had to complete during each 15-second segment was determined by previous individual testing, and it was intended to constitute a heavy effort. Working up to that particular exercise at that intensity "takes quite a bit of training," says John Porcari, who was one of the study's investigators and is an exercise scientist and executive director of the La Crosse exercise and health program. An exercise physiologist from the American Council on Exercise designed a beginner's program for those new to kettlebells, which is included in the study; the routine includes dead lifts, single-arm swings, push-ups, and lunges—all with the kettlebell. (Just follow the link above.) Many gyms feature kettlebell classes; see if yours does.
You may wonder (I did, at least) how lifting a kettlebell is any different from lifting a dumbbell of equivalent weight. "The whole balance point is different," given the main weight and the handle, says Porcari. The result: It's a heck of a lot harder to snatch a 20-pound kettlebell than a 20-pound dumbbell.