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In the fitness business, hype about new training techniques often outpaces objective evidence about how well they actually work. That was certainly true for kettlebells, the cannonball-with-a-handle training tools that started showing up on lists of fitness trends about three years ago.

But, eventually, the evidence catches up. A pair of studies published last month in the Journal of Strength and Conditioning Research take a hard look at how well kettlebells fulfill their promise of better strength, power and aerobic fitness. The results are generally positive, but also serve as a reminder of an important training principle: The more benefits you try to squeeze from a single workout, the less effective it will be for each individual goal.

The current enthusiasm for kettlebells, whose murky origins can be traced back to 18th-century Russia, stems in part from their simplicity. A few basic movements with a single piece of equipment can raise heart rate and recruit muscles throughout the body, in contrast to traditional weight training, which uses specific exercises to isolate individual muscles.

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One of the challenges in evaluating the effectiveness of kettlebells is figuring out what to compare them to. For strength and power, exercise physiologist Jared Coburn and his colleagues at California State University in Fullerton chose three standard kettlebell moves – the kettlebell swing, accelerated swing and goblet squat – and matched them to three traditional weight-lifting exercises: the high dead lift, power clean and back squat.

The researchers randomly assigned 30 volunteers to follow identical programs using either kettlebells or barbells for six weeks, then measured their strength and power. As expected, both groups improved. The gains were similar in measures of power, like vertical jump, but for strength the gains were bigger with barbells. In the maximal back squat, for example, the barbell group increased by 13.6 per cent, while the kettlebell group increase by only 4.5 per cent.

One explanation for the difference is that kettlebell movements emphasize speed and explosiveness, but are less suited to dealing with very heavy weights, Dr. Coburn says: "My advice would be to incorporate them into a training program alongside more traditional methods, not as a permanent replacement."

At the other end of the fitness spectrum, researchers at Truman State University in Missouri pitted a 10-minute kettlebell swing routine (35 seconds of swinging alternating with 25 seconds of rest) against a 10-minute treadmill run. In order to get a fair comparison, they had their volunteers repeatedly estimate their perceived exertion during the kettlebell routine on a standard numerical scale from 6 to 20. Then they continuously adjusted the treadmill speed so that the perceived effort of the two workouts stayed the same.

On the surface, the results were clear: The treadmill workout burned more calories and consumed more oxygen than the kettlebells, by 25 to 39 per cent. Still, the kettlebell routine maintained heart rates up above 85 per cent of maximum, enough to produce gains in cardiovascular fitness. And there was an interesting wrinkle: To keep the effort levels matched, the subjects had to keep adjusting the speed of the treadmill to go faster and faster – a pattern that, in practice, most gym-goers don't follow.

The verdict on the aerobic benefits of kettlebells, then, is very similar to the verdict on their strength-training benefits. In an appropriately matched head-to-head contest, kettlebells don't quite stack up in either category, but are nonetheless decent in both categories. That means you can tailor your workout to favour one extreme or the other, or choose a middle ground.

"If it's a heavier kettlebell that's lifted only a few times, it's probably a strength workout," says Jerry Mayhew, the senior author of the Truman State study. "If it's a lighter kettlebell that's lifted in rhythm for an extended time, like three to five minutes, it's probably more of a cardiovascular workout."

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In other words, it's not the fitness equipment that matters; it's what you do with it.

Safe kettlebell usage

Kettlebells put less compression but more lateral force on your vertebrae compared to conventional barbells, according to research by the University of Waterloo's Stuart McGill. As a result, proper form is important – master the motions with no weight before picking up a kettlebell. Dr. McGill recommends starting with the "shortstop squat" to practice keeping the spine in a neutral position: hands on knees, bending with the hips and looking straight ahead.

Alex Hutchinson blogs about research on exercise at sweatscience.com. His latest book is Which Comes First, Cardio or Weights?

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