Denyse Jones grimaces while lifting a kettlebell above her head during a kettlebell class taught by Mike Diamond at St. Louis Self-Defense and Fitness in St. Louis, Missouri.

ST. LOUIS — The regulars at the Monday evening class stood in a circle, feet wide apart, swinging kettlebells from between their legs up to shoulder level. In the blink of an eye, they switched to repeatedly whirling the heavy iron balls around their hips passing them from hand to hand. A half-minute later they were lunging forward on one leg and passing them beneath the knee.

On and on it went like this for more than 20 minutes, nonstop and smooth as a high-speed rail system.

Mike Diamond, certified kettlebell instructor and co-owner of St. Louis Self Defense and Fitness in Brentwood, Mo., stood in the middle of the circle with a stopwatch. Every 30 seconds he called out a different exercise, using names like two-hand swing, high-pull and right-handed clean.

"You guys are working like champions. Good job. Five more seconds," he'd say, egging them on.

Kettlebells, an old staple of Russian strongman workouts, have gained a new following at some fitness studios.

The kettlebell — which looks like a cannonball with a handle — is often incorporated into otherwise traditional exercises such as lunges, squats and burpees, turning them into full-body movements that improve strength and endurance, balance and agility.

Kettlebells can do everything dumbbells can do, but they're easier to transfer from hand to hand and around the body to create more of a continuous swinging motion.

It's a total body workout that uses arm, shoulder, chest and leg strength to lift the ball as the muscles of the core work hard to stabilize the body.

Because the motion is continuous, the workout combines cardio exercise with strength training. The idea is a compound workout in a compact time, Diamond said.

"All you need is 20, 25 minutes a day working with the kettlebell and you're going to get your strength and endurance," he said.

In June, Shawn Mozen of kettlebell manufacturer Agatsu in Canada came to St. Louis to re-certify Diamond, who turns 51 today, and several of their students as kettlebell instructors.

USING YOUR REAR END

During kettlebell class, the students, fresh and raring to go when they arrived, were sweating and panting within minutes. By the end of the session, most had slowed down noticeably and several seemed a tad wobbly. Including this reporter.

When I tried the workout, Diamond literally kicked my rear end.

He wanted to make sure my gluteus maximus muscles were tight, and that I was using my core, hips and butt muscles to thrust the 22-pound kettlebell up and outward. So he tapped my buttocks with his shoe.

Proper form and technique are everything when working with kettlebells.

Diamond tells his clients to squat like they do over toilets in public restrooms. Women understand this right away, he says. Men — well, it sometimes takes them a little more time to get it.

From there you swing the kettlebell up so your arms are parallel to the ground, doing it by thrusting the hips forward and tightening your butt muscles.

"I hear people say that they go to the gym and ride the stationary bike while reading the newspaper," Diamond said. "Shawn (Mozen) says that's not a cardio workout, that's sweaty reading."

Because of the strength, coordination and concentration needed to use a kettlebell, Diamond calls the it "a gym in your hand."

Wilhelm Lowell, 52, of Ladue, Mo., has worked out at the Brentwood studio for four years and wanted to try some resistance training. He initially balked when Diamond recommended the kettlebell, because he didn't think he had enough body mass.

"But he had me start with 25-pound weights and now I'm up to 35-pound weights," Lowell said. "If he sees it getting too easy for you, he makes you increase the weight."

Diamond always recommends that beginners take his Sunday morning classes at least twice to learn proper technique and basic swings. Jumping right into the 30-minute more advanced classes could cause injuries and severe muscle breakdown, he says.

Many of the students have bought their own kettlebells, which cost up to $60. Stephanie Branch, a tall blond, has worked her way up to 36-pound kettlebells. She started taking the classes six months ago when her boyfriend talked her into it.

"It was hard to come to class at first, but once you get over that it's fun," said Branch, 25, of Brentwood. "I didn't want to get bulky so I was worried about that. I lost 2 1/2 inches around my waist after six classes. And you lose weight proportionately and you tone proportionately."