The concept of re-purposing — developing secondary uses for objects — didn’t exist a few centuries ago, but that’s what happened when 18th-century Russian weightlifters and soldiers began incorporating common market grain weights in their fitness regimens.

Who would have thought the cast-iron weights, which look like spoutless teapots or cannonballs with handles, would also hold the key to balancing your 21st-century fitness routine?

Movement and lifestyle coach Trevor Spring became acquainted with kettlebells in 2003 while training Wall Streeters at Equinox Gym and Peak Performance NYC, one of the nation’s top gyms, according to Men’s Fitness magazine.

Whereas traditional weights have a fixed resistance point, the beauty — and danger — of kettlebells is in the variable resistance caused by displacement of weight from the handle during a swing.

Spring said dealing with that force requires the strength and coordination of muscles throughout the body.

"It jacks up your nervous system in a way that’s more complex than stationary movement," he said.

Getting started requires a knowledge of body mechanics and core strength, and while it might appear to be simple swinging of a weight, no one should attempt to try it without guidance from a certified kettlebell trainer.

"A lot of people try the YouTube style of learning, where they see it on a video and think they know how to replicate it, but the body needs to work hard not to injure itself," Spring said. "People who do it wrong over time end up with repetitive stress injuries."

An avid athlete who teaches at Honolulu Club, with his next session beginning in November, Spring is the founder of Nature’s Design Movement and Lifestyle Coaching, created to promote holistic self-care. Kettlebells are part of the equation because they can be added to any fitness regimen to achieve diverse aims. These include weight loss, strengthening core muscles, building endurance and increasing agility, making them popular with athletes. The benefits are borne out by research from the American Council on Exercise, outlined in a report, "Kettlebells: Twice the Results in Half the Time."

Local trainer Chris Ranes said, "ACE is not interested in fads/gimmicks, and they are a pretty conservative bunch."

Spring said working out with kettlebells has improved his performance in everything from surfing to paddling and snowboarding.

Although small weights of 5 or 10 pounds can be found for beginners, individuals typically work with weights of 18 to 20 pounds.

"Twenty pounds can be quite a load, but you’re using momentum and gravity, the idea of fulcrums and levers, and it becomes a different animal," said Kristin "KC" Carlberg, founder of Try Fitness Hawaii, dedicated to women’s health and fitness. She introduced kettlebells to her training last November after receiving instruction from a Canadian coach. The kettlebells are now part of her Boot Camp for Women, a four-week cross-training program incorporating a variety of activities for those looking for a gym alternative.

Kettlebells, she said, are a great way to maximize one’s workout in a short time span.

"People have time constraints with their workdays, their lives. They don’t have a whole lot of time to dedicate to working out, so when they do, they want it to really count.

"Sports Authority continues to sell out of kettlebells. A lot of people are getting into it because it’s something you can take with you and do a short 20-minute workout and feel like you’ve done a full workout.

"Lance Armstrong is one of the known athletes who’s incorporated kettlebells in his core training to prepare him to race. He’s not even 40, but in the world of cycling he’s considered old," Carlberg said. "If you look at the population across the board, people are getting older and are looking for things that will help them maintain their strength. That’s why you see them getting into yoga, Pilates and so forth. They’re mixing it up so their bodies keep adapting."

Unlike fitness classes that allow individuals to jump in at any time, kettlebell classes are usually offered in a series of sessions that start with core training and fundamentals. "By getting to know what it feels like to be in alignment, you can feel when you’re out of alignment," Carlberg said.

"Swinging the weight is a very typical movement. The power comes from your lower body and hinge of your hips. When the bell is moving, the momentum makes it easier to move the weight."

"We want to keep everybody safe," she said. Those deemed too weak to tackle the kettlebells might be started with free weights.

Knowing the basics helps in every aspect of one’s lifestyle, she said.

"People are surprised to learn how many ordinary activities can cause injuries, whether making a bed or picking up golf balls. It helps when you have training so you’re aware of when you might be straining your back or to know when you should be engaging your abs. It helps incredibly as we move about our lives."

Trainer Chris Ranes of My Fitness With Chris is able to train others in use of kettlebells in spite of motorcycle accident injuries six years ago that left his right arm paralyzed.

Certified by the International Kettlebell & Fitness Federation (IKFF), one of a handful of accepted certification programs, he said his injury and partial recovery helped him earn the respect and trust of his clientele. His website details his saga, from couch potato to would-be competitive bodybuilder, to accident victim and his current endeavors. He recently opened a fitness studio, Hawaii Kettlebell, on McCully Street.

"They read my story and they see that I went from being really, really fat and obese to decent shape, to bodybuilding, to what I am now. Most people can relate to at least one part of what I’ve been through. I had to take a big step back and start with nothing."

His rehab started with relearning how to wiggle his fingers, and he now trains others in kettlebell techniques by demonstrating with his left arm, although he’s able to grab its handles with both hands in swings.

"I can’t lift it to my shoulder and do presses with my right arm, but the power generated through the hips and strength needed to hold on with my hands creates a stimulus that helps. It’s very much a full-body exercise, and I feel like I’ve been healing from my hand upward. It’s been a long, slow process. I still can’t lift up my right arm, but I’m now able to bend my elbow."

His progress reinforced the reasons he was able to continue with his health-improvement regimen despite the odds against him after his debilitating accident.

"The main thing is having the right mindset and positive attitude," Ranes said. "It helped me to never lose sight of why I’m doing the things I do and not let anything hold me back. I want to make a difference in people’s lives."