As Knight put it, kettlebells are a tool to use for strength training, if they are heavy enough. They can improve core stability, if your posture while using them is good enough, or can be used for power training, if you swing them safely. They even offer aerobic conditioning, if they are light enough so that you do frequent repetitions.
"They may be part of a program, but not the end-all-be-all," he said.
Other fitness experts note that kettlebells have some unique features, from their design to the way they are used, that make them worth including in an exercise or injury rehabilitation program.
Kettlebells teach you to use full-body movement from your head to your toes, and they teach people to be much more powerful, said Tim Brewster, a physical therapist and the owner of Train Boston in Wellesley, Mass., a fitness training facility that uses kettlebells for individualized sessions and in group classes.
"They are one of the best ways to teach proper lifting technique to individuals trying to recover from chronic backs injured due to weakness and improper lifting," Brewster said.
And there's another good reason to give them a try, he added: "They're fun."
"We're starting to recognize that kettlebells are a form of lifting that involve range of motions that other forms of exercise don't allow," said Mandla Nkosi, a certified instructor and owner of Boston Kettlebells in Brookline, Mass.
According to Nkosi, the fitness industry makes it easier for people to strength train because machines stabilize the weight for you. Kettlebells, on the other hand, are an awkward tool to lift because the center of mass is displaced from the handle.
Yet it's the very awkwardness of hoisting a kettlebell that Nkosi believes is a good thing.
With most everything you lift -- whether it's the groceries, a child, or snow on a shovel -- the object is never in the optimum place for leverage, he explained. So training with a kettlebell mimics the same kind of lifting you do in everyday life.
Whether you're pressing the kettlebell over your head or swinging it forward, you have to learn how to position yourself correctly and manipulate the load. And this, said Nkoski, involves a greater range of motion than conventional forms of strength training.
He said another advantage is that people who use kettlebells stay more supple and loose, whereas people who do other forms of weight training might develop bodies that are really strong, but also really tight and not so good at movement.
And kettlebells differ from other kinds of strength training in yet another key way: Because they involve doing numerous repetitions of exercises where you are swinging a weight, there's lots of cardio work, Nkorski said. More cardio, of course, often means more calories and fat burned during the activity, so "endurance people love it."
Despite the many benefits, not all fitness facilities are jumping on the kettlebell bandwagon or singing their praises.
At the University of Wisconsin Hospital's athletic training facilities, Knight preferred dumbbells to kettlebells because, he said, they are less expensive, take up less space, and people could do a lot of the same -- or more – exercises with them.