Swinging heavy kettlebells may offer an aerobic workout in addition to strength improvements, a small study suggests.

Female soccer players who trained with kettlebells for one month significantly increased their aerobic capacity, while traditional circuit weight training did not have the same effect, California researchers found.

Theirs is one of the first studies to find the effects of kettlebell training over time, said study leader J. Asher Falatic, of the Kinesiology department at San José State University.

Kettlebells are circular weights with handles at the top. They've been used for centuries but have gained popularity recently, according to Alexander Koch, program coordinator for exercise science at Lenoir-Rhyne University in North Carolina.

“Because of its shape, it differs from a traditional dumbbell and can be used to help train multiple parts of the body," Falatic said in an email.

He and his colleagues measured the effect of kettlebell exercise on aerobic capacity, which is the ability of the heart and lungs to consume oxygen and deliver it to the muscles.

“Aerobic capacity, also known as fitness, is an important predictor of health and longevity,” said Lars Andersen, a professor of musculoskeletal disorders at The National Research Centre for the Working Environment in Denmark, who was not involved in the study.

Seventeen female NCAA Division I college soccer players participated in the study. Nine were placed in the kettlebell group and eight in a comparison group that did circuit training instead.

The groups were not randomly assigned. Women were included in the kettlebell group only if they showed proper technique for the “snatch” exercise - swinging the weight down between the legs and then lifting it up over the head, alternating between arms. The researchers write in the Journal of Strength and Conditioning Research that this was done to prevent injury.

Both groups trained for 20 minutes, three days a week, in addition to their usual off-season training.

The circuit training involved multiple free-weight and body-weight exercises including squats, sit-ups and sprints. The kettlebell group did the snatch exercise.

After four weeks, aerobic capacity in the kettlebell group rose by 6 percent but didn't change significantly in the circuit training group.

The authors say the results might only apply to athletes or people with some kettlebell experience. Earlier studies in relatively inactive people did not show significant improvement.

Still, Koch said, kettlebells can be used at any fitness level. A popular recommendation is a starting weight of 16 kilograms (about 35 pounds) for men and 12kg (about 26 lbs) for women of “average fitness level.”

He warns, however, “Kettlebells are not a magic bullet, they are simply one viable option people have to exercise with to increase their aerobic capacity.”

Kettlebells are an alternative to traditional aerobic fitness such as running, he said, and “finding exercise modes one enjoys is the key to building a lifelong habit of movement and optimal health.”