While strength training was once doubted to benefit kids, a new research review confirms that children and teenagers can boost their muscle strength with regular workouts.

The findings, researchers say, support recent recommendations from the National Strength and Conditioning Association (NSCA) that kids strength train two or three times a week — though only under professional supervision.

Studies in recent years have shown that kids’ risk of injury from strength training is no greater than — and is often less than — that from other types of exercise or sports. And experts now say that the potential benefits of such training — such as increased bone density, decreased body fat, boosting performance and curbing injury risk in sports — generally outweigh any risks.

In most of the studies, kids used free weights or resistance-training machines anywhere from one to five times a week for an average of 40 minutes per session. The duration of the training ranged from one month to just over a year