Many exercises can be tough on the knees. But move training sessions into a pool and you’ve got a safe and therapeutic way for people with arthritis or knee injuries to work out.

When someone walks in water, the force of drag that opposes his or her movements leads to an intense workout because water provides 12 times more resistance than air. In addition, water is buoyant and supports body weight, acting to cushion the joints and reduce strain. For those plagued with aching knees, this makes exercise safer and movements less likely to hurt. Plus, water’s hydrostatic pressure—the pushing force exerted by fluids due to gravity—can lessen joint and tissue swelling.

Another benefit of exercising in the pool is that people can work out for longer periods of time because movements are easier to execute in the water. According to experts, for the best results from pool workouts, try different types of exercises.