"The body has 90 percent support when under water. That means the joints only have to offer 10 percent support," says Theodor Stemper, a sports scientist at the Bergischen University in Wuppertal, Germany.

The buoyancy adds comfort and alleviates the pressure on the joints. "That lets a person participate ... even if they're overweight or have never done it before," says Stemper. Aqua fitness is also great for pregnant women or people suffering from a joint injury.

Apart from buoyancy, the water pressure can prove beneficial. For every extra meter of water depth, the pressure rises by 0.1 bar.

"That ... promotes the circulation of blood," says Stemper. "It also promotes the metabolism."

All these effects can be achieved in aqua relaxing and aqua balancing, which involve minimal physical exertion.

Other forms of aqua fitness require some form of activity. And all have positive effects on health.

"Aqua-jogging, aqua-running ［and］ aqua-aerobics all help train heart and circulatory endurance," adds Peter Freyer, a sports instructor in Duisburg, Germany.

Training in water always has a bigger impact than training in the air, as water is 1,000 times denser.

"That means I'm working against significantly greater resistance," says Freyer. "A lot more is being demanded of the body."