Pool therapy (also known as aquatic therapy or hydrotherapy) basically means "therapy in water," and has been around for a while. However, its use may not be as popular as it should be.
Water has two great properties which can be used in therapy. One is the buoyancy and the other is its isokinetic resistance.
Because the human body is slightly less dense than water, our body's weight is supported in a pool. The more the immersion, the more supported it will be. This "low-impact" activity is very helpful for those people who cannot walk because of pain upon weight bearing activity, such as those with arthritis in the knee or hip.
Buoyancy can also be used to support a limb so that the limb can exercise in a condition without the effect of gravity. Early movement is important especially in the shoulder. People with frozen shoulder or impingement syndrome will find it easier to mobilize in water, without aggravating the pain.
Usually the water in a therapeutic pool is kept warm. The heat and buoyancy from the water will help to relax muscles and make stretching more effective. The heat can also improve circulation in the muscles and joints. A stiff joint will find it a lot easier to mobilize in the pool.
Water can also help to strengthen muscles. By providing a turbulent "drag," water provides resistance to a movement called isokinetic resistance.
An isokinetic resistance is different than that of a dumbbell. The resistance from a dumbbell is constant and downward (due to the force of gravity), but isokinetic resistance varies throughout the movement, depending on the force being exerted. In other words, the quicker you want to move in the water, the more resistance you will experience. This property is very important when it comes to strengthening a painful muscle or joint. When patients cannot exert much effort at a certain point in the range of motion because of pain, the resistance patients experience will decrease accordingly. Therefore, the risk of getting injured during exercises will be reduced to a minimum. In fact, this property of water has been widely used in building high caliber weight training machines.