Water environment greatly different from the land, studies have shown that people in water activity resistance is 883 times the air. The huge resistance to water increases the difficulty and total amount of exercise, giving you more than 6 times the strength you normally use to accomplish an exercise, thus achieving a multiplier effect.
In addition, the thermal conductivity of the water environment is 28 times the air, even stay in the water will consume a lot of energy stationary. Just like a hot egg, put it in water to cool it much faster than in the air. So, the energy consumed by a person in water for 20 minutes equates to more than one hour of exercise on the ground with the same intensity. 40 minutes of exercise in the water can consume 400 calories easily reach the purpose of exercise, and will not cause muscle pain.