The best swimmers tend to be tall and thin with long arms, long legs, long feet, and long hands. The size of their hands gives them great "water grasp", and only a very small hand movement keeps them afloat. They also have great strength, endurance, insulation  against heat loss while in water below body temperature, thus better energy conservation, low resistance  when in the water, and swimming stroke mechanics . Part of the reason why some people are better swimmers than others has to do with body density. The average person's body density is slightly less than that of water. Muscle has greater density than fat. Therefore, very muscular people tend to be poor at staying afloat. Bone has greater density than fat. Therefore, very skinny people tend to be poor at staying afloat. Good buoyancy  is not necessarily the most important factor in good swimming, but it certainly helps. For example, great competitive swimmers--tall, thin, and usually more muscular than average--tend to have greater body density than average and less buoyancy. Many would not float very well if they remained motionless in the water. Their much greater "water grasp" makes up for it.  www.ks5u.com The average woman contains a higher proportion  of fat in her body than the average man, and the average man contains a higher proportion of muscle in his body than the average woman. Therefore, in general, women are better floaters than men. Compared with other competitive sports, the performance of women is closer to that of men in competitive swimming. What can we learn from the passage about body density?
Higher body density reduces swimming ability. A person's body density decides one's swimming ability. Fat people can always swim faster than thin people. People with a lower proportion of muscle can hardly swim.