Biomechanics is the branch of science that is concerned with understanding the relationship between a living body's structure and function relative to movement. When looking at swimming one must first look at how the human body acts and generates forces in water. Water is a unique environment. It possesses qualities that will assist the swimmer, but it also has qualities that will impede the swimmer's progress through the water. This upward force is known as the buoyant force and acts through the center of the displaced water.
Every object has an absolute center position where all forces exerted by the body equal zero.
This central point of an object's mass is known as the center of gravity and is the point around which it balances. The location of the center of buoyancy which is the center of the water displaced by the body, is actually closer to the head than in the location of the center of gravity. The chest has a lower density than the water because of its lung capacity. The term specific gravity is used to describe the ratio between an object's density to that of water's density. Therefore anything placed in water will float or sink in accordance to it's own specific gravity value.