Boxing punch bags are normally filled with shredded fiber that is enclosed in a 1-inch closed cell foam liner. They can also be filled with cloths, rags, various grains and sand.
Some punching bags can be filled with water or air, as long as they have an internal bladder that can contain it. The differences between various fill types reflect the way the bags affect the body of the boxer. A hard-fill bag retains its shape and provides the most resistance. On the other hand, a soft-fill bag lessens the impact of punching on the boxer's hands and wrists. Water-filled bags provide a lot of elasticity while retaining their shape.