Any part of the body that is likely to bump into something.
Kids often fall down and bump into things causing scratches and bruises on their leading edges. When a child frequently has bruises on parts of their body that are not leading edges, child abuse is sometimes suspected as possible cause.
Clothing, especially sportswear and protective clothing are often reinforced in leading edge areas because those are the parts that are most likely to suffer damage. Some jackets for motorcycle riders are reinforced with kevlar on the leading edges.