A

A load applied perpendicular (normal) to a surface produces a normal stress; when that load pushes the material inward it shortens the dimension in the load direction, creating compressive strain rather than shear or tensile effects. Experiments that applied forces perpendicular to equine metatarsal bone, for example, showed the largest response as a negative (compressive) principal strain [Schamhardt, 1989, PMID 2729717].