Tensile stress is produced when an object or material is axially loaded in tension with forces pulling on either side. For example, the humerus is loaded axially in tension when a sit-up is done; very large tensile loads may sprain or rupture ligaments and tendons, tear muscles and cartilage, and fracture bones. Compressive stress is the axial stress that results when a load tends to push or squash the molecules of a material more tightly together at the analysis plane. For example, the femur and tibia are under compression when you are standing, as a result of your body weight pushing down; large compressive loads may cause bruising of soft tissue and crushing fractures of bones.
Shear stress is a transverse stress that acts parallel to the analysis plane as a result of forces acting parallel to this plane; these forces tend to slide the molecules of the object past each other. Scissors are also referred to as shears due to creating large shear stresses in the material; however in the human body shear loads can cause blisters of the skin, joint dislocation or shear fractures of the bone. Strain is the quantification of the deformation of a material. Linear strain is produced by compressive or tensile stresses; molecules being pulled apart or pushed together, as a result some change in length accompanies this stress. However, shear strain occurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other.