The unique relationship between stress (intensity of force) and strain (measurement of deformation) is evident. The external forces that act on the body impose loads that affect the internal structures of the body. In biomechanics, the understanding of mechanical properties internal forces is important for preventing injury and evaluating the causes of injury. It starts with mechanical stress, the internal force divided by the cross-sectional area of the surface on which the internal force acts. The three principle stresses are tension, compression, shear in biomechanics it’s known as the three basic methods of loading. Tension involves pulling the structure, compression involves a stress that compacts the structure and shear involves pushing the structure eccentrically.
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.