Biomechanics is a study of force and motion and the interrelationship between them. Our activities of daily living depend on our ability to move. These movements in turn depend on our capabilities of producing muscle forces that can move the bones around the joints. The muscle forces, gravitational forces, and ligamentous forces in our body have both internal and external effects. The external effect is to cause an acceleration in the body. The internal effect is to develop a state of strain, that is to change the shape of the body.
Every time a step is taken, the hip abductors are needed to decelerate the trunk from a lateral movement. This is the external effect of the abductors. The internal effect of the abductors is to produce a state of strain in the head of the femur.
In applying biomechanical thinking to orthopedic surgery, one must constantly keep in mind
Frankel VH. Biomechanics of the Musculoskeletal SystemIntroduction. Arch Surg. 1973;107(3):405. doi:10.1001/archsurg.1973.01350210041013