Kevin A. Kirby, DPM, demonstrates how the hip joint can store and release elastic strain energy. When a patient is lying completely relaxed in a supine position, manually rotating the hip into an internally rotated position and then releasing the lower extremity shows how the hip joint may store and release transverse plane elastic strain energy. When the capsular ligaments and muscles that cross the hip joint are stretched by internal hip rotation, these soft tissue structures will store elastic strain energy or potential energy. When the limb is suddenly released, the stored elastic strain energy is converted into kinetic energy that causes a rapid external rotation acceleration of the thigh, leg and foot due to rapid shortening of these ligaments and muscles. This storage and release of transverse plane elastic strain at the hip is an important contributor to the gait phenomenon known as abductory twist. With the abductory twist, rapid external rotation of the foot occurs at the time of heel off in individuals whose hip has been excessively internally rotated during the late midstance phase of gait by excessive pronation of the subtalar joint. Elastic strain energy is also an important energy conservation mechanism in many forms of animal locomotion.
For a related video on the abductory twist, see http://www.podiatrylive.com/_biomechanical_abductory_twist .