BIOMECHANIC PRINCIPLES OF THE FOOT AND ANKLE
The primary role of the foot and ankle is locomotion. It is critical for those treating disorders of the foot and ankle to not only have knowledge and understanding of the anatomy but also the biomechanical principles of the foot and ankle. The following is a brief description of the biomechanics of the foot and ankle during the gait cycle. Understanding and being familiar with these principles aids in diagnosing problems that affect the anatomy and function of the foot and ankle.
Gait is the orderly progression of the body through space while expending as little energy as possible. The phases of gait are described in more detail in Chapter 13. As the body moves through a gait cycle, muscle forces are generated actively to counteract the passive effects of gravity on the body. To accommodate these forces, the foot is flexible at the time of heel strike to allow for absorption of impact of the body against the ground. However, at the end of stance phase, the foot becomes rigid at toe-off, when it must assist in propelling the body forward. The magnitude of the forces on the foot increases significantly as the speed of gait increases. For example, when an individual is walking, the initial force with which the foot meets the ground is approximately 80% of body weight. With jogging, the force increases to approximately 160%. The peak force against the foot during walking is 110% of body weight, and for jogging 240%. This marked increase in force contributes to some of the injuries seen in runners.
An important component of the physical examination is observation of the patient during the walking cycle (Figures 10–1 and 10–2). This helps in determining the cause of a gait anomaly. For example, an equinus deformity from spasticity or contracture will cause the forefoot to make initial contact with the ground rather than the heel. Furthermore, at 7% of the gait cycle, the foot is usually flat on the ground, but spasticity or contracture of the Achilles tendon causes this to be delayed. At 12% of the cycle, the opposite foot toes off and the swing phase begins. Heel rise of the standing foot begins at 34% of the cycle as the swinging leg passes the standing limb. However, with spasticity, heel rise may occur earlier, whereas with gastrocsoleus weakness, heel rise will be later. Heel strike of the opposite foot occurs at 50% of the cycle, ending the period of single-limb support, and may occur sooner if there is weakness of the contralateral gastrocsoleus muscle. Toe-off of the opposite foot occurs at 62% of the cycle, at the beginning of the swing phase. These markers of the gait cycle should be kept in mind when observing gait, so pathologic conditions may be ...