Chapter 9

The following is a limited discussion of the biomechanic principles governing the foot and ankle during the gait cycle. The physician must have a clear understanding of these principles to evaluate problems affecting the foot and ankle accurately. Once normal biomechanic function is understood, anatomic and functional abnormalities are more easily detected.

### Gait

Gait is the orderly progression of the body through space while expending as little energy as possible. As the body moves through a gait cycle, forces are generated actively, by action of the body’s muscles, and passively, by the effects of gravity on the body. To accommodate these forces, the foot is flexible at the time of heel strike, when it must absorb the impact of the body against the ground, and rigid at the time of toe-off, when it must assist in moving 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, whereas when an individual is jogging, it is approximately 160%. The peak force against the foot during walking is approximately 110% of body weight, whereas for jogging it is approximately 240%. This marked increase probably contributes to some of the injuries seen in runners.

### The Walking Cycle

The walking cycle is discussed more extensively in Chapter 1, but pertinent aspects relating to the foot are discussed here (Figure 9–1).

###### Figure 9–1.

Phases of the walking cycle. Stance phase constitutes approximately 62% and swing phase 38% of the cycle.

(Reproduced, with permission, from Mann RA, Coughlin MJ: The Video Textbook of Foot and Ankle Surgery. Medical Video Productions, 1991.)

Observation of the patient while walking may give the clinician insight into the cause of a gait anomaly (Figure 9–2). For example, equinus deformity resulting from spasticity or contracture may cause the toe to make initial contact with the ground rather than the heel. At 7% of the gait cycle, the foot is usually flat on the ground, but spasticity or tightness 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. Heel rise may be premature in spasticity or prolonged in weakness of the gastrocsoleus muscle. Heel strike of the opposite foot occurs at 50% of the cycle, ending the period of single-limb support; this may occur sooner if there is weakness of the contralateral calf muscle. Toe-off of the opposite foot occurs at 62% of the cycle, at the beginning of the swing phase. These markers ...

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