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 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 is discussed more extensively in Chapter 1, but pertinent aspects relating to the foot are discussed
here (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
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 ...