KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 125

segment distal to the joint are positive; clockwise moments are negative.
Thus, a plantarflexor moment (acting on its distal segment) is negative, a
knee extensor moment is shown to be positive, and a hip extensor moment
is negative.
The moments are plotted during stance, with heel contact at 0 and toe-off
at 680 ms. The ankle muscles generate a positive (dorsiflexor) moment for
the first 80 ms of stance as the pretibial muscles act eccentrically to lower
the foot to the ground. Then the plantarflexors increase their activity during
mid- and late stance. During midstance they act to control the amount of
forward rotation of the leg over the foot, which is flat on the ground. As
the plantarflexors generate their peak of about 60 N m, they cause the foot

to plantarflex and create a pushoff. This concentric action results in a major
generation of energy in normals (Winter, 1983), but in this patient it was
somewhat reduced because of the pathology related to the patient's hip joint
replacement. Prior to toe-off, the plantarflexor moment drops to 0 because
that limb is now unloaded due to the fact that the patient is now weight
bearing on his good limb and, for the last 90 ms prior to toe-off, the toe is
just touching the ground with a light force. At this same point in time, the
hip flexors are active to pull this limb upward and forward as a first phase
of lower limb swing.
The knee muscles effectively show one pattern during all of stance. The
quadriceps are active to generate an extensor moment, which acts to con-
trol the amount of knee flexion during early stance and also extends the
knee during midstance. Even during pushoff, when the knee starts flexing in
preparation for swing, the quadriceps act eccentrically to control the amount
of knee flexion. At heel contact, the hip moment is negative (extensor) and
remains so until midstance. Such activity has two functions. First, the hip
muscles act on the thigh to assist the quadriceps in controlling the amount
of knee flexion. Second, the hip extensors act to control the forward rota-
tion of the upper body as it attempts to rotate forward over the hip joint
(the reaction force at the hip has a backward component during the first half
of stance). Then, during the latter half of stance, the hip moment becomes
positive (flexor), initially to reverse the backward rotating thigh and then, as
described earlier, to pull the thigh forward and upward.
The fourth curve, M s , bears some explanation. It is the net summation
of the moments at all three joints, such that extensor moments are positive.
It is called the support moment (Winter, 1980) because it represents a total
limb pattern to push away from the ground. In scores of walking and running
trials on normal subjects and patients, this synergy has been shown to be
consistently positive during single support, in spite of considerable variabil-
ity at individual joints (Winter, 1984). This latter curve is presented as an
example to demonstrate that moment-of-force curves should not be looked at
in isolation but rather as part of a total integrated synergy in a given move-
ment task. A complete discussion and analysis of this synergy is presented
in Section 11.1.

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