Biomedical Engineering Reference
In-Depth Information
is continuing to be accelerated forward and now away from the stance foot.
During this single-support phase, the stance limb plantarflexors increase their
activity, causing the COP to move rapidly forward to an initial “pushoff.”
Simultaneously, the right swing limb is swinging forward and RHC1 occurs
at 35% of this first gait cycle. Then, during the double-support phase, the
COP moves very rapidly forward toward the right foot. The trajectory of the
COM has now moved forward about 25 cm and (not shown here) is headed
to pass forward along the medial border of the right stance foot (Jian, et al.
1993). Thus, by the end of the first step, the trajectory of the COM has already
reached the steady state walking pattern, as shown previously in Figure 11.7.
The initiation of gait in the young, the elderly, and Parkinson's patients
was analyzed in considerable detail (Halliday et al., 1998). The basic find-
ing in virtually all the kinematic and kinetic variables was that the temporal
patterns were the same but that they were altered by a scaling factor related
to their final steady state walking velocity. The young adults had the highest
velocity; the fit and healthy elderly were slower, and the Parkinson's patients
were slowest. All of the several significant differences disappeared after the
variables were divided by the subjects' walking velocity. Thus, we can con-
clude that the synergistic patterns are still present but are reduced by some
tonic gain control that decreases with aging and disease.
11.3.3 Gait Termination
The challenge to the balance control system during termination of gait is
even more critical during termination of steady state walking. The forward
momentum of the body must be removed within the last two steps, and the
COP must be controlled to a position slightly ahead of the COM trajectory
as the COM comes to a near stop. Figure 11.9 depicts the COM and COP
trajectories during the contact of the right foot on force plate #2 and then the
left foot on force plate #1; the command to stop was made by a flashing light
when the previous left heel made contact with force plate #3 (not shown).
The percent of that stride period began at that heel contact and at 64% of that
stride was LTO1 when 100% of body weight is supported by the right foot.
Prior to this 64% point was the double-support period, and the trajectory of
the COP moved rapidly forward from the left foot to the right foot (shown
is the 56 - 64% period). During this double-support period, the COP moves
rapidly forward so that the COP-COM vectors (shown as arrows) indicate
a rapid deceleration of COM. Then, during single support of the right foot
(64 - 100%), the right plantarflexors' activity increases dramatically, causing
COP to move forward to increase the COP-COM deceleration vector, and by
100%, the COM forward velocity is reduced by about 70%. During this right
single-support period, the left swing limb is rapidly decelerated by its hip
extensors/knee flexors, resulting in a step length about half of normal. The
rapid loading of the left foot after LHC causes a rapid lateral and forward
shift of the COP until at 120% the COP stops directly ahead of the trajectory
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