BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 294

Top View Total Body COM versus Resultant COP
Walking Direction

Termination

− 80

LTO1

FORCE PLATE #2

LHC

64 *

100 *

− 70

70 + 64 +

80 + 60 +

56 + 90 +

100 +

105 *

110 + 151 *

151 +

QUIET STANDING

COM

COP

− 60

Arrow :COP → COM

FORCE PLATE #1

* COP % of Gait Cycle
+

120 *

COM % of Gait Cycle

34 44 54 64

X-Coordinate (cm)

Figure 11.9 Trajectories of the COP and COM during termination of gait. The final
two steps are shown; the right foot on force plate #2 and the left foot on force plate #1.
The last gait cycle, from left heel contact (LHC) to LHC, was 0 - 100%, and shown here
is 56 - 100%. From 56 - 64% is double-support, from 64 - 100% is right single support,
and 100 - 151% is the final double-support. The COP trajectory moved rapidly forward
toward the right foot during double-support and continues forward under the right foot
until LHC, when the left foot begins to load, moving the COP rapidly laterally until
it reaches 120%, when it stops ahead of the COM trajectory. See the text for the
synergistic motor patterns to achieve the desired COM trajectory.

of the of the COM. This loading of the left foot was achieved simultaneously
by increased left hip abductor activity and unloading of the right foot by
decreased right hip abductor activity. The final phase, from 120% to 151%,
sees the final deceleration of the COM as the COP moves posteriorly (by
reduced plantarflexors of both limbs) and slightly to the right (by increased
right hip abductor and decreased left hip abductor activity) to come to rest
at quiet standing. The most significant aspect of this termination synergy is
the proactive control of the both hip abductors to arrest the later trajectory
of the COP at a point directly ahead of the COM trajectory. About 85% of
the forward velocity was arrested by the right plantarflexors and 15% by the
left plantarflexors.
Interestingly, on two unpublished termination trials, one on a patient with
peripheral sensory loss and one on subject with ischemic leg block, the COP
trajectory in this final stage when both feet were loaded was very erratic: the
COP trajectory overshot the COM trajectory in both the medial/lateral and
anterior/posterior directions. This indicates that the peripheral sensory system
plays a major role in monitoring and feedback for both the COP and COM
positions.