BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 292

Top View Total Body COM versus Resultant COP
Walking Direction

Initiation

− 79

RHC1

35 *

25 *

20 *

0 *

RTO

− 5

− 69

10 +

20 +

25 +

− 10

0 +

FORCE PLATE #2

69 +

−

QUIET STANDING

69 *

−

COM

− 15

− 20

COP

Arrow : CUP COM

→

RELEASE PHASE

*COP % of Gait Cycle
+ COM % of Gait Cycle

FORCE PLATE #1

27 37

X-Coordinate (cm)

47

Figure 11.8 Trajectories of the COM and COP during initiation of gait. Subject was
standing quietly with the left foot on force plate # 2 and the right foot on force plate
#1. From the quiet standing position ( - 69% of the initial gait cycle), the COP moves
posteriorly and laterally toward the swing limb, reaching the release position at - 20%
of the gait cycle, then moving rapidly laterally toward the stance foot, reaching it at
right toe-off (RTO). The arrows from the COP to the COM are acceleration vectors
showing the acceleration of the COM as predicted by the inverted pendulum model.
See the text for details of the motor control of the COP to achieve the desired COM
trajectory.

origin of both axes is located over the quiet standing position. The initial task
of gait initiation is to accelerate the COM forward and toward the stance limb.
Thus, the first small movement of the COP must be in the opposite direction:
posteriorly and toward the swing limb. This posterior shift is accomplished
by a sudden decrease in plantarflexor activity; the lateral shift toward the
swing limb results from a momentary loading of the swing limb via increased
activity of the swing limb abductors and decreased activity in the stance limb
abductors. As predicted by Equation (11.3), this momentary shift of the COP
accelerates the COM in the desired direction forward and toward the stance
limb. This constitutes the release phase. Interestingly, this initial increase in
swing limb ground reaction force and decrease in stance limb force appeared
in graphs considerably earlier (Herman, et al., 1973) but no comment was
made. Then the COP moves rapidly laterally towards the stance limb as the
stance limb abductors turn on, the swing limb abductors turn off, and the
swing limb hip flexors/knee extensors accelerate the swing limb upward and
forward. Then, at RTO, the COP is under the left foot and is controlled by the
left leg plantarflexors. The start of this single-support phase is labeled 0%. At
this time, the COM has moved forward about 6 cm, but from its curvature, it