BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 290

*

0.8

*Peak R xy 0.766

at T = 0.06 s

=

0.6

0.4

16

0.2

14

0

12 − 1 − 0.5 0 0.5 1

τ (sec)

10

8

R Glut Med
L Glut Med

6

4

2

0

0 10 20 30

Time (sec)

40 50 60

Figure 11.6 A 60-sec linear envelope record of the left and right gluteus medius
muscles showing an antagonistic cocontraction pattern. R xy (upper right) is plotted for
τ =± 1 sec and shows a peak of 0.766 at τ = 0 . 06 sec; this shows that these left and
right muscles were virtually in phase, indicating an inefficient coactivation. See the text
for more details on the reported low back pain of those subjects exhibiting this pattern.

is decided by the vector joining the COP to the COM (review the inverted
pendulum equations in Section 5.2.9). Thus, from LTO the medial-lateral
trajectory of the COM is headed toward the right foot, but from the curvature
of this trajectory, it is evident that it is being accelerated medially toward
the future position of the left foot. In the plane of progression (shown by the
center line), there is a deceleration of the COM while it is posterior of the
COP and an acceleration of the COM after the COM moves forward along
the foot and is anterior of the COP. This would be seen in the velocity of
the COM, which is decreasing during the first half of stance and increasing
during the latter half of stance. The challenge to the CNS is that the human
is never more than about 400 ms away from falling, and it is the trajectory of
the swinging foot that decides its future position and, therefore, its stability
for the next single-support period.

11.3.2 Initiation of Gait
The complete collaboration of both the A/P and M/L muscle groups is never
more evident in the initiation of gait. Going from a very stable balance con-
dition during quiet standing to a walking state in about two steps requires