BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 286

11.2 MEDIAL/LATERAL AND ANTERIOR/POSTERIOR
BALANCE IN STANDING

11.2.1 Quiet Standing
Standing has been the subject of considerable research with posture and bal-
ance being the dominant tasks in both medial-lateral (M/L) and anterior-
posterior (A/P) directions (Horak & Nashner, 1986; Winter et al. 1996 ; Gage
et al. 2003). The primary outcome measures are center-of-pressure (COP) and
center-of-mass (COM), and it has been shown that as an inverted pendulum
model (see Section 5.2.9) that for sway angles of less than 8 ◦ the COP and
COM are related to the horizontal acceleration of the COM in either the A/P
or M/L directions (Winter et al. 1996):

COP − COM =− I x /Wd =− K x (11.3)

where: I = is the moment of inertia of the total body about the ankle in the

direction of interest.

x = is the horizontal acceleration of the COM in the direction of

interest.

d = is the vertical distance from the ankle joints to the COM.

W = is the total body weight above the ankle joints.

Thus, we can think of COP - COM as an error signal in the balance
control system for controlling the horizontal acceleration of the COM, and
it forces us to focus on how the CNS controls the COP to achieve a sta-
ble balance. In quiet standing in the A/P direction COP is controlled by the
ankle dorsiflexors/plantorflexors (Horak & Nashner, 1986), whereas in the
M/L direction, COP is controlled by the hip abductors/adductors in what
has been described as a “load/unload” mechanism (Winter, et al. 1996).
Figure 11.4 summarizes this mechanism during quiet standing and shows
when two force platforms are used the right and the left vertical ground
reaction forces along with the M/L COP. Note that the these vertical ground
reaction forces oscillate about 50% of body weight and the fluctuations about
this 50% are virtually equal in magnitude and also exactly out of phase. The
M/L COP (cm) is a weighted average of the left and right vertical reaction
forces and with the convention shown is in phase with the right vertical reac-
tion force. The horizontal ground reaction forces are negligible; thus, from
our inverse dynamics the left and right hip moments are respectively in phase
with the left and right vertical reaction forces. Thus, this “load/unload” mech-
anism is accomplished by hip abductor/adductor moments that are exactly
equal in magnitude and also 180 ◦ out of phase. A detailed analysis of the
COP and COM waveforms has attributed these motor patterns to a simple
stiffness control (Winter at al., 1998). This stiffness control is not reactive
because the COP is virtually in phase with the COM except the COP oscil-
lates with larger amplitude either side of the COM and the COP - COM