Biomedical Engineering Reference
In-Depth Information
addressed in static or quasi-static conditions by assuming that the projection of the
COM on the ground remains in the contact area. However, this assumption becomes
false when velocity or accelerations cannot be neglected, such as in walking. Some
authors have extended the idea of maintaining the projection of COM inside the
base of support to such a dynamic situation [
27
](seeFig.
3.7
). The key idea is to
take velocity into account in the inverted pendulum model. The resulting so-called
extrapolated center of mass xCOM is thus defined by:
l
C OM
ω
0
xCOM
=
COM
+
with
ω
0
=
where l stands for the length of the pendulum (the distance between the ankle joint
and the COM).
This point has been used to demonstrate a simple control strategy to deal with
balance while walking [
28
] and could thus be evaluated in immersive environments.
This control strategy states that the COP should be placed at a certain distance
behind and outward of the xCOM when foot-strike occurs. Hence, a disturbance in
COM velocity change
v can be compensated by a change in COP position equal to
/
ω
0
. This control strategy is also available when starting, stopping, or turning.
This point is nowwidely used in biomechanics and neurosciences, but it has not been
used in VR protocols to analyze the balance status of users who have to navigate in
virtual environments.
However, understanding dynamic balance in walking is still a challenge. Indeed,
the inverted-pendulummodel which globally can generally explain energy exchanges
in walking (as explained in the previous section), could demonstrate forward falling
phases while walking, which naturally leads to stepping forward to prevent the user
from falling. Falling forward would also be a good representation of what is hap-
pening in step initiation. This idea has been reused recently in VR. The “Joyman”
v
Fig. 3.7
Inverted pendulum
model in static condition, with
length l (distance between the
ankle joint and the COM),
mass m. COPmin and COP-
max stand for the minimal
and maximal limits that the
COP can take to remain in the
base of support. In the static
condition, xCOM is simply
the projection of the COM on
the ground. Adapted from [
27
]
