Biomedical Engineering Reference
In-Depth Information
where W
0
is the initial body weight measured prior to an important strike, t
s
is
the frame number when the strike occurs, and W
ts
is the body weight after foot
trike. It is thus possible to deduce oscillations of the center of mass. It could be
used in Walk-In-Place interfaces based on GRF measurements (such as using a
Nintendo Wii Balance Board). Information about other axes provides interesting
information for navigation, such as the velocity in the forward direction or lateral
displacements involved in rotations. However, they are more difficult to use in virtual
reality as they require the subject to move in all the directions which generally
leads to going outside the measurement area in a very near future (generally the
next step).
One has to notice here that the two feet must be placed over the forceplate surface
to enable correct estimation of the above displacements. It is also possible to use
footscan devices (from RSscan company
http://www.rsscan.com
)
to measure the
pressure below the feet. It consists in introducing a sole in the shoe which enables the
user to move freely in the real environment. This type of device delivers the pressure
in space and time which enables us to compute the resulting vertical component of
GRF. The other components cannot be accurately deduced but the location of the
center of pressure (COP) may help to guess how this force is oriented in the other
main directions.
COP can also be deduced with a forceplate if this latter provides 3D GRF and the
corresponding global momentum M over the ground. M and GRF are linked by:
M
x
=
GRF
z
×
y
COP
−
GRF
y
×
z
0
M
y
=−
GRF
z
×
x
COP
+
GRF
x
×
z
0
where x
COP
and y
COP
stand for the local coordinates of the COP in the forceplate
reference frame, and
z
0
stands for the vertical coordinate of the forceplate surface.
It becomes:
x
COP
=−
(
M
y
−
GRF
x
×
z
0
)
GRF
z
y
COP
=
(
M
x
+
GRF
y
×
z
0
)
GRF
z
In quasi-static condition, it is possible to assume that the average value of COP
is the projection of the center of mass on the ground. Hence if COP moves the
pose of the user has changed, for example to lean in a given direction. This type
of information has been used in biomechanics to determine if a subject was turning
while walking [
24
,
34
]. It can thus be used to indicate a main direction so that the
user could be viewed as a kind of joystick. It is used in some Nintendo Wii games
based on the Balance Board device (one has to notice that this device is not a force
plate as it does not provide the systemwith 3DGRF andMomentums ; it provides the
vertical component of the GRF and the location of the COP). The Joyman interface
[
19
,
26
] is an extension of the idea of using the user's body as a joystick.
