Biomedical Engineering Reference
In-Depth Information
fight against gravity and produce external forces to propel his body while maintain-
ing balance. To this end, one has to produce forces and torques at each joint (resulting
from the muscle activation patterns) in order to satisfy the external constraints. In
this section, we provide a brief description of the available knowledge about these
points. Many devices have been introduced in order to measure dynamic constraints
in walking such as force plates and could be used to control avatars or cameras. The
Wii balance board from Nintendo has been used to immerse users in videogames by
measuring the instantaneous ground reaction force and the center of pressure of the
user. This type of device is strongly associated with the idea of balance control which
is also a key point in walking. We thus provide the reader with some knowledge about
balance control in walking and give some examples of how it could be used in VR.
We also address the problem of energy expenditure which is a key point in bipedal
locomotion and which could be used to evaluate the relevance of immersive systems
for navigating in virtual environments.
3.3.1 Forces and Torques Description
From the mechanical point of view, the external forces and torques exerted on the
body while walking are due to gravity g and ground reaction force GRF. It comes:
m
γ
+
=
BW
GRF
where BW is body weight, m is the total mass and
γ
is the center of mass acceleration.
Hence, GRF is very important in order to understand motor strategies and to analyze
human walking. It is generally analyzed using a force plate through the three main
axes. Even if GRFs vary [
5
] depending on speed, style, mechanical properties of the
ground, the type of shoes worn by the subject, etc., it exhibits a stereotyped shape
[
87
], as shown in Figure
3.4
.
During walking, the vertical component of GRF exhibits two peaks, which corre-
spond to heel-trike (associated with the absorption of the downward COM velocity)
and toe-off (associated with propulsion of the body forward and upward). These two
peaks reach higher values than body weight. From the muscle point of view, “absorp-
tion” is linked to gathering elastic potential energy and “propulsion” is associated
with release of this elastic potential energy.
Females have a higher vertical ground reaction force (expressed as a percentage of
body weight) in heel-strike and toe-off stages than males [
15
,
16
]. Indeed females
tend to increase their walking speed by increasing step length instead of frequency,
which results in a higher vertical ground reaction force than males. Walking speed
also has an effect on vertical ground reaction force so that in VR it could be possible
to adapt feedback associated with force according to speed and gender.
Some authors have used information about the shape of the ground reaction force to
provide vibration feedback to the user [
73
] and sound feedback could also be tuned
according to this type of information [
37
]. This type of feedback has positive effects
