Biomedical Engineering Reference
In-Depth Information
Fig. 3.6
Energy transfer
between kinetic (
bold curve
)
and potential energy (
dashed
curve
) which demonstrate
an anti-phase coordination
(adapted from [
13
])
Another solution to indirectly evaluate energy expenditure without these types
of devices consists of computing the mechanical internal work. Using the Angular
Momentum Theorem and knowing joints kinematics it is possible to approximate
the internal mechanical work W
int
[
86
]:
n
1
2
m
i
1
2
I
i
ω
G
i
2
i
W
int
=
+
+
m
i
g
h
i
i
=
1
where m
i
is the mass of the ith segment, G
i
its center of mass position, I
i
its inertia, h
i
its height and
ω
i
its angular velocity. g stands for gravity. The absolute values of the
instantaneous work (computed at each time) is known to be well correlated with the
energy expenditure measured using measures of oxygen consumption [
86
], although
there exist some debate about this relation. It could thus be used in VR with motion
capture devices to evaluate the difficult of or the performance on a navigation task.
3.3.3 Balance
In bipedal locomotion balance is a key point as the supported area on the ground is
very small compared to the body surface. As stated above, energy expenditure is a key
point in human walking because of transfers between kinetic and potential energy
suggesting that walking could be viewed as a sequence of forward falls. Hence, it must
have an impact on several multisensory inputs, especially on vestibular information.
It has been shown that changes in walking conditions, such as walking on a treadmill,
could affect stability and balance, compared to ground walking [
29
]. To maintain
balance people increase step width and generally reduce step length for a given
speed. Hence, balance status is used by some researchers to evaluate the naturalness
of treadmill walking in immersive environments [
61
].
To address balance in walking, the human body is generally considered as an
inverted pendulum with the contact point being the ankle joint and for which the
total mass is placed on the COM. This problem of unstable balance is generally
