Biomedical Engineering Reference
In-Depth Information
•
to animate avatars of the user with only little information, such as gait speed [
12
],
by computing the corresponding joint angles, so that the user could embody the
virtual human which is walking in VR,
•
and to compute relevant multisensory feedback such as forces, sounds, vibra-
tions… in accordance with the current walking state; the goal is to improve
immersion and the feeling of being there in the virtual environment.
3.2.1 Global Description
3.2.1.1 Linear Walking
A gait cycle is defined as the time interval between two identical movement events
during gait. The heel contact to the floor (heel strike) is widely accepted as the starting
point and the next contact of the same heel as the end of a gait cycle. Similarly, a
stride is defined as a period of time between two successive heel contacts of the same
foot. A step is defined as a period between successive heel contacts of the opposite
feet during double support. Bipedal walking is defined as a quasi-cyclic motion and is
generally characterized by a sequence of single (one foot in contact with the ground)
and double support (two feet on the ground) phases.
Within a stride, the double-support phase represents approximately 20 % of the
total time [
3
,
31
] . For a given leg, the stride is composed of a contact phase (60 %
of the total time) and a swing phase (40 % of the total time). The ratio between the
contact and the swing phase changes according to speed. Indeed, the duration of the
contact phase decreases when speed increases.When the duration of the contact phase
decreases down to 50 %, the double-support phase disappears, which corresponds
to the walk-run transition [
46
,
76
]. Intra-individual variability of these durations is
low (below 4 %), while inter-individual variability is greater than 10 % [
69
].
The leg walking cycle could be decomposed in a more accurate manner according
to relevant events: left (resp. right) foot-strike, right (resp. left) toe-off, right (resp.
left) foot-strike and left (resp. right) toe-off.
The walking cycle is globally represented by three quantitative parameters: the
step length (L), the step frequency (F) and the walking speed (V), which are linked
through the relation:
V
=
L
∗
F
In order to increase walking speed (V), humans jointly increase the step length
and frequency. When continuously increasing walking speed on a treadmill, people
generally increase step length first until a limit is reached and then increase step
frequency [
74
]. However the results change when analyzing ground walking.
While people are walking at a freely chosen step-rate it has been reported that
step-length divided by step-rate (called 'the walk ratio' [
45
,
62
]) does not vary over
wide ranges of speed [
5
,
62
,
88
] during ground walking. The walk ratio seems to
