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3.1 Introduction
Biomechanical laboratories for the investigation of gait have been designed
in various ways. One approach is to use a motor driven treadmill with various
number of cameras placed around it. The use of a treadmill has some advantages
and some disadvantages. The advantages are that it may reduce the space
requirements considerably and that the gait velocity can be strictly controlled.
The disadvantages are that there is no actual translation of the subject and
that you cannot calculate net joint moments by inverse dynamics. It is possible
to obtain a "fictive" translation of the subject by digitizing the
movement of the belt, but this will require specially designed software. Several
people have made attempts to build a force plate into a treadmill to calculate
net joint moments by inverse dynamics. In most cases the only available ground
reaction force was the vertical component, in a few cases the center of pressure
was available, but to the best of our knowledge nobody has been able to measure
the horizontal force component or the transversal for that matter. To calculate
net joint moments by inverse dynamics in two dimensions you need to measure both
the vertical and the horizontal ground reaction force and you need to align the
center of pressure to the spatial positions of the foot. Therefore, the majority
of gait labs consists of some sort of walkway containing zero, one or two force
platforms (figure 4.1.1).
FIGURE 4.1.1
A gait laboratory must be sufficiently large to fulfill two requirements:
-
The subject must be able to reach a "steady pace" of walking
before the analysis takes place. The cameras set up to record the movements will
normally record a volume of space in the middle of the laboratory. It is
therefore essential that the subject is walking at a steady velocity when
passing through the recorded volume. It is known that normal subjects reach a
steady pace after 1.5 step cycle and accordingly it is required to ask the
subject to continue this pace at least 1.5 step cycle after the recorded volume.
This means that a walkway with a length of 7-8 meters will normally be
sufficient. On the other hand, if the walkway is too long, the subject may
experience difficulties keeping a constant velocity and if force plates are
used, there may be problems hitting the plates correctly.
-
The cameras have to be positioned at a distance that will allow a
recording of the spatial volume of interest. If a whole body analysis over a
full step cycle is required, each camera must cover a volume of about 2 m height
and 2 m length. If only the pelvis and the lower extremities are of interest,
the cameras may be zoomed or positioned closer to the recorded volume, which
will give a better resolution. If the room is too small the only solution is to
use "wide-angle" lenses, which will result in numerous calibration
problems. For a three-dimensional analysis the cameras should be positioned so
that each marker on the subject can be seen by at least two cameras, except for
very brief periods when e.g. the arm swings past a hip marker. If the analysis
can be limited to two dimensions, the walkway should be placed next to a wall
with a single camera positioned as far away as possible but perpendicular to the
walkway (see further below).
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register your lab setup in APAS/Gait. |
