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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:

  1. 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.

  2. 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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