Five concepts are very important in this measurement system: i) the four minimum points of gait (and foot-line), ii) using the heel-point for measurements, iii) the time point of the snapshot as heel-contact, iv) the 5 straight lines over the step, and v) the standard start position.

i) The identification of the 4 minimum points of gait: 1) the step-heel-point, 2) step-pelvic joint, 3) rear-pelvic joint and 4) start-heel-point, is the main principle. Foot-line is included, if possible, because it adds a great deal of detail for such a simple factor, but it's not essential. These 4 points (and line) are all that's necessary to describe distance and direction for a person walking, wrt any 2D plane.

The entire gait measurement system, and all the terms and definitions, are based on the projection of the 4 points (and line) onto a specific 2D plane. Their relative positions are what's represented by the 8 fundamental parameters of gait. But, they don't define how people walk. They describe a series of consistent, relevant measurements, based on body segments, which will be useful to investigate how people walk.

Only the two heel-points are required to take measurements like step, stride and walking base, but adding the pelvic joints allows the identification of 6 of the 8 fundamental parameters, with foot and push-off angles being seen as a single direction change. Step, stride and walking base are products of these parameters.

Adding the foot-line allows the separate measurement of foot and push-off angles, as well as the identification of 3 of the 5 straight lines over the step.

Fortunately, because the body can also be represented as vectors, analysis is much easier since vector techniques, equations and properties are relevant. This validates the 2D projection, and allows for the definition of the standard start position, among many other things.

Now, direction and distance changes over the step can be separated into contributions from specific joints and body segments, so changes in each parameter can be related to variations in more strictly definable muscle and joint sets.

Other reference points or lines can be tracked by incorporating them into the model, but the projection of the 4 points (and line) is the base reference.

ii) The heel-point is the point on the bottom of the heel that wouldn't move if you spun around on your heel on one leg, assuming your leg was a stationary vertical axis. When this is used as the point for measurements, distance and direction variations introduced when the edge of the heel is the reference point are removed, and it also eliminates the requirement for contact with the ground.

iii) The time point chosen for the snapshots, the instant of heel-contact, allows the isolation of the 8th parameter, aberrations, which is very important. Aberrations are slides, spins, etc, - anything that changes the heel-point and/or foot-line position between sequential heel-contacts of opposing feet. It's a large and complex set of movements and may have controlled and/or non-controlled elements.

But, though the time of the snapshot is heel-contact, the measurements are still to and from heel-points.

That the heel-point is usually off the ground at heel-contact is an annoyance, but a much smaller one than choosing the time as heel-point contact, since that position may include all or part of an aberration. For eg, if there was a slide just at heel-contact, but before heel-point contact, among others.

And, heel-contact is better in that it makes the system universal by removing the requirement of actual contact of the heel-point with the floor. This measurement system could track the 2D path a person would make on the ground if they were pretending to walk while floating in space, with arbitrary designation of the times of heel-contact. The snapshots would define 7 of the 8 parameters, and any shifts and rotations in between would be considered an aberration, and so measured as a separate entity, independent of the other parameters. Variations in the parameters due to the 3rd dimension can be correlated as a separate factor.

It's just a matter of geometry, mathematics and interpretation. But, it's only 2D progression. The 3rd dimension can be integrated, but it should be treated as an extra, like time and many others, by showing variations in the 2D wrt the 3rd.

(iv) Body segments define the 5 straight lines over the step, and they are:
    1) step-foot-line of the previous step,
    2) foot-line after aberrations,
    3) rear-leg-line,
    4) step-out-line and,
    5) step-foot-line of the current step.
These allow the continuous determination of direction changes over the entire step and path. (See Fig. 2)

v) The standard start position is a purely theoretical, and arbitrary, "body" position which is established at the instant of heel-contact. Only 3 of the minimum points of gait are needed to define it, the step and rear-pelvic joints and the start-heel-point.

To visualize it, imagine yourself frozen at the instant of heel-contact. Now draw yourself back along a straight line, until you're standing straight up at a stop, with the step-foot in the air, and the left and right feet at a distance of straddle-line apart (not pelvis line). In the Step Model, the reference-foot model represents the position of the foot that's in the air (the step-foot), in the standard start position.

Aberrations and push-off angle change the position of the standard start position, but foot offset and foot angle don’t.

This provides a separate, consistent measurement standard, the reference-heel-point, which is still defined by the heel-contact snapshots. It allows the measurement of the step and carry lines by taking advantage of the vector nature of all the measured distances.

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Limb & Eye Dominance Techniques Gaits Awareness