(A) Introduction
A2. Five Central Concepts
Five concepts are very important in this measurement system: i) the four
minimum points of gait (and footline), ii) using the heelpoint for
measurements, iii) the time point of the snapshot as heelcontact, 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 stepheelpoint, 2) steppelvic joint, 3) rearpelvic joint and
4) startheelpoint, is the main principle. Footline 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
heelpoints 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 pushoff angles being seen as a single
direction change. Step, stride and walking base are products of these
parameters.
Adding the footline allows the separate measurement of
foot and pushoff 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 heelpoint 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
heelcontact, allows the isolation of the 8th parameter, aberrations, which
is very important. Aberrations are slides, spins, etc,  anything that
changes the heelpoint and/or footline position between sequential
heelcontacts of opposing feet. It's a large and complex set of movements
and may have controlled and/or noncontrolled elements.
But, though
the time of the snapshot is heelcontact, the measurements are still to and
from heelpoints.
That the heelpoint is usually off the ground at
heelcontact is an annoyance, but a much smaller one than choosing the time
as heelpoint contact, since that position may include all or part of an
aberration. For eg, if there was a slide just at heelcontact, but before
heelpoint contact, among others.
And, heelcontact is better in that
it makes the system universal by removing the requirement of actual contact
of the heelpoint 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 heelcontact.
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) stepfootline
of the previous step, 2) footline after aberrations,
3) rearlegline, 4) stepoutline and,
5) stepfootline 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
heelcontact. Only 3 of the minimum points of gait are needed to define it,
the step and rearpelvic joints and the startheelpoint.
To
visualize it, imagine yourself frozen at the instant of heelcontact. Now
draw yourself back along a straight line, until you're standing straight up
at a stop, with the stepfoot in the air, and the left and right feet at a
distance of straddleline apart (not pelvis line). In the Step Model, the
referencefoot model represents the position of the foot that's in the air
(the stepfoot), in the standard start position.
Aberrations and
pushoff 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 referenceheelpoint, which is still defined by
the heelcontact 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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Part I Part II
Part III
Part IV
Part V Copyright
© 2008
