| The Plot Model The plot model
attempts to geometrically display the critical measures and rotation points
needed for the creation of realistic footfall patterns. It exactly defines
foot placement, not the body’s real movement during a stride.
Distance relationships observed in the plots should exactly reflect those
in real footfall patterns.
Making The Plot Model
While there are only foot angles and offsets to change direction, various
distances are needed to set up the feet for placement. Since everyone is
different, these values will be different for everyone. This time, though,
that’s a good thing.
Building the Standard Model
The plot model I use consists of two vertical step-line models (see Figs.
3 and 4), one vertical DOT line and two horizontal lines of straddle length
(see Fig. 5). I use CorelDraw8 with a scale of 1:10, the screen resolution
at the highest and hair-line lines.
I find it easiest to make lines and then dimension and position them
exactly by typing in the X-Y co-ord. Also, the proper order of grouping in
the model will make it much easier to use.
- These are the values for my standard model, and they are the minimum
number of measurements needed.
- Step out length = 15”
- Rear stretch length = 15”
- (Step length = step out length + rear stretch length = 30”)
- Straddle length = 3”
In the field, you could estimate equal values for step out and rear
stretch lengths, if you just had step length, and distance errors would be
far too small to be a problem. The separation of step into step out and rear
stretch lengths is necessary in order to define the proper rotation point
for the foot offset. This is set at the connection point of the two lines,
usually near the middle of the step line.
If a person walked without any side-side movement, the straddle length
would be very nearly the distance between the right and left pelvic joints.
Since people do have lateral sway, the straddle length represents the shifts
in the step lines at the instant of foot placement, and is equal to the
perpendicular distance between the shifted step lines for a person walking
straight with no foot angles or offsets.
The value for straddle length was estimated from literature walking base
measurements. Since walking base changes with angular changes in the DOT
(even if walking straight), but straddle length doesn’t, the two aren’t
exactly comparable. But, for the first set of plots, it’s good enough.
Please refer to Part IIc - Plotting Results for an explanation of the
shorthand notation.
| |
Walking base (in) |
|
| str3 |
3.00 |
walking straight, walking base (WB) = straddle length (str) |
| L4dR:R4dLstr3 |
4.13 |
walking straight, both feet internal rotations, WB > str |
| L4dL:R4dRstr3 |
2.12 |
walking straight, both feet external rotations, WB < str |
For my standard model, I also include the foot models to make analyzing
the plot more intuitive, but it isn’t required. I could just as easily be
using a dot or cross, since the heel point position is the only thing that
matters for distance and direction. But it’s very easy to do and gives a
much more realistic graphic representation. It also makes it possible to see
DOT relationships, and much easier to see plotting errors and distance
relationships.
The foot model length is from the heel point (which is also the rotation
point) to the tip of the toe, measured along the foot line.
When the straight foot angle is zero deg, as in all my plots so far, the
arrow for the foot line is parallel to the DOT at the start of every step.
The hidden line (see Fig.3) is 2” long and has its origin on
the heel point. Once ungrouped, the top end is extended and placed on the
next heel point of interest. Dimensioning this line gives the heel point to
heel point distance (or heel point to any reference point or line).
The horizontal line is 1” and the foot line is 6”. I use
gray step foot models and black start foot models. (see Fig. 4.)
Group the horizontal line and foot line first, and change
the rotation point to the heel point. Then, group this with the hidden line
and change the rotation point for the entire figure to the heel point.
The rotation point for the foot offset is at the origin of
the step out line (see Fig.4), and is the connection point of the rear
stretch and step out lines. All step out and rear stretch lines are
independent of each other, so for a real subject these could be four
different values. Group the step out line and the step foot
model and change the rotation point to the bottom of the step out line. Then
group this figure with the rear stretch line and start foot model.
For Fig.5, the plot model, the rotation point for foot angle
is the appropriate start foot model’s heel point. However, since this
rotates the entire figure anyway, it’s much easier just to type in the
rotation for the entire figure and then move it to the proper position over
the last heel point. That way, leg angle and foot angle values can be added,
and the entire figure rotated once to account for both angular deviations.
Type in exact values for dimensioning and positioning. The
body centerline averages the body position to a line perpendicular to the
step lines, and is set at the start of the step out line. If right and left
rear stretch lengths were different, there would be two body lines, one for
each step. For now, this line is only to help visualization, since it
represents the body’s position at the end of the step. Group
the whole thing after it’s built. What’s left out
Some factors can be simplified, since the observed action or length results
from a complex interplay of apparently independent variables. Step length is
an example. Though the leg length (thigh and shank) is constant, the
observed step length is changed by changing body segment angles via pelvic
rotation, pelvic list, ankle rockers, and knee flexion. None of these need
to be measured, though, because the observed, or effective, step length is
all that matters. A circle for the head would not be placed
on the DOT line, and it’s easier to leave it off for now.
Rotations at joints can change the rear stretch and step out lengths, and
the exact position of rotation points (and hence, step out and straddle arc
characteristics). Distance relationships would be the same, but length
values would change. The model generalizes these to single lines. I believe
deviations from these would be so small as to not be worth the trouble.
All the excluded elements could be incorporated into a few models to see if
they affect rotations, lengths, etc. enough to worry about them. Previous
Next
Part I (Page
1, Page 2) Part IIa Part IIb
(Page 1, Page 2)
Part IIc Part III
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