| OVERVIEW It can
be shown with 100% certainty that differences in step length alone can not
lead to path deviation.
The only ways a person can change direction when walking is to step off
the straight line to one side, walk off the line by stepping out at an
angle, or a combination of the two.
Each step has its own deviation from the straight line. The total
deviation from the target is the summation of the individual deviations in
each step. Theoretically, if there was a way to exactly know every deviation
in every step, the exact location of the person would be known.
It’s observed that people wander in a regular way, suggesting a regular
(average) wander per step. But, since a lost person makes changes of
direction that can’t be predicted, this method would be relevant only across
areas of wander.
The basic analysis method uses the standard of the “perfect” step. By
using a shifting frame of reference, deviations in each step can be related
to offset and angle changes, since these are the only ways a person can
change direction.
Part II will outline system parameters and the plot model. The plot model
generates “perfect” footfall patterns to help understand subtle distance
relationships impossible to interpret from field patterns.
Part III will describe the analytical model and details for experiments.
Standard and control experiments will shed light on how people walk and
correct their direction of travel (DOT) normally. Changing the conditions
will then show how various stresses change a person’s walking patterns, and,
hence, wander patterns.
Generalities in wander patterns would likely arise, which could be
outlined for field use. This could generate a data base of predicted wander
patterns covering different ages and conditions, with ways of combining the
information for the specific conditions encountered. A series of “rules of
thumb” for field use would be the result.
PROBLEMS WITH PREVIOUS WORK
The current discussions of path deviation are extremely encumbered. There
are several fundamental problems with previous measurements and definitions
which cripple any attempts at further understanding. Several are outlined
below.
Lack of an accurate picture of the system:
Much of the confusion regarding measured differences in stride length for
a person apparently walking straight is likely because no one can actually
walk a straight line. The path is “sinuous”. It wanders about the straight
line. A person walking normally is almost always CONSTANTLY ENGAGED IN
TURNING. So, it’s not surprising that even accurate measurements might be
apparently amiss. Stride lengths, when accurately measured, would usually
show differences in length.
When a person steps out from the planted foot to take a step with the
opposite foot, they try to normalize over the planted foot in order to make
the step as comfortable as possible. If the planted foot was to one side of
the straight line (foot offset) or the pitch angle of the foot was not for a
straight step (foot angle), there will be a slight change of direction when
the step is taken with the other foot. People don’t go around curves like
they had wheels. They step out in a different direction.
Also, there is no accurate description of a stride. Stride = carry +
step. The “carry length” (as defined later) is virtually always disregarded,
but it is absolutely critical that it be considered. This is the distance
equalization mechanism for the total distance traveled by each foot. It must
be included.
Previous measurement techniques were inaccurate:
Generally, any measurements that start or end at the toe are not accurate
measurements of distance traveled and can not be used in any accurate
calculations. When the toe is used as one of the points, the measured
distance changes with changes in foot angle and foot offset. Total distance
traveled is independent of these two values.
A heel to heel measurement of step length is not accurate. It would be
longer than the real distance traveled, and varies with foot offset without
any change in the actual step length.
Any measurements that are not taken parallel to the DOT are not accurate
(that is, the vector sum DOT).
Comparison of right and left heel to heel measurements of stride length
are not valid. The right stride is related to different parts of two
consecutive left strides (The step of the first plus the carry of the
second.), and vice versa.
Conclusions based on these kinds of measurements can only be considered
“rules of thumb” which are very useful for field work, but are not accurate
measurements of distance traveled.
Dominance factors:
Step length is influenced by dominance, but this will not affect DOT. It
is only coincidence.
There are many dominance factors that affect how the feet are planted in
walking. Dominance likely shows great influence on wander paths due to its
asymmetric control of muscles. Dominant limbs are generally stronger and
much better controlled when the foot is planted. Variations in the placement
of the non-dominant foot may be an important directional determinant.
The brain may attend to directional and other cues differently depending
on dominance factors.
All these things would show through how the foot is placed on the ground.
Everything filters through that. Good experiments would shed a lot of light
on the dominance elements.
Conflicting definitions:
While I’d like to conform to definitions that are in current use, I don’t
think that’s possible. The same terms are used to describe different things,
and vice versa, depending who you talk to. Unfortunately, literal terms like
step length and stride length have been used to describe inaccurate
measurements, even in clinical work. In the research arena, literal
terminology is usually reserved for accurate descriptions.
In order to have any kind of discussion, the terminology has to be
standardized. I don’t care if anyone else uses these. But, this is what they
mean when I talk about them. See also, Figs. 1 and 2, below.
Heel point – the point of contact with the ground if we had peg
legs that went to points. Heel points exactly define direction for a person
walking and total distance traveled for each foot. It’s the point that
wouldn’t move if you spun around on your foot. The foot line must go through
the heel point.
Stride length – the distance from the heel point on one foot to
the same point on the next footfall of the same foot, measured parallel to
the DOT. Any stride goes through two phases, first the carry, then the step.
ie. stride length = carry length + step length. Stride lengths change with
angular changes to the DOT. Left and right stride lengths are equal for a
person walking straight and unequal for a person turning. This is an
accurate measurement of distance traveled.
Step length – For the left foot step length, the distance from the
heel point of the left foot when it’s directly adjacent to the opposite foot
(when a line connecting the heel points of both feet is perpendicular to the
local DOT, left foot is usually in the air at this point) to the heel point
of the next left footfall, measured parallel to the DOT. Step length = step
out length + rear stretch length and is constant with angular changes to the
DOT. This is an accurate measurement of distance traveled.
Carry length – For the left carry length, the distance from the
heel point of the last left footfall (at the end of its last step) to a
point directly adjacent to the right foot (the left foot is in the air, as
for step length) after the right foot has been planted in its next stride,
and the body rotated for foot angle deviations, measured parallel to the
DOT. Carry lengths change with angular changes to the DOT. This is an
accurate measurement of distance traveled.
Foot line - Usually taken as a line connecting the center of the
calcaneus and the second metatarsal head. The line must go through the heel
point, so the heel point position doesn’t move with foot angle changes.
Since it’s the changes that are important, the exact position isn’t
critical, as long as it goes through the heel point and is the same for all
measurements.
Foot angle – the foot’s pitch angle. Also called toe out or in.
The angle the foot line makes with the current DOT. (see also, Part II,
General Description of Walking)
Push-off angle – the angle the new DOT makes with the foot line at
the start of the step. (see also, Part II, General Description of Walking)
Foot offset – the distance, measured perpendicular to the DOT,
that the heel point of the foot is planted off of the straight line for that
foot. This measurement must include the designation “left” or “right”. A
perfect straight step would have zero foot offset. A foot offset has a
corresponding leg angle.
For a 15” step out length (30” step length), a 1” foot offset induces an
approx. 1.9 deg leg angle change in the same direction, which will add to
any foot angle changes. The measurement is actually of the arc, but the arc
length can be approximated using the perpendicular measure for small
offsets, and calculated exactly for larger.
Leg angle – when the leg is extended to plant the foot at the end
of a step, the angle the leg makes with the local DOT. Any foot offset has a
corresponding leg angle, which will add to any foot angle changes. Leg angle
can be calculated from the step out length and foot offset value.
Direction of travel (DOT) – the current direction of forward
movement. The DOT is completely defined by the heal point pattern when taken
in the proper frame of reference. Local and current DOT refers to the DOT at
a specific place or instant. DOT changes with each step for a person walking
normally. (We should be using vectors, but let’s leave them out right now)
Straddle length– the distance between the heel points of the left
and right feet, measured perpendicular to the DOT, when the person is
walking straight with no foot angles or offsets. For these conditions,
straddle length = walking base. This value depends only on the person’s
physical characteristics, and is constant with angular changes to the DOT.
Changes in straddle length change the walking base, but not vice versa.
Walking base – could probably be called the apparent straddle
length. The sum of the perpendicular distances from the points of initial
contact of the left and right feet to the center line of forward
progression. However, this value varies with angular changes to the DOT,
unlike straddle length, which is constant.
Step out length – step out length + rear stretch length = step
length. At the end of a step, just as the front foot is planted, the body is
nearly in the center of the step length line. Step out length is the
distance from the centerline of the body to the front foot’s heel point,
measured parallel to the local DOT. The lower end of this line is the
rotation point for foot offsets. See Part II for a more detailed discussion.
Rear stretch length – rear stretch length + step out length = step
length. Distance from the heel point of the rear footfall to the centerline
of the body when just at the start of the double stance phase, measured
parallel to the local DOT. See Part II for a more detailed discussion.
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Part I (Page
1, Page 2) Part IIa Part IIb
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Part IIc Part III
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