The Mouck Method for Path Deviation Study

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The Mouck Method for Gait Analysis and Path Deviation Study
Part V
Applications and Acknowledgements
Written by Mike Mouck
A) Human Tracking
B) Clinical Gait Analysis
1) 2D Classification System
2) Time Dependant Values
3) Application to Real Data
4) Correction Mechanisms
5) First Real Application
6) Various Points
C) Quadrupeds
D) Acknowledgements
A) Human Tracking
When a person walks in a desert-like scene, the lack of visual direction cues makes it more likely that natural left/right differences in
walking pattern will cause the overall direction of travel to show regular, minor changes. The path often shows as a large arc.
Since a person wandering even a short distance can lead to relatively large direction changes, the analysis of how people walk while
wandering could develop into a much more significant factor in the human tracking arena, now that direction changes over a single step
are better understood.
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Every possible 2D path characteristic can be recreated using the Step Model, so it allows for the creation of correlation tables to relate
changes in one or more parameters to variations in field available measurements, like step length (line), or distance and direction from a
standard point or line(s), etc.
This method will facilitate the creation of a system to predict wander paths for lost people (path deviation), given a few standard physical
measurements and a current sample footfall pattern. Since a person walking is a vector system, it's perfect for a computer program.
The study of limb dominance, and its effect on walking pattern, would be a large part of this application. But, how parameters change
with external stresses, such as fatigue or carrying something heavy, would be the most substantial area, as well as correction mechanisms
(see below).
Co-operation with 3D gait analysts should provide much of the data required to investigate path deviation, and there could be a lot done
with what’s already on file.
Unfortunately, the 4 points of gait (and foot-line), at the proper time point of heel-contact, are not measurable from a real footprint pattern,
since they’re in the air. But, that doesn’t mean the system can’t be applied. At least now we know the correct body segments and joints
which are involved in distance and direction when walking, and have an idea of their individual variations.
And, the direction changes over a step may be large enough were estimated positions of heel-point pelvic joints are adequate. Since the
parameters are fundamentally related due to the physical connection of the 4 points of gait, muscular and/or skeletal limitations may make
it possible to limit the possibilities for each step, perhaps with error estimations.
This method is important to human tracking for 2 main reasons. It allows the production of realistic footfall plots, in order to study
distance and direction relationships between footfalls which couldn’t be studied without it, and it shows how the different body segments
and joints contribute, via the parameters, to the observed footfall pattern.
This means that tables can be created which show how each parameter affects the position of the footprint, so variations for each can be
correlated in order to produce a “most likely” Step Model for each step. A part of this is lab study of how people walk, in order define
limits of motion and show how each parameter is related to the others.
The relationship of pelvic stretch and straddle-line doesn’t have to be studied, since they’re the sides of a right triangle with the pelvis-line
as hypotenuse, but all others have to be determined. The step-out-line and rear-leg-line, for eg., have a relationship through ground
contact and the pelvis-line. How one changes wrt the other should provide greater insight into basic walking controls, and help determine
what simplifications can be applied, for analysis.
Application to SAR should involve 2 main areas, one of which requires real data input.
The first area involves 2 computer programs and a database. The initial program would be for creating Step Models from specific input,
taking relevant measurements on the Model, using the Model (with any others) to create footfall plots, taking relevant measurements on
the plot, showing the acquired data via figures and graphs, and tabulating the data in a standard format.
The stage 2 program would use the previous for the base, but be enhanced to facilitate SAR in the field (see Foreword, Application to
SAR - One Plausible Scenario). This would require the input of physical data and it’s use to estimate Step Models, the plotting of wander
paths, the evaluation of terrain if such data is available (popular areas could have a detailed topographical map), the calculation and
display of potential error path regions, allow input of field determined factors like rockiness or moisture, create logs of input and all
analyses, etc.
The world database would be to tabulate real data.
The second main area of application is the study of normal walking patterns, as well as changes in the parameters due to specific stresses
(such as wearing a heavy backpack) and limb dominance. This would require the definition of a standard method to determine limb
dominance (which there currently isn’t, I believe), as well as the analysis of parameter data. This requires technical data input, such as 3D
marker tracks.
B) Clinical Gait Analysis
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Each step is an individual. A person walking is a mechanical instrument. A person walking is a vector system. Walking is a controlled
stagger. The primary goals of walking are distance and direction. Walking is a complex, personal attribute. Walking is the manipulation
of the 8 fundamental parameters of gait. Etc., etc.
Walking can be described in many ways, and each different perspective highlights specific factors which uniquely contribute to a person's
gait pattern.
This method is a new way to describe gait.
Changes in step and stride-line and walking-straddle can now be related to foot offset, foot angle, push-off angle, heel-point shifts and
foot angle changes over aberrations, step-out, step, carry, L/R, rear-leg and straddle lines and pelvic-stretch, and all of these for every step
over the path.
And, all the fundamental parameters have a unique physical basis, and the other measurements are just products of them.
This approach is very flexible, and the general strategy is valid even if different points are tracked. Study of different aspects of gait may
be easier if other reference points or lines are incorporated into the Step Model. Then, variations in these can be directly related to the
specific direction and distance changes over a step, as well as rotations of the basic grid.
This method can be applied immediately for the tracking of changes over treatment. But, application to diagnosis will require some
research into normal and known deviant step patterns. Once described, however, the data, graphs and correlation diagrams which can be
derived using this measurement system will make for a very powerful diagnostic tool.
1) 2D Classification System
One of the main difficulties in current gait work, besides the problems outlined in Part II, is the lack of critical co-relations. Any complex
study, such as gait analysis, requires the recognition and definition of central threads which can be used to organize the many and varied
facets of modern research.
For gait analysis, the 8 fundamental parameters provide those central threads, and they are the foundation for a primary classification
system to describe point and line movements, wrt effects on distance and direction over a single step.
All movements for a person walking will do one or more of only 4 things, and each category will be further subdivided, potentially all the
way to specific muscles and joint rotations:
1) direction change:
a) foot angle,
b) foot offset,
c) push-off angle
2) distance change:
a) rear-leg-line,
b) pelvic-stretch,
c) step-out-line,
d) straddle-line
Note: b) and d) have a fixed relationship, since they are the sides of a right triangle, with the pelvis-line as the hypotenuse.
3) distance and direction change:
a) aberrations leading to heel-point shifts and foot-line rotations
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4) no effect on distance or direction:
a) mass shift,
b) comfort,
c) desire,
d) balance,
e) etc.
2) Time Dependant Values
Time dependant parameters can be tracked the same as is currently done. Comparisons of relevant snapshots, wrt elapsed time, will give
all the currently studied values, since this is just tracking points from a different perspective.
The rotating reference grid is a major benefit to this area. Since point trajectories are vectors, knowing their relationships to the Step
Model’s grid is essential for accurate, consistent interpretation.
Also, time dependant analysis wrt the fundamental parameters should provide a way to better isolate and define the specific factors
affecting gait. Deviation factors could compare linear and angular deviation wrt the final or standard value, and this should lead to a readout profile indicative of specific ailments. Other points and lines could be tracked in the same way. This could be a very important
But, time analysis entails specific variations that require correction factors to account for limitations in all current measurement systems.
The main requirement is the identification of the 4 points and foot-line, but, since the pelvic joints are inside the body, and the heel-point
is on the sole of the foot, the exact positions of these must be estimated using available position marker data.
Fortunately, the individual direction changes over a step appear to be very large, in general, and minor inaccuracies in pin-pointing the
exact positions of the 4 points and line will likely be insignificant for most applications.
3) Application to Real Data
There has to be a way to identify the four points (and foot-line, if possible); start-heel-point, rear-pelvic joint, step-pelvic joint and stepheel-point, at the instant of heel contact.
An overhead picture may be enough to get quick measurements. The required points should be identifiable by inspection. It has to be the
projection onto the floor anyway. This might not be very accurate, but should be good enough to use at the beginning, to get used to the
measurement system, and help judge what kind of accuracy is needed to get meaningful results.
To create the Step Model, draw a line from the start-heel-point to rear-pelvic joint, then extend this past where the straddle-line from the
step-pelvic joint connects with it.
Draw a line from the step-heel-point to the step-pelvic joint. This is the step-out-line, and the angle from the straight line is the leg angle.
Connect the pelvic joints, and fill in the rest of the figure as shown.
**INSERT FIG 15, application
Note: In Fig. 15, the foot offset is caused by rotation at the rear-pelvic joint alone, ie. there is no rotation at the step-pelvic joint. Also, it
appears the reference line for the leg angle is "outside the step". The reference is actually the rear-leg-line, but any line parallel to it can
also be used.
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If data was available for a person walking over several strides, a Step Model could be made for each step, and the path recreated with the
This method can also be applied under any conditions.
One study could be how a person's step characteristics change when, for eg., they're carrying something heavy. Others might show
important aspects of balance control vs direction, descriptions of standard "control" characteristics, etc. Many intriguing experiments
could study the effects of physical activity or injury on step patterns. One person could be studied over a period of time, with new tests
whenever they report a change in their physical, or even mental, condition. The list could go on and on and on...
Another compelling advantage is that sequential Step Models can be made for both feet over the whole path, then compared side by side.
And, Step Models from different times, even months or years apart, are easily comparable, as well as from other individuals (perhaps to
compare the effects of the same affliction).
Fig. 16 shows a sample analysis set for a patient with a gait abnormality. This shows the level of detail inherent in the system, but the
linear parameters are held constant for clarity, and there's no pelvic-stretch or aberrations.
In practice, each step could (and almost certainly does) have unique values for every parameter. With an overhead view, all of them could
be determined for every step.
The main line description is L21{0}[21]R21{0}[21]str14, both feet 21cm step-out line, 0 pelvic-stretch, 21cm rear-leg-line and 14cm
The line description for R1(below) of R(5)L-20L-<20>R reads as right foot (5 deg leg angle)offset left-20deg foot angle left- 20deg pushoff angle right. This results in a net angular shift of 5 deg L over the step, as well as a small extra component left due to the side-step
character of the foot offset.
INSERT Figure 16, sample clinical*
Following are the Step Model line descriptions for 4 steps of each foot, from Fig. 16.
1L L(15)R-5L-<20>R
Step, stride, etc. measures can be taken wrt every heel-point. There's a great deal of information in Fig 16.
If there was a difference in straddle-line (and pelvic-stretch) for adjacent steps, two models would be aligned at the heel-point, and each
step would use the model with its straddle-line as the reference for that step. The reference heel-points for each model can then be related
wrt distance and direction, similar to an aberration.
Again, this is only an example to show how easy it is to compare steps. For a real case, each step would also include the main line
description of step-out-line, pelvic-stretch, rear-leg and straddle lines.
The original footfall plot can be manipulated in many ways. How these relate to other gait factors, like different joint rotations, velocity,
forces, etc., are matters to be discovered.
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INSERT Figure 17, plot manipulations for sample clinical*ref
4) Correction mechanisms
Correction mechanisms refer to how people cope with errant distance and/or direction changes, such as introduced by injury, fatigue or
For eg., in the clinical example, Fig 16 and 17, the subject has consistent deviations in the right foot of (5)L and 20L, due to a physical
abnormality. This leads to a constant 25 deg deviation with each right step. Attempts to correct for the straight path result in erratic foot
offset in the left foot, and also push-off angle, as well as push-off angle in the right foot.
It may turn out that direction in walking is mainly determined by correction mechanisms, and that most steps could be considered an
attempt to correct previous direction deviations, to maintain a straight path to the objective.
This could be a very large and interesting area of study.
5) First Real Application
All real data generously provided by Gammon Earhart, PhD, PT, Washington University in St. Louis.
This first real application suggests many of the benefits of the measurement system.
Figures 18 - 25 show the results from 3 separate tests; straight walk, 90 deg right turn, and anomaly; performed by the same subject, in the
same session.. The anomaly is an emulation of a common gait anomaly for stroke victims, where the affected leg goes through the carry
and step by swinging out and around, circumducting at the hip, rather than being carried through.
**INSERT FIG 18, real-straight
**INSERT FIG 19, plots-straight
**INSERT FIG 20, real-turn
**INSERT FIG 21, plots1-turn
**INSERT FIG 22, plots2-turn
**INSERT FIG 23, real-anomaly
**INSERT FIG 24, plots-anomaly
**INSERT FIG 25, all Step Models
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**INSERT FIG 25, all Step Models
There are a great many details of similarities and differences in these Figures, but most will be described in subsequent articles. The main
thing now is to see how the method is able to describe and distinguish important new gait parameters, and changes can be easily tracked
and independently evaluated.
Also, intra-step time analysis, using 0.05s intervals, shows many characteristics not evident in the heel-contact snapshots. This may be
one of the most important applications. But, these will also be explored in future work.
6) General Points
-should foot-line be a fundamental parameter?
-allows for the expression of step/stride/walking base as a 2D and 3D vector equation.
-“induced stress” experiments may help in diagnosis
-static analysis of movements outside points and lines, like lateral knee joint movement and step-out-line, may be revealing
-the point and line projections must be purely vertical wrt the plane of interest
-if time profile-some joint and other rotations give equivalent values in the 2D
-computer modeling-rigid frame and introduce rotations sequentially
-degrees of freedom wrt parameter changes
-standard orientation not along foot-line, but "push-off-line", even though the start and reference foot models' foot-lines are parallel to the
straight line at heel-contact.
-for modeling, show how variations in each fund. parameter affects other measured distances like, step, stride, etc.
-have to determine how it fits in, how it can be applied effectively to each area
-the 2D projection of points other than the minimum 4 may show periodicity with respect to changes in the primary elements.
-at first, do measure at heel-strike and heel-point contact, if possible, heel-point not at the bone, but is contact point with the floor. Any of
several choices for heel-point, but contact with the floor may be best, since that's most easily measurable without equipment
- for different str in sequential steps, 2 models at heel-point with the different str values, both in standard orientation, each the reference
for it's appropriate step, heel-points can then be related like an aberration
-body motions, momentum, etc. have to be studied to determine effect on parameters
-negative step?, no-no step, shortened to 0 carry
-there may be some simplifications which can be applied for quicker analysis, if the required equipment is unavailable. This would have
to be determined.
-there are physical constraints relevant which may limit the ranges of one or more of the parameters wrt one or more of the others.
-applied to running - last heel-point position when rear-foot leaves the ground to the rear-pelvic joint at the next heel-contact (two
snapshots needed per step, step-heel contact and rear-foot leave ground (ie. the start/stop for aberrations).
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- though new, can be immediately applied to all levels of gait analysis because it can track walking patterns over any period of time,
without the need to compare to standards, since it's only a measurement system. But, comparison to standards will be important in the full
analysis, so a relevant database would be very useful.
-the orientation of the upper body greatly confuses the interpretation of direction changes over the path. But, the upper body pointing
"straight ahead" doesn't mean the person is walking a straight line. And, a person who places each foot on it's appropriate straight line for
every step isn't necessarily walking straight in each step, and probably usually isn't.
-The lower frame defines both direction and distance through the varied relationship between the 4 minimum points (and foot-line).
C) Quadrupeds
The biped is the simplest normal gait system to model. The same general strategy should apply to quadrupeds, or any multi-ped, but the
system is a fair bit more complicated. Modeling would be much more complicated, but the measurements very similar.
It may be best to think of the quadruped as two connected "biped sets". All the parameters would be derived from tracking the 8
minimum points of quadruped gait, the front and rear would both have the same minimum 4 points as for biped gait.
The line of connection, the spine-line, is from the center of the front "shoulder-line" to the center of the rear pelvis-line. Though this
represents the spine, it should be a straight line connecting the shoulder and pelvis-line centers, and it may be desirable to define the
measures related to this connection as fundamental parameters.
Spine curvature would be seen as an apparent rotation of the spine-line, as well as apparent rotation of the shoulder and /or pelvis-line at
the centers, both leading to direction change, as well as a decrease in apparent spine length.
The development of this application would be much better using experimental data. It's easy enough to choose points to track in the 2D
plane, but the order of heel-contact may vary, and organizing the direction relationships more complicated.
Comparison of snapshots at each heel-contact, with identification of the 8 points, as well as the time of each foot contact, will make it
much easier to determine the proper procedure for application. But, it's still just tracking points in 2D. Each snapshot is a reference grid,
and how the various lines and angles are related to overall gait, within each step and over sequential snapshots, requires consideration of
what factors are considered important for that study, and what generalizations, standard positions and/or values are desired or available.
The establishment of the 2 “standard start positions” should be similar to biped, but there would be many more “straight lines over the
step”, representing sequential direction changes.
This would be a very challenging area of research at the beginning. Once standard procedures are established, though, it should be as
easily applied as for biped gait.
D) Acknowledgements
Very many thanks to Walter Muma, Jean Hurrle and Gammon Earhart for their contributions to this effort. Jean and Gammon for very
productive discussions to focus and clarify the work on the trapper and clinical sides, resp, and Walter for putting up with a lot to publish
the article and revisions on his Wildwood Survival web-site.