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“The Mouck Method for Gait Analysis and Path Deviation Study”
Part I
Introduction, Definitions and Shorthand Notation
Copyright2008
Written by Mike Mouck
A) Introduction
1) Overview
2) 5 Central Concepts
3) The Step Model and Footfall Plots
B) Understanding the System
1) Clinical Descriptions of Walking
a) Gait Cycle (= Stride)
b) 6 Determinants of Normal and Pathological Gait
2) Dispelling the Step Length Myth
C) Definitions
D) Shorthand Notation
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A) Introduction
1) Overview
This method is derived using the 4 minimum points of gait (step-heel-point, step-pelvic joint, rear-pelvic joint, start-heel-point) and the foot-line.
Lines and angles created by connecting these points and line, defined at specific time points (snapshots) and projected onto a 2D plane (usually the plane of the floor, but can be any), describe 8 fundamental parameters relevant to distance and direction for a person walking. 3 define direction changes (foot offset, foot and push-off angles), 4 for distances (step-out, straddle and rear-leg lines, and pelvic-stretch) and 1 both a distance and direction change (aberration).
Walking is the manipulation of these parameters.
These measurements are based on the different parts of the body, and the specific actions, responsible for distance and direction changes during a step. Any change of distance or direction when walking must show as a change in at least one of the fundamental parameters, so any movement that does not cause a change in at least one will have no effect on distance or direction, in the relevant plane.
This provides the basis for a primary classification system to compare point, line, mass, etc. movements wrt their relationship to specific distance and/or direction elements. Also, vertical movements are removed, but they can be re-correlated separately, as can many other "outside" factors, including time.
The true nature of many currently used measurements is also uncovered:
1) Stride length and walking base (straddle) are both products of the same 9 distance and 5 direction elements, as well as 3 extra distance and direction elements, if the measurement was taken at the point of contact of the heel, instead of the heel-point.
2) Current step length (if left to right heel) is a product of 4 distance elements and one direction element, and it does not accurately define the total distance traveled by the foot over the step. If step length is defined as stride length/2, then this is a measure of the stride, not the step.
3) Etc., etc.,...
It's not surprising changes in step and stride lengths, etc. may be difficult to correlate with changes of state for a subject. Each contributing parameter is affected by different physical factors, and most are independent of each other. That's at least 14 different body segment lengths and joint rotations for stride and walking base.
This method allows the measurement of all the fundamental parameters for every step. All of the 14 values can be separately evaluated.
And, any path can be re-created exactly using the individual Step Models. Then, comparison of point and line movements with "standard" positions may show unique information, or provide some other analytical aid.
All possible 2D step patterns can be represented using the 8 parameters and described via a very informative line description, for eg. L15{2}[15]str8: (2)L-2L -<4>R (see Shorthand Notation) and/or a graphic Step Model. Every step, over a single path or from different paths at different times, can be easily compared side by side, or up and down, on a piece of paper (see Fig 16).
This is very, very useful for clinical applications. Patient progress can be tracked via the 3 direction, 4 distance, and 1 distance and direction defining elements. With relatively high accuracy and over any period of time or conditions.
It also helps narrow the possibilities during diagnosis. For eg., nothing changes straddle length (line) but rotations (real or apparent) at the rear-pelvic joint. But, rotation at that joint also affects pelvic-stretch, a distance change, and induces a foot offset, a direction change. Foot offset is mainly changed by real or apparent rotations at the step- and/or rear-pelvic joints. Etc., etc. Correlation of changes in the various parameters will help pin-point and track the problem areas, and evaluate treatment options.
Clinical application has extraordinary potential, but virtually all other facets of gait analysis should benefit as well. An entire level of critical detail is being added.
There's also great flexibility and it's universal. It's just measuring the distance and direction between projections of connected points, and is valid for any arbitrary orientation of the 2D step-plane, at any point in time, even if different minimum points are chosen.
How can it be universal? Because it takes all the points and lines of reference from the body itself. The reference frame moves with the person. There's no need to be touching anything , no application of arbitrary, external references like "line of progression."
This method removes the vertical component of motion to provide a detailed, 2D picture of every step, which is easily applied to a person walking on a treadmill, a rotating disk, around and/or over objects, climbing stairs and inclines, in any physical condition (including using a prosthesis), and on any surface; even floating in space, crouching while walking or walking on the hands.
It allows the direct comparison of how a person walks, for eg., on a rotating disk to how they subsequently walk on a stationary plane, and how distance and direction parameters change as the conditioning wears off. It can even be used to study the detailed effects of multiple conditioning, since all the distance and direction elements can be separately measured, and their variations independently evaluated.
The minimum requirements are an overhead view (or equivalent), and a way to identify the projection of the 4 points and 1 line onto the desired 2D plane, at specific times. With current technology, this should be almost trivial. One overhead camera, with visual identification of the points, may be enough. 3D provides everything, assuming the appropriate time co-ordinates can be extracted.
Also, measuring changes in the parameters vs time over the path may be revealing. It doesn't matter if one (or even both) heel-point is in the air at the snapshot, since the projection takes out the vertical part. After all, the pelvic joints are always in the air. It just has to be interpreted properly.
And, the Step Model can also be used to produce "perfect" footfall plots. Any or all of the fundamental parameters can be varied to see distance and direction relationships between footfalls which couldn't be studied without some kind of controlled model. Every possible 2D step and path characteristic, wrt footfall position, can be plotted by varying the parameters in the Step Model. This has been a very fruitful endeavor.
It shows, among other things, that a person can be walking a straight line while turning with every step, and that measured equality of stride length doesn't necessarily mean the person is walking straight, when compared to another person who is also walking straight (see Fig 14).
2) 5 Central Concepts.
5 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.
3) The Step Model and Footfall Plots
The Step Model shows the body orientation at the instant of heel-contact, at the end of the step. It can be used to make footfall plots which re-create any 2D step or path characteristic.
Also, many things are shown in the Step Model itself, so much (most) is gained just looking at the model, without the need to make plots.
But, the footfall plots are a very revealing part of the system. And very easy to make, despite the apparent weight of the instructions. They're used to predict path characteristics, given specific input, and should help greatly in the design of experimental protocols.
If you start at a standard position on the page and type in all the dimensions and co-ordinates for making the specific models, it gets to be a very simple process. And, if the same base model is used (ie., identical linear parameters), it's a matter of seconds to produce test models.
The only rotation points are the step-heel-point for rotating the step-foot model (for foot angle), and the step-pelvic joint for the rotation of the step-out-line/step-foot model (for foot offset). 3 models per series can give most of the information, and one standard model is all that's required for the 3, if only looking at direction parameters. All of the direction changes are accounted for by rotating the whole model, then aligning appropriate heel-points, and any changes of linear parameters are taken care of when making the model.
Modeling using the Step Model could be a big area. Since the procedure is exactly the same for all plots, a computer program would be ideal to generate standard plots. This would be an extraordinary project. And, actually relatively simple, though a lot of details.
The final program would be very useful to facilitate application to the diverse areas of gait research. It will allow the generation of correlation tables for variations of the fundamental parameters, wrt each other as well as other factors, like step and stride-line. This will simplify the analysis of real footfall patterns. It would also form the base program for integration of the 3rd dimension, and other elements, in multi-D gait analysis programs.
When applied to real data, it’s the reverse of standard plotting. The data gives the Step Models for each step, then each model is examined in detail.
B) Understanding the System
A person walking is a mechanical instrument. The brain uses muscles to manipulate a lever (vector) and point system to carry the body in any direction. Though the brain is in control of all the muscles, the lever and point system defines distance and direction.
As with any system, there are disadvantages and advantages to its study. Some are outlined below:
Disadvantages
- since human subjects – averages and trends rather than exact
- could be large variations in step characteristics within a few strides or with every step
- not easy to measure angles and distances in field experiments
- everyone has distinct physical characteristics and learning which make the way they walk unique
- angle and offset values could be small – a few degrees, or less than 1 inch offsets, possibly
- knee, ankle and hip joint rotations complicate the picture
Advantages
-a person walking is a vector system
- the path of the foot in the air doesn’t matter, only the final foot placement.
- know starting point each time, the footfall
- feet are attached through the leg and pelvis.
- even though everyone’s walk is unique, all direction and distance changes are definable using the 8 parameters.
- not necessary to identify and understand every control factor for leg movement, many can be generalized to standard influences.
- all the upper body is irrelevant to the distance and direction measurement