The Mouck Method for Path Deviation Study
by Mike Mouck
Part IIa – Understanding the System
Copyright © 2007. Free for non-profit use

Part I showed that differences in step length can not cause changes of direction when walking. It further concluded that the only ways for a person to change direction is by changing foot angles and/or foot offsets.

Part IIa defines and discusses the system and its parameters.

Part IIb outlines the Plot Model. This model geometrically represents the important factors for direction control when walking by relating all direction changes to foot angle and offset deviations. It is used to produce standard footfall patterns which show subtle distance relationships impossible to interpret from field footfall patterns.

Part IIc presents some plotting results and describes several distance relationships which may be important.

Part IIa - UNDERSTANDING THE SYSTEM

A person walking is a mechanical instrument. The brain uses muscles to manipulate a lever and pivot point system to carry the body forward. Though the brain is in control of all the muscles, the lever and pivot point system totally defines direction and the footfall pattern totally describes it.

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
• impossible to predict conscious direction changes for a lost person
• everyone has distinct physical characteristics and learning which make the way they walk unique
• angle and offset values small – a few degrees, or less than 1 inch offsets, probably
• knee, ankle and hip joint rotations complicate the picture

Advantages

• the path of the foot in the air doesn’t matter, only the final foot placement
• know starting point each time (the footfall), and can get DOT (using the model)
• feet are attached through the leg and pelvis
• even though everyone’s walk is unique, all deviations result from foot angle and offset changes
• wander patterns are generally regular (but don’t need to be, they can still be studied)
• not necessary to identify and understand every control factor for leg movement, many can be generalized to standard influences
• footfall patterns are linear, continuous and sequential
• can define a “perfect” model system
• the model can be used to produce two useful variants, the analytical and plot models.
  • The analytical model allows the investigation of field footfall patterns.
  • The plot model allows the production of perfect footfall patterns for any combination of step length, foot angle, etc.

Clinical Descriptions and Discussions

A stride, or gait cycle, can be discussed in several ways. One is to describe limb movement as in a swing or stance phase for each leg/foot. The swing phase is when the foot is in the air (carry and step) and the stance phase when the foot is planted. The stance phase is further separated into single (one foot planted) and double (both feet planted) stance phases.

Gait Cycle (= Stride)

Swing phase:

• one/stride, 40% of the gait cycle
• foot is in the air and moving forward through the carry and then step
• other foot is in single stance phase

Stance phases:

Single stance phase:

• one/stride, 40% of the gait cycle
• foot is planted and the leg used to vault forward
• other foot is in swing phase

Double stance phase:

• two/stride, 10% of the gait cycle each
• both feet are planted
• one foot going toe-off to swing phase, the other heel-strike to toe-off

These phases are further sub-divided with respect to forces, etc., but there’s no need to go into that.

Clinical work also describes six determinants of normal and pathological gait in an attempt to describe the most important physical parameters involved in walking.

Six Determinants of Normal and Pathological Gait

These are generally (though not universally) accepted as describing the factors important for affecting gait, and are usually explained as an attempt to minimize the energy cost of walking by reducing the movement and position of the body’s center of gravity (COG). The COG moves side-side (lateral sway), and up-down in a normal gait cycle.

1. Pelvic rotation – the pelvis rotates forward at heel strike and backward at toe off to increase the effective leg length and decrease the drop of the COG. This doesn’t affect direction, but could influence the straddle, stride, step and carry lengths.
2. Pelvic list – the pelvis tilts down at toe-off and heel-strike to increase the effective leg length and decrease the drop of the COG. This doesn’t affect direction, but could influence stride, step and carry lengths, and very slightly the straddle length.
3. Stance phase knee flexion – flexing the knee of the planted leg decreases the rise of the COG over the single stance phase. This doesn’t affect direction or any lengths unless the knee is not normally extended at toe-off and/or heel-strike.
4. Ankle rockers – stretching the foot out at toe-off and contracting it back at heel-strike increases the effective leg length and decreases the drop of the COG. This doesn’t affect direction but could influence stride, step and carry lengths.
5. Transverse rotation of leg segments – ie. changing foot angle. This is one of the two ways a person changes direction when walking and it also influences the lateral sway of the COG. Internal rotations (CW for left foot, CCW for right) increase lateral sway and walking base, and decrease stride length for the opposite foot, while external rotations (CCW for left, CW for right) do the opposite.
6. Genu valgum – the anatomy of the knee allows sideways motion at the joint. When “knock-kneed,” with the feet moving closer as well, there is a smaller lateral sway of the COG. This doesn’t affect direction, but could influence the straddle and R/L lengths.

Point #5 is the only one directly related to direction.

All the others would affect direction if deviations caused the foot to be planted differently than for a straight step.

For non-pathological gait, all contribute to the walking characteristics leading to normal foot offsets and angles.

For anyone interested in looking farther into the clinical aspects of walking, a good place to start is Dr. Chris Kirtley’s web-site discussing observational gait analysis. It gives some background information with diagrams. Or, just use “gait analysis” as the internet search term to find a list of others.

Most of the work into normal and pathological gaits relate joint angles, the positions of other parts of the body, forces on the floor, etc. I didn’t find any references to direction elements as I describe, but I couldn’t look very far.

Even in clinical research, they use the inaccurate definition for step length as the distance from one foot strike to the next (L-R and R-L). This gives a value greater than the real step length, and it can vary without a change in the real step length.

A foot offset does not change step length, but changes the R/L, stride and walking base lengths.

A foot angle does not change R/L or step lengths, but changes stride and walking base lengths.

So, some step length deviations interpreted from R-L, L-R or stride length changes may actually be caused by foot angles and offsets. I haven’t seen these factors discussed with reference to step and stride lengths, so I don’t know if they where taken into account.

Also, they compare footfall to footfall measurements of stride length, apparently without taking into account that a stride length for one foot is equal to different parts of two consecutive strides of the other foot. These errors may not be important for them, since they are not looking at factors affecting direction.

However, these studies give clues to important factors for the control of direction in walking, and where to look to help understand deviations.

General Description of Walking

Start from standing with both feet planted normally, side by side, and hips perpendicular to DOT. The first step is taken with the left foot. The movements of the knee, ankle and hip joints have been excluded for brevity. The body is moving forward continuously.

1. The left foot is picked up and the body goes to maximum lateral shift to the right, to balance over the right foot.
2. The left leg is extended and the body begins to laterally shift to the left.
3. The vault continues to the end of the step, where the left foot is planted with a certain foot angle and foot offset The body is continuing to laterally shift to the left. This begins a double stance phase, right leg to the rear, and just before the right carry. If there was non-zero foot angle or offset in the left foot, the DOT will change by these values.
4. The right foot is raised and carried to a standard start position (directly adjacent to the left foot, straddle distance apart) while still in the air. This is the left leg single stance phase and the carry part of the right leg swing phase.


The DOT has changed by the leg angle value of the previous left foot’s offset, ie. the new DOT follows the previous left step’s leg line, and has changed by the value of the previous step’s leg angle. The right foot’s current stride length will increase if the left foot was placed to the left of the line (turning left) and decrease if the foot was placed to the right (turning right), and it would be the standard start/stop position if there was no foot angle deviation.

This is the first of the two possible angular changes to the DOT.
 

5. If there was a foot angle deviation, the body rotates on the left foot to normalize for a more comfortable forward step, changing the DOT by an amount related to foot and push-off angles. The body is continuing the vault on the left leg, and has maximum lateral shift to the left.

This is the second of the two possible angular changes to the DOT.

If the right foot was held directly adjacent to the left foot, normal straddle distance apart, its path would describe an arc as the body rotates on the left foot.
The rotation increases the current right stride length if the left foot angle change was counter-clockwise (CCW, turning left) to the zero foot angle, and decreases it if the foot angle change was clock-wise (CW, turning right).
If the right foot was planted after the rotation, it would define the end of the carry and start of the step. It’s the standard start/stop position which is used for the plot model, and to define all angles, carry, step and stride lengths.
 
Even if the foot never actually passes the start position point, it can be used to measure standard distances. This is because the path of the foot in the air doesn’t matter, only the final placement. So, I could do figure-8’s in the air with my foot, put it down anywhere along that path and use that position as a standard start/stop position, as long as that point is used as the reference for all the steps. It would be a very inconvenient reference, since it’s distances and angles would be related to elements from more than one stride.
 
The start/stop position I use means that all deviations are accounted for within the same stride
 

6. The right foot then passes the planted left foot and extends to at or near the step out length for that leg. The body is moving forward by vaulting on the planted left leg, and is laterally shifting to the right.

7. At the end of the step, the right foot is planted with a certain foot angle and offset. This is the second double stance phase, with the right foot in front, and the end of the right step.

8. Continue as from Step 4), above.

For one stride, each foot goes through a double stance (foot at rear), then swing, then double stance (foot at front), then single stance.

There are 2 possible angular direction changes with each step, and so 4 possible over the stride.

Foot Angle vs. Push-off Angle

Push-off angle uses the foot line as its zero mark, and accounts for anomalies when stepping out from the planted foot. Foot angle and push-off angle are partly independent, and it’s the difference in the two values that determine direction change.

For example,

For a person with straight foot angle of 0 deg, when taking a straight step, the foot angle is 0 deg and the push-off angle is 0 deg.
ie. foot angle = push-off angle; this is the only condition where that’s true. The change in DOT is 0 deg.

For a person with straight foot angle of 6 deg CCW, when taking a straight step, the foot angle is 6 deg CCW
and the push-off angle is 6 deg CW. The change in DOT is 0 deg.

For a person with straight foot angle of 0 deg, but when on the planted foot pushes off at a 4 deg angle CW, the
foot angle is 0 deg and the push-off angle is 4 deg CW. The change in DOT is 4 deg CW.

For a person stepping out straight with straight foot angle of 6 deg CCW, but for this step has a push-off angle of
only 4 deg CW, the DOT will change by 2 deg CCW.

When straight foot angle is 0 deg, for a straight step out, push-off angle = foot angle. However, if there is a value for the normal straight foot angle or there is different push-off, the two are not equal.

In analyzing field patterns and pathology studies, foot angle and push-off angle would have to be separated. For the current preliminary discussions, it’s only excess weight.

For this entire discussion, I assume that the straight foot angle and all push-off angles are 0 deg, unless otherwise stated. All such deviations can be accounted for, but they can be described after the establishment of the basic method.

Summary of Potential Direction Changes during a Normal Step

When taking a step, changes in direction manifest through two variables: 1) foot offset and 2) foot angle.

When the front foot is planted at the end of its step,

• EITHER the foot (heel point) is placed on the line for a straight step or it isn’t, and
• EITHER the foot angle is such for a straight step in the next step or it isn’t.

All directional deviations can be described using a short series of simple questions:

This simple analysis forms the basis for the development of the Plot Model.

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Part I (Page 1, Page 2)    Part IIa    Part IIb (Page 1, Page 2)    Part IIc    Part III

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