Part III - Various Discussions
- The Four Minimum Points of Gait and Foot-line
- The Eight Fundamental Parameters of Gait
- The Five Straight Lines Forward for a Single
Step
- Real vs Apparent Rotation
- Walking is a Controlled Stagger
- The Primary Goals of Walking are Distance and
Direction
- The Standard Start Position
- Heel vs Heel-point Contact
- Direction Changes Over the Step
- Step, Carry and Stride
- Straddle-line, Straddle, Straddle Width, Stride
Width, Step Width, Walking Base and Walking Straddle
- Aberrations
- Vectors
- Accuracy vs Precision
- Limb Dominance
- The Rotating Reference Grid
- Balance
- Movements Affecting the Eight Parameters
1. The Four Minimum Points of Gait and
Foot-line
The four minimum points of
gait are:
1) step-heel-point,
2) step-pelvic
joint,
3) rear-pelvic joint,
4) start-heel-point.
Deconstruction and simplification of the skeleton
leaves the two heel-points as contact with the ground,
and the pelvic joints as the only necessary rotation
points, with 3 straight lines connecting the points.
Measurements based on the projection of the 4 points
onto any 2D plane gives all the distance and direction
information for a person walking, wrt that plane.
The foot-line should be included if possible, but it
isn't required. Without it, foot and push-off angles are
recorded as a single angular change, and foot-line
rotations over aberrations can't be determined.
Also, the heel-points are normally the points of contact
with the ground if we had peg legs that went to points.
So if a person was wearing shoes, the heel-point would
be on the bottom of the shoe, not on the sole of the
foot. However, this method allows for the choice of
different points, eg. using the sole of the foot even
though wearing shoes, as long as the interpretation
takes it into account.
The entire measurement
system is derived from the detailed analysis of the
relationships between these 4 points and foot-line.
2. The Eight Fundamental Parameters of Gait
The 8 fundamental parameters of gait are lines and
angles derived from the projection of the 4 minimum
points of gait and foot-line, onto a specific 2D plane
(usually the floor). They organize the contributions to
distance and direction from specific body segments and
rotation points (joints).
There must be a change
in at least one of the fundamental parameters for there
to be any change in distance and/or direction, and a
change in any one must show as a change in distance
and/or direction.
Distance:
1) rear-leg-line
2) step-out-line
3) straddle-line
4)
pelvic-stretch
Direction:
4) foot offset
5)
foot angle
6) push-off angle
Distance and
direction:
7) aberration
Walking is the
manipulation of these parameters.
1) and 2) are
the 3rd and 4th straight lines over the step, resp.
Note: Step, stride and walking base are not
fundamental parameters, they are products of them.
Also, 3) straddle-line and 4) pelvic-stretch, are
the sides of a right triangle, with the pelvis-line as
the hypotenuse. Technically, the pelvis-line should be
the fundamental parameter (since it’s directly based on
a body segment), not the other 2. However, since pelvic
stretch and straddle-line are far more descriptive wrt
the important elements of gait, I prefer to use them
instead.
3. The Five Straight Lines Forward for a
Single Step
The 4 minimum points and
foot-line define a series of 5 "straight lines forward"
which are relevant over the course of a single step.
These lines describe sequential direction changes.
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.
The rear-leg and step-out lines are
fundamental parameters, the foot-line isn't.
Foot-lines represent 3 of the straight lines, but, since
it’s not a required element, only very useful. If there
was no foot-line, there would be 2 straight lines. The
first would be the rear-leg-line, and the second the
step-out-line. This shows why adding the foot-line is
desirable, it adds quite a bit of info because each
foot-line is affected by different factors.
Each
line represents what would be the straight line forward
if there were no further direction changes. 1) would be
the straight line if there were no foot offsets, foot or
push-off angles, or aberrations. When there's an
aberration, the foot-line, 2), becomes the straight
line. When there's push-off angle, the rear-leg-line, 3)
then becomes the straight line and, if there’s foot
offset 4) becomes the straight line forward. 5) becomes
the straight line if there’s foot angle. 5) is 1) for
the next step. (4 and 5 occur at the same time.)
These represent a continuum which is defined by body
segments, and allows the accurate determination of
direction changes within the step, and over the entire
path. This should help with the interpretation of data
in many areas of gait research.
4. Real vs Apparent Rotation
Rotations at either pelvic joint or along the
step-out-line axis result in direction changes, as well
as others, but it doesn't have to be real rotation.
Since the step-out-line is a vector sum, the
specific orientation of the component vectors could lead
to a lateral heel-point shift without any actual
rotation at the step-pelvic joint. (The specifics of
this would have to be studied.) This would be measured
as a rotation at the step-pelvic joint, even though it
had nothing to do with it. Also, since this is a 2D
projection, other movements along the z-axis may lead to
apparent rotations.
Real vs apparent rotation is
irrelevant to the measurements. It would be very
important, though, to other aspects of a full clinical
analysis. This highlights the fact that this method,
though extensive, is still only one part of the greater
analysis.
5. Walking is a Controlled Stagger
When a person walks, they are manipulating the 8
parameters. The degree of control over this manipulation
is the factor which defines when a person is
"staggering".
The distance parameters aren't as
important, but variation of any of the direction
parameters leads to a change in direction.
Distance deviations don't have to be compensated for,
but if at any time there's a change to one of the
direction parameters, there isn't only that turn, but
there must be a future, compensating turn in order to
stay on the straight path. This is by manipulation of
the next parameters in that step, as well as the
parameters for the next steps. If the compensating turn
isn't exactly right, there has to be another
compensating turn farther along the path, etc.
Someone who's drunk has less control over the direction
changes, and the compensating turns, and shows greater
lateral movement than normal, as well as other gait
abnormalities. The classic drunken stagger.
For a
sober person, each step virtually certainly has
direction deviations and/or compensating turns. The
upper body may be perfectly stable, but the lower frame
is changing direction by small amounts. A well
controlled stagger.
6. The Primary Goals of Walking are Distance
and Direction
Humans evolved the ability
to walk in order to better get from point A to point B.
Since a person's body is directional, the path to B will
not only include a distance to be traveled, but also a
direction change from straight ahead.
Physical
processes developed to accomplish these goals, using the
available framework, which also required the balancing
of a large, gangly mass over the lower locomotion
framework, the minimization of energy expenditure, the
versatility necessary to traverse any terrain, and the
breadth of control to quickly alter the walking pattern
to suit virtually any immediate choice (such as jumping,
changing speed or direction).
Extremely complex
muscle controls have evolved, but, if distance and
direction are the primary goals, then it’s reasonable to
assume muscular and other controls developed to
facilitate these, and, so, should be definable as
specific sets associated with each.
That's why
the plane of the floor is the most revealing, and why
the 8 parameters should be the central correlation for
all areas of gait research, since they directly show
distance and direction changes involved during the step,
defined wrt body segments and rotation points (ie.,
specific movements and muscle action).
Also,
since each step is an individual, the ability to
separate the unique direction and distance variations
over each step, which are directly related to various
mass movements, should aid in the analysis of vector
data such as force, velocity, etc.
7. The Standard Start Position
Definition of the theoretical standard start
position is required for the separation of step and
carry lines. Only 3 of the minimum points of gait are
needed to define it, the step and rear-pelvic joints and
the start-heel-point, and it's the position when all
changes due to the previous step are accounted for by
rotations and/or heel-point shifts, and after aberration
and push-off angle shifts and rotations.
The
first 2 direction changes, aberrations and push-off
angle, change the Step Model grid orientation, and,
hence, the position of the standard start position.
To visualize it, imagine yourself frozen at the
instant of heel-contact. Now draw yourself back along a
straight line, keeping the same straddle-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.
If you had any pelvic stretch in that step (which
everyone probably has), since this would decrease the
straddle-line but have no effect on the pelvis-line, you
couldn’t stand at the standard start position in real
life. But, it's still a valid standard reference because
of the vector nature of all the measured distances.
Also, the foot never has to pass over the standard
position, and the person never has to take up the
orientation of the standard grid except at heel-contact.
This provides a separate, consistent measurement
standard, the reference-heel-point, which is still
defined by the heel-contact (or any other) snapshots. In
the Step Model, the reference and start foot-models
define the standard start position.. The reference-foot
model represents the foot that's in the air (the
step-foot), and the reference-heel-point is the
stop/start point for carry/step lines, resp.
8. Heel vs Heel-point Contact
The time of the snap-shot which defines the
parameters is the instant of heel-contact. But,
heel-edge is not the point used for measurements. All
measurements are to and from heel-points.
The
heel-contact time point is chosen so movements due to
aberrations can be isolated. Usually, the heel-point is
still in the air at heel-contact, but that doesn't
matter to the measurements. Aberrations are highly
variable, and the ability to separate the distance and
direction changes they cause greatly simplifies the
analysis and allows far greater accuracy.
The
position of the heel-point contact is important, though,
since this would be the best start/stop point for field
measurements. This position, which can be estimated from
a footprint, is the most accurate field determination of
heel-point position. It includes all or part of an
aberration, but may still be very close to it's position
at heel-edge contact. A more detailed footprint analysis
could estimate deviations.
As long as it's
consistent, and since changes are the most important,
any discrepancy in position of heel-point at heel-edge
vs heel-point contact may be insignificant, or at least
tolerable. This would be a matter for study.
Also, since the heel-point is a point on the body, not
the floor, there's the potential for time dependant
analysis of all the parameters. The 4 points and line
are always definable, whether in the air or not, but the
interpretation would have to be modified for movement
through the air and angular transition wrt the pelvis
and leg-lines.
Heel contact is considered the end
of the current step and start of the next step.
9. Direction Changes Over the Step
The 4 direction parameters are not expressed at the
same time, or in the same way, over the step.
Aberrations and push-off angle are due to movements
associated with the planted (opposite) foot, over single
and part of double stance phases.
Foot offset and
foot angle occur over swing phase of the step foot, and
are established at heel-contact.
So, aberrations
is the 1st, push-off angle the 2nd, and foot angle and
offset occur at the same time and are the 3rd and 4th
direction changes.
7 of the 8 fundamental
parameters are defined by a single snapshot, but
aberrations require the comparison of consecutive
snapshots, wrt foot angle change and heel-point shift of
the planted foot.
Foot offset, aberrations and
push-off angle require linear translation to express
normally, while foot angle can be expressed while
stepping in place.
Within the same step, foot
offset and foot angle can compensate for aberration and
push-off angle deviations, but not vice versa. Push-off
angle can compensate for aberrations.
Only
certain (real or apparent) actions can change direction:
1) aberrations – spins, slides and all other movements
of the planted foot, between sequential heel-contacts
2) push-off angle - appropriate muscle action in the
planted foot's leg and foot, as well as momentum and
other path specifics
3) foot angle - rotation of the
foot-line around the 3D axis of the step-out-line
4)
foot offset - step and/or rear pelvic joint rotation
(lateral and/or vertical), and rotation at the step-knee
and/or ankle joint which leads to heel-point shift.
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Index
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Part I
Part II
Part III
Part IV
Part V
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© 2008
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