Part III  Various Discussions
 The Four Minimum Points of Gait and Footline
 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 Heelpoint Contact
 Direction Changes Over the Step
 Step, Carry and Stride
 Straddleline, 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
Footline
The four minimum points of
gait are: 1) stepheelpoint, 2) steppelvic
joint, 3) rearpelvic joint, 4) startheelpoint.
Deconstruction and simplification of the skeleton
leaves the two heelpoints 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 footline should be included if possible, but it
isn't required. Without it, foot and pushoff angles are
recorded as a single angular change, and footline
rotations over aberrations can't be determined.
Also, the heelpoints 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 heelpoint 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 footline.
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 footline, 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) rearlegline
2) stepoutline 3) straddleline 4)
pelvicstretch
Direction: 4) foot offset 5)
foot angle 6) pushoff 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) straddleline and 4) pelvicstretch, are
the sides of a right triangle, with the pelvisline as
the hypotenuse. Technically, the pelvisline should be
the fundamental parameter (since it’s directly based on
a body segment), not the other 2. However, since pelvic
stretch and straddleline 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
footline 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) stepfootline of the previous step, 2)
footline after aberrations, 3) rearlegline, 4)
stepoutline and, 5) stepfootline of the current
step.
The rearleg and stepout lines are
fundamental parameters, the footline isn't.
Footlines represent 3 of the straight lines, but, since
it’s not a required element, only very useful. If there
was no footline, there would be 2 straight lines. The
first would be the rearlegline, and the second the
stepoutline. This shows why adding the footline is
desirable, it adds quite a bit of info because each
footline 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
pushoff angles, or aberrations. When there's an
aberration, the footline, 2), becomes the straight
line. When there's pushoff angle, the rearlegline, 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
stepoutline axis result in direction changes, as well
as others, but it doesn't have to be real rotation.
Since the stepoutline is a vector sum, the
specific orientation of the component vectors could lead
to a lateral heelpoint shift without any actual
rotation at the steppelvic joint. (The specifics of
this would have to be studied.) This would be measured
as a rotation at the steppelvic joint, even though it
had nothing to do with it. Also, since this is a 2D
projection, other movements along the zaxis 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 rearpelvic joints and
the startheelpoint, and it's the position when all
changes due to the previous step are accounted for by
rotations and/or heelpoint shifts, and after aberration
and pushoff angle shifts and rotations.
The
first 2 direction changes, aberrations and pushoff
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 heelcontact. Now draw yourself back along a
straight line, keeping the same straddleline, until
you're standing straight up at a stop, with the
stepfoot in the air, and the left and right feet at a
distance of straddleline apart, not pelvis line.
If you had any pelvic stretch in that step (which
everyone probably has), since this would decrease the
straddleline but have no effect on the pelvisline, 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 heelcontact.
This provides a separate, consistent measurement
standard, the referenceheelpoint, which is still
defined by the heelcontact (or any other) snapshots. In
the Step Model, the reference and start footmodels
define the standard start position.. The referencefoot
model represents the foot that's in the air (the
stepfoot), and the referenceheelpoint is the
stop/start point for carry/step lines, resp.
8. Heel vs Heelpoint Contact
The time of the snapshot which defines the
parameters is the instant of heelcontact. But,
heeledge is not the point used for measurements. All
measurements are to and from heelpoints.
The
heelcontact time point is chosen so movements due to
aberrations can be isolated. Usually, the heelpoint is
still in the air at heelcontact, 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 heelpoint 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
heelpoint position. It includes all or part of an
aberration, but may still be very close to it's position
at heeledge 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 heelpoint at heeledge
vs heelpoint contact may be insignificant, or at least
tolerable. This would be a matter for study.
Also, since the heelpoint 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 leglines.
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 pushoff 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 heelcontact.
So, aberrations
is the 1st, pushoff 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 heelpoint shift of
the planted foot.
Foot offset, aberrations and
pushoff 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
pushoff angle deviations, but not vice versa. Pushoff
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 heelcontacts
2) pushoff 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
footline around the 3D axis of the stepoutline 4)
foot offset  step and/or rear pelvic joint rotation
(lateral and/or vertical), and rotation at the stepknee
and/or ankle joint which leads to heelpoint shift.
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Part I
Part II
Part III
Part IV
Part V Copyright
© 2008
