Part II - Problems with Current Gait
Analysis Work
Problems with Current Gait Analysis Work:
- Carry-line has not been recognized
- Foot offset has not been recognized
- Stride specifics are not understood
- Inaccurate measurement of the step
length
- Line of progression is not an
appropriate reference line for angular measurements
- No standard reference position
- Conflicting and inaccurate
definitions
- Measurements are taken at the
heel-edge
- Aberrations aren't measured as a
distinct entity
1. Carry-line has not been recognized.
Stride = carry + step (vectors).
When a
person takes a step with the left foot, as they walk the
right foot has to be brought up to and then pass the
left foot while in the air, in order to take its own
step. This action is called the "carry".
Many
discussions question how a person can walk with
radically different step lines and still show no
problems in gait. Disregarding the carry is the critical
error which leads to the confusion. Carry is the
distance equalization mechanism for the total distance
traveled by each foot.
Thus, when the left foot
takes a 15" step, the right foot is "carried" for 15".
Then, the right foot moves forward with a step of 20",
followed by the 20" carry of the left foot. For one full
stride, the left foot moves forward by a step of 15"
PLUS a carry of 20", for total of 35", and the right
moves forward with a carry of 15" PLUS a step of 20",
for a total of 35". (Note: These numbers come out even
because I assume there are no turns over the step. If
there are, as is probably the usual case, opposing step
and carries are not equal, and accounts for left/right
foot distance variations when turning.)
Step-line
can be easily controlled, but carry-line changes as a
consequence of the other foot's previous step-line, foot
offset and foot angle, as well as the current step’s
push-off angle, aberrations, pelvic stretch (and
straddle-line). So, distance and direction variations in
one foot are reflected by changes in the corresponding
carry-line for the other foot.
The carry-line
must be included as a distinct distance unit.
2. Foot offset has not been recognized.
Foot offset describes how far a person puts their
foot to the side of the straight line for that foot,
entails a rotation of the step-out line, and shows the
contribution from pelvic joint rotations to direction
control. Rotation at either pelvic joint results in foot
offset.
It's a direction parameter with a
side-step part, resulting in a lateral mass shift, as
well as a direction change via rotation of the foot-line
along with the step-out line. This foot-line rotation,
however, is not a foot angle change, since foot angle
uses the step-out line as its 0 reference.
This
may be why foot offset hasn't been defined until now. It
took a logical analysis, not a measurement, to discover
it. Measures using the "line of progression" read all
foot-line rotations as from foot angle, with no way to
distinguish the contribution from foot offset.
Foot offset and foot angle are separate direction
parameters which have different characteristics and
controls. Foot offset is changed by real or apparent
lateral rotation at either pelvic joint, as well as
lateral movement at the knee and ankle joints. Foot
angle is changed by real or apparent foot-line rotation
around the 3D step-out-line axis.
Foot offset
must be included as a separate direction entity.
3. Stride specifics are not understood.
Comparison of right and left heel to heel stride
measurements is not valid as the sole determinant of
direction change. The right stride is related to
different parts of two consecutive left strides (The
step of the first plus the carry of the second.), and
vice versa. Several consecutive measurements would
minimize this error.
And, measured equality of
stride lines doesn't guarantee the person is walking
straight.
a) The paths of 2 people both "walking
straight" may diverge if they're not walking straight in
the same way.
b) A person walking a straight
stride may be turning with every step ( I think this is
how everyone normally walks. Walking is a controlled
stagger. A person is virtually always engaged in
turning, according to the lower frame, even if walking
“straight”.)
Stride-line (and walking base) is a
product of 9 distance and 5 direction elements, and is
also the vector sum of the step and carry lines. It is
not equivalent to 2 steps. Stride = carry + step
(vectors).
4. Inaccurate measurement of the step length.
First, it has to be clarified what's being described
with the step length (line). To this method, it's an
accurate measurement of the distance traveled by the
foot over the step, measured from heel-points.
Current literature shows 2 main ways to measure "step
length":
1) Left heel-edge to right heel-edge,
and vice versa. (This measurement does not accurately
describe the total distance traveled by the foot over
the step. It includes a part from the pelvis, and shows
a different variation with respect to foot offset than
the accurate measurement. I call this L/R-line (but,
taken from heel-points) see Fig.6)
2) Step =
Stride/2. This is a measure that splits stride length in
two. And, that's all it is.
Measurements for
step-(carry-) line are wrt the appropriate heel-point in
the standard start position. This accurately defines the
distance traveled by the foot over the step (carry).
Also, even in the clinical literature there's the
occasional reference to step length with respect to
direction change, saying that different right vs left
step lengths will cause a person to change direction.
This does not happen. Differences in step length for the
left and right feet can not change direction, only
distance.
However, since L/R-line is changed by
foot offset (as is the step-line), and L/R-line is what
many currently define as "step length", direction
changes associated with foot offset may appear to result
from the change in the L/R-line. But, change in the
L/R-line is a consequence of the foot offset, not the
cause.
5. "Line of progression" is not an
appropriate reference line for angular measurements.
The "line of progression" is a decision about
direction based on the motion of a mass unit or the
position of footfalls. It's a very inaccurate reference
line for the measurement of direction parameters like
foot angle.
If based on a mass unit, it's motion
is what it shows, and if based on footfalls, it varies
with foot offset and aberrations.
It also
inhibits the recognition of foot offset, since foot-line
rotations due to foot angle vs foot offset appear as all
from foot angle.
There are 5 consecutive
"straight lines forward" which are used as references
over the course of a step. These allow the tracking of
direction change over the entire step, as contributions
from each of the 4 parameters relevant for direction.
6. No standard reference position.
There are two separate distance units involved when
the foot is in swing phase, the carry and step lines.
Since the foot is in the air, and it could have any
number of different paths while in the air, how can the
two be separated?
This requires the application
of an arbitrary, theoretical reference position. The
most logical choice for this is the "standard start
position”, which is based on the pelvic joint positions
and start heel-point at heel-contact.
The
"reference-heel-point", where the step-foot is in the
air in the standard start position, is the stop/start
point for carry/step lines, resp.
But, 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 the snapshot. Those details are
matters for observation, not definition.
This
reference position is valid because of the vector nature
of all the measured distances.
7. Conflicting and inaccurate definitions.
While I’d like to conform to definitions that are in
current use, it's not possible. The same terms are used
to describe different things, and vice versa, depending
who you talk to. Unfortunately, literal terms like step
length and stride length have been used to describe
inaccurate measurements.
In order to have any
kind of discussion, terminology has to be standardized,
even for just this paper. All the definitions I've
created are unique, and many are accurate versions of
currently used terms. So, there was a bit of a dilemma.
Question: How can the new terms be distinguished
from current, inaccurate versions of the same ones?
Answer: I use the designation "line" when referring
to an accurate quantity.
Any measured distance
can be connected with a straight line. Every line has a
length. So, reference to the step "line" is a reference,
ipso facto, to the step "length", and vice versa. This
avoids the vast majority of conflicts, and highlights
the vector nature of all the measured distances.
8. Measurements are taken at the heel-edge.
Heel-edge measurements are not accurate.
They're easier, but always introduce a variation in both
distance and direction with respect to the accurate
heel-point measurement. Heel-edge measures do not
accurately describe the distance the foot travels over a
step or stride, though some may be very close in number.
Accurate measures must be to and from heel-points,
even though the time of the snapshot is the instant of
heel-contact.
9. Aberrations aren't measured as a distinct
entity.
Aberrations are anything that
changes the heel-point position and/or rotates the
foot-line after heel-contact of the current foot, but
before the next heel-contact of the other foot.
These are slides, jumps, foot (toe) extensions and
rotations on the planted foot, etc. A spin turn is an
aberration, but a step turn isn't.
This is a
complex and variable set of movements which is separated
from the other 7 parameters by the choice of the time
snapshots as the time of heel-contact, rather than
heel-point contact.
Aberrations is a fundamental
parameter which accounts for all relevant changes in the
position of the planted foot.
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Part I
Part II
Part III
Part IV
Part V
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© 2008
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