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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