Differences in step length for the left and right feet can not lead, by themselves, to path deviation.

When a person takes a 20 in. step with the left foot, as they walk the right leg has to be brought up to and then pass the left foot. This action I have termed the "carry".

Thus, when the left foot takes a 20 in. step, the right foot is "carried" for 20 in. Then, the right foot moves forward with a step of 15 in., followed by the 15 in. carry of the left foot. The total distance moved forward by each foot is not just the 20 or 15 in. step, but also includes the corresponding carry. For one full stride, the left foot would move forward by a step of 20 in. PLUS a carry of 15 in., for total of 35 in., and the right moves forward with a carry of 20 in. PLUS a step of 15 in., for a total of 35 in.

Disregarding the carry is the critical error which leads to the confusion. The carry equalizes the total distance traveled for each foot.

Try walking with extremely exaggerated differences in the left and right step length (eg. left-25 in. and right-2 in.). If step length had anything to do with direction, after 1 or 2 strides you should be pointing off of the straight line. You don't because the carry of 2 in. for the left step and 25 in. for the right step means the total distance traveled is still 27 in. for both feet. (Stride = carry + step)

Try this. Draw two lines on the floor. Make them parallel and at a distance apart that is comfortable when you walk, your normal walking base. Now step on the lines. Take a few steps, keeping each foot to its line. This is the normal starting point for all of the discussions that I have seen. This is a person who is walking straight. Do you agree?

Now, while keeping your feet on the lines, walk along it with any combination of step lengths that you like. Look at the lines, or not, as long as you keep stepping on them. Have you stepped off the line at the end of the room? No, you haven’t.

Now, imagine that the lines are in a room that is twice as long as the first one. And do the same thing, taking whatever step lengths you want for each foot (try 0” step length with one of the feet, or 2”). At the end of that room, did you step off the line? No, you didn’t.

Do you think that at the end of a room a million times longer you would be off the line? No, you wouldn’t. Because that’s not how humans change direction when they walk. It just doesn’t happen that way. If step length had anything to do with direction, whether we’re controlling the step lengths or not is irrelevant, the result on path characteristics would be the same.

It’s said that the height of foolishness is to continue to do exactly the same thing while each time expecting something different to happen.

The only way to deviate from the straight path is for one or the other foot to be placed off of the straight line during the step.

SO THEN, “WHAT IS THE CAUSE OF PATH DEVIATION?”

The study of path deviation is the study of direction changes during walking.

With each step there are 4 possible direction changes, in this order: 1) aberration, 2) push-off angle, and 3) and 4) foot offset and foot angle occur at the same time.

That means 8 potential direction changes per stride. These changes almost certainly won't be equivalent.

Each relevant part of the body contributes it's direction change to the step, and each step contributes its direction change to the body DOT. It’s the summation of the changes that determine overall direction. For example, if each left step gave a 2 deg turn left, and each right step gave a 2 deg turn to the right, the person would be walking straight, but turning with each step.

ie. If the net direction change over a stride is zero, the person will be walking straight (ie. measured equality of stride lines), even if they’re turning with each step. (How a person is walking straight is also critical to the overall path characteristics.)

If the left gave 3 deg left and the right 2 deg right, the person would be turning left at 1 deg per stride, and would be at 90 deg to the original DOT in 90 strides.

For a 30” normal step with a 15” step-out line, an increase in foot angle of only 2 deg for one foot will cause a person to go at 90 deg to the original DOT in 45 strides, or about 40 yds. The same deviation occurs with a 1/2” foot offset. That’s barely perceptible.

So, when a person deviates from a straight line, the exact path is a line punctuated by angle changes at each step, rather than a smooth curve, and the exact characteristics depends on how all the parameters are being manipulated in each step.

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