I am examining periodic nature of gait signal through linear displacement gait parameter.I am using treadmill data collected through marker based system. In frontal and transverse plane the time series is showing periodic nature but in sagittal plane it is showing non-periodic.why it is happening.Does this sagittal plane data does not represent gait?
Gait is not purely periodic nor purely non-periodic, but lies somewhere in between. I wonder why do you state that the mediolateral (or, as you said, "right-left", or "sagital") displacement during gait is not periodic. I mean, it is periodic, even though it also shows a trend according to the position the subject is on the treadmill (just like Luca Ardigò said, "if subject does not walk straight". In fact, no one does).
I guess the most remarkable thing on mediolateral displacement is that its fundamental cycle repeats every stride, while the anteroposterior (frontal) and vertical (transverse) displacement repeats every step. Just make sure to consider that displacement have a trend (it fluctuates), even on treadmill. You can remove this trend, revealing its cyclic nature by converting it in velocity, thus removing the trend.
The sagital plane data does represent gait well. In fact, some researchers consider this the most critical data to evaluate gait stability or variability, due to invert pendulum models.
My best to you!
I surely agree general walking lies somewhere between periodic and non-periodic. Treadmill is surely more periodic due to its un-ecological specificity. right-left direction=medio-lateral, horizontal (or transverse) plane
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