2.4.4. Events Detection

To detect gait events avoiding the need of determining angular rate or acceleration in body coordinates (where the alignment of the sensor on the body would need to be known [8,15,16]), we used acceleration magnitude and the vertical component of acceleration (in global coordinates).

We observed that FO events can generally be found in an acceleration magnitude perturbation before swing. To approximate this instant, we filtered acceleration magnitude using a zero-lag bidirectional 2nd order Butterworth low-pass filter with a cutoff of 7 Hz. The cutoff frequency was experimentally chosen to ensure that (i) the perturbations were smoothed resulting in a single peak value, and (ii) the resultant peak approximates the acceleration magnitude perturbation where annotated FO events are observed. The FO event was considered the first maximum peak above the average of the moving interval appearing between two ZVIs (i.e., the first value above the average surrounded by two values with lower magnitude), as illustrated in Figure 3.

Events detection from low pass filtered acceleration magnitude and low pass filtered vertical acceleration (FO—Foot off; FC—Initial foot contact; MS—Mid-stance; Ann-FO—Annotated FO; Ann-FC—Annotated FC).

FCs are detected between FOs and the beginnings of the next ZVIs. FCs were considered the absolute minimum of vertical acceleration measured between these two instants (Figure 3). Before detecting FC events, vertical acceleration was low-pass filtered using a zero-lag bidirectional 1st order Butterworth filter with a cutoff of 30 Hz. The cutoff frequency was experimentally chosen to ensure the attenuation of high frequency noise that could hinder the detection of FC events.