Why don't I always get APA measures from my Walk trials?

The anticipatory postural adjustment (APA) algorithm measures the (sometimes very) subtle shift in a subject's posture prior to step initiation. Detection requires that this shift can be picked out from the baseline movement of the subject. If the baseline movement of the subject is on the same scale as the APA, it cannot be confidently detected, and no metrics will be reported. Furthermore, some subjects exhibit a very subtle or non-existent APA, making detection more difficult and increasing the chance that baseline movement or the limits of the Opal sensors will not allow for successful APA detection and analysis. This does not necessarily imply a poorly administered test or out of calibration sensor.