The sit-to-stand and stand-to-sit algorithms first estimate the vertical displacement of the lumbar sensor within a trial, and look for displacement above a particular threshold. This threshold needs to be low enough to detect these events, but needs to be high enough such that movement common during gait is not mis-detected as sit-to-stand or stand-to-sit events. This constraint in the algorithm makes it impossible to detect these events for young or short subjects, because the lumbar sensor is not displaced as much while seated as it is for taller subjects. The height cutoff is difficult to strictly state due to different subject behaviors, but in a small study of 38 subjects ranging in age from 5-21 and in height from 41-71 inches found the threshold to be around 52 inches (4'4").
We will be investigating ways to improve this algorithm in the future, perhaps taking the subject's height into account for the detection phase.