A novel method for determining ground-referenced contacts during stair ascent: comparing relative hip position to quiet standing hip height.

Stair climbing can be measured using body-fixed sensors, whereby the origin and axes of the coordinate system are fixed with respect to the geometry of a body segment. These sensors can be part of a portable system, which provides the possibility to collect data in complex real-life environments. However due to the fact that the sensors are body-fixed, difficulties in determining the ground-based parameters of stair ascent can occur. The purpose of this study is to present a new approach for determining initial contacts based on a multi-chain biomechanical model combined with a new analysis method, in which relative hip height is compared to hip height during normal standing. Initial contacts obtained from the proposed method were compared to those obtained using an optical tracking device. An average absolute timing difference ranging from 0.04 (SD + or - 0.03) to 0.06 (+ or - 0.03) s and a root mean square error ranging from 0.05 to 0.07 s were found between the two techniques. This shows that the new approach presented in this study can be used to accurately determine initial contacts during stair ascent using portable equipment.