Detection of medio-lateral weight-shifting strategy based on a double inverted pendulum

Abstract

Introduction Medio-lateral weight-shifting (MLWS) is a crucial ability in many functional tasks [1] . It is known that people at risk of falling have difficulties performing MLWS and use compensatory strategy [2] , but, MLWS strategy has rarely been described. Therefore, we propose a method to identify postural strategies based on a double inverted pendulum (DIP) during the different phases of the MLWS. Material and methods Ten healthy adults performed MLWS with a home-made video game coupling a Kinect and a Wii Balance Board to assess their postural strategy [3] , [4] . The body was modelled as a DIP using the midpoint between the ankle joints, the spine base joint and the mid shoulder joint from the Kinect's skeleton. We define the orientation of the lower body (θ1) from the vertical and the trunk (θ2) from the orientation of θ1. Natural oscillations in quiet standing were used as a threshold (θnul). If θ1 or θ2 is outside this threshold, it is either set to θMLWS or θopp if it's in the direction of the MLWS or the opposite. Based on two segments orientation and three possible angle states we can identify five postural strategies: quiet standing, ankle strategy, trunk strategy, hip strategy in-phase (HSin-p) or hip strategy out-of-phase (HSout-p). Results Results show that on average, participants naturally adopt a HSout-p during the MLWS, and even more during the deceleration phase. Discussion/conclusion This method can identify strategies throughout the movement based on a DIP and will help to provide meaningful feedback during training.