Could Postural Strategies Be Assessed with the Microsoft Kinect v2?

Quantification of body movement strategies to maintain balance may be useful to understand changes in postural control. Some methods for this purpose require special preparations, such as placement of inertial sensors, goniometers or EMG electrodes. In this study, the capability of the Microsoft Kinect v2, a markerless motion sensor, to assess postural control strategies was tested. Forty-six young healthy subjects had the trajectories of 25 "joints", provided by a Kinect v2, recorded during upright stance with eyes open or close, on rigid (force platform) and soft (foam pad) surfaces. Postural strategies were characterized by a strategy index (SI) based on the phase difference between the accelerations of upper (trunk) and lower (hip) segments of the body, measured by the Kinect in anterior-posterior and medial-lateral direction. Ankle and hip strategies were identified by in-phase or counterphase accelerations respectively, the phase being estimated from the covariance between 2-s sliding windows of the two signals. The trajectories of center of mass (COM) and center of pressure (COP) were also computed from the Kinect and the force plate, respectively. The SI and the velocities of COP and COM were significantly different between conditions (Friedman p < 0.001 for SI), suggesting effects of sensory information. These results are in line with other studies, showing coexistence of both strategies during stance and the predominance of ankle rather than hip strategy on foam or with closed eyes instead of on rigid surface with open eyes. These results support using the Kinect v2 to assess postural strategies.