Reliability and validity of kinect RGB-D sensor for assessing standing balance

Microsoft Kinect uses a built-in RGB-D sensor and the skeleton tracking algorithm to capture 3-D movements of the human body. It also has the potential for assessing postural stability, which is fundamental for most motor activities. The aim of this paper was to investigate whether standing balance can be evaluated reliably and validly by this low-cost device. Nine healthy subjects were required to maintain balance during three standing positions (double limb stance with feet apart, double limb stance with feet together and single limb stance). The center of mass (COM) was calculated from the body's kinematic data acquired by the Kinect system and Optotrak Certus motion capture system. The position variability and average velocity of the COM in the horizontal plane were calculated and used to evaluate the subject's balance. These COM parameters from the two systems showed excellent and comparable test-retest reliability (intraclass correlation coefficient ICC >0.75. In addition, although the average velocity of the COM calculated from Kinect was significantly lower, each COM parameter showed excellent concurrent validity ICC > 0.88 and a significant linear relationship (p<0.001,r>0.930) existed between the two systems, which meant that biases may be corrected using linear calibration equations. Therefore, Kinect may be a valid, reliable, and convenient device for assessing standing balance when its measured COM parameters are properly calibrated.