The camera is able to track up to six people under normal conditions with two people being active at the same time. Based on the depth information and triangulation it tracks movements of the human extremities by computing the position of various skeleton joints. The origin of the skeleton structure is the hip centre, from which the skeletal tracking algorithm builds the whole skeleton. The position of all joints can be accessed by calling the respective functions provided in the Kinect SDK.
In total, the position of 20 joints or body parts are available. To evaluate accuracy of the Kinect motion tracking, we employed a high resolution 3D motion tracking system. The detection of the same marker by different cameras allows its location in three-dimensional space through spatial triangulation.
Recording of camera data and computation of marker positions was done using the Vicon Nexus Software Vicon. Using the Cartesian coordinates provided by both the Kinect and the Vicon system, the length of extremities and the angles between those can be calculated as described in Eq.
Figure 1 provides an example on how to calculate the angle in which the elbow is abducted. The length between joints A and B is calculated as:. Figure 2 provides a schematic overview of the experimental setup. The Vicon cameras were placed on tripods and positioned around the test subject. Tripod height and camera angle were adjusted to provide an optimal vision of the reflective markers.
Three equally distanced markers were placed each on the right wrist, elbow, shoulder, hip, knee and ankle of the test subject. Figure 3 shows the markers on the test subject. Test subject with reflective markers attached to wrist, elbow, shoulder, hip, knee and ankle joints.
Thus, for the elbow the angle between humerus and ulna, for the shoulder the angle between humerus and right body side, for the hip the angle between femur and right body side was calculated. The skeletal joint coordinates were estimated at the centre of the triangular area spanned by the markers. The calculated angles were then averaged over the period in which the test subject was holding the position. The Kinect camera was placed in front of the test subject with a distance of three meters.
At each of the postures executed by the test subject, a snapshot containing the current joint coordinates was taken. Table 1 provides a comparison between angles calculated with the Vicon system and the Kinect system.
Angles provided by the Kinect system showed a maximum deviation of Elbow and hip angles were all overestimated by the Kinect system, while shoulder angles were underestimated. Mean of the absolute deviations was 8. The use of low-cost motion tracking systems with computer game background in a medical environment has been increasing majorly over the last few years. They allow high-quality training support at home without stressing health insurance costs. However, being intended to be used as a tool for gesture recognition, the accuracy might not be suflicient for medical applications.
The presented results show that the Kinect system has a deviation in calculating the angle between two joints of at least 2. However, there are several limitations to this study to be mentioned. Firstly, the presented results were acquired with one single test subject. The Kinect system builds the skeleton structure based on the on the outer shape recognized through the dual infrared sensors. Thus, in obese patients, positional accuracy of the skeletal joints might decrease.
An extended test with a greater number of test subjects, preferably with different body sizes and shapes would be necessary. Secondly, the joint positions recorded with the Vicon system were defined by three markers equidistantly placed around the skeletal joints of the test subject, thus not necessarily being in accordance with the joint positions calculated with the Kinect system.
Therefore, the angles calculated in both systems comprise a natural difference. In the Kinect calculations, the hip joints are usually positioned more towards the hip centre compared to the placement of the Vicon markers. Logically, the highest deviation was recorded there. As the compared systems employ different reference coordinates, comparing the position of joints is not possible, thus preventing an analysis on which skeleton joint is tracked with best and which is tracked worst by the Kinect system.
Concluding, we believe that using a motion tracking system such as the Microsoft Kinect may be used as a low-cost solution for home-based physiotherapy as the recorded deviations are — despite the mentioned limitations — within a reasonable range.
For professional applications the tracking accuracy might however be not suflicient. Conflict of interest : Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Lloyd-Jones, R. Adams, T. Brown, M. Carnethon, S. Dai, G. De Simone, T. Ferguson, E. Ford, K. Furie, et al. Search in Google Scholar.
Kisner and L. Colby, Therapeutic Exercise - Foundations and Techniques, 6 ed. Philadelphia: F. Davis Company, Clark, Y. Pua, K. Fortin, C. Ritchie, K. Each connection bone links the parent joint with a child joint. The figure illustrates the joint locations and connection relative to the human body. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.
Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Joints Joint position and orientation are estimates relative to the global depth sensor frame of reference. Joint coordinates The position and orientation of each joint form its own right-handed joint coordinate system.
Note The choice of flipped axis orientation for corresponding joints across the two sides of the body is intended to simplify mirror movement e. Is this page helpful? Yes No.
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