Head (not face) detection and tracking in kinect depth maps

Head (not face) detection and tracking in kinect depth maps

I am looking for the best way to track human heads (not faces) in a stream
of depth maps from the kinect using OpenCV. I want to use only depth data,
no color info. Any ideas? I've tried using Haar cascade classifiers for
upper body provided in OpenCV but they do not give robust performance.
Thanks