In this paper we present a system for human body model acquisition and tracking of its parameters from voxel data. 3D voxel reconstruction of the body in each frame is computed from silhouettes extracted from multiple cameras. The system performs automatic model acquisition using a template based initialization procedure and a Bayesian network for refinement of body part size estimates. The twist-based human body model leads to a simple formulation of the extended Kalman filter that performs the tracking and with joint angle limits guarantees physically valid posture estimates. Evaluation of the approach was performed on several sequences with different types of motion captured with six cameras.