A computing system and method to implement a three-dimensional virtual reality world with avatar posture animation without user posture tracking devices. A position and orientation of a respective avatar in the virtual reality world is tracked to generate a view of the virtual world for the avatar and to present the avatar to others. In response to input data tracking a position, orientation, and motions of a head of a user of the virtual reality world, the server system uses a posture model to predict, from the input data, a posture of an avatar of the user in the virtual reality world, and computes an animation of the avatar showing the posture of the avatar in the virtual reality world.