1. A method of tracking the position of the upper body of a user from a video image stream, said upper body comprising the torso and the head of the user, the method comprising a determination of the position of the torso (T x0 , T y0 ) in a first image, wherein: a virtual reference frame is associated with the torso in said first image; and in that, for a second image: a new position of the virtual reference frame is determined in said second image; and a relative position of the head (El, Az, P x , P y ) relative to said new position of the virtual reference frame is measured by comparison with the position of the virtual reference frame in said first image, in order to determine independently the movements of the head and of the torso.