This paper deals with the movement of virtual signers, and more particularly with the notion of biological motion and the issue of anonymization. Animating an avatar from motion models as biological as possible is important to ensure perceptual acceptability. Recording real movements thanks to motion capture systems and mapping them to an avatar can ensure this acceptability. However, such accurate systems may convey extra-linguistic information such as the signer's identity. For some applications, it will be necessary to determine the parameters of the movement that carry identity in order to control them.