The whales' swimming is mainly ensured by their horizontal caudal peduncle formed by two lobes and two unequal and distinct propulsive muscles : the hypoaxial or flexor muscle and the epiaxial or extensor muscle. The first ensures the downward movement of the tail as well as the variation of its angle of attack according to the orientation of the displacements, the second - at least twice as powerful - ensures the upward movement.