Respective role of the axial and appendicular systems in relation to the transition to limblessness.

In lower quadrupedal vertebrates locomotor efficiency seems to result from the associate movements of the axial and appendicular systems, which are totally independent in structure and embryological origin. The curvature of the trunk, produced by a standing wave, magnifies the propulsive action of the limbs. In intermediate forms, the association of an elongate trunk with limbs reduced in size brings about functional consequences which may be noticeably diverse according to the degree of trunk elongation and limb reduction. According to environmental constraints, animals search for better locomotor efficiency, which implies the maintenance or breakage of this association of both locomotor systems. In some cases, limb action on the ground is added to the axial wave action through a perfect mutual adjustment of rhythmic activity, until mechanical inefficiency of the limbs is reached by possible loss of contact with the ground. In other cases, the limbs dragged on the ground during the stance phase act against the axial action or, on the contrary, are inhibited by the axial system. A review of available data tries to contribute to an understanding of the respective roles of both systems in the transition to limblessness.