1. Structural differences between the forelimbs and the hindlimbs have been well documented among vertebrates; the pectoral girdle is generally more mobile than the pelvic girdle and structural differences between the glenoid cavity and the acetabulum lead to the potential for a greater range of motion in the forelimb than in the hindlimb. 2. Although the hindlimbs are the key propulsors in terrestrial vertebrates, with the forelimbs absorbing collisional energy and acting primarily as brakes.