Kinematic analysis of the cat shoulder girdle during treadmill locomotion: an X-ray study

A quantitative kinematic analysis of the movements of the shoulder girdle in the three dimensions of space during treadmill locomotion (velocity range 0.33-1.2 m/s) was performed in two cats. Since the movement patterns of the scapula and the humeroscapular joint can only vaguely be estimated through the overlying skin we used implanted metal spheres placed on the scapula in combination with three-dimensional pulsed X-ray cinematography (time resolution 20 ms) to reconstruct the excursions of the scapula, the humerus and the elbow and to calculate the respective angular amplitudes and velocities. The movements of the scapula relative to the Th4 spinous process consist of four major components:(i) a monophasic flexion (caudocranial movement of glenoid fossa during swing)/extension (craniocaudal movement of the glenoid fossa during stance) sequence, the fulcrum for which sequence is situated near the vertebral border of the scapula at the medial elongation of the scapular spine; (ii) a vertical monophasic up/down sequence of the fulcrum relative to the trunk, the highest vertical position being reached during mid-stance and the lowest vertical position during mid-swing; (iii) a biphasic abduction/adduction sequence during swing and during stance respectively; and (iv) small rotations along the scapular spine. The trajectory recordings of the scapula indicate that the scapula yields relative to the trunk under the body weight after ground contact. The angular excursions of the humeroscapular joint consist of : (i) a flexion/extension sequence during swing, a yield after ground contact and a final extension at the end of stance; (ii) an adduction and outward rotation during the early swing phase flexion; (iii) an abduction and inward rotation during the late swing phase extension; and (iv) an adduction during the yield with only minor rotations during the whole stance phase. The movement patterns are discussed in view of the muscular synergies necessary to guide the scapula and the humerus during stance and swing, and in relation to the implications for the organization of these patterns in spinal neuronal systems.