X-ray kinematic analysis of shoulder movements during target reaching and food taking in the cat

Co-ordinate movements-around the shoulder are essential during reaching mov ements. We performed a quantitative kinematic analysis of movements of the shoulder girdle: three-dimensional X-ray frames (time resolution 20 ms) wer e recorded during the target-reaching and food-taking paradigm in five cats either sitting (n = 4) or standing (n = I) in front of a food well. Moveme nts of the scapula consisted of a flexion of the scapula (anteversion of th e glenoid) followed by flexion of the gleno-humeral joint (decrease in the angle between the scapular spine and humerus). In the sitting animals, the gleno-humeral flexion reversed to extension some 120 ms before object conta ct, while in the standing animal flexion continued during the ongoing scapu lar flexion. In both cases, the scapula was nearly horizontal at the end of target reaching. The fulcrum for scapular movements was located near the v ertebral border of the scapula at the medial elongation of the scapular spi ne. No major translational components of the fulcrum with respect to the tr unk were found during reaching. Together with full flexion of the scapula, this reduces the number of degrees of freedom considerably and thereby prob ably simplifying the specification of the end-point of the limb chain. End- point specification is further supported by rotational movements of the sca pula. In the sitting animal, the amplitude of inward rotation along the lon g axis of the scapula was around 20 degrees, while it was much more variabl e in the standing animal, reflecting more variable starting positions. We h ypothesize that the glenoid is used to 'foveate' the target abject.