Coordination of the shoulder complex for pointing in the peripersonal workspace
Mrs Alexandra RORENa, Dr Marie-Martine LEFEVRE-COLAUa, Mrs Johanna ROBERTSONb, Prof Serge POIRAUDEAUa, Dr Agnès ROBY-BRAMIc
a CHU Cochin, université Paris-Descartes, INSERM U1153, b CHU Raymond Poincaré, 104 Boulevard Raymond Poincaré, c Institut des systèmes intelligents et de robotique (ISIR), Université Pierre et Marie Curie, CNRS UMR 7222, Paris
Objective: The shoulder complex allows a large redundancy for the control of goal directed movements. Our aim was to investigate the contribution of scapula-thoracic motion to pointing movements in the peripersonal workspace.
Methods:10 healthy participants performed pointing movements with both sides toward nine targets. The kinematics of the shoulder was recorded using a Polhemus Fastrak 6 DoF electromagnetic device with 4 sensors fixed on the trunk, scapula, upper arm and wrist. The 3D rotations between trunk, scapula and humerus and the 3D displacement of the center of the scapula were measured at rest and for each pointing movement. Ratios of the global angles were used to compute a scapula-humeral rhythm. We performed a principal component analysis to study the coupling between scapula movements. A direct kinematic procedure was used to compute the contribution of scapula motion to the workspace of the hand.
Results: Trunk and shoulder (glenohumeral (GH) and scapulothoracic (ST)) movements were finely tuned to target position, with small asymmetries. For all pointing movements, 3D global rotation angles of the GH and ST joints varied according to a scapulohumeral rhythm approximately equal to 2/1. Three principal components (1: internal rotation and lateral and anterior displacement, 2: posterior tilt and inferior displacement, 3: lateral rotation) could explain 76.5% of the variability of scapula motion. The direct kinematics showed that the contribution of ST motion to the hand workspace was major (reaching ~15 cm) and depended on the target.
Discussion/conclusion: ST motion contributes significantly to the workspace of the hand. The scapulo-humeral rhythm classically described in the planar movements of the shoulder can be generalized in 3D and remains constant during pointing movements. ST motion should be taken into account in the evaluation and understanding of the movements of the upper limb.
Keywords : coordination, scapula, pointing, scapulohumeral rhythm