In vivo assessment of scapulohumeral rhythm during unconstrained overhead reaching in asymptomatic subjects.
BACKGROUND
The contribution of scapulothoracic and glenohumeral motion to overall shoulder motion remains difficult to determine. We sought to determine the exact ratio between these two motion components in order to better understand overall shoulder kinematics in asymptomatic individuals in unconstrained reaching.


METHODS
This study assessed shoulder motion using bone-fixed sensors to quantify scapulohumeral motion during unconstrained raising and lowering of the arm. Electromagnetic tracking devices rigidly fixed to bone pins recorded active scapular and humeral motion.


RESULTS
We found a significant difference in the ratio of glenohumeral elevation to scapular upward rotation during arm raising (2.3) and lowering (2.7). Each degree of glenohumeral elevation yielded scapular upward rotation of 0.43 degrees (raising) compared with downward rotation of 0.37 degrees (lowering), across the motion arc. Until 125 degrees of glenohumeral elevation, the scapula internally rotated and then externally rotated with further elevation. Scapular upward rotation and posterior tilting progressively increased until maximal elevation. Scapulohumeral rhythm was greatest in the first increment of raising the arm and higher overall when lowering the arm.


CONCLUSIONS
Understanding these data allows improved evaluation of potential motion abnormalities in patients with shoulder pathology and may improve treatment for restoration of normal shoulder motion.
Braman
Jonathan P
JP
Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, MN 55454, USA. brama011@umn.edu
Engel
Sean C
SC
Laprade
Robert F
RF
Ludewig
Paula M
PM
D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D006811:Humerus / Q000502:physiology; D008297:Male; D012016:Reference Values; D012540:Scapula / Q000502:physiology; D012785:Shoulder Joint / Q000502:physiology*