Arm abduction strength and its relationship to shoulder geometry Krystyna Gielo-Perczak a, * , Simon Matz a , Kai-Nan An b a Liberty Mutual Research Institute for Safety, 71 Frankland Road, Hopkinton, MA, 01748, USA b Biomechanics Laboratory, Mayo Clinic, Rochester, MN, USA Received 8 September 2004; received in revised form 10 May 2005; accepted 12 May 2005 Abstract This study was conducted to test whether glenohumeral geometry, as measured through MRI scans, is correlated with upper arm strength. The isometric shoulder strength of 12 subjects during one-handed arm abduction in the coronal plane, in a range from 5° to 30°, was correlated with the geometries of their glenoid fossas. Seven parameters describing the glenohumeral joint geometry in the coronal plane were identified as having expected influence on shoulder strength. In addition to these, a new geometric parameter, named the area of glenoid asymmetry (AGA), was considered to reflect the concavity-compression mechanism as well as the incli- nation of the glenoid surface. As a result of the high correlation between the AGA and mean force and mean moment (0.80, p 6 0.01 and 0.69, p 6 0.05, respectively) at the glenohumeral joint in a coronal plane, the AGA was distinguished as a factor having influ- ence on shoulder strength when an arm was abducted in a range from 5° to 30°. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: One-handed arm pushing; Arm abduction; Glenohumeral joint; Maximum acceptable load; Geometry of glenoid fossa; Area of glenoid asymmetry 1. Introduction Shoulder strength was found by Mital and Sanghavi [16] to be a limiting factor in upper extremity strength exertion capability. They further deduced that the gleno- humeral joint is the most critical joint in the shoulder complex and appears to limit the torque exertion capa- bility of individuals. Anatomical dimensions of the gle- nohumeral joint, or their combination, could be a key factor in strength predictability. Thus, occupational ergonomics and safety issues require a biomechanical approach that takes into consideration the complex geo- metric nature of the human joints with the demands of the workplace in order to explain the mechanism of musculoskeletal disorders and optimize performance taking strength into account. In addition to arm length, muscle cross-sectional area and muscle moment arms interactions of the glenohu- meral joint elements have influence on potential abduc- tion strength. The glenohumeral joint consists of the interrelated joint surfaces, capsuloligamentous structures and muscles. The glenohumeral joint is the most mobile articulation of the human body, involving muscles and their effective moment arms during arm abduction. The articulating surfaces and capsuloligamentous structures provide basic static constraint. The muscles provide dy- namic balance and stability. Elevation of the humerus re- quires a combined action of the deltoid and rotator cuff muscles [19]. The torque produced by the joint is approx- imately the same throughout the range of abduction. However, the role of the supraspinatus and deltoid mus- cles are particularly distinguished in the initiation of an abduction movement. Approximate involvement of these 1050-6411/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2005.05.004 * Corresponding author. Tel.: +1 508 497 0218; fax: +1 508 435 0482. E-mail address: Krystyna.Gielo-Perczak@libertymutual.com (K. Gielo-Perczak). www.elsevier.com/locate/jelekin Journal of Electromyography and Kinesiology 16 (2006) 66–78