Total shoulder and relative muscle strength in the scapular plane

Strength profiles of the shoulder joint are measured experimentally for two arm positions in "the scapular plane" in order to present quantitative dat a on the shoulder strength. Apart from yielding the actual force a subject can exert in various directions, these measurements also exhibit e.g. the s trongest and weakest directions, in fact the relative strength in all direc tions. The inter-individual variation of the direction of maximal force was at most 14 degrees (sd). The experimental profiles are compared with the c orresponding theoretical profiles, obtained by using a shoulder model. The calculations were made both with default muscle parameters and individually adapted parameters. The results show that the employed shoulder model, whi ch is based on data from an elderly population. may be adapted to other pop ulations and that the necessary changes in relative muscle strength are tho se expected on biomechanical grounds. Without model changes the difference between measured (in the mean) and predicted maximal force directions was a t most 50 degrees. Muscle parameter adjustment reduced this difference to 2 3 degrees. The strength profiles clearly indicate in what direction a perso n can produce larger forces and which muscles that contribute. (C) 1999 Els evier Science Ltd. All rights reserved.