Effect of different arm loads on the position of the scapula in abduction postures

Objective. The objective of this study is to determine the relation between arm load and the three-dimensional shoulder orientations. Background Analysis of a musculo-skeletal system by means of an inverse dyn amical simulation requires postural data of the bony elements involved. For the shoulder, the positions of the clavicle and the scapula are difficult to record due to the skin displacement. It would therefore be useful to pre dict the three-dimensional relation between the orientation of the arm, the clavicula and the scapula, i.e. the three-dimensional shoulder rhythm, wit h respect to the thorax under different load conditions. Methods. The orientation of the clavicula and the scapula was determined wi th respect to the thorax by means of palpation of skeletal landmarks, for s even postures of arm elevation in the frontal plane at four load conditions : 0, 0.9, 1.9 and 2.9 kg at the wrist. The data were expressed by Cardan an gles and analyzed by means of repeated measurements analysis of variance. Results. While the clavicular and scapular angles were significantly relate d to the arm elevation, no significant relation was found with the load in the hands for the seven arm postures. Conclusions. The three-dimensional shoulder rhythm does not change under di fferent gravitational load conditions on the arm.