Deltoid muscle contribution to shoulder flexion and abduction strength – An experimental approach

ABSTRACT Background The rotator cuff (RC) and the deltoid muscle are two synergistic units that enable the functionally demanding movements of the shoulder. A number of biomechanical studies assume similar force contribution of the force couple (RC and deltoid) over the whole range of motion, while others propose position dependent force distribution. There is a lack of in vivo data regarding the deltoid`s contribution to shoulder flexion and abduction strength. This study`s aim is to create reliable in vivo data, quantifying deltoid`s contribution to shoulder flexion and abduction strength throughout range of motion. Methods Active range of motion and isometric muscle strength of shoulder abduction and flexion in 0, 30, 60, 90 and 120 degrees of abduction/flexion as well as internal and external rotation in 0 and 90 degrees of abduction were obtained in 12 healthy volunteers on the dominant arm before and after an ultrasound-guided isolated axillary nerve block. Needle electromyography was performed before and after the block to confirm deltoid paralysis. Radiographs of the shoulder and an ultrasound examination were used to exclude relevant shoulder pathologies. Results Active range of motion showed a minimal to moderate reduction to 94% and 88% of the pre-intervention value for abduction and flexion. Internal and external rotation amplitude was not impaired. The abduction strength was significantly reduced to 76% at 0° (p=0.002) and to 25% at 120° (p Conclusion The deltoid shows a linear contribution to maximal shoulder strength depending on the abduction or flexion angle, ranging from 24% in 0° to 75% in 120° of abduction and from 11% in 0° to 70% in 120° of flexion, respectively. The overall contribution to abduction strength is higher than to flexion strength. The combination of deltoid muscle and teres minor contributes about 50% to external rotation strength in 90° of abduction. The internal rotation strength is not influenced by a deltoid paralysis. This study highlights the position-dependent contribution of the shoulder muscles to strength development and thereby provides a new empirical approach to better understand human shoulder kinematics. Level of evidence Basic Science Study; Kinesiology