Upper-limb, scapular, and thoracic spine motions during hand-behind-back movements in healthy young adults

Many daily activities involve hand-behind-back (HBB) movements, which are often difficult for patients with chronic shoulder pathologies. HBB movements involve upper-limb and scapular motions. However, thoracic spine motions during the HBB movements and the effects of thoracic spine alignment on the HBB movement kinematics remain uninvestigated. This study aimed to examine the upper-limb, scapular, and thoracic spine motions during HBB movements by using a three-dimensional motion analysis system. Twenty healthy young adults were included (10 women and 10 men, mean age = 22.8 years). HBB movement measurements started while the participants stood with their palms facing forward. They then placed their hands behind their buttocks, with their palms facing backward. Almost all participants exhibited elbow flexion, shoulder extension, abduction, internal rotation, and scapular anterior tilt during the HBB movement. The scapular upward/downward rotation differed between the participants. Scapular, shoulder, and elbow motions showed significant correlations, suggesting that the HBB movements from the arms positioned on the side to the hands placed behind the buttocks involve complex coordinated motions of the shoulder, elbow, and scapula. Several participants exhibited little thoracic motion, whereas the others flexed their thoracic spine. Individuals with a smaller thoracic flexion motion tended to have a larger initial thoracic flexion angle before the HBB movement. The initial thoracic flexion angle and thoracic flexion motion did not correlate with the scapular, shoulder, or elbow motion; thus, healthy young adults with little thoracic flexion did not use specific compensation strategies during the HBB movements until the hands reached the buttocks.