Shoulder joint movement during reaching to the contralateral and ipsilateral lumbar back

In activities of daily living, hand-behind-back (HBB) motion often requires reaching not only to the spinal column but also to the contralateral or ipsilateral lumbar back. However, the specific shoulder joint kinematics involved in HBB motion toward these directions remain unclear. This study aimed to understand the shoulder joint motions that occur during HBB motion when reaching the contralateral or ipsilateral side. The participants were 18 healthy men (age, 29.8 ± 6.8 years). The measurement task comprised three types of HBB motions: placing the thumb at the first lumbar vertebra and placing the thumb on the contralateral and ipsilateral lumbar back. Shoulder joint extension, abduction, internal rotation angles were investigated using a three-dimensional motion analyzer. For the reaching to the contralateral lumbar back in this study, shoulder abduction was 17.7° ± 7.1° and shoulder extension was 28.8° ± 6.6°, internal rotation was 34.8° ± 10.5°; to reach the ipsilateral lumbar back, shoulder abduction was 43.0° ± 8.9° and shoulder extension was 37.3° ± 6.1°, internal rotation was 30.4° ± 9.0°. The contralateral reach showed greater internal rotation of the shoulder joint and lesser shoulder joint extension and abduction than the ipsilateral reach. The contralateral reach showed a significant negative correlation with shoulder joint internal rotation and extension. In the HBB motion, the shoulder joint movements differ when reaching the contralateral and ipsilateral lumbar back. As the HBB motion may involve reaching the contralateral or ipsilateral side, the movements of the shoulder joint should be evaluated in relation to daily activities.