Neck-Shoulder Main Musculature is the Major Cervical Compression Producers during Single-Hand Lifting

Objective: Cervical spine injuries have been associated with work tasks requiring heavy arm and shoulder exertions. Current biomechanical models established to estimate compressive forces acting on the cervical spine do not include the contributions of the muscles surrounding the neck in supporting the shoulder and arm during these exertions. This paper presents a biomechanical analysis of compressive force as a result of single-hand lifting. Method: It was hypothesized that contraction of the three main shared muscles are the major producers of the cervical compression. To test this hypothesis, bilateral Electromyography data of major shared musculature (upper trapezius, sternocleidomastoid, and levator scapula) and posture data were collected from twenty subjects preforming lifting of five different weights from twenty different locations produced by the interaction of varying heights, reach distance, and angles simulating the work done by assembly line workers. All lifting tasks were done by the right hand. Analysis of the moment produced by these muscles forces and the head’s mass at the C4/C5 cervical spine intervertebral disc were performed to investigate moment balance. A coordinate system was set on segment at the C4-C5 disc center and the moment was calculated around two axes; the X, which divides the segment into front and back, and around Y, which divides the segment into right and left. Results: A t-test of the summation of moments around each axis was performed and results showed that the moment was balanced. Conclusion: A balance in moment indicates that the shared muscles considered are the major producers of cervical compression during single hand lifting. These findings also demonstrate that any attempt to develop a biomechanical modeling to estimate the cervical compression must include the contribution of hand lifting activities