Modeling the risk factors associated with the neck disorders during manual material handling tasks

Work-related musculoskeletal disorders (MSD) of the neck or cervical spine result in longer sick leaves, substantial levels of human suffering, and high costs for society. Epidemiological studies clearly indicate strong associations between MSD of the neck and the work activities requiring forceful arm exertions and heavy lifting. However, studies evaluating the loading of the cervical spine during forceful arm exertions and heavy lifting tasks are limited. Major neck muscles, the sternocleidomastoid and the upper trapezius, run parallel to the cervical spine and couple the shoulder to the skull. It was hypothesized that such anatomical orientation may require these muscles to play an active role in supporting the shoulder during lifting activities and thus affecting the compressive forces acting on the cervical spine. The loading of the cervical spine during a variety of manual material handling tasks was studied using electromyography (EMG) and biomechanical modeling techniques. In the EMG study, thirty healthy participants simulated isometric lifting, pushing, and pulling tasks at different heights (e.g., knuckle, elbow, shoulder, and overhead) exerting 25%, 50%, and 75% of their respective maximum static strengths in different neck postures (e.g., neutral, fully flexed, and fully extended neck postures). An increase in the weight significantly affected the activation of neck muscles (P<0. 001). Independent of the weight lifted, the sternocleidomastoid showed the highest activation at the extended neck posture, while the upper trapezius showed the highest activation at the flexed neck posture (P<0. 001). The activities of the neck muscles increased significantly with an increase in lifting height from elbow to shoulder to overhead (P<0. 001). A biomechanical model of the neck consisting of four bilateral pairs of muscles was formulated and a double optimization procedure was used to determine the forces generated by the neck muscles. The total compressive forces exerted by the four neck muscles at the C4-C5 level during isometric lifting task at elbow height were 72. 6(19. 4), 128. 5(37. 7), and 184. 4(56. 1) N, corresponding to the 25%, 50%, and 75% weight conditions. The results of this study demonstrate that the neck muscles play an active role during lifting activities and may influence MSD development due to resulting physiological changes.