Objective: Work-related musculoskeletal disorder is rapidly growing every year. Maintaining an inappropriate posture for a long time is a cause of musculoskeletal disorders and such a static posture may be maintained in an industrial field and daily life. The purpose of this study was to investigate the effects of the back muscle workload in variable static work.
Method: Sixteen male participated in the study. We performed test to subjects keeping continuous posture for 10 minutes. Seven different postures were tested. 1) Standing; upright standing posture, 2) Twist; trunk axial twisting posture, 3) Reaching, 60º; upright standing with arm flexion, 60º between arm and trunk, 4) Reaching, 120º; upright standing with arm flexion, 120º between arm and trunk, 5) Stoop, 20º; stooped posture with 20º trunk flexion in sagittal plane, 6) Stoop, 40º; stooped posture with 40º trunk flexion in sagittal plane, 7) Stoop, 60º; stooped posture with 60º trunk flexion in sagittal plane. Back muscle workload was studied using surface electromyography (sEMG) at lumbar multifidus, longissimus thoracis, iliocostalis lumborum, quadratus lumborum, latissimus dorsi, external oblique, internal oblique and rectus abdominis muscles from each subject during the whole task. Portable respiratory gas analyzer was used to measure the physiological response and the quantitative oxygen consumption in a static posture. Rating of Perceived Exertion (RPE) and Numeric Rating Scale (NRS) were used to monitor exercise intensity and pain.
Result: Root mean square (RMS) of sEMG data was calculated using initial 204 seconds from 16 subjects. NRS was significantly increased in each posture and the highest in the 40 degree stooped posture (7.4 ± 1.02, P < 0.001)). NRS difference between pre- and post-test was also greatest in the 40 degree angle stooped posture (5.7 ± 1.66, P < 0.001). (Table 1) In 120 degree (between body and arm) reaching posture, mean RPE was the highest (17.7 ± 1.49, P < 0.001) and the posture holding time was the shortest (351.9 ± 111.22 seconds, P < 0.001). The RMS has a positive correlation with NRS and RPE among back muscle, it was most relevant in lumbar multifidus (r=0.478, P < 0.001) and latissimus dorsi muscle (r=0.256, P < 0.05), respectively. (Table 2) Both VO2 and VCO2 showed significant differences in all postures (P < 0.001) and the highest value in 60º stooped posture. In stooped posture, VO2 and VCO2 were higher in stooped angle of 40 and more degree than less than 40 degree. (Table 3)
Conclusion: The posture associated with back pain was the 40 degree stooped posture. Among back muscle, activity of lumbar multifidus muscle was related with back pain and stooped posture. Maintaining a posture bent 40 and over degrees may cause back pain and workload can be increased. |
