Analysis of muscle fatigue of erect spinae caused by treatment table height in ultrasound therapy

[Purpose] The objective of this study was to propose a work environment that could reduce musculoskeletal workload. Accordingly, spinal muscle fatigue caused by ultrasound therapy at various treatment table heights was examined and compared. [Subjects and Methods] Twenty-five healthy subjects participated in this experiment. The table height was set to 100%, 125%, and 150% of the stool height (45 cm). The electromyographic signals of the erector spinae at the thoracic (T10, T12) and lumbar (L2, L4) levels were collected by an electromyography (EMG) system during the performance of ultrasound therapy. The median frequencies were then calculated and compared. [Results] The lower the table height was, the smaller the median frequencies of thoracic and lumbar erector spinae on both sides were. The T10 and T12 levels on both sides and the left L2 region showed significant differences among the table heights. At every spinal level, the median frequency of the left erector spinae was lower than that of the right: T10, T12, L2, and L4 at 100%, L4 at 125%, and T10, T12, L2, and L4 at 150% showed significant differences. [Conclusion] During ultrasound therapy muscle fatigue increased at higher table heights and the muscle fatigue of the left erector spinae was greater than that of the right side. To reduce muscle fatigue, we recommend the table height work is raised to an appropriate height, and that is shared between left and right arms.