| Sumario: | BACKGROUND: Previous research has suggested eliminating certain types of manual material handling (MMH) work by recommending specific arm angles and postures to avoid, such as arm flexion or abduction over 90°. MMH with arm flexion over 90° can require lifting objects as well as lowering objects. However, few studies have evaluated MMH work while lowering objects in detail. OBJECTIVE: This study investigated the effects of different lifting and lowering heights on upper arm, shoulder, and back muscle activity during a MMH task. METHODS: The participants performed a MMH task that involved stoop lowering and lifting. The participants transferred the box to shelves positioned 30 cm in front of them under various conditions. Conditions 1 to 4 involved transferring the box to 1) ankle-, 2) knee-, 3) waist-, and 4) shoulder-high shelves, respectively. Surface electrodes were attached to the biceps brachii, upper trapezius, rhomboid minor, and L4 erector spinae. RESULTS: The activity of the biceps brachii was decreased significantly in Conditions 1 and 2 compared to Conditions 3 and 4. The upper trapezius activity was increased significantly in Conditions 1 and 4 compared to Conditions 2 and 3. The rhomboid minor activity increased significantly in Condition 1 compared to Conditions 2 to 4. The L4 erector spinae activity decreased significantly in Condition 1 compared to Conditions 2 to 4 CONCLUSIONS: A low-lowering MMH work could contribute to neck, shoulder, and back pain. Therefore, further studies must examine a height below-knee MMH work in detail.
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