Effects of different lifting and lowering heights on upper arm, shoulder and back muscle activity during a manual material handling task.

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.