Industrial workers frequently perform to the one-handed manual material handling for a long duration. It makes that using the arm and shoulder muscles, subsequently these muscles will occur with the excessive load and accumulation fatigue. Shoulder musculoskeletal system will be occurs with co-activation between shoulder, shoulder girdle and rotator cuff muscles. It needed to be research how inter-correlation between muscles of the shoulder during dynamic motions in the various directions. The purpose of this study is to suggest that the prime mover by directions and the ergonomic work design based on the muscle activity and duration during one-handed manual material handling by means of dynamic electromyography(EMG). Ten male subjects volunteered to participate in this experiment. The independent variables was considered to the six directions(upward, downward, leftward, rightward, forward and backward) and fourteen muscles(biceps brachii, triceps brachii long head and lateral head, anterior, middle and posterior deltoid, upper, middle and lower trapezius, pectoralis major, rhomboideus major, teres major, latissimus dorsi, serratus anterior) for the dependent variables of muscle activity and duration. The subject ten repeated lifting a dumbbell, moving toward terminal point at 60cm on a standing workstation according to the direction. This study analyzed to the muscle activity and duration using linear envelope detection by dynamic EMG signals. The ANOVA result shows that main effects and interaction effect were significant difference for the muscle activity and duration. And also, correlation analysis was significant positive correlation between the muscle activity and duration. The upward direction and biceps brachii muscle were analyzed significantly higher muscle activity and duration. The rightward direction was analyzed higher muscle activity but, the activity duration analyzed lower than other directions. The muscle activity and duration significantly difference for the directions. Overall result shows that the upward direction and the biceps brachii muscle were analyzed significantly higher muscle activity and duration.
It may have derived because the arm and shoulder muscles were affected by the sustained gravitational force. And the shoulder may be increased the contact pressure to caused deformation of coracoacromial ligament during the shoulder flexion. Downward, leftward and backward directions caused featured lower activity and duration because moving the arm to the center had a concentric ballistic effect of using the elastic energy stored in the eccentric phase. The biceps brachii muscle was analyzed higher than the other muscles because why one-handed handling of the six directions frequently occurred flexion and extension of the elbow joint. Finally, the selection criteria of prime mover of the one-handed manual material handling by directions are as follows: Upward - biceps brachii, triceps long head, anterior·middle deltoid, upper·middle·lower trapezius, pectoralis major, Downward - biceps brachii, anterior deltoid, upper·lower trapezius, rhomboideus major, pectoralis major, Leftward - biceps brachii, triceps long head, anterior deltoid, upper·middle·lower trapezius, pectoralis major, Rightward - biceps brachii, anterior deltoid, posterior deltoid, upper·middle·lower trapezius, rhomboideus major, pectoralis major, serratus anterior, Forward - biceps brachii, triceps lateral head, anterior·middle·posterior deltoid, upper·lower trapezius, rhomboideus major, serratus anterior, teres major, Backward - biceps brachii, triceps lateral head, anterior·posterior deltoid, upper·middle·lower trapezius, rhomboideus major, teres major. And the ergonomic safety grade suggested that leftward, downward, rightward, backward, forward, upward in this order. This study results could be used as ergonomic work design by considering one-handed manual material handling for the industrial field. Furthermore, this study could be help recommended appropriate the control of robot arm and the shoulder rehabilitation based on the EMG load data of arm and shoulder muscles.
