Net shoulder joint moment and muscular activity during light weight–handling at different displacements and frequencies

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Abstract
The purpose of this study was to calculate net shoulder (gleno–humeral) joint moments from inverse dynamics and to measure muscular activity from six shoulder muscles (supraspinatus, infraspinatus, middle deltoid, anterior deltoid, trapezius, and pectoralis major) during light weight–handling at two different displacements (horizontal and vertical) and frequencies (40 and 60 cycles/min), to simulate an occupational cervicobrachial working task (light weight displacement). Ten normal adult male subjects were asked to move a known weight, representing 15% of the maximal lifted weight, in both horizontal and vertical conditions at frequencies of 40 cycles/min and 60 cycles/min. Raw EMG signals from six shoulder muscles were recorded and synchronized with the cinematographic data during three trials of 6 s each. The raw EMG signals of each muscle were full wave rectified and filtered at 3 Hz. The linear envelope (LE EMG) signals were normalized by time (% cycle) and by amplitude (% MVC), and for the analysis of variance, the normalized LE EMG signals were integrated (IN LE EMG). The average shoulder angular velocities, joint moments, and moment powers were computed from cinematographical data. No significant differences were observed between both tasks for the supraspinatus, infraspinatus, and pectoralis major IN LE EMG data as well as for integrated normalized shoulder joint moment for the whole cycle of movement. IN LE EMG data from middle deltoid, anterior deltoid, and trapezius muscles were significantly higher (p<005) when performing the vertical displacement task for the whole cycle of movement. This muscular activity difference between vertical and horizontal tasks indicated that the vertical displacement conditions induced higher muscular loads on the shoulder than the horizontal weight displacement conditions, although the vertical displacements were approximately 15% longer than the horizontal displacements. The non–significant difference of IN LE EMG between frequencies obtained for all muscles indicated that neither frequencies induced more muscular activity.