Effects of precision and force demands, grip diameter, and arm support during manual work: an electromyographic study

Musculoskeletal disorders in the neck, shoulder, and arm are common in some occupational groups, and have been ascribed to high precision demands and sustained static load in the neck-shoulder region. In order to evaluate the influence of precision and force demands in manual work related to arm support, instrument grip size, the muscular activity in neck, shoulder, and arm muscles was recorded by electromyography. This EMG and perceived exertion were estimated during a simulated work situation where 12 subjects followed a rotating track, using a handheld dental instrument. Normalized mean EMG amplitude levels (% reference maximal contraction) were calculated. The analysis of variance of the results showed that (a) the precision factor affected significantly the muscular load of the two dominant muscles with postural stabilization function (extensor carpi radialis and infraspinatus); (b) the force factor itself had no specific influence on the muscular load of the investigated muscles; (c) arm support, but not hand support, was of significant importance for the load of three dominant shoulder muscles (trapezius, supraspinatus, and anterior deltoid); and (d) the two different hand grip diameters did not change the activity of any muscle investigated.