Examining the low, high and range measures of muscle activity amplitudes in symptomatic and asymptomatic computer users performing typing and mousing tasks.

Past studies on work-related musculoskeletal disorders (WMSD) have reported increased median muscle activities in terms of 50th% of amplitude probability distribution function (APDF), and this was thought to be a manifestation of altered motor control--an important mechanism contributing to WMSD. The present study aimed to examine whether such altered motor control was also present in other parameters of APDF--the 10th and 90th% values, which can be considered indicators of the low and high measures of muscle activity. The difference between 10th and 90th% APDF can be considered an indicator of the variation in muscle activity amplitude (the "APDF range"). Surface electromyography was examined in female office workers as Case (n = 21) and Control (n = 18) subjects. The APDF variables were measured in cervical erector spinae (CES) and upper trapezius (UT) muscles during typing, mousing and type-and-mouse, for 20 min each. The Case Group had significantly higher CES activity in the 10th, 90th% and APDF range compared to Controls. The UT muscles showed similar trends but the between-group differences were not statistically significant. These results have demonstrated the robustness of the APDF variables as sensitive indicators of motor control variations in symptomatic subjects with musculoskeletal disorders.