Single Motor Unit Activity in the Trapezius Muscles of Elderly Female Computer Users with and without Neck-shoulder Pain during Computer Work

INTRODUCTION Musculoskeletal disorders in the neck and shoulder regions of female computer users are a major occupational concern in the European countries and the incidence of disorders increases with age and hours of work per week. It has been reported that neck shoulder cases (NS-cases) compared to neck shoulder controls (NS-controls) have a significantly lower muscle strength and a lower muscle activation measured as a lower EMGrms value at the same relative force demand [1]. This finding supports the pain adaptation model supposed to protect the painful muscle from further activity by an inhibitory effect of pain. In contrast, another study has reported an increased trapezius activity during computer tasks in the NS-cases compared to controls evaluated from the total number of motor unit action potentials (MUAP) per second [2]. Such an activity pattern would be in agreement with the vicious circle theory suggesting a pain induced increase in activity in the painful muscle. The aim of the present study was to investigate if the results of a decreased muscle strength and lower activation in a large field study of subjects with and without neck-shoulder pain can be supported by an analysis on single motor unit (MU) level during a typing and editing task in a sub sample of the study population.