Motor-Unit Recruitment in the Trapezius Muscle during a Computer Typing Task

Abstract

The possible linkage between intense computer work and chronic shoulder pain was studied on the basis of the motor-unit overuse theory. By using a four-lead wire-electrode intramuscular electromyography was recorded from the upper trapezius muscle of seven healthy subjects during a computer typing task. The task consisted of text entry from a manuscript. Motor unit action potentials (MUAPs) were tracked using an automatic classification program with possibilities for manual corrections. The duration of the work task was 10 min. After 5 minutes typing, one surface and three intramuscular EMG channels were sampled in periods of 20 seconds each minute (5 periods in total). For the surface EMG, the maximum voluntary electrical activation was measured, for normalization, in the end of the experiment. One to twelve units were identified per subject (median 6, totally 40). In six of the seven subjects at least half of the motor units were identified in all five minutes (in total, 22 of 34). In the seventh subject, four units that were identified in minutes 6 and 7 were substituted by five others that were active in the remaining three minutes. The surface EMG RMS values ranged from 1.3 to 8.5 %MVE (mean, 4.7). Since many of the motor units were active in all periods, the basic hypothesis was supported, but since others were active in a subset of the periods, it is was concluded that long time measurements are needed to anatomize the recruitment pattern during enduring work with computer input devices.