Effects of exposure to excessive drafts on myoelectric activity in shoulder muscles.

Surface electromyography was recorded from the lateral and the cervical portion of the descending trapezius muscle and the levator scapulae muscle in 12 word processor operators. All operators were exposed to air flows with velocities ranging from 0.96 to 1.96 m/s during three continuous 6-min work periods. The skin temperature measured on the descending trapezius muscle dropped from 31.8 to 30.2°C. The air temperature remained constant at 22°C. For the lateral part of the descending trapezius muscle, a significant increase in both mean power frequency (MPF) and root-mean-square (RMS) amplitudes were observed. It was suggested that this finding was a reflex recruitment of motor units. Similar but not as distinct changes were observed for the levator scapulae muscle. For the cervical part of the descending trapezius and the levator scapulae muscles, the main finding was a significant increase in RMS values with a significant decrease in MPF values. This may be regarded as temperature effects, since it was unlikely that muscular fatigue developed.