Motor-unit recruitment in the trapezius muscle during arm movements and inVDU precision work

It has been hypothesized that shoulder myalgia, which is common in many occ upations involving light-to-moderate manual work, is due to an overuse of l ow threshold muscle fibers, causing damage at the muscle cell level. The pr esent study was undertaken to investigate in the trapezius muscle, (a) to w hat extent it is possible to track low threshold motor-units in non-isometr ic conditions, and (b) to study the effect of arm movements on recruitment and firing patterns. Intramuscular four lead fine wire electrodes were inse rted in the trapezius muscle of four volunteers, who performed arm movement s (shoulder abduction-adduction 0-90-0 degrees in the scapula plane), as we ll as standardized work using computer mouse. Three channels of electromyog raphy (EMG) were used for motor-unit action potential (MUAP) identification and signal decomposition. Results showed (a) that it was possible to perfo rm signal tracking during slow movements, and (b) that there were motor-uni ts that were active over a wide range of shoulder abduction movement, as we ll as in work with computer mouse. The findings indicate that stereotypic r ecruitment patterns exist in the trapezius muscle not only in static work, but also in work situations involving arm and shoulder movements. Relevance to industry Occupationally related chronic shoulder pain is an increasing problem in in dustry, in manual work as well as in computer work. This paper presents res ults of studies on muscular activity patterns during movements which suppor t the notion that chronic pain may be evoked by overuse syndromes at the mu scular fiber level. Practical precautions to reduce such risks are discusse d. (C) 1999 Elsevier Science B.V. All rights reserved.