Motor unit recruitment in the trapezius muscle with special reference to coarse arm movements

Chronic shoulder pain is common in a variety of occupations. The "Cinderell a hypothesis" suggests that the pain originates from damaged type I muscle fibres driven into degenerative processes as a result of too long activatio n and too short recovery time. The main purpose of this study was to invest igate if the same motor units are active during all phases of coarse arm mo vements. Eight healthy volunteers participated in the study. Intramuscular electromyographic signals were picked up with a four-lead fine wire electro de, during a unilateral straight arm movement. The movement started with ei ther (part 1) an abduction or a flexion, then (2) a movement in the horizon tal plane from the sagittal to the frontal plane or vice versa, and finally (3) an adduction or an extension to the start position. The movement cycle was performed in three different speeds, slow, medium, and high, with one, two or five cycles per 20 s, respectively. On an average, the motor unit a ction potentials (MUAPs) of 6 motor units (range, 1-15) were identified per trail. In total 94% of the MUAP trains that were identified showed firings in all 3 parts of the movements. The findings support the Cinderella hypot hesis, although there is a need to further investigate the temporal pattern of long-term motor unit activity. (C) 2001 Elsevier Science Ltd. All right s reserved.