Recruitment of low threshold motor-units in the trapezius muscle in different static arm positions

A prevailing hypothesis for development of myalgia symptoms in the trapeziu s muscle is based on observations that in stereotypic activation of muscle the same low-threshold motor-units are the first ones to be recruited, and that these units are active throughout the contraction, until total relaxat ion. The theory suggests that these units are the first ones in monotonous repetitive work to be subject to degenerative processes, causing pain. The present project was undertaken to evaluate to what extent recruitment of mo tor-units in the trapezius muscle is position-specific, i.e. if there are m otor-units that are recruited over a wide range of arm postures. Fine wire electrode pairs, inserted 2 cm lateral to the midpoint between the acromion and the C7 spinous processus, were used for signal acquisition. Methods fo r decomposition of the interference patterns were developed, allowing ident ification of single motor-units in signals registered in different arm posi tions. Voluntary recruitment of motor-units in the descending portion of th e trapezius muscle was studied in 24 different arm positions (0-90 degrees shoulder flexion, 0-45 degrees humeral abduction, and 45 - 135 degrees elbo w flexion) in three subjects. The results showed that the wire electrode te chnique and the signal processing algorithm employed allowed motor-unit ide ntification in nonisometric conditions. It was found that low threshold mot or-units in the trapezius muscle, active over a wide range of arm positions , could be identified in all three subjects. We refer to those as Cinderell a, or C, units. These results support the research hypothesis addressed.