Fibre type proportion and fibre size in trapezius muscle biopsies from cleaners with and without myalgia and its correlation with ragged red fibres, cytochrome-c-oxidase-negative fibres, biomechanical output, perception of fatigue, and surface electromyography during repetitive forward flexions

In the literature enlarged/increased cross-sectional area (CSA) of type I m uscle fibres has been reported as a morphological mark of work-related loca lised myalgia in the descending part of the trapezius muscle of women. Thes e studies did not use enough subjects or lacked an adequate control group. The recording of surface electromyograms (EMC) is central to the research f ield of work-related myalgia. However, the influence of intrinsic muscle pr operties - such as the effect of muscle fibre distribution upon surface EMG has to be better understood in order properly to evaluate this method as a possible diagnostic and preventive tool. This study had two aims. Firstly, it investigated the muscle fibre distribution and CSA in work-related myal gia in trapezius muscles. Secondly, the multivariate relationships among mu scle morphology and histochemistry [ragged-red (RR) fibres, and cytochrome- c-oxidase-negative-fibre changes] EMG, perceived fatigue, and biomechanical output of shoulder flexions were analysed. The raw data have been presente d in an earlier study. The participants in this study were 25 female cleane rs with work-related myalgia of the trapezius muscle and 25 female cleaners not experiencing work-related myalgia of this muscle. The control group co mprised 21 healthy female teachers who had not been exposed to highly repet itive work or static muscle work. Smaller CSA of type II fibres were found in cleaners compared to teachers. In this study the CSA of type I fibres of the trapezius muscles associated with myalgia were no greater than in musc les without myalgia. The prevalence of RR fibres together with age, fibre t ype proportions, CSA and working as a cleaner correlated with the ability t o relax as recorded electromyographically. The relative mean frequency of t he EMG of the trapezius muscle correlated with the prevalence of RR fibres. but it did not correlate with the proportions and CSA of different fibre t ypes. Low biomechanical outputs and low signal amplitude increases of the E MG during the test were associated with high proportions of type IIB fibres . The smaller CSA of type II fibres in cleaners might have reflected a diff erent muscle activation pattern due to different occupational demands in cl eaners than in teachers, Morphological or histopathological variables can i nfluence the three EMG variables investigated.