Muscle performance and fibre type composition were investigated in women with fibromyalgia, work-related trapezius myalgia and healthy volunteers. Each subject performed 100 repetitive shoulder flexions using an isokinetic dynamometer during simultaneous registration of surface electromyography. A biopsy from the trapezius muscle was obtained. The groups differed neither in mechanical performance nor in fibre type proportions. An inability to relax between contractions was found in all registered muscles in patients with fibromyalgia. The patients with work-related myalgia displayed an inability to relax only in the myalgic trapezius muscle. An inability to relax during repetitive movements might play an important role both in initiating and upholding muscle pain.

This study is an investigation of the relationship between muscle morphology and surface electromyographic (EMG) parameters [mean frequency of the power spectrum (MNF), signal amplitude (root mean square, RMS) and the signal amplitude ratio (SAR; i.e. the ratio between the RMS level during the passive part of the contraction cycle and the RMS level during the active part of the contraction cycle)] during 100 maximal dynamic knee extensions at 90 degrees. s(-1). Each contraction cycle comprised of 1 s of active knee extension and 1 s of passive knee flexion. The surface EMG was recorded from the vastus lateralis muscle. Twenty clinically healthy subjects participated in the study, and muscle biopsy samples of the vastus lateralis were obtained from 19 of those subjects. The relationships between muscle morphology and EMG were investigated at three stages of the test: initially, during the fatigue phase (initial 40 contractions), and at the endurance level (the final 50 contractions). Major findings on correlations are that SAR and MNF tended to correlate positively with the proportion of type 1 fibres, and RMS correlated positively with the proportion of type 2 muscle fibres. The muscle fibre areas showed little correlation with the EMG variables under investigation. The results of the present study showed that the three EMG variables of a dynamic endurance test that were investigated (RMS, MNF and SAR) were clearly correlated with the proportions of the different fibre types, but only to a small extent with fibre areas. These findings contradict some of the theoretical models of the EMG, especially for parameters in the frequency domain.

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In the literature enlarged/increased crosssectional area (CSA) of type I muscle ®bres has been reported as a morphological mark of work-related localised myalgia in the descending part of the trapezius muscle of women. These studies did not use enough subjects or lacked an adequate control group. The recording of surface electromyograms (EMG) is central to the research ®eld of work-related myalgia. However, the in¯uence of intrinsic muscle properties ± such as the eect of muscle ®bre 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 ®bre distribution and CSA in work-related myalgia in trapezius muscles. Secondly, the multivariate relationships among muscle morphology and histochemistry [ragged-red (RR) ®bres, and cytochromec-oxidase-negative-®bre changes] EMG, perceived fatigue, and biomechanical output of shoulder¯exions were analysed. The raw data have been presented in an earlier study. The participants in this study were 25 female cleaners with work-related myalgia of the trapezius muscle and 25 female cleaners not experiencing workrelated myalgia of this muscle. The control group comprised 21 healthy female teachers who had not been exposed to highly repetitive work or static muscle work. Smaller CSA of type II ®bres were found in cleaners compared to teachers. In this study the CSA of type I ®bres of the trapezius muscles associated with myalgia were no greater than in muscles without myalgia. The prevalence of RR ®bres together with age, ®bre type proportions, CSA and working as a cleaner correlated with the ability to relax as recorded electromyographically. The relative mean frequency of the EMG of the trapezius muscle correlated with the prevalence of RR ®bres, but it did not correlate with the proportions and CSA of dierent ®bre types. Low biomechanical outputs and low signal amplitude increases of the EMG during the test were associated with high proportions of type -IIB ®bres. The smaller CSA of type II ®bres in cleaners might have re¯ected a dierent muscle activation pattern due to dierent occupational demands in cleaners than in teachers. Morphological or histopathological variables can in¯uence the three EMG variables investigated.