Morphological and electromyographical studies of trapezius myalgia in cleaners

Abstract: Pathomechanisms of work-related myalgia are poorly understood. Myalgia is caused by exitation of nociceptors present in the muscular tissue but not in the muscle fibres themselves. Morphological analysis can contribute to knowledge of excitation of nociceptors. Surface EMG is a widely used method in the research field of ergonomics. Knowledge of influence of myalgia, occupation, age and intrinsic properties on the surface EMG is limited. Twenty-five female cleaners with long standing work related myalgia, 25 female cleaners without trapezius myalgia and 21 healthy teachers participated in the study. All participants were clinically examined to reach the diagnosis trapezius myalgia. Surface EMG was recorded during an isokinetic endurance test of shoulder forward flexors. Biomechanical output and the EMG parameters mean power frequency and root mean square (RMS) were calculated. The RMS ratio between passive and active phases of contractions were calculated to assess the ability to relax between contractions. Surgical trapezius muscle biopsies were obtained from all subjects. Enzym- and immunohistochemical stainings were performed to visualize different fibre types, capillary density, fibres with subsarcolemmal accumulations of mitochondria, cytochrome oxidase deficient fibres and fibres with a disorganization of mitochondrial distribution. Occupational cleaning work, myalgia and age contributed to the total variation of biomechanical output and EMG parameters. Histopathological findings and the relative distribution of myosin heavy chains in the trapezius muscle influenced the surface EMG signal. The capillary supply but not the morphological characteristics and functional properties of the trapezius muscle may be associated with myalgia. Histochemical and enzym histochemical alterations indicating a disturbed oxidative metabolism were linked to heavy work load on the trapezius muscle.