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

Abstract

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.

Mechanical exposure concepts using force as the agent

“…In the case of bone and articular cartilage, the process of mechanotransduction, or how cells detect strain and transduce it to a cellular response is central. Not all musculoskeletal disorders can be understood through the use of this approach; reduced blood flow with resulting localized muscle hypoxia (20) or increased tissue shear stress in hand-arm vibration resulting in damage (21,22) may need modified approaches.…”

Inflammation and the pathophysiology of work-related musculoskeletal disorders

“…Trapezius muscle biopsies from male and female workers with either continuous or intermittent trapezius myalgia of at least 12 months duration show evidence of myopathic changes such as moth eaten and ragged, red type I muscle fibers, increased frequency of type II myofibers and atrophic myofibers consistent with muscle injury, and denervation/ischemic loss of muscle fibers, but no evidence of inflammation (Larsson et al, 2001). In contrast, Dennet and Fry (1988) examining the first dorsal interosseous muscle collected from 29 patients with painful chronic overuse syndrome found increased inflammatory cells as well as myopathic changes.…”

Effect of contrasting physical exercise interventions on rapid force capacity of chronically painful muscles

“…In painful conditions, the rate of torque development is a more sensitive measure than maximal muscle strength and may consequently be highly responsive to rehabilitation. Although rapid as well as maximal force capacity is influenced by both neural and muscular factors, strength deficits in women with myalgia is not caused by muscular atrophy (12,13,31).…”