Decreased grip strength, muscle pain, and atrophy occur in rats following long-term exposure to excessive repetitive motion.

Mitsuhiro Fujiwara,Takayuki Inoue,Shigeyuki Suzuki,Yosuke Aizawa,Natsumi Yoshito,Masahiro Iwata,Kazuhiro Hayashi

doi:10.1002/2211-5463.12315

Abstract

Work‐related musculoskeletal disorders (WMSD) are caused by the overuse of muscles in the workplace. Performing repetitive tasks is a primary risk factor for the development of WMSD. Many workers in highly repetitive jobs exhibit muscle pain and decline in handgrip strength, yet the mechanisms underlying these dysfunctions are poorly understood. In our study, rats performed voluntary repetitive reaching and grasping tasks (Task group), while Control group rats did not perform these activities. In the Task group, grip strength and forearm flexor withdrawal threshold declined significantly from week 2 to week 6, compared with these values at week 0 (P < 0.05). Relative muscle weight and muscle fiber cross‐sectional area of flexor digitorum superficialis (FDS) muscles decreased significantly in the Task group, compared with the Control group, at 6 weeks (P < 0.05 and P < 0.01, respectively). Nerve growth factor, glial cell line‐derived neurotrophic factor, and tumor necrosis factor α‐expression in FDS muscles were not significantly different in Control and Task groups at 3 and 6 weeks. At 6 weeks, the Task group had elevated MuRF1 protein levels (P = 0.065) and significant overexpression of the autophagy‐related (Atg) proteins, Beclin1 and Atg5–Atg12, compared with in the Control group (both P < 0.05). These data suggested that long‐term exposure to excessive repetitive motion causes loss of grip strength, muscle pain, and skeletal muscle atrophy. Furthermore, this exposure may enhance protein degradation through both the ubiquitin‐proteasome and autophagy‐lysosome systems, thereby decreasing skeletal muscle mass.

Highlights

Work-related musculoskeletal disorders (WMSD) are caused by the overuse of muscles in the workplace
In our study, we investigated the hypothesis that long-term exposure to excessive repetitive motion would cause loss of grip strength, muscle pain, and skeletal muscle atrophy and, that such atrophy would involve enhanced protein degradation through the ubiquitin-proteasome and autophagy-lysosome systems
For the first time, that the decline in grip strength caused by the repetitive tasks occurred earlier than the previous study was able to discern

Summary

Introduction

Work-related musculoskeletal disorders (WMSD) are caused by the overuse of muscles in the workplace. At 6 weeks, the Task group had elevated MuRF1 protein levels (P = 0.065) and significant overexpression of the autophagy-related (Atg) proteins, Beclin and Atg5–Atg, compared with in the Control group (both P < 0.05) These data suggested that long-term exposure to excessive repetitive motion causes loss of grip strength, muscle pain, and skeletal muscle atrophy. Abbreviations Atg, autophagy-related; Atrogin-1, muscle atrophy F-box/Atrogin-1; CBB, coomassie brilliant blue; CSA, cross-sectional area; DOMS, delayedonset muscle soreness; EDL, extensor digitorum longus; FDS, flexor digitorum superficialis; GDNF, glial cell line-derived neurotrophic factor; IL, interleukin; LC3, microtubule-associated protein 1 light chain 3; MuRF1, muscle RING finger 1; NGF, nerve growth factor; PVDF, polyvinylidene difluoride; qRT-PCR, quantitative reverse transcription PCR; SEM, standard error of the mean; TNF, tumor necrosis factor; WMSD, work-related musculoskeletal disorders. Women seeking medical care for WMSD of the upper extremities showed decreased handgrip strength, and these symptoms were related to their levels of perceived physical exertion [2]

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