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

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.

Keywords: autophagy‐lysosome system; grip strength; muscle atrophy; muscle pain; ubiquitin‐proteasome system; work‐related musculoskeletal disorders.

Figure 1

Voluntary repetitive reaching and grasping for food pellets. During the task, the rat repeatedly performs the action shown in the photograph once every 15 s (A–D). (A) The food pellet is dispensed on the shelf attached to the test box placing the rat. (B,C) The rat reaches and grasps for the food pellet placed on the shelf. (D) The rat eats the food pellet grasped in its paw.

Figure 2

Mechanical hyperalgesia analysis measuring forearm withdrawal threshold. Two photographs illustrate the procedure, as described in detail in the Materials and methods. Each rat, with head and trunk covered, was suspended in a homemade hammock (left). This position allowed the forelimbs to freely move, with the arrow indicating the direction of mechanical forces applied to the forearm flexors (right).

Figure 3

Changes in grip strength. Data are means ± SEM. *P < 0.05, **P < 0.01, compared with week 0. ^aa P < 0.01, compared with time‐matched rats from the Control group (n = 6 per group).

Figure 4

Change in forearm flexor withdrawal thresholds. Data are means ± SEM. *P < 0.05, **P < 0.01, compared with week 0. ^a P < 0.05, ^aa P < 0.01 compared with time‐matched rats from Control group (n = 7 in Control group and n = 10 in Task group).

Figure 5

Morphological changes in FDS muscles at 3 and 6 weeks. (A) Relative muscle weight, (B) muscle fiber CSA. Data are means ± SEM. *P < 0.05, **P < 0.01, compared with time‐matched rats from Control group (n = 6 per group).

Figure 6

Expression of neurotrophic factors in FDS muscles. (A) NGF and GDNF mRNA expression at 3 weeks (n = 6 per group); (B) NGF and GDNF mRNA expression at 6 weeks (n = 5 per group); (C) NGF and GDNF protein expression at 6 weeks. Representative blots depicting NGF and GDNF are shown. Quantitative analysis is shown in the lower panel. Results are reported as fold changes with respect to control levels, which were arbitrarily set to 1. Data are means ± SEM.

Figure 7

Expression of TNF‐α protein in FDS muscles at 6 weeks. Data are means ± SEM (n = 5 per group).

Figure 8

mRNA expression of E3 ubiquitin ligases and Atg genes in FDS muscles. (A) At 3 weeks. (B) At 6 weeks. Results are reported as fold changes with respect to control levels, which were arbitrarily set to 1. Data are means ± SEM. *P < 0.05, compared with time‐matched rats in Control group (n = 6 per group). MuRF1, muscle RING finger 1.

Figure 9

Levels of MuRF1, Beclin1, and Atg 5–Atg12 proteins in FDS muscles at 6 weeks. Representative blots depicting MuRF1, Beclin1, and Atg5–Atg12 are shown. Quantitative analysis is shown in the lower panel. Results are reported as fold changes with respect to control levels, which were arbitrarily set to 1. Data are means ± SEM. *P < 0.05, compared with time‐matched rats in Control group (n = 6 per group).