Chronic repetitive reaching and grasping results in decreased motor performance and widespread tissue responses in a rat model of MSD

Abstract

This study investigated changes in motor skills and tissues of the upper extremity (UE) with regard to injury and inflammatory reactions resulting from performance of a voluntary forelimb repetitive reaching and grasping task in rats. Rats reached for food at a rate of 4 reaches/min, 2 h/day, and 3 days/week for up to 8 weeks during which reach rate, task duration and movement strategies were observed. UE tissues were collected bilaterally at weekly time points of 3-8 weeks and examined for morphological changes. Serum was tested for levels of interleukin-1alpha (IL-1) protein. The macrophage-specific antibody, ED1, was used to identify infiltrating macrophages and the ED2 antibody was used to identify resident macrophages. Rats were unable to maintain baseline reach rate in weeks 5 and 6 of task performance. Alternative patterns of movement emerged. Fraying of tendon fibrils was observed after 6 weeks in the mid-forelimb. After 4 weeks, a general elevation of ED1-IR macrophages were seen in all tissues examined bilaterally including the contralateral, uninvolved forelimb and hindlimbs. Significantly more resident macrophages were seen at 6 and 8 weeks in the reach limb. At 8 weeks, serum levels of IL-1alpha increased significantly above week 0. Our results demonstrate that performance of repetitive tasks elicits motor decrements, signs of injury and a cellular and tissue responses associated with inflammation.

Fig. 1

Behavioral outcomes. (A) Note the significant decrease in weeks 5 and 6 for reach rate and (B) in week 5 for task duration ( ^*: p ≤ 0.05). These changes coincide with the emergence of raking (C), which is present in 100% of the animals by week 7.

Fig. 2

Photomicrographs illustrating evidence of tendon microfray and localized infiltration of macrophages. (A,B) Type I collagen-IR (green) in the proximal tendon of flexor digitorum superficialis (M). A: At week 0, type I collagen-IR (green) in this tendon shows a neat transition between the myofibers (unstained) and tendon fibrils (T). B: A kink, or microfray, is visible in type I collagen-IR tendons of flexor digitorum superficialis of a 6 week rat. (C,D) ED1 immunoreactivity in loose CTs of the palm. C: No ED1-IR macrophages were visible in the loose CT of the palm in a control rat (week 0). D: By 6 weeks of task performance, many ED1-IR macrophages (brown staining cells) had infiltrated the palmar CT. (E,F) ED1 immunoreactivity in the flexor digitorum superficialis muscle at the junction of myofibers and tendon fibrils, located mid-forearm. E: No ED1-IR macrophages were present in the musculotendinous juncture site in a control (week 0) rat. F: By 6 weeks of task performance, ED1 immunoreactive macrophages (arrows) had infiltrated a comparable site in the flexor digitorum superficialis muscle of the reach limb. The inset illustrates a tendon from ipsilateral, proximal forearm and illustrates the presence of infiltrating ED1-IR macrophages in that tissue after 6 weeks of task performance. Bar = 50 μm.

Fig. 3

Quantification of immature, infiltrating macrophages (ED1-IR) in tissues from palmar (A,B) and forelimb (C,D) regions in the reach and non-reach limbs. The mean number of ED1-IR cells was quantified in loose CT, tendons (T), and muscles (M) in the anterior palms and flexors of distal forelimbs. There were significantly more ED1-IR macrophages in all of the tissues examined in the palm after week 3, and in the distal forelimb after week 4 (reach limb) and week 5 (non-reach limb) compared to controls ( ^*: p < 0.05).

Fig. 4

Quantification of resident, tissue macrophages (ED2-IR) in tissues from the distal forelimbs of reach limbs. The mean number of ED2-IR cells increased significantly above controls levels at weeks 6 and 8 ( ^*: p < 0.05; ^** : p < 0.001). MTJ = myotendinous junction.

Fig. 5

Inflammatory response in regions distant from anterior forelimb and palm of reach limb. The number of ED1-IR macrophages was quantified in tissues (musculotendinous and associated CTs) of the (A) forearm extensor, (B) brachial, (C) rotator cuff, and (D) tibial regions. Weeks 5 and 6 were combined and compared to week 0. The number of ED1-IR macrophages was significantly higher in weeks 5 and 6 than week 0 ( ^**: p ≤ 0.001).

Fig. 6

Serum levels of IL-1α. (A) Protein levels of IL-1α increased significantly in the serum of 8 week rats compared to controls ( ^*: p < 0.05). (B) Photomicrograph illustrating ED1-IR macrophages (green) that are co-expressing IL-1α (red).