Abstract
We sought to determine if tendon inflammatory and histopathological responses increase in aged rats compared to young rats performing a voluntary upper extremity repetitive task, and if these changes are associated with motor declines. Ninety-six female Sprague-Dawley rats were used in the rat model of upper extremity overuse: 67 aged and 29 young adult rats. After a training period of 4 weeks, task rats performed a voluntary high repetition low force (HRLF) handle-pulling task for 2 hrs/day, 3 days/wk for up to 12 weeks. Upper extremity motor function was assessed, as were inflammatory and histomorphological changes in flexor digitorum and supraspinatus tendons. The percentage of successful reaches improved in young adult HRLF rats, but not in aged HRLF rats. Forelimb agility decreased transiently in young adult HRLF rats, but persistently in aged HRLF rats. HRLF task performance for 12 weeks lead to increased IL-1beta and IL-6 in flexor digitorum tendons of aged HRLF rats, compared to aged normal control (NC) as well as young adult HRLF rats. In contrast, TNF-alpha increased more in flexor digitorum tendons of young adult 12-week HRLF rats than in aged HRLF rats. Vascularity and collagen fibril organization were not affected by task performance in flexor digitorum tendons of either age group, although cellularity increased in both. By week 12 of HRLF task performance, vascularity and cellularity increased in the supraspinatus tendons of only aged rats. The increased cellularity was due to increased macrophages and connective tissue growth factor (CTGF)-immunoreactive fibroblasts in the peritendon. In conclusion, aged rat tendons were overall more affected by the HRLF task than young adult tendons, particularly supraspinatus tendons. Greater inflammatory changes in aged HRLF rat tendons were observed, increases associated temporally with decreased forelimb agility and lack of improvement in task success.
Highlights
Upper extremity work-related musculoskeletal disorders (WMSDs) are common and economically burdensome, and accounted for 29 percent of all workplace injuries and illnesses in the U.S requiring time away from work in 2010 [1]
In a rat model of upper extremity WMSDs, we found that aging enhanced a pro-inflammatory serum cytokine response that was greater in aged rats performing a high repetition low force (HRLF) task, compared to young adult HRLF rats [13]
Aged HRLF rats showed no differences with task performance, but young adult HRLF rats had a significant increase in the percentage of successful reaches in week 12, compared to their week 1 (p,0.05)
Summary
Upper extremity work-related musculoskeletal disorders (WMSDs) are common and economically burdensome, and accounted for 29 percent of all workplace injuries and illnesses in the U.S requiring time away from work in 2010 [1]. Our work and others have found a link between inflammation and decreased sensorimotor function in patients with WMSDs and in our rat model of WMSDs, these studies were performed in young adult patients or rats [8,9,10,11]. In a rat model of upper extremity WMSDs, we found that aging enhanced a pro-inflammatory serum cytokine response that was greater in aged rats performing a high repetition low force (HRLF) task, compared to young adult HRLF rats [13]. The combination of aging and HRLF task performance did not lead to greater declines in grip strength than that found in young adult HRLF rats [13]. We speculate that increased tissue inflammatory cytokines occurring as a consequence of both aging and repetitive task performance will contribute to declines in motor abilities and tissue pathology
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