Functional and Histopathological Changes in Muscle after 6-weeks Repetitive Strain Injury: A 10-week Follow Up of Aged Rats
ABSTRACT: Repetitive eccentric contractions are associated with repetitive strain injury (RSI) of muscle and tendon and were accompanied by an increase in extracellular matrix (ECM), atrophy, and reduce force. However, a research gap exists regarding the effect of aging on injury susceptibility and recovery to repetitive strain exposures. In this paper, we examined the response of gastrocnemius of aged rats to 6weeks chronic strain injury followed 10 weeks without specific rehabilitation. 16 elderly male rats divided to two groups: control (n=8) and RSI (n=8). RSI group underwent 6weeks (5 days/week) of fast velocity submaximal eccentric contractions. After 4 and 10 weeks' post-injury non-active rest, isometric force, muscle wet mass, and histopathological changes of gastrocnemius muscle in RSI-model and control groups were measured. After 4 weeks' post-injury raw and relative (percent to body weight) measures of isometric force and wet muscle mass of gastrocnemius in control group are significantly greater than RSI group. force deficit was reduction Masson Trichrome and Hematoxylin & eosin stains also showed histopathologic changes were present only in RSI group that included increase in fibrosis and non-contractile area, and decrease of myoﬁber area. After 10 weeks of injury protocol, decrease in IF of gastrocnemius (8% and 6% for raw and relative measures respectively) were remained in RSI-Re group, but muscle wet mass was recovered. Also, myoﬁber area and non-contractile area were not fully recovered after 10-week rest in RSI-Re group (+2.77% and -3.6% respectively). Six weeks repeated bouts of moderate eccentric contractions caused in the rat gastrocnemius muscle decreases in muscular size and strength and myoﬁber area, whereas the non-contractile area and fibrosis was markedly increased. These results suggest that in aged rat force deficit and histopathological changes of gastrocnemius muscle after chronic strain injury were reminded after 10 weeks' rest. These observations are consistent with the hypothesis that structural damage (e.g. fibrosis) was not recovered after prolong rest.
KEY WORDS Repetitive strain injury, Eccentric contractions, Aging, Histopathological changes
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