The objective of this study was to identify acute responses and chronic adaptations of supraspinatus tendon to non-injurious exercise. We hypothesized that chronic exercise increases tendon mechanical properties, and a single exercise bout increases MMP activity acutely. Rats were divided into acute or chronic exercise (EX) or cage activity (CA) groups. Animals in acute EX groups were euthanized 3, 12, 24, 48, or 72 hours upon completion of a single bout of exercise (10 m/min, 1 hour) on a flat treadmill. Animals in chronic EX groups walked on a flat treadmill for 3 days, 1 week, 2 weeks, or 8 weeks. Tendon histology, MMP activity, and mechanics were measured. A single bout of exercise trended toward reducing tendon mechanical properties, but 2 or 8 weeks of chronic exercise increased tendon mechanics. Cell density was not affected. Cells became rounder with chronic exercise. All tendons were highly organized. MMP activity decreased after a single bout of exercise and returned to baseline by 72 hours. MMP activity decreased after 8 weeks of chronic exercise. Decreased MMP activity may indicate an anabolic instead of catabolic response, in contrast to injury. Results suggest that mild, acute decreases in MMP activity and tendon mechanics following a single exercise bout lead to enhanced tendon mechanical adaptations with repeated exercise bouts. This study defines acute and chronic changes MMP activity, mechanical properties, and histology of the rat supraspinatus tendon in response to beneficial exercise and proposes a mechanism by which acute responses translate to chronic adaptations.
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