Early-life exercise effects on Achilles tendon in mice selectively bred for high voluntary wheel-running behavior.

Exercise increases muscle and bone strength and mass, but effects on tendons are less documented. We investigated the impact of voluntary exercise (wheel running) during early-life exercise (weanling; 3 weeks old) compared to post-skeletal maturity (young adult; 9 weeks old) on tendon morphology and material properties. We utilized a selectively bred High Runner (HR, N = 40) mouse line and a control line (N = 40). Mice underwent 8 weeks in cages either with or without wheels. HR mice ran ~3-fold more and were smaller than controls, but exercise reduced body mass in both lines. Tendon cross-sectional area was unaffected, but tendon length showed a line*exercise interaction (p = 0.0410) and a near-significant line*age interaction (p = 0.0866). HR mice broadly had greater yield stress (p = 0.0262) and tended toward higher failure stress (p = 0.0676) than controls. Work to failure was greater in younger cohort mice (p = 0.0435), and marginal age-related interactions were observed for modulus (line*exercise, p = 0.0632) and yield strain (line*age, p = 0.0535). HR mice were more responsive to exercise; older exercised HR mice had shorter tendons (p = 0.0282), and younger exercised HR mice showed lower yield and failure strains than sedentary counterparts (p = 0.0445, 0.0246). Exercise and its relative timing produced slight but complex effects on tendon properties, with HR mice showing the strongest structural and mechanical responses.