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.