Intensity-dependent effect of treadmill running on rat Achilles tendon

It is understood that mechanical loading may affect tendon properties. However, how different mechanical loading conditions may affect tendons remains unknown. The present study aimed to investigate the effect of treadmill running at various intensities on rat Achilles tendon. A total of 18 male Wistar rats were randomly assigned to one of three groups: Control (CON), medium-intensity running (MIR), and high-intensity running (HIR). Following 8 weeks of treadmill running protocols, all Achilles tendons were harvested for histological observation and gene expression analysis. Significant morphological changes were observed with regular and large diameter collagen fibrils in the MIR group, whereas irregular and small diameter collagen fibrils were observed in the HIR group. Collagen type I was significantly upregulated in the MIR group compared with the CON group, and downregulated in the HIR group compared with the CON or MIR groups (P<0.05). However, collagen type III was significantly upregulated in the HIR group in comparison with the CON or MIR groups (P<0.05). Furthermore, the expression of matrix metallopeptidase-13 was significantly increased in the MIR and HIR groups compared with the CON group (P<0.05). The expression of tissue inhibitor of metalloproteinases-1 was increased in the MIR group compared with the CON group, but decreased in the HIR group compared with the CON and MIR groups (P<0.05). Additionally, decorin expression was significantly higher in the MIR group compared with the CON group, and significantly decreased in the HIR group compared with the CON or MIR groups (P<0.05). A converse pattern of changes in biglycan expression was identified among the three groups. Aggrecan expression was significantly higher in the HIR group compared with the CON or MIR groups (P<0.05). These findings indicated that moderate exercise may induce increased collagen synthesis and organize regular and large collagen fibers, thus benefiting the Achilles tendon. However, overuse during exercise may result in collagen degradation and disturbance, which predisposes individuals to injury.