Secondly, we examined the influences of forced running and MIA injection on cartilage (Fig. 2a, b). All rats completed 15 km of running even after MIA injection. Macroscopically, 15 km of strenuous running affected neither the femoral nor tibial articular cartilage (Fig. 3a). Intra-articuar injection of 0.1 mg MIA did not affect the femoral articular cartilage surface, but induced erosion at the tibial cartilage irrespective of the forced 15 km of running (Fig. 3b).Fig. 3Macroscopic observation for articular cartilage. a Macroscopic images of the femoral and tibial articular cartilages stained with India ink. Cartilage erosion is surrounded by a yellow dotted line. b Quantification of degenerated area of the tibial cartilage (n =4, * p < 0.05 by Steel-Dwass test)