Thirdly, we examined the influences of forced running and MIA injection on synovium. In the PBS injected knee (Fig. 5a), thickness of the synovial cell layer at the infrapatellar fat pad did not increase after 7 days, the synovial thickness appeared to be unchanged after 15 km of running (Fig. 5b) and synovitis scores were similar between rats not forced to run and rats subjected to forced running (Fig. 5c). In MIA treated knees (Fig. 5d), even though the thickness of the synovial cell layer at the infrapatellar fat pad increased 7 days after MIA injection, the synovial thickness decreased irrespective of running at 21 days (Fig. 5e). The synovitis score in rats injected with MIA at 21 days with or without running was significantly lower than that in rats injected with MIA at 0 days (Fig. 5f). There was no difference between synovitis scores with running and without running in MIA treated knees.Fig. 5Analyses for synovitis of the knee. a Protocol for running without MIA injection. b Synovial sections stained with HE. Whole knee joints were sectioned in the sagittal plane for synovium of infrapatellar fat pad. c Quantification of synovitis evaluated by Krenn’s synovitis scoring system (n = 4). d Protocol for running with MIA injection. e Synovial sections stained with HE. Synovial cell layer is indicated by arrows. f Quantification of synovitis evaluated by Krenn’s synovitis scoring system (n = 4, * p < 0.05 by Steel-Dwass test)