The objective of the study was to determine whether strenuous running exercise produces a significant amount of fibrosis in the infrapatellar fat pad (IFP) of the knee and whether intraarticular injections of hyaluronan (HA) protect fibrosis in the IFP in a rat running model. Strenuous running exercise was performed on a rodent treadmill with 5% inclination using Wistar rats. After the exercise, hyaluronan was injected into the right knee joint once per week in the HA injection group. In the treatment control group of the left knee, no injection was done. Also, the running exercise group and the sedentary control group received no HA injection. The IFP was analyzed histologically and immunohistochemically after 1 week (5 km), 2 weeks (10 km), 3 weeks (15 km) and 6 weeks (30 km). The inflammatory reaction in the IFP was not prominent at each running stage. A slightly, but significantly increased amount of inflammatory cell infiltration appeared during strenuous running. The amount of collagen fibers increased significantly at 30 km stage. Histological scores showed less fibrosis of the IFP in the HA injection group than in the control group at 30 km run. Strenuous running exercise may cause fibrosis in the IFP of runners. Intraarticular HA injection will inhibit arthrofibrosis of painful runners.