This biomechanical study was performed to measure tissue pressure in the infrapatellar fat pad and the volume changes of the anterior knee compartment during knee flexion-extension motion. Knee motion from 120 degrees of flexion to full extension was simulated on ten fresh frozen human knee specimens (six from males, four from females, average age 44 years) using a hydraulic kinematic simulator (30, 40, and 50 Nm extension moment). Infrapatellar tissue pressure was measured using a closed cell sensor. Infrapatellar volume change in the anterior knee compartment was evaluated subsequent to removal of the fat pad using a water-filled bladder. We found a significant increase of the infrapatellar tissue pressure during knee flexion, at flexion angles of <20 degrees and >100 degrees . The average tissue pressure ranged from 343 (+/-223) mbar at 0 degrees to 60 (+/-64) mbar at 60 degrees of flexion. The smallest volume in the anterior knee compartment was measured at full extension and 120 degrees of flexion, whereas the maximum volume was observed at 50 degrees of flexion. In conclusion, the data suggest a biomechanical function of the infrapatellar fat pad at flexion angles of <20 degrees and >100 degrees , which suggests a role of the infrapatellar fat pad in stabilizing the patella in the extremes of knee motion.