A biomechanical model of the lower limb with active threedimensional (3D) muscles was developed in the open-source Artisynth platform (www.artisynth.org) to simulate knee movement. This model is going to be used in particular to broaden our knowledge about the biomechanical effects of high tibial osteotomy surgery and to assist the surgical planning procedure. The model geometry was reconstructed from MRI and CT data of a volunteer. The joint movement is acquired by activating various muscle groups. The joint kinematics as well as contact pressure distribution is monitored.