Knee osteoarthritis often occurs in medial and patellofemoral compartments. A bicompartmental knee replacement system replaces these two affected knee compartments and keeps the lateral compartment and cruciate ligaments intact. It is yet to be determined whether limbs with bicompartmental knee systems can demonstrate frontal-plane knee mechanics and hamstring coactivation similar to healthy control limbs during daily activities requiring the weight-bearing knees to bend through a large range of motion (e.g., stand-to-sit). Three-dimensional knee mechanics and quadriceps and hamstring electromyographic data were collected from 8 patients with a unilateral bicompartmental knee system and 10 healthy control participants. No differences in frontal-plane knee mechanics and hamstring coactivation were found among the surgical, contralateral, and control limbs during stand-to-sit (p greater than 0.05). (Contains 3 tables.)