Chronic occupational lower limb injuries such as osteoarthritis require an understanding of mechanical loading conditions in joints during occupational activities. We aimed to develop a human musculoskeletal model and to estimate knee contact force and ground reaction force (GRF) simultaneously during walking and squatting. GRF and joint kinematics were obtained from a subject with electronic total knee replacement during walking and squatting. Force reaction elements were embedded in the knee and foot of a full-body musculoskeletal model. The joint kinematics and ground vertical force were applied to the musculoskeletal model to estimate ground shear force and knee contact force using an inverse dynamics-based optimization. Ground shear force and knee contact force could be estimated simultaneously with root mean square (RMS) error less than 1.6% body weight (BW) and 35% BW, respectively. Simultaneous estimations could be accurately conducted but the RMS error for the knee contact force increased by approximately 10% BW.