Traditional total knee replacement (TKR) implants have two components: a tibial component consisting of a metal tibial tray with a stem inserted into the tibia and a polyethylene bearing surface inserted into the tray, and a metal femoral component attached to the end of the femur. While these components replicate the functionality of the knee, they are passive and offer no ability to provide data on the status of the knee replacement unit. In this work, piezoelectric materials are considered for both sensing and energy harvesting in TKR implants. This paper focuses on the development of a biomechanical force model that can be used to determine the forces exerted by the human body, which must be endured by the piezoelectric harvester/sensor once it has been implanted in the knee. This model has been developed using OpenSim, an open source software that is used for biomechanical modeling of musculoskeletal systems. The forces calculated in this model are applied directly to the piezoelectric material to determine the power generated during a typical step. Simulation results show that 3-35 mW of average power can be harvested depending on the piezoelectric material used in the stack. Compared to typical low power sensors, which operate in the μW - mW range, the generated power is sufficient to power an embedded sensor in the TKR unit.