Energy harvesting inside human body is crucial for powering implantable long-term biomedical devices. In this paper, the performance of a piezoelectric energy harvester embedded within custom-designed knee implant in powering RF transmitter was evaluated through simulations. This power harvester is composed of four piezoelectric generators along with four off-the-shelf power conditioning circuits. It can harvest electric power from dynamic forces dissipated inside knee implant during walking. The results demonstrate the possibility to deliver a constant electric power of 59.4mW during a simulated walking cycle. This conditioned power can operate an off-the-shelf ultra-low power RF transmitter. This may eliminate the need for rechargeable batteries to power instrumented knee implants. The proposed piezoelectric generator along with the ultra-miniature power conditioning and telemetry circuits may be integrated within the knee implant without increasing its physical size or changing its geometrical shape significantly with respect to the conventional one. Therefore, its dynamic functionality and mechanical longevity may not be affected.