The stiffness of muscle-tendon units (MTUs) influences many aspects of human movement from athletic performance to injury risk. Presently the controlled-release technique of measuring MTU stiffness has been applied almost exclusively to the distal joints of the body, i.e., the ankle. This is primarily because of the mechanical limitations of implementing this technique. However, in order to better understand how the elastic properties of the MTU affect both performance and injury potential, measurements of MTU stiffness of the more proximal joints must be made. The knee flexors are a logical choice because of the integral role of MTU stiffness of this muscle group in both hamstring strains and knee injury. The purpose of this study was to modify a commercial ergometer so that it could be used to measure the musculotendinous stiffness of the knee flexors. Data are presented for a representative participant to illustrate the feasibility and capability of this ergometer, and the measured MTU stiffness was 519 N·m·rad-1 at a knee flexion moment of 100 N·m. Our results indicate that it is indeed possible to modify a commercial ergometer and measure musculotendinous stiffness of large muscle groups crossing proximal joints.