The Flextensor helps in "re-educating" the lower limbs via the use of a controlled elastic force in an open kinetic chain. The technology is obtained from the possibility of strengthening the femoral quadriceps and the knee flexor muscles. It can also switch from light heavy exercises with a high angular speed in the final stages and once the recovery has been met. Flextensor helps in stimulating the knee extensor and the flexor muscles through isometric and isotonic activities by arranging a series of exercises and repetitions. This device can also be customized for patients to performance gestural movements at great speeds.