stores energy during early stance knee flexion from 1 to 3 here anterior knee valve k is closed or locked throughout early stance knee flexion and extension from 1 to 5 this stored energy together with an applied extensor hip moment from either a robotic or biological hip result in an extensor moment at the knee forcing the knee to extend and compress posterior pneumatic spring f from 3 to 5 it is important to note that posterior knee valve g is open during early stance knee flexion so that posterior pneumatic spring f exerts little force knee valve g is then closed during knee extension so that energy is stored in the posterior pneumatic spring f when the ankle is maximally dorsi flexed and the knee fully extended leg configuration 5 posterior pneumatic spring f is maximally compressed when the leg assumes this posture knee ankle transfer valve changes from a closed state to an open state and anterior ankle valve n changes to a closed state allowing all the energy stored in spring f is be transferred through the ankle to power ankle plantar flexion from 6 to 7 during late stance from 5 to 6 the knee of the supporting leg begins to flex again in preparation for the swing phase for this.