The elastic energy stored in your tendons can contribute up to 35% of your energy needs when running. sports scientists have studied the change in lenght of the knee extensor tendon in sprinters and nonathletes. they find (on average) that the sprinters' tendons stretch 43mm, while nonatheletes' stretch only 30mm. the spring constant for the tendon is the same for both groups, 33 n/mm. what is the difference in maximum stored energy between the sprinters and the nonathletes?