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 length of the knee extensor tendon in sprinters and nonathletes. They find (on average) that the sprinters' tendons stretch 40mm , while nonathletes' stretch only 31mm. 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?