If, during a stride, the stretch causes her center of mass to lower by \(10 \mathrm{~mm}\), what is the stored energy?

A. \(3.0 \mathrm{~J}\)

B. \(6.0 \mathrm{~J}\)

C. \(9.0 \mathrm{~J}\)

D. \(12 \mathrm{~J}\)

We saw that a runner's Achilles tendon will stretch like a spring and then rebound, storing and returning energy during a step. We can model this as the simple harmonic motion of a mass-spring system. When the foot rolls forward, the tendon spring begins to stretch as the weight moves to the ball of the foot, transforming kinetic energy into elastic potential energy. This is the first phase of an oscillation. The spring then rebounds, converting potential energy to kinetic energy as the foot lifts off the ground. The oscillation is fast: Sprinters running a short race keep each foot in contact with the ground for about 0.10 second, and some of that time corresponds to the heel strike and subsequent rolling forward of the foot.