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Q: Do runners waste any energy output by raising their bodies vertically
every step? If so, what percentage is wasted, and why do they do it?

A: Running is a series of leaps, in which we rise and then fall by about 6
centimetres (at a good marathon speed), gaining and then losing potential
energy. Also, we accelerate as we take off and slow down as we land, gaining
and then losing kinetic energy. Potential energy fluctuations predominate at
low speeds, and kinetic energy fluctuations at high speeds. Most of the work
required of our muscles serves to supply these components of energy.

The work referred to is wasted, in that there would be no need to do it if
we ran smoothly on wheels. Measurements of oxygen consumption show that
cycling needs less than half as much energy as running at the same speed. But
we don’t run on wheels – and would be poor at crossing rough ground if we
did.

We avoid part of the cost of running on legs instead of wheels by bouncing
along like a rubber ball. Each time a foot lands on the ground, springs are
stretched, and they recoil elastically as the foot leaves the ground. About
half of the (kinetic plus potential) energy lost and regained is stored up as
elastic strain energy and returned, halving the work that the muscles have to
do.

The springs involved are the Achilles tendon and the ligaments of the arch
of the foot. The tendon stretches and recoils by about 5 per cent of its
length. Stretching and recoil of the ligaments allow the arch of the foot to
flatten, and then arch again.