Breaking Down the Stride

The energetic costs of running and the underlying physiological
mechanisms have been studied for decades. However, relating the
energetics to underlying mechanics has relied on “black-box” approaches.
In an experimental study of guinea fowl, Marsh et al. (p.
80; see the Perspective by Heglund) used blood flow to the muscles
as a measure of how the energy is distributed. Contrary to previous
predictions, the energy used by muscles that swing the upper limbs
was not negligible—it was about a third of the amount used by the
lower limbs that transmit force to the ground.