McMahon & Greene 1979, the influence of track compliance on running

Journal of Biomechanics study behind Harvard’s “tuned track”. It showed that a surface of intermediate compliance, springy rather than hard or too soft, reduces the peak foot-strike force and shortens ground-contact time, and within an optimal stiffness range can actually increase running speed; overly soft surfaces lengthen contact and slow the runner. Later work (Kerdok et al. 2002) corroborated that running on more compliant surfaces can lower the metabolic cost. The basis for the point that an engineered, energy-returning track surface runs faster than hard road, independent of the bends.