Ferris, Louie & Farley 1998, adjusting leg stiffness for different surfaces

Proceedings of the Royal Society B study showing that runners adjust the stiffness of the leg “spring” to match the surface they run on. On a more compliant (softer) surface they increase leg stiffness, and on a stiffer surface they decrease it, so that the total stiffness of leg-plus-surface, and the path of the centre of mass, stay roughly constant regardless of surface. A follow-up (Ferris et al. 1999, J Biomech) found the adjustment is made on the very first step onto a new surface. The mechanistic basis for why surface hardness alone is a weaker injury and performance variable than intuition suggests: the body compensates.