Sassi et al. 2011, energy cost of running on grass and turf

Study in the Journal of Strength and Conditioning Research comparing the energy cost of running on natural grass, artificial turf and a hard asphalt surface. Running on natural grass and on artificial turf cost about 5% more energy than the hard surface (4.22 and 4.21 versus 4.02 J·kg⁻¹·m⁻¹). Grass and turf were metabolically near-identical, so the extra cost tracks the compliance and shock absorption of the surface rather than the material. Firm grass alone therefore adds only a small penalty; the larger cost penalties of cross-country ground come from softness (mud and sand), unevenness and gradient. Sets a lower bound for the terrain cost of cross country, which combines these factors.