Pinnington & Dawson 2001, energy cost of running on grass versus sand

Journal of Science and Medicine in Sport study comparing the energy cost of running on grass with soft dry beach sand. Running on soft sand cost roughly 1.6 times the energy of running on grass at matched speeds, for both barefoot and shod running. Consistent with earlier work (Zamparo et al. 1992; Lejeune et al. 1998) showing soft, compliant surfaces raise metabolic cost by reducing the recovery of elastic and mechanical energy each stride. The evidence that surface compliance changes the energy cost of running, with very soft surfaces being markedly more demanding.