Fig. 1. Simplified view of the forces acting on a runner. The subject is accelerating forward while running on flat terrain (A) or running uphill at constant speed (B). The subject's body mass is assumed to be located at the centre of mass (COM); a_f=forward acceleration; g=acceleration of gravity; g'=(a_f²+g²)^0.5 is the acceleration resulting from the vectorial sum of a_f plus g; T=terrain; H=horizontal; (=arctan g/a_f) is the angle between runner's body and T; the angle between T and H is '=90–. (Modified from di Prampero et al., 2002.)