Fig. 1. Theoretical comparison of dynamics of running on level ground (A) vs climbing using two different models. In the first model (B), legs produce deceleratory fore–aft forces, F_–x, as an unavoidable consequence of foot attachment. Thus larger acceleratory forces, F_+x, are required to counteract the combined deceleration of the legs and gravity g. In the second model (C), legs do not produce deceleratory forces. Thus, acceleratory forces are reduced since only gravity decelerates the animal, and total mechanical energy (E_tot) required to climb approaches potential energy (E_P).