Fig. 2. Experimental records of running on Earth (A) and during a flight profile simulating 1.3 g (B). (A) Fore–aft (F_f) and vertical (F_v) components of the force exerted by the foot on the force platform during running between the two photocells at the indicated speeds on Earth. (B) From top to bottom at each speed are shown the vertical (a_v), lateral (a_l), and fore–aft (a_f) components of the acceleration recorded on the aircraft during the run; the force signals F_f and F_v from the force platform (noise is due to vibrations of the aircraft) and the output of the photocell circuit. The vertical dotted lines delimit the time interval corresponding to the steps illustrated in Fig. 3, with expansion of the energy records to include the previous valley of potential energy as described in the Materials and methods. Subject A, mass 72 kg.