Figure 1. Experimental records of walking during parabolic flight simulating a gravity of 0.4 g.

The upper panel shows the vertical (a_v) and fore-aft (a_f) components of the acceleration recorded during one parabola of the KC-135 aircraft; the lateral component was less than the fore-aft component, and is not shown. The middle panel shows the vertical (F_v) and fore-aft (F_f) force exerted by the subject on the force platform during one passage. The vertical dashed lines delimit the time interval corresponding to the duration of the one step analysed in Fig. 2. The bottom panel shows the output of the photocell circuit; the amplitude of the signal corresponds to the photocell number when the subject interrupted the light beam, indicating the subject’s speed and direction. The offsets in the force traces, clearly seen at the beginning and the end of the trial when the subject is not on the platform, are due to the suspended mass of the force plates times the changes in the aeroplane acceleration (traces above). As described in Methods, these offsets were automatically corrected during the analysis using the accelerometer records. Subject mass 77 kg, walking at a speed of 3 km h⁻¹.