Fig. 5. The magnitude and orientation (length and direction of the blue arrows) of the peak ground reaction force (GRF) from representative runs of different inclination. (A) Peak GRF during fast walking on the level. During this trial the animal was decelerating slightly, and not maintaining a steady velocity. (B-E) Peak GRF for the illustrated runs at 60°, 70°, 80° and 90°, respectively. During wing-assisted running (C-E) the magnitude of the GRF is less than at 60°, where wings are generally not used. Despite the pattern shown between wing position and the instance of peak GRF shown in C-E, during sustained bouts of WAIR on an inclined treadmill we found no relationship between footfall and wing position. M_b, body weight.