Fig. 4. During ascents (body postures shown above), both the normal (squares) and parallel (circles) components of the ground reaction force (GRF) show large increases over level walking values (A). The relative importance of the parallel axes to the peak GRF increases with the angle of ascent (B). The high forces in the normal plane during WAIR are consistent with the development of high frictional forces, which permit hindlimb propulsion. The magnitude of the GRF decreases during climbs when the wings are used to assist locomotion, generally angles greater than 60°. In B the sum of the percentage of peak GRF for each component does not equal 100%; rather vector addition requires the squares of the components to sum to the square of the GRF.