Fig 11. The association, or lack thereof, between t(F_z,peak) or t(F_net,peak) and t(F_x = 0).

(A, B) t(F_z,peak) versus t(F_x = 0). (C, D) t(F_net,peak) versus t(F_x = 0). (E, F) F_z(F_x = 0) compared to F_z,peak for each trial; the blue lines are lines of parity. (A), (C) and (E) are for birds, (B), (D) and (F) are for humans. In (A), the regression line has a slope of 0.7718 and an r² of 0.1289; in (C), the regression line has a slope of 0.7627 and an r² of 0.1194. In human walking, peak vertical or net force either occurs early or late in the stance; in running, they occur largely before mid-stance (t < 0.5), yet t(F_x = 0) largely occurs after mid-stance (t > 0.5).