The mean (length of the bar) and standard deviation (the error bar, n = 10) of the Impact Factor A, which is the change in the stability (Δs) caused by a per unit increment in the hip extensor, knee flexor, and ankle plantar-flexor moments of the stance limb (Δτ, one bm×g×bh Nm, bm represents the body mass, g indicates the gravitational acceleration, and bh the body height), and is measured by the slope of the line best fitting the simulation results as shown in Fig 4.