Fig. 1. A portion of a sperm flagellum that is propagating a bending wave, shown at two points in time in A and B. If the bending wave is moving from left to right, a short segment (dark bar) will be moving up in A and down in B, changing its tilt as the velocity changes. The velocity, V, of any segment can be resolved into a longitudinal component, V_L, and a normal component, V_N. Resistive force theory states that the force components pushing on the fluid will then be F_L=C_LV_L and F_N=C_NV_N, where C_L and C_N are resistance constants. If C_N>C_L, the total force F=F_N+F_L will always have a component pushing on the fluid in the direction of wave propagation, and this will provide a thrust in the opposite direction, propelling the spermatozoon to the left.