(a) The relationship between externally applied force per motor F_ext and the velocity of the backbone for different motor stiffness with ΔE = 64 pN·nm, l = 10 nm, b/l = 0.2, Δz/b = 0.5, ω_on = 40 s⁻¹, Ω = 1000 s⁻¹, and D = 10⁶ nm²/s. (b)–(d) The motor stiffness can affect the sign of v_c, which denotes the velocity corresponding to the extreme value of F_ext(v): (b) k_m = 1.4 pN/nm, v_c < 0; (c) k_m = 3.0 pN/nm, v_c = 0; (d) k_m = 80.0 pN/nm, v_c > 0.