(a) The motor system is connected with its environment via a spring K_s. (b) Position X versus time t for different motor systems with k_m = 1.5 pN/nm (v_c < 0), k_m = 3.0 pN/nm (v_c = 0), and k_m = 80 pN/nm (v_c>0), respectively, where Δz/b = 0.1 and k_s = 0.5 pN/nm. The other parameters are the same as those in Fig. 2.