(A–D) Histograms of steps sizes and example trace segments measured under 1 pN (n = 716), 3 pN (n = 1104), 6 pN (n = 1314), and 10 pN (n = 642) constant rearward load (force-feedback). The gray-shaded histograms correspond to the combined step size data and the red and blue histogram bars indicate the steps assigned to the advancing and nonadvancing modes, respectively. The raw stepping data are shown in black and the steps detected by the step-finding program in red (advancing mode) and blue (nonadvancing mode). The probability

for taking a backward step in the advancing mode (

, with

being the probability for taking a forward step in the advancing mode) at 1, 3, 6, and 10 pN load is 0.26, 0.34, 0.4, and 0.75, respectively (calculated from advancing step size histograms shown in red). The probability

*p*^{non-adv} for taking any step in the non-advancing mode (

*p*^{adv} +

*p*^{non-adv} = 1, with

*p*^{adv} being the probability for taking any step in the advancing mode) is 0.19, 0.42, 0.5, and 0.31 at 1, 3, 6, and 10 pN, respectively.

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