The critical phase for visual control of human walking over complex terrain

Abstract

To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upcoming ground surface without interfering with the exploitation of passive mechanical forces. We propose that walkers use visual information to initialize the mechanical state of the body before the beginning of each step so the resulting ballistic trajectory of the walker's center-of-mass will facilitate stepping on target footholds. Using a precision stepping task and synchronizing target visibility to the gait cycle, we empirically validated two predictions derived from this strategy: (1) Walkers must have information about upcoming footholds during the second half of the preceding step, and (2) foot placement is guided by information about the position of the target foothold relative to the preceding base of support. We conclude that active and passive modes of control work synergistically to allow walkers to negotiate complex terrain with efficiency, stability, and precision.

Keywords: biomechanics; foot placement; human locomotion; inverted pendulum; visual control.

Fig. 1.

(A) A conceptual diagram of the steady-state gait cycle. In the step-to-step transition, positive and negative work from the trailing and leading legs (red and blue arrows, respectively) redirect the COM and establish the conditions leading into the next step. (B) The ballistic trajectory of the COM during a step is defined by two factors—the location of the planted foot and the magnitude of the push-off force from the trailing limb. (C) By adjusting these factors, a walker can tailor the passive trajectory of the COM in an oncoming step to facilitate a step onto a target foothold. (D) The critical control phase for targets 3–6 (see also Movie S1).

Fig. 2.

(Left) Depiction of four target visibility manipulations used in experiment 1 (see Movie S2). Horizontal bars indicate the location of the walker’s COM when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. (Right) Mean absolute stepping error and 95% CI for each condition in experiment 1.

Fig. S1.

(Left) Depiction of four target visibility manipulations used in experiment 1 (see Movie S2). Horizontal bars indicate the location of the walker’s COM when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. (Right) Mean walking speed (m/s) and 95% CI for each condition in experiment 1.

Fig. 3.

(Left) Depiction of target visibility manipulations used in experiments 2A (Top) and 2B (Bottom). See Movies S3 and S4. Horizontal bars indicate the location of the walker’s COM when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. The same three visibility manipulations were used in both experiments but were applied to targets 3–6 in experiment 2A and to targets 4 or 5 in experiment 2B. (Right) Mean absolute stepping error relative to full vision control condition and 95% CI for each condition in experiments 2A and 2B.

Fig. 4.

(Left) Depiction of target visibility manipulations used in experiment 3. Horizontal bars indicate the location of the walker’s COM when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. Target visibility manipulation was applied to either target 4 or target 5. (Right) Mean absolute stepping error relative to full vision control condition and 95% CI for each condition in experiment 3.

Fig. 5.

Experimental set-up (shown with all targets visible). Reprinted with permission from ref. (Matthis et al.), copyright Association for Research in Vision and Ophthalmology.

Fig. 6.

Schematic depiction of the target visibility manipulation used in the Just in Time condition applied to target 4. (A) Target 4 becomes visible when the subject’s foot crosses the visibility trigger surrounding target 3 (blue dashed circle) and remains so until the foot crosses the invisibility trigger (red dotted circle). (B) The steps to target 3 (blue arrows) and target 4 (red arrows) shown with time progressing from left to right. The visibility of target 4 is shown on the black and red arrow, along with the trigger number associated the changing visibility.

Fig. S2.

(Left) Depiction of three additional target visibility manipulations used in experiment 1 that were not reported in the original manuscript. Horizontal bars indicate the phase of the gait cycle when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. (Right) Mean absolute stepping error and 95% CI for each condition. Note that stepping error is less than it is in the Just in Time and Needlessly Early conditions (see Fig. 2). This is consistent with the two targets hypothesis, as there is an overlap in the visibility of the two upcoming targets in all three conditions shown in this figure.

Fig. S3.

(Left) Depiction of target visibility manipulations used in experiment 2C with two active targets. Horizontal bars indicate the phase of the gait cycle when the target of the corresponding color was visible. Vertical bands indicate hypothesized critical control phase for each target. (Right) Mean absolute stepping error relative to full vision control condition and 95% CI for each condition in experiment 2C. Note that results are nearly identical to those in experiment 2A with four active targets but different from those in experiment 2B with one active target (see Fig. 3). This is consistent with the two targets hypothesis, as the degree of overlap in the visibility of consecutive targets varies across conditions as it does in experiment 2A.