The biomechanics of walking shape the use of visual information during locomotion over complex terrain

Abstract

The aim of this study was to examine how visual information is used to control stepping during locomotion over terrain that demands precision in the placement of the feet. More specifically, we sought to determine the point in the gait cycle at which visual information about a target is no longer needed to guide accurate foot placement. Subjects walked along a path while stepping as accurately as possible on a series of small, irregularly spaced target footholds. In various conditions, each of the targets became invisible either during the step to the target or during the step to the previous target. We found that making targets invisible after toe off of the step to the target had little to no effect on stepping accuracy. However, when targets disappeared during the step to the previous target, foot placement became less accurate and more variable. The findings suggest that visual information about a target is used prior to initiation of the step to that target but is not needed to continuously guide the foot throughout the swing phase. We propose that this style of control is rooted in the biomechanics of walking, which facilitates an energetically efficient strategy in which visual information is primarily used to initialize the mechanical state of the body leading into a ballistic movement toward the target foothold. Taken together with previous studies, the findings suggest the availability of visual information about the terrain near a particular step is most essential during the latter half of the preceding step, which constitutes a critical control phase in the bipedal gait cycle.

Keywords: biomechanics; bipedal gait; foot placement; locomotion; visual control.

Figure 1

Experimental setup.

Figure 2

Schematic depiction of the invisibility-trigger manipulation for four conditions. In the 0.5 invisibility-trigger condition (A), each target became invisible when the toe marker on the relevant foot entered a circular region centered on the target and with a radius equal to 0.5 of the stride length (SL) needed to reach that target (0.5 SL). Similarly, in the 0.75 (B) and 1.0 (C) invisibility-trigger conditions, targets became invisible when the relevant foot entered within 0.75 SL and 1.0 SL, respectively. When the invisibility trigger was larger than 1.0, targets were made invisible when the relevant toe marker intersected the invisibility-trigger radius of the previous target. For example, in the 1.5 invisibility-trigger condition (D), target₄ became invisible when the toe was within 0.5 SL of target₃. Note that the invisibility-trigger manipulation was applied to target₃ through target₆ although, for clarity, only the trigger affecting target₄ is shown here.

Figure 3

Scatterplot of every recorded step showing stepping error versus calculated (lag-adjusted) invisibility trigger. Markers are color-coded according to invisibility-trigger condition.

Figure 4

Results for stepping error in the different invisibility trigger conditions. (A) Overall stepping error measured in two dimensions. (B) The contribution of variable error in the AP dimension to overall error, and (C) the contribution of constant error (bias) in the AP dimension. (D and E) The contributions of variable and constant error, respectively, in the ML dimension. The invisibility-trigger values shown along the abscissa are the mean calculated (lag-adjusted) visibility for each step taken in that condition (i.e., the mean of the horizontal position of each colored dot in Figure 3). Bars show ± 1 SEM. Asterisks denote significant difference from the full-vision condition.

Figure 5

Difference between actual and predicted step length in each invisibility-trigger condition. Predicted step length was based on Equation 1. Error bars show ± 1 SEM. Thick horizontal line corresponds to the difference in step length in the free-walking condition (dotted lines are ± 1 SEM).

Figure 6

Based on the results of the current experiment and the results reported in Matthis and Fajen (2013, 2014), it appears that the critical phase for the visual control of foot placement occurs during the latter half of the preceding step.