Sean Barton

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

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.

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

Significance The physical dynamics of the body are central to the generation and maintenance of the human gait cycle. The ability to exploit the force of gravity and bodily inertia increases the energetic efficiency of locomotion by minimizing the need for internally generated muscular forces and simplifies control by obviating the need to actively guide each body segment. Here we explore how these principles generalize to situations in which foot placement is constrained, as when walking over a rocky trail. Walkers can exploit external forces to efficiently traverse extended stretches of complex terrain provided that visual information about the upcoming ground surface is available during a particular (critical) phase of the gait cycle between midstance of the preceding step and toe-off. 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.

Visual regulation of gait: Zeroing in on a solution to the complex terrain problem.

We examine the theoretical understanding of visual gait regulation that has emerged from decades of research since the publication of Lee, Lishman, and Thompson’s (1982) classic study of elite long jumpers. The first round of research identified specific informational variables, parameters of the action system, and laws of control that capture the coupling of perception and action in this context, but left unanswered important questions about why visual information is sampled in an intermittent manner and how the strategies that actors adopt ensure stability and energetic efficiency. More recent developments lead to a refined view according to which visual information is used at a specific phase of the gait cycle to modify the parameters that govern the passive dynamics of the body. We then present the results of a new experiment designed to test the prediction that when the terrain offers multiple foothold options for a given step, walkers’ choices will be constrained by a strong preference for not interfering with the natural, ballistic movement of the body throughout the single support phase of that step. The findings are consistent with this prediction and support a view of visual gait regulation that is concordant with contemporary accounts of how actors use both active and passive modes of control.

Visual control of precise foot placement when walking over complex terrain

1) Motivation When a person walks over complex terrain such as a rocky trail, obstacles and other impediments may render desirable foothold locations unavailable, so visual information from the upcoming terrain must be utilized in order to select safe footholds from among the available options. Our goal is to understand the way in which humans use visual information to guide foothold selection and foot placement when walking over complex terrain.

The pickup of visual information about size and location during approach to an obstacle

The present study investigated differences in the pickup of information about the size and location of an obstacle in the path of locomotion. The main hypothesis was that information about obstacle location is most useful when it is sampled at a specific time during the approach phase, whereas information about obstacle size can be sampled at any point during the last few steps. Subjects approached and stepped over obstacles in a virtual environment viewed through a head-mounted display. In Experiment 1, a horizontal line on the ground indicating obstacle location was visible throughout the trial while information about obstacle height and depth was available only while the subject was passing through a viewing window located at one of four locations along the subject’s path. Subjects exhibited more cautious behavior when the obstacle did not become visible until they were within one step length, but walking behavior was at most weakly affected in the other viewing window conditions. In Experiment 2, the horizontal line indicating obstacle location was removed, such that no information about the obstacle (size or location) was available outside of the viewing window. Subjects adopted a more cautious strategy compared to Experiment 1 and differences between the viewing window conditions and the full vision control condition were observed across several measures. The differences in walking behavior and performance across the two experiments support the hypothesis that walkers have greater flexibility in when they can sample information about obstacle size compared to location. Such flexibility may impact gaze and locomotor control strategies, especially in more complex environments with multiple objects and obstacles.

Towards understanding visually guided locomotion over complex and rough terrain: A phase-space planning method

As legged robots maneuver over increasingly complex and rough terrains, designing motion planners with the capability of predicting future footsteps becomes imperative. In turn, these planners provide a valuable tool for understanding the fundamental principles underlying human locomotion [2, 3]. In this study, we use our previously proposed phase-space planning framework [1] to analyze human walking over complex terrain. In particular, we highlight (i) the center of mass (CoM) apex-state-based feature of the phase-space planning, and (ii) the role of vision in CoM apex state selection during human walking over complex terrain [2].

Visual information for the control of walking over complex terrain.

Which regions of the ground surface do humans need to see to control walking over complex terrain? Previously, we offered an answer to this question rooted in the biomechanics of walking: To efficiently exploit their inverted-pendulum-like structure, walkers should use information about potential target footholds for an upcoming step during the last part of the preceding step. That is, the last part of each step is the critical phase for the visual control of the upcoming step. The aim of the present study was to determine the nature of the visual information used during this critical control phase. To efficiently exploit their biomechanical structure, walkers must initialize each step with a pushoff force from the trailing foot that is properly tailored to the position of the next target relative to the previous target. This leads to the hypothesis that walkers rely on information about the relative position of pairs of consecutive targets. To test this hypothesis, we instructed subjects to walk along a path of irregularly spaced target footholds (small circular patches of light projected onto the floor) while their movements were tracked by a motion capture system. On some trials, the visibility of a subset of targets was manipulated such that they were only visible for a brief period. The duration of the period of visibility varied such that consecutive targets were simultaneously visible in some conditions (leaving relative position information intact) but not others. We found no significant differences in stepping accuracy between conditions in which relative position information was available and a control condition in which all targets were always visible. However, stepping accuracy degraded in conditions in which relative position information was unavailable. We conclude that walkers rely on relative position information and that such information facilitates energetically efficient walking over complex terrain. Meeting abstract presented at VSS 2015.