Visual control of foot placement when walking over complex terrain.

Abstract

The aim of this study was to investigate the role of visual information in the control of walking over complex terrain with irregularly spaced obstacles. We developed an experimental paradigm to measure how far along the future path people need to see in order to maintain forward progress and avoid stepping on obstacles. Participants walked over an array of randomly distributed virtual obstacles that were projected onto the floor by an LCD projector while their movements were tracked by a full-body motion capture system. Walking behavior in a full-vision control condition was compared with behavior in a number of other visibility conditions in which obstacles did not appear until they fell within a window of visibility centered on the moving observer. Collisions with obstacles were more frequent and, for some participants, walking speed was slower when the visibility window constrained vision to less than two step lengths ahead. When window sizes were greater than two step lengths, the frequency of collisions and walking speed were weakly affected or unaffected. We conclude that visual information from at least two step lengths ahead is needed to guide foot placement when walking over complex terrain. When placed in the context of recent research on the biomechanics of walking, the findings suggest that two step lengths of visual information may be needed because it allows walkers to exploit the passive mechanical forces inherent to bipedal locomotion, thereby avoiding obstacles while maximizing energetic efficiency.

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

“…1D). ‡ Indeed, restricting a walker's ability to see upcoming terrain to less than two step lengths results in decreases in stepping accuracy and walking speed (43). Furthermore, the trajectory of the COM is less like that of a passively moving inverted pendulum compared with when walkers can see two or more step lengths ahead, suggesting that walkers need at least two step lengths of visual lookahead to exploit their inverted pendulum dynamics when walking over complex terrain (44).…”

Humans exploit the biomechanics of bipedal gait during visually guided walking over complex terrain

“…In a previous study with a similar experimental design, we found that some subjects accommodated smaller visibility windows by decreasing their walking speed and maintaining a low collision rate with the obstacles while others maintained a normal walking speed at the cost of increased collisions [43]. To encourage more homogeneous behaviour in this study, subjects were only allowed to hit one obstacle within each trial.…”

“…Virtual obstacles were displayed by a Sanyo PLC-XP45 projector at a resolution of 1024 Â 768 and a brightness of 3500 ANSI lumens. Details of the experimental set-up and the procedure to calibrate and synchronize the projector with the motion capture system can be found in [43].…”

“…The position of each subject's COM was found by calculating the weighted average of the position of each body segment, based on anthropometric data reported by Winter [45]. Walking speed and collisions per trial were calculated in the method described by Matthis & Fajen [43].…”

“…Subjects performed this task in the full vision condition and with nine different sizes of visibility window, which provided visual information about upcoming obstacles from 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 step lengths ahead. One step length was defined as 0.7 of the subject's leg length, which was found to be subjects' preferred step length during unconstrained steady-state walking [9,43]. If humans use visual information about the upcoming path to choose footholds An LCD projector displayed obstacles on the ground.…”