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When walking over stable, complex terrain, visual information about an upcoming foothold is primarily utilized during the preceding step to organize a nearly ballistic forward movement of the body. However, it is often necessary to respond to changes in the position of an intended foothold that occur around step initiation. Although humans are capable of rapidly adjusting foot trajectory mid-swing in response to a perturbation of target position, such movements may disrupt the efficiency and stability of the gait cycle. In the present study, we consider whether walkers sometimes adopt alternative strategies for responding to perturbations that interfere less with ongoing forward locomotion. Subjects walked along a path of irregularly spaced stepping targets projected onto the ground, while their movements were recorded by a full-body motion-capture system. On a subset of trials, the location of one target was perturbed in either a medial–lateral or anterior–posterior direction. We found that subjects were best able to respond to perturbations that occurred during the latter half of the preceding step and that responses to perturbations that occurred during a step were less successful than previously reported in studies using a single-step paradigm. We also found that, when possible, subjects adjusted the ballistic movement of their center of mass in response to perturbations. We conclude that, during continuous walking, strategies for responding to perturbations that rely on reach-like movements of the foot may be less effective than previously assumed. For perturbations that are detected around step initiation, walkers prefer to adapt by tailoring the global, pendular mechanics of the body. [ABSTRACT FROM AUTHOR]

Adjustments to locomotion to avoid an obstacle require a change to the usual pattern of foot placement, i.e. changes to step length and/or step width. Previous studies have demonstrated a difficulty in individuals with Developmental Coordination Disorder (DCD) in controlling stability while both stepping over and while circumventing an obstacle. In a previous study, we have considered the way in which individuals with DCD prepare for the possibility of an obstacle appearing (Wilmut and Barnett in Exp Brain Res 235:1531-1340, 2017). Using a parallel data set from this same task on the same individuals, the aim of the current study was to investigate the exact nature of changes in foot placement during obstacle avoidance, as this was not clear from previous work. Children and adults aged from 7 to 34 years of age took part in the study. Forty-four met the criteria for a diagnosis of DCD and there were 44 typically developing (TD) age and gender-matched controls. Participants walked at a comfortable pace down an 11 m walkway; on 6 out of 36 trials a 'gate' closed across their pathway which required circumvention. These 6 'gate close' trials were analysed for this study. The number and magnitude of step length and step width adjustments were similar across the DCD and TD groups, however, the younger children (7-11 years) made a greater number of early adjustments compared to the older children and adults (12-34 years of age). In contrast the adults made a greater number of adjustments later in the movement compared to the children. In terms of foot placement adjustments a clear preference was seen across all participants to use adjustments which resulted in reducing step length, stepping away from the obstacle and a combination of these. Apart from subtle differences, the individuals with DCD make step placements to circumvent an obstacle in line with their peers. It is suggested that the choice of foot placement strategy in individuals with DCD, although in line with their peers, may not be optimal for their level of motor ability. [ABSTRACT FROM AUTHOR]

During locomotion in a cluttered terrain, certain terrain surfaces such as an icy one are not appropriate for foot placement; an alternate choice is required. In a previous study we showed that the selection of foot placement is not random but systematic; the dominant choices made are not uniquely defined by the available or predicted sensory inputs. We argued that selection is guided by specific rules and involves minimal displacement of the foot from its normal landing spot. The experimental protocol involved implicit spatial constraint by requiring individuals to step on the force plate that could trigger a lighted area to be avoided, thereby requiring individuals to respond within one step-cycle. Alternate foot placement was visually identified, but not measured. The purpose of this study was to directly measure foot placement, validate and/or refine the rules used to guide selection, and identify whether the alternate foot placement choices are influenced by spatial and temporal constraints on response selection. The area to be avoided was visible from the start and therefore individuals could plan and implement appropriate avoidance strategies without any temporal constraint. Spatial constraint introduced in this experiment included requirement both to step on a specific location and to avoid stepping on a specific location on the next step. The results provide support for the rules previously identified in guiding foot placement to an alternate location. Minimal displacement of the foot from its normal landing spot was validated as an important factor for selecting alternate foot placement. When several choices satisfied this factor, additional factors guide alternate foot placement. Modifications in the plane of progression are preferred while stepping wide is avoided. When no temporal constraints are imposed on the response selection, enhancing forward progression of the body becomes the dominant determinant followed by stability and lastly by energy costs associated with the modifications. A decision algorithm for selecting foot placement is proposed based on these findings. It is clear that while visual input plays a critical role in guiding foot placement, it is not entirely based on reactive control. This has implications for implementing visually guided adaptive locomotion in legged robots. [ABSTRACT FROM AUTHOR]

Our goal was to understand the bases for selection of alternate foot placement during locomotion when the normal landing area is undesirable. In this study, a light spot of different shapes and sizes simulated an undesirable landing area. Participants were required to avoid stepping on this spot under different time constraints. Alternate chosen foot placements were categorised into one of eight choices. Results showed that selection of alternate foot placement is systematic. There is a single dominant choice for each combination of light spot and normal landing spot. The dominant choice minimises the displacement of the foot from its normal landing spot (less than half a foot length). If several response choices satisfy this criterion, three selection strategies are used to guide foot placement: placing the foot in the plane of progression, choosing to take a longer step over a shorter step and selecting a medial rather than lateral foot placement. All these alternate foot-placement choices require minimal changes to the ongoing locomotor muscle activity, pose minimal threat to dynamic stability, allow for quick initiation of change in ongoing movement and ensure that the locomotor task runs without interruption. Thus, alternate foot-placement choices are dependent not only on visual input about the location, size and shape of the undesirable surface, but also on the relationship between the characteristics of the undesirable surface and the normal landing area. [ABSTRACT FROM AUTHOR]