Locomotion control during curb descent: Bilateral ground reaction variables covary consistently during the double support phase regardless of future foot placement constraints

Abstract

During community ambulation, anticipatory adaptations in gait are key for navigating built, populated and natural environments. It has been argued that some instability in gait can be functionally beneficial in situations demanding high maneuverability, and while the mechanisms utilized to maintain locomotor balance are well understood, relatively less is known about how the control of gait stability changes to facilitate upcoming maneuvers in challenging environments. The double support phase may be important in this regard; since both feet can push off the ground simultaneously, there is greater control authority over the body's movement during this phase. Our goal was to identify how this control authority is exploited to prepare for upcoming maneuvers in challenging environments. We used synergy indices to quantify the degree of coordination between the ground reaction forces and moments under the two feet for stabilizing the resultant force and moment on the body during the double support phase of curb descent. In contrast to our expectations, we observed that the kinetic synergy indices during curb descent were minimally influenced by expected foot targeting maneuvers for the subsequent step. Only the resultant moment in the frontal plane showed reduced stability when targeting was required, but the synergy index was still high, indicating that the resultant moment was stable. Furthermore, the synergy indices indicated that the main function of the ground reaction variables is to maintain stability of whole-body rotations during double support, and this prerogative was minimally influenced by the subsequent foot targeting tasks, likely because the cost of losing balance while descending a curb would be higher than the cost of mis-stepping on a visual target. Our work demonstrates the salience of stabilizing body rotations during curb negotiation and improves our understanding of locomotor control in challenging environments.

Fig 1

(A) Experimental Setup. Two force plates were embedded in the walkway, one in the elevated curb, and one in the ground. A projector was used to present visual stepping target on the walkway. (B) Baseline condition. No target was presented. (C) Fixed target condition. The target was presented at the preferred foot landing position and remained stationary. (D, E) Target shift conditions. A target shift in the anterior (D) or lateral (E) direction was triggered with 50% probability when the lead foot contacted the ground force plate (LFC_Ground). Kinetic synergies were quantified at the double support phase while stepping down using the trials where the target did not shift, as illustrated by the footprints in the initial targets in (D) and (E).

Fig 2. Mean and standard error for foot placement RMSE for each task.

RMSE is computed separately for the no-shift and shift trials for the Anterior-shift and Lateral-shift tasks. Means with different letters are significantly different from one another.

Fig 3

(A) Mean and standard error for gait speed during double support phase, and (B) double support duration for the four tasks. Means with different letters are significantly different from one another.

Fig 4

(A) Mean and SE of foot placement locations relative to the curb edge (horizontal line) and the midpoint of the walkway for the four tasks. Error bars represent across-participant standard error. The dashed footprints are a zoomed-in view of the two foot placement locations with a common scale for provided on the right. (B, C) Mean and standard error for step length and step width. Means with different letters are significantly different from one another.

Fig 5

The mean and SE for synergy index (ΔVz) for stabilizing resultant force along the (A) AP, (B) ML and (C) vertical axis during the double support phase for the four tasks. The horizontal lines in each plot show the discriminating values for each synergy index.

Fig 6

Mean and standard error for the synergy index (ΔVz) for stabilizing resultant moment about the (A) AP, (B) ML, and (C) vertical axes during the double support phase. The horizontal lines in each plot show the discriminating values for each synergy index. ΔVz greater than the corresponding discriminating value indicates presence of synergy. Means with different letters are significantly different from one another.