An Exploratory Approach to Manipulating Dynamic Stability: Investigating the Role of Visual Control during a Precision Foot Placement Task

ABSTRACT Background: The visual system provides the body with an accurate sensory system; designed to gather information at a distance and acts as a feedforward control mechanism during human locomotion. By doing so, visual information contributes coordination of the head-arm-trunk (HAT) segment and modulating foot placement. The purpose of this study was to examine the effects of a constrained pathway during a complex navigational stone-stepping task on HAT segment control and how the visual system guides locomotion during a complex foot placement task. Methods: Nine university-aged females (Mean age: 22.5 years old +/-1.75) participated in this study. Participants were instrumented with four rigid bodies (4x3 IRED markers) on the head, trunk and feet and two IRED markers on the wrists in order to measure kinematic data, collected by Optotrak system (NDI, Waterloo, Canada). Additionally, each participant was outfitted with an ASL H7-HS High Speed Head Mounted Optics (ASL, Bedford, USA) eye tracking unit to assess gaze behaviours. The experimental protocol required participants to perform 40 walking trials across four conditions (i.e., constrained and self-selected pathways; starting with either the left or the right foot), on a 7.2m x 1.2m raised-target platform. The platform consisted of 60 sloper-style rock climbing holds, whose location was designed to satisfy one of three criterion: 1) in line with natural footfall locations (e.g. normal step length and/or width dimensions of 60cm by 10cm); 2) greater or less than one of the dimensions of a natural step length or width; or 3) to act as a possible option/distractor on the pathway. The two constrained pathways were indicated with a high-contrasting moldable material placed inside each hold’s screw hole. Measurements were compared across conditions (i.e., constrained versus unconstrained), time points (e.g. first, middle, and last trial performed of each condition), and segment (Segment 1: first 3m of path or Segment 2: last 3m of path). The measurements included: horizontal and vertical pupil velocity RMS; average walking speed; trunk rotations about the hip (i.e., pitch and roll), and whole-body movement (i.e., ML COM variability). Results: Findings revealed that there was a significant difference between conditions such that: 1) the constrained vertical pupil RMS velocity was higher than the unconstrained (F(3,24)=4.71; p= .04; d=.46); 2) the unconstrained horizontal pupil RMS velocity was higher than the unconstrained (F(3,24)=4.40; p= .03; d=.36); 3) the constrained average walking speed was greater than the unconstrained (F(3,24)=23.27; p=0.04; d=.30); 4) the constrained trunk roll was greater than the unconstrained (F(3,21)=4.84; p=0.01; d=.45); and 5) the unconstrained dynamic stability margin minimum (DSMmin) was greater than the constrained (F(3,21)=4.89; p= .01; d=.41). Conclusions: The complex nature of the raised-target foot placement task challenged individuals from the start of each condition, forcing participants to learn how to control body movements—especially in the AP direction. During constrained condition, there was evidence to suggest that there was a greater regulation of trunk control than during unconstrained trials. This was attributed to the conditional demands of predetermined pathway to follow. However, during unconstrained trials, individuals were able to choose footholds, which were most likely based on their current state of stability. And thus, conditional demands of the pathway influenced gaze behaviours, such that during the constrained condition participants used a scanning behaviour (i.e., greater vertical pupil velocity RMS) whereas participants used more of a sampling behaviour (i.e., greater horizontal and slower vertical pupil velocities) during the free choice pathway condition. Therefore, the finding from this study suggest that gaze behaviours are influenced by stepping characteristics and these different gaze behaviours have different effects on trunk control