Anticipatory Postural Control of Stability during Gait Initiation Over Obstacles of Different Height and Distance Made Under Reaction-Time and Self-Initiated Instructions

Abstract

Despite the abundant literature on obstacle crossing in humans, the question of how the central nervous system (CNS) controls postural stability during gait initiation with the goal to clear an obstacle remains unclear. Stabilizing features of gait initiation include anticipatory postural adjustments (APAs) and lateral swing foot placement. To answer the above question, 14 participants initiated gait as fast as possible in three conditions of obstacle height, three conditions of obstacle distance and one obstacle-free (control) condition. Each of these conditions was performed with two levels of temporal pressure: reaction-time (high-pressure) and self-initiated (low-pressure) movements. A mechanical model of the body falling laterally under the influence of gravity and submitted to an elastic restoring force is proposed to assess the effect of initial (foot-off) center-of-mass position and velocity (or "initial center-of-mass set") on the stability at foot-contact. Results showed that the anticipatory peak of mediolateral (ML) center-of-pressure shift, the initial ML center-of-mass velocity and the duration of the swing phase, of gait initiation increased with obstacle height, but not with obstacle distance. These results suggest that ML APAs are scaled with swing duration in order to maintain an equivalent stability across experimental conditions. This statement is strengthened by the results obtained with the mechanical model, which showed how stability would be degraded if there was no adaptation of the initial center-of-mass set to swing duration. The anteroposterior (AP) component of APAs varied also according to obstacle height and distance, but in an opposite way to the ML component. Indeed, results showed that the anticipatory peak of backward center-of-pressure shift and the initial forward center-of-mass set decreased with obstacle height, probably in order to limit the risk to trip over the obstacle, while the forward center-of-mass velocity at foot-off increased with obstacle distance, allowing a further step to be taken. These effects of obstacle height and distance were globally similar under low and high-temporal pressure. Collectively, these findings imply that the CNS is able to predict the potential instability elicited by the obstacle clearance and that it scales the spatiotemporal parameters of APAs accordingly.

Keywords: anticipatory postural adjustments; gait initiation; human; mechanical modeling; motor coordination; obstacle clearance; stability; temporal pressure.

Figure 1

Schematic illustration of the experimental set-up. Key: (1) walkway; (2) force-plate; (3) obstacle; (4) reflective marker; (5) Vicon camera; (6) visual target; (7) obstacle distance marker; and (8) obstacle height marker.

Figure 2

Mechanical model. The mechanical model is represented as a conic inverted pendulum which pivots about a fixed point 0. Body displacement during the swing phase (from toe off to foot contact) presents five degrees of freedom on the absolute referential (0; x; y; z). (0; x₁; y₁; z₁) are the main axes of the inertia momentum of the solid body after precession ψ around z and nutation θ around x₁. The center of mass m falls under the influence of the gravity force P and the elastic restoring force T. The initial position and velocity of the cone correspond to the position and velocity of the subject’s center of mass at toe off.

Figure 3

Example of biomechanical traces and representation of the main experimental variables obtained for one representative subject initiating gait (one trial) in the reaction-time condition with the high height/long distance condition (left) and the small height/small distance condition (right). Anteroposterior (AP) direction x′M: center of mass (COM) velocity; x′M_TO, x′M_FC: COM velocity at foot off and at foot contact. xP: center of pressure (COP) displacement; xPmax: peak of COP displacement during APAs; F: forward; B: backward. Mediolateral (ML) direction y′M: ML COM velocity; y′M_TO, y′M_FC: COM velocity at foot off and foot contact; yM: ML COM displacement; yM_FC: COM displacement at foot contact; yP: ML COP displacement; yPmax: peak of COP displacement during APAs; and ST: stance limb; SW: swing limb. Vertical dashed lines SO: Go signal onset (in the reaction-time condition only); t₀ onset variation of biomechanical traces; HO: swing heel off; FO: swing foot off; FC: swing foot contact. Horizontal arrows: RT: time-windows for reaction-time; APA: anticipatory postural adjustments FL: foot lift; SWING: swing phase.

Figure 4

Effects of obstacle height and distance on stability parameters. Reported are mean values (all participants together) ± 1 SD. MOS, margin of stability. *Indicates a significant difference between bars.

Figure 5

Effects of obstacle height on selected ML and AP postural parameters. Reported are mean values (all participants together) ± 1 SD. APAs, anticipatory postural adjustments; TO, toe off; COP, center of pressure; COM, center of mass. *Indicates a significant difference between bars.

Figure 6

Validation of the mechanical model. Typical experimental (full line) and theoretical (dash line) time-course traces of the ML center of mass shift (A) and velocity (B) are superimposed during the swing phase (from swing foot off to foot contact). Traces are obtained from one representative subject in the reaction-time condition with the medium height and short distance obstacle. (C) Example of linear regression between experimental vs. theoretical data obtained in the obstacle free (control) condition. Each point represents the average value of the ML center of mass position (Y_M) and velocity (Y′_M) at foot contact in the control conditions (self-initiated and reaction-time conditions pooled together) for each of the 14 subjects. Note the excellent fit between the experimental and theoretical data.

Figure 7

Effects of obstacle height on the experimental (MOS_exp) and theoretical (MOS_th) “margin of stability” (MOS) values computed in the conditions with no APA scaling. The height of the obstacle is indicated in the abscissa (small, medium and high). In the control condition, there was no obstacle. Reported are mean values (all participants together) ± 1 SD. Note that the experimental MOS values remained unchanged, while theoretical MOS values decreased with obstacle height.