Body weight support impacts lateral stability during treadmill walking

Abstract

Body weight support (BWS) systems are a common tool used in gait rehabilitation. BWS systems may alter the requirements for an individual to actively stabilize by 1) providing lateral restoring forces that reduce the requirements for the nervous system to actively stabilize and 2) decreasing the stabilizing gravitational moment in the frontal plane, which could increase the requirements to actively stabilize. The goal of the current study was to quantify the interaction between BWS and lateral stability. We hypothesized that when able-bodied people walk with BWS: 1) the lateral restoring forces provided by BWS would reduce the requirements to stabilize in the frontal plane when comparing dynamically similar gaits, and 2) increasing BWS would decrease the stabilizing gravitational moment in the frontal plane and increase the requirements to stabilize when speed is constrained. Our findings partly support these hypotheses, but indicate a complex interaction between BWS and lateral stability. With BWS, subjects significantly decreased step width variability and significantly increased step width (p<0.05) for both the dynamically similar and Speed-Matched conditions. The decrease in step width variability may be attributable to a combination of lateral restoring forces decreasing the mechanical requirements to stabilize and an enhanced sense of position that could have improved locomotor control. Increases in step width when walking with high levels of BWS could have been due to decreases in the gravitational moment about the stance limb, which may challenge the control of stability in multiple planes.

Keywords: Balance; Biomechanics; Body weight support; Foot placement; Gait; Margin of stability; Rehabilitation; Stability.

Figure 1

(A) Schematic of the lateral restoring force, F_lateral, provided by the BWS support system as the subject deviates laterally. We expect this force to stabilize the subject, and reduce the requirements to actively stabilize in the frontal plane. (B) Schematic of the frontal plane balance control when walking with BWS (adapted from MacKinnon and Winter, 1993). During normal walking, the whole body lateral inertial moment about the subtalar joint (Iα) is balanced by a gravitational moment about the subtalar joint (M_g), ankle joint center accelerations (a_z, a_y), and inversion/eversion muscle moments (M_sub). With BWS, the gravitational moment is reduced due to the vertical force of the BWS. We expect that to control whole body lateral inertial moments when M_g decreases will require active control, and increase the requirements to stabilize in the frontal plane.

Figure 2

Mean estimated lateral force provided by the BWS system for (A) Dynamically Similar and (B) Speed-Matched conditions. Conditions ‘20’, ‘40’, and ‘60’ represent the amount of BWS provided. Significance (p < 0.05) is denoted by * and indicates each level is significantly different than the other two.

Figure 3

Lateral center of mass trajectory and foot placements for a representative subject walking in three different BWS conditions: 0% BWS (A), 60% Dynamically Similar (B), and 60% Speed-Matched (C). DS is Dynamically Similar, SM is Speed Matched.

Figure 4

Mean + SD step width comparisons for (A) Dynamically Similar and (B) Speed-Matched conditions. (C) Mean + SD step width variability comparisons for Dynamically Similar and (D) Speed-Matched conditions. The units for all graphs are reported in leg length (L). Significance (p < 0.05) from 0% BWS is denoted by *.

Figure 5

Mean + SD step length comparisons for (A) Dynamically Similar and (B) Speed-Matched conditions. Mean + SD step length variability comparisons for (C) Dynamically Similar and (D) Speed-Matched conditions. The units for all graphs are reported in leg length (L). Significance (p < 0.05) from 0% BWS is denoted by *.

Figure 6

Mean + SD for Minimum lateral Margin of Stability for (A) Dynamically Similar and (B) Speed-Matched conditions. There were no significant comparisons for this measure.