Where to Step? Contributions of Stance Leg Muscle Spindle Afference to Planning of Mediolateral Foot Placement for Balance Control in Young and Old Adults

Abstract

Stable gait requires active control of the mediolateral (ML) kinematics of the body center of mass (CoM) and the base of support (BoS) in relation to each other. Stance leg hip abductor (HA) muscle spindle afference may be used to guide contralateral swing foot placement and adequately position the BoS in relation to the CoM. We studied the role of HA spindle afference in control of ML gait stability in young and older adults by means of muscle vibration. Healthy young (n = 12) and older (age > 65 years, n = 18) adults walked on a treadmill at their preferred speed. In unperturbed trials, individual linear models using each subject's body CoM position and velocity at mid-swing as inputs accurately predicted foot placement at the end of the swing phase in the young [mean R² = 0.73 (SD 0.11)], but less so in the older adults [mean R² = 0.60 (SD 0.14)]. In vibration trials, HA afference was perturbed either left or right by vibration (90 Hz) in a random selection of 40% of the stance phases. After vibrated stance phases, but not after unvibrated stance phases in the same trials, the foot was placed significantly more inward than predicted by individual models for unperturbed gait. The effect of vibration was stronger in young adults, suggesting that older adults rely less on HA spindle afference. These results show that HA spindle afference in the stance phase of gait contributes to the control of subsequent ML foot placement in relation to the kinematics of the CoM, to stabilize gait in the ML direction and that this pocess is impaired in older adults.

Keywords: aging; balance; gait; muscle spindles; proprioception; stability.

FIGURE 1

Schematic illustration of the expected illusory effect of stance leg HA muscle vibration, with the solid figure illustrating the actual state in mid swing and the dashed figure illustrating the perceived HA lengthening and associated hip adduction corresponding with an ipsilateral shift of the body CoM (circle), in response to which the contralateral foot will be placed more inward.

FIGURE 2

(A) Results of a single subject showing the time series of the mediolateral position of the COM during gait in part of an unperturbed reference trial with the actual foot-placement positions and their duration indicated as colored horizontal lines and model predictions of foot-placement based on the COM position and velocity at preceding mid-swing as circles (o). The arrows represent the prediction and connect the COM state at mid-swing (^∗, the predictor variable) to the predicted foot-placement at the instant of foot contact. Data for the left foot are presented in blue, data for the right foot in red. (B) Quality of the model prediction illustrated by scatter plots of actual and predicted foot-placements of the complete reference trial. The blue circles in the top graph represent left foot placement and red circles in the bottom graph represent right foot placement. The diagonal lines are identity lines.

FIGURE 3

The adjustment of left and right foot placements in non-vibrated and vibrated steps in one young and one older subject over the vibration trial. Only steps taken into consideration in the analysis are represented as symbols. Zero adjustment refers to the foot placement predicted by the model based on the unperturbed reference trial.

FIGURE 4

Boxplot of adjustments in foot placement relative to the predictive model derived from the reference trials, when applied to non-vibrated and vibrated steps in the vibration trials. In vibrated steps, adjustments relative to predicted foot placement were significantly more toward inward in the young group, in line with the expected illusory inward movement of the CoM associated with lengthening of the HA muscles. Zero adjustment refers to the foot placement predicted by the model based on the unperturbed reference trial.