Ankle Exoskeletons May Hinder Standing Balance in Simple Models of Older and Younger Adults

TL;DR

It is shown that an ankle exoskeleton moderately reduces feasible stability boundaries in users who have full ankle strength, suggesting that well-established control strategies must still be experimentally validated in older adults.

Abstract

Humans rely on ankle torque to maintain standing balance, particularly in the presence of small to moderate perturbations. Reductions in maximum torque (MT) production and maximum rate of torque development (MRTD) occur at the ankle with age, diminishing stability. Ankle exoskeletons are powered orthotic devices that may assist older adults by compensating for reduced muscle force and power production capabilities. They may also be able to assist with ankle strategies used for balance. However, no studies have investigated the effect of such devices on balance in older adults. Here, we model the effect ankle exoskeletons have on stability in physics-based models of healthy young and old adults, focusing on the mitigation of age-related deficits such as reduced MT and MRTD. We show that an ankle exoskeleton moderately reduces feasible stability boundaries in users who have full ankle strength. For individuals with age-related deficits, there is a trade-off. While exoskeletons augment stability in low velocity conditions, they reduce stability in some high velocity conditions. Our results suggest that well-established control strategies must still be experimentally validated in older adults.

Figures (7)

• Figure 1: Illustration of a target set (green) and the set of all states from which it can be reached. This set is called the backward reachable set (BRS, in purple). The state $\hat{x}_0$ is in the BRS, because there exists at least one controller, $u_1$, that drives the system to the target set. No such controller exists for $\Tilde{x}_0$ so it is not in the BRS. If the target set is controlled invariant then the union of the target set and the BRS is also invariant. Here, the BRS was computed with model parameters corresponding to a young male, using our method described in Section \ref{['sec:methods']}. The gray region is computed using the extrapolated center of mass (XCoM) method from HOF20051, which we have converted here to angular coordinates.
• Figure 2: Free body diagram of the combined human-exoskeleton system.
• Figure 3: Continua of equilibrium points in the zero-velocity plane for the older female model with and without an exoskeleton. The shaded green region indicates the bounds of the foot along the horizontal axis, and the human torque bounds on the vertical axis. To generate sets representing quiet standing, we construct ellipsoids centered at the ankle and at the point closest to the toe corresponding to maximum feasible static support. These points are marked with blue circles.
• Figure 4: Panel (4a) shows the stabilizable region computed for the young male model (top) and weak older female (bottom). The baseline regions without the exoskeleton are shown in orange, while the region with exoskeleton assistance is shown in purple. Stabilizable regions when MT and MRTD are reduced independently are shown in the panels (4b) and (4c). In the top row, stabilizable regions are computed without an exoskeleton. The largest sets are baseline, followed by 30% reduction and 50% reduction in each respective quantity. In the bottom row, the stabilizable regions with exoskeleton assistance are shown in purple, while the reference baseline (full strength, without exoskeleton) is shown in light orange.
• Figure 5: Trends in stabilizable region area when maximum torque (MT, circular markers) and maximum rate of torque development (MRTD, diamond markers) are independently reduced, with (filled/purple) and without the exoskeleton (open/orange). 5a) At forward velocities, the addition of the exoskeleton amplifies MRTD deficits, while it mitigates MT deficits, at least with respect to the % SR area measure. 5b) At backward velocities, the exoskeleton tightens the foot-ground interaction constraints. This results in a large reduction in % SR area, compared to the more subtle changes related to reduced MT or MRTD.

Theorems & Definitions (5)

• Definition 1
• Definition 2
• Definition 3
• Definition 4
• Definition 5