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

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 during aging, diminishing stability. Ankle exoskeletons are powered orthotic devices that may assist older adults by compensating for reduced muscle force and power capabilities. They may also be able to assist with ankle strategies used for balance. However, no studies have investigated their effect on balance in older adults. Here, we model the effect these devices have on stability in physics-based models of healthy young and old adults, focusing on 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 portions of the phase plane, they reduce stability in others. Our results suggest that well-established control strategies must still be experimentally validated in older adults.