- The paper demonstrates that exoskeletons can both increase and decrease stability, with up to 17.5% gains in low-velocity conditions and 19.56% reductions in high-velocity conditions.
- The study employs physics-based models using Hamilton-Jacobi Bellman reachability to define stabilizable regions and provide formal stability guarantees.
- Implications stress the need for personalized tuning of exoskeleton assistance to address age-related deficits in maximum torque and rate of torque development.
Overview of "Ankle Exoskeletons May Hinder Standing Balance in Simple Models of Older and Younger Adults"
This paper by Raz et al. investigates the effects of ankle exoskeletons on the standing balance of older and younger adults through physics-based modeling. The study focuses on mitigating age-related deficits, particularly reductions in maximum torque (MT) and the maximum rate of torque development (MRTD), which contribute to decreased stability in older adults. While ankle exoskeletons are generally perceived as devices that could aid in maintaining balance by compensating for reduced muscle force, their potential impact, especially in unstable conditions, hasn't been thoroughly investigated prior to this study.
Key Findings and Numerical Results
The models constructed in this study demonstrate that the use of ankle exoskeletons results in a complex trade-off:
- For individuals with full ankle strength, exoskeletons moderately reduced the feasible stability boundaries.
- For individuals with age-related MT and MRTD deficits:
- Exoskeletons augmented stability in low-velocity conditions.
- Exoskeletons reduced stability in certain high-velocity conditions.
The authors explored these effects using controlled invariant and backward reachable sets, providing strong formal guarantees about the human-exoskeleton dynamic system's stability. Notably, the addition of exoskeleton torque did not uniformly increase or decrease the stabilizable region (SR) areas:
- In young adult models, the exoskeleton showed slight to moderate reductions in SR area.
- In older adult models, especially a weaker older female model, the exoskeleton assistance increased the SR area in low-velocity conditions by up to 17.5% but reduced it in high-velocity conditions by as much as 19.56%.
Moreover, the paper provides a detailed analysis showing the independent contributions of reduced MT and MRTD to the overall stability. When the MRTD and MT were independently reduced, the reduction in SR area was more significant with exoskeleton assistance, particularly for reduced MRTD, suggesting that the exoskeleton may exacerbate MRTD deficits.
Theoretical and Practical Implications
The theoretical implications of this study lie in the formulation and characterization of stabilizable regions under biological constraints and exoskeleton assistance. The work provides:
- A framework for computing SR through Hamilton Jacobi Bellman reachability, which includes invariance and stability guarantees.
- Insights into how exoskeletons alter the domain of invariant stable motions available to users, factoring in joint-level changes due to aging.
From a practical perspective, these findings suggest caution when deploying ankle exoskeletons for older adults. The assistance provided by exoskeletons may need to be finely tuned to individual needs to avoid inadvertently decreasing stability in more dynamic or perturbed conditions. This underpins the necessity for experimental validations of well-established control strategies in older adults before widespread clinical deployment.
Future Developments
Future developments in this domain may include:
- More sophisticated exoskeleton control strategies that adapt dynamically to the user's state and environmental perturbations.
- Integrating detailed muscle actuation models and considering neural delays for a more realistic interaction between the human user and the exoskeleton.
- Experimental validation of the model-based predictions to assess real-world interactions and outcomes.
Additionally, extending this work to include various perturbation scenarios and testing with populations suffering from diverse mobility impairments could provide deeper insights into the optimal design and utility of such assistive devices.
Overall, this paper adds a nuanced understanding of the complexities involved in using ankle exoskeletons to assist with standing balance, highlighting both the potential benefits and risks depending on the conditions and user characteristics.