Simulation Framework to Develop Active Posterior Walker to Aid People with Pathologic Gait

In Summary, I will

• Quantify how gait changes while walking with a posterior walker.

• Develop a predictive simulation framework to characterize how assistive forces change the efficiency of gait.

• Design assistance methods that can be implemented through a powered posterior walker.

• Validate predictive simulations by applying a subset of the developed assistance methods to a physical powered posterior walker.

RESEARCH STATEMENT

Human gait has been studied extensively. How assistive devices, such as posterior walkers, affect gait is much less well defined. Many individuals with walking disorders, due to neuromuscular disabilities, such as cerebral palsy (CP), use assistive devices, such as walkers or crutches, to aid their mobility. These devices are prescribed and utilized because they increase the stability of the user, and can act as weight support for the upper body to offload the lower limbs. The stability gained by the user comes at the additional metabolic cost of manipulating the device for the duration of ambulation. This increases the workload of walking on the user. There is a critical gap in the understanding of the interface between the walker and the user. Individuals with CP or other disabilities can have compromised motor control, impaired balance, and weakness, all of which can make ambulating difficult – even while using a walker. Currently, a major obstacle to designing devices that optimally aid users is the lack of understanding of how the user interacts with the walker.

The goal of this work is to first understand the effect of using a walker on both the stability and efficiency of the gait of individuals with walking disorders, then devise and implement a control scheme that can add assistive forces to the user to improve the efficiency of gait, while maintaining the stability benefits of the device. Previous work has shown that application of a forward assist force equal to 8% body weight results in a 34.7% reduction in metabolic cost (Zirker 2013). In this work, we will explore if an assist force applied to the human through the implementation of motorized wheels on a posterior walker can lower the cost of walking with a walker. First, we will quantify the cost of walking with the walker, then model methods to decrease this cost. Finally, we will test possible assistance methods for children with CP by implementing these assists on a powered posterior walker. We expect the powered walker to reduce the metabolic cost of walking, improving gait efficiency across a variety of pathologic impairments, while continuing to allow the user to rely on it for stability and safety. This allows users to walk more before they tire, resulting in greater independence and improved quality of life for the user, as well as relieving some of the impact of their condition from their caretakers.

Hypothesis 1: Walking with a posterior walker increases the dynamic stability of the user, compared to walking unassisted, and this increased stability comes at the cost of decreased efficiency of walking.

Aim 1. Understand how gait changes while walking with a posterior walker.

Aim 1.1. Quantify kinematics and kinetics of walking with and without a passive posterior walker. 3D kinematic data, ground and handle reaction forces will be collected utilizing an instrumented posterior walker with six degree-of-freedom load cells in the handles. We will then develop a 21-segment, multibody model of the human pulling a posterior walker in OpenSim. Using inverse dynamics methods, we can use the recorded motion and external forces to calculate the joint angles and moments during gait to compare the two walking conditions.

Aim 1.2. Quantify changes in stability between walking with and without a passive posterior walker. Stability is the necessary condition to remain in motion and not fall over. We propose to measure how close you are to the critical point between walking and falling using angular momentum analysis as this measure can be related to both biological control processes and observed movement patterns. Beyond this, angular momentum analysis can also be decomposed to each of the anatomical planes and segments of the body, giving us deeper insights into the development of stability not possible using other dynamic stability metrics.

Aim 1.3. Quantify changes in efficiency between walking with and without a passive posterior walker. Efficiency of gait can currently be assessed via indirect calorimetry and modeled muscle energy expenditure methods. We propose to develop a method to assess the efficiency of gait from a mechanical perspective, as opposed to a physiological one, in order to investigate the efficiency of gait of users that cannot tolerate collection methods required for indirect calorimetry, or whose muscle physiology differs significantly from available muscle models.

Joint kinematics, joint kinetics, efficiency of movement, and whole-body stability through the gait cycle completely describe a human walking with a posterior walker.

Hypothesis 2: We can model how assist forces change the kinetics of gait to predict trends in how assistance changes efficiency of gait.

Aim 2. Develop predictive simulation framework to characterize how assistive forces change gait.

Aim 2.1. Develop foot-floor contact model. In order to simulate realistic ground reaction forces, we will develop and implement a contact model between each foot and the floor. This ground contact model will allow us to investigate how this foot-floor interaction changes with the addition of assistive forces.

Aim 2.2. Define motion tracking optimization method and cost function. The human body musculoskeletal model in this framework will generate motion though coordinate actuators controlling every degree of freedom in the model. We will predict the kinematics and kinetics inside of this framework using a direct collocation optimization method. This optimization will be formulated such that deviation from motion being tracked and combined total actuation are minimized over the whole time simulated.

Aim 2.3. Determine sensitivity of optimization scheme. To determine the robustness of this framework, we will perform a sensitivity analysis to determine how sensitive results from this method are to cost function weightings and contact model parameters.

Hypothesis 3: We can apply assistance to the user via the posterior walker to keep the stability increase of using the device, while also increasing the efficiency of walking with a posterior walker.

Aim 3. Develop assistance methods and apply subset of assistance methods to physical posterior walker to validate trends from predictive simulations.

Aim 3.1. Apply assistive forces to human musculoskeletal model. We will apply assistive forces of various magnitudes at various timing patterns with respect to the gait cycle to predict how this assistance changes the kinetics of gait. We will use these predicted kinetics to predict trends in efficiency under each assistance method.

Aim 3.2. Validate predictive framework through application of subset of developed assist methods to physical powered walker. Under each assistance condition, the performance metric to assess efficiency developed in Aim 1 will be executed to determine how each assistance method affects the efficiency of the user. To validate that this framework predicts trends in efficiency that reflect real life, a subset of these proposed assistance methods will be implemented on our physical active posterior walker. We will then collect motion capture and external loading under these assistance conditions and compare the results from inverse methods to our predictions.

Ultimately, this work will result in a validated predictive simulation framework to assess how possible assistance methods may alter the efficiency of a person’s gait. In the use case presented here, this framework is used to develop a posterior walker with motorized rear wheels driven by a control scheme optimized to increase efficiency of ambulation without losing stability benefits of the device.

It is important to keep individuals with walking disabilities ambulatory for as long as possible. The significance of the proposed effort is to provide a posterior walker that reduces the workload of walking, keeping this population walking longer, providing critical exercise and continued muscle development. This will help the individual stay active longer, and perhaps prevent the use of a wheelchair. This will greatly improve their quality of life, providing key physiological, mental, and social benefits.