Team IHMC at the 2020 Cybathlon: a user-centered approach towards personal mobility exoskeletons.

BACKGROUND: The past few decades have seen rapid advancements in exoskeleton technology, with a considerable shift towards applications involving users with gait pathologies. Commercial devices from ReWalk, Ekso Bionics, and Indego, mainly designed for rehabilitation purposes, have inspired the development of many research platforms aimed at extending capabilities for use as safe and effective personal mobility devices. The 2016 Cybathlon featured an impressive demonstration of exoskeletons designed to enable mobility for individuals with spinal cord injury, however, not a single team completed every task and only two completed the stairs. Major improvements were showcased at the 2020 Cybathlon, with seven of the nine teams completing a similar set of tasks. Team IHMC built upon its silver-medal success from 2016 with an upgraded device, Quix. METHODS: Quix features several notable improvements including an additional powered degree of freedom for hip ab/adduction to laterally shift the device and reduce user effort while walking, custom-tailored cuffs and soft goods based on 3D body scans to optimize user comfort, and a streamlined testing pipeline for online tuning of gait parameters. RESULTS: Team IHMC finished in fourth place behind the teams from EPFL and Angel Robotics. Although we suffered from a considerably slower flat-ground walking speed, our pilot reported marked improvements in overall effort, comfort, and ease-of-use compared to our previous device. CONCLUSIONS: Clear progress in exoskeleton development has been exhibited since the inaugural Cybathlon, with tasks involving rough terrain, stairs, and ramps now posing little threat to most of the competitors. As a result, the layout of the powered exoskeleton course will likely undergo significant modifications to further push the devices towards suitability for personal everyday use. The current tasks do not address the issue of donning and doffing, nor do they simulate a scenario similar to maneuvering a kitchen to prepare a meal, for example. An additional limitation that may be more difficult to test in a competition setting is the required upper-body effort to manipulate the device in an effective manner.

Switched Tracking Control of the Lower Limb During Asynchronous Neuromuscular Electrical Stimulation: Theory and Experiments.

Neuromuscular electrical stimulation (NMES) induces muscle contractions via electrical stimuli. NMES can be used for rehabilitation and to enable functional movements; however, a fundamental limitation is the early onset of fatigue. Asynchronous stimulation is a method that can reduce fatigue by utilizing multiple stimulation channels to segregate and switch between different sets of recruited motor units. However, switching between stimulation channels is challenging due to each channel's differing response to stimulation. To address this challenge, a switched systems analysis is used in the present work to design a controller that allows for instantaneous switching between stimulation channels. The developed controller yields semi-global exponential tracking of a desired angular trajectory for a person's knee-joint. Experiments were conducted in six able-bodied individuals. Compared to conventional stimulation, the results indicate that asynchronous stimulation with the developed controller yields longer durations of successful tracking despite different responses between the stimulation channels.

Switched Control of Cadence During Stationary Cycling Induced by Functional Electrical Stimulation.

Functional electrical stimulation (FES) can be used to activate the dysfunctional lower limb muscles of individuals with neurological disorders to produce cycling as a means of rehabilitation. However, previous literature suggests that poor muscle control and nonphysiological muscle fiber recruitment during FES-cycling causes lower efficiency and power output at the cycle crank than able-bodied cycling, thus motivating the investigation of improved control methods for FES-cycling. In this paper, a stimulation pattern is designed based on the kinematic effectiveness of the rider's hip and knee joints to produce a forward torque about the cycle crank. A robust controller is designed for the uncertain, nonlinear cycle-rider system with autonomous, state-dependent switching. Provided sufficient conditions are satisfied, the switched controller yields ultimately bounded tracking of a desired cadence. Experimental results on four able-bodied subjects demonstrate cadence tracking errors of 0.05 ±1.59 and 5.27 ±2.14 revolutions per minute during volitional and FES-induced cycling, respectively. To establish feasibility of FES-assisted cycling in subjects with Parkinson's disease, experimental results with one subject demonstrate tracking errors of 0.43 ± 4.06 and 0.17 ±3.11 revolutions per minute during volitional and FES-induced cycling, respectively.

Identification-Based Closed-Loop NMES Limb Tracking With Amplitude-Modulated Control Input.

An upper motor neuron lesion (UMNL) can be caused by various neurological disorders or trauma and leads to disabilities. Neuromuscular electrical stimulation (NMES) is a technique that is widely used for rehabilitation and restoration of motor function for people suffering from UMNL. Typically, stability analysis for closed-loop NMES ignores the modulated implementation of NMES. However, electrical stimulation must be applied to muscle as a modulated series of pulses. In this paper, a muscle activation model with an amplitude modulated control input is developed to capture the discontinuous nature of muscle activation, and an identification-based closed-loop NMES controller is designed and analyzed for the uncertain amplitude modulated muscle activation model. Semi-global uniformly ultimately bounded tracking is guaranteed. The stability of the closed-loop system is analyzed with Lyapunov-based methods, and a pulse frequency related gain condition is obtained. Experiments are performed with five able-bodied subjects to demonstrate the interplay between the control gains and the pulse frequency, and results are provided which indicate that control gains should be increased to maintain stability if the stimulation pulse frequency is decreased to mitigate muscle fatigue. For the first time, this paper brings together an analysis of the controller and modulation scheme.

Closed-Loop Asynchronous Neuromuscular Electrical Stimulation Prolongs Functional Movements in the Lower Body.

Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitative settings and is also used for assistive purposes to create functional movements, where it is termed functional electrical stimulation (FES). One limitation of NMES/FES is early onset of muscle fatigue. NMES-induced fatigue can be reduced by switching between multiple stimulation channels that target different motor units or synergistic muscles (i.e., asynchronous stimulation). However, switching stimulation channels introduces additional complexity due to the need to consider the switching dynamics and differing muscle response to stimulation. The objective of this study was to develop and test a closed-loop controller for asynchronous stimulation. The developed closed-loop controller yields asymptotic tracking of a desired trajectory for a person's knee-shank complex despite switching between stimulation channels. The developed controller was implemented on four able-bodied individuals with four-channel asynchronous stimulation as well as single-channel conventional stimulation. The results indicate that asynchronous stimulation extends the duration that functional movements can be performed during feedback control. This result is promising for the implementation of asynchronous stimulation in closed-loop rehabilitative procedures and in assistive devices as a method to reduce muscle fatigue while maintaining a person's ability to track a desired limb trajectory.

Comparing the Induced Muscle Fatigue Between Asynchronous and Synchronous Electrical Stimulation in Able-Bodied and Spinal Cord Injured Populations.

Neuromuscular electrical stimulation (NMES) has been shown to impart a number of health benefits and can be used to produce functional outcomes. However, one limitation of NMES is the onset of NMES-induced fatigue. Multi-channel asynchronous stimulation has been shown to reduce NMES-induced fatigue compared to conventional single-channel stimulation. However, in previous studies in man, the effect of stimulation frequency on the NMES-induced fatigue has not been examined for asynchronous stimulation. Low stimulation frequencies are known to reduce fatigue during conventional stimulation. Therefore, the aim of this study was to examine the fatigue characteristics of high- and low-frequency asynchronous stimulation as well as high- and low-frequency conventional stimulation. Experiments were performed in both able-bodied and spinal cord injured populations. Low frequency asynchronous stimulation is found to have significant fatigue benefits over high frequency asynchronous stimulation as well as high- and low-frequency conventional stimulation, motivating its use for rehabilitation and functional electrical stimulation (FES).

A novel modulation strategy to increase stimulation duration in neuromuscular electrical stimulation.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) has been shown to be an effective treatment for muscular dysfunction. Yet, a fundamental barrier to NMES treatments is the rapid onset of muscle fatigue. The purpose of this study is to examine the effect of feedback-based frequency modulation on the closed-loop performance of the quadriceps during repeated dynamic contractions. METHODS: In the first experiment, subjects completed four different frequency modulation NMES protocols utilizing the same amplitude modulation control to compare the successful run times (SRTs). A second experiment was performed to determine the change in muscle response to high- and low-frequency stimulation. RESULTS: Compared with constant-frequency stimulation, results indicate that using an error-driven strategy to vary the stimulation frequency during amplitude modulation increases the number of successful contractions during non-isometric conditions. CONCLUSION: Simultaneous frequency and amplitude modulation increases the SRT during closed-loop NMES control.