Ryan J. Downey

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. Muscle Nerve 44: 382–387, 2011

Stationary cycling induced by switched functional electrical stimulation control

Functional electrical stimulation (FES) is used to activate the dysfunctional lower limb muscles of individuals with neuromuscular disorders to produce cycling as a means of exercise and rehabilitation. In this paper, a stimulation pattern for quadriceps femoris-only FES-cycling is derived based on the effectiveness of knee joint torque in producing forward pedaling. In addition, a switched sliding-mode controller is designed for the uncertain, nonlinear cycle-rider system with autonomous state-dependent switching. The switched controller yields ultimately bounded tracking of a desired trajectory in the presence of an unknown, time-varying, bounded disturbance, provided a reverse dwell-time condition is satisfied by appropriate choice of the control gains and a sufficient desired cadence. Stability is derived through Lyapunov methods for switched systems, and experimental results demonstrate the performance of the switched control system under typical cycling conditions.

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 persons 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 persons ability to track a desired limb trajectory.

Tracking control for FES-cycling based on force direction efficiency with antagonistic bi-articular muscles

A functional electrical stimulation (FES)-based tracking controller is developed to enable cycling based on a strategy to yield force direction efficiency by exploiting antagonistic bi-articular muscles. Given the input redundancy naturally occurring among multiple muscle groups, the force direction at the pedal is explicitly determined as a means to improve the efficiency of cycling. A model of a stationary cycle and rider is developed as a closed-chain mechanism. A strategy is then developed to switch between muscle groups for improved efficiency based on the force direction of each muscle group. Stability of the developed controller is analyzed through Lyapunov-based methods.

Tracking control of a human limb during asynchronous neuromuscular electrical stimulation

Neuromuscular electrical stimulation (NMES) is defined as the use of an electrical stimulus to elicit muscle contractions and is commonly used in rehabilitative settings. NMES 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 fatigue due to the nonselective, spatially fixed, synchronous activation of motor units. Asynchronous stimulation can reduce NMES-induced fatigue; however, one limitation of asynchronous stimulation is that switching between stimulation channels may introduce discontinuities due to a differing response to stimulation by each group of recruited motor units. Thus, there is a need to design a controller which considers the switching dynamics and muscle response to stimulation during asynchronous stimulation. A closed-loop feedback controller is developed in this paper to yield semi-global asymptotic tracking of a desired trajectory for a persons knee-shank complex during asynchronous stimulation. The result is promising for the implementation of asynchronous stimulation in assistive devices as a method to reduce fatigue while tracking a desired trajectory.

Isometric Torque Control for Neuromuscular Electrical Stimulation With Time-Varying Input Delay

Previous results have shown experimental evidence that the muscle response to neuromuscular electrical stimulation (NMES) is delayed; the time lag is often referred to as electromechanical delay. NMES closed-loop control methods have been developed to compensate for a known constant input delay. However, as a muscle fatigues, this delay increases. This paper develops a feedback controller that robustly compensates for the time-varying delay of an uncertain muscle model during isometric contractions. The controller is proven to yield global uniformly ultimately bounded torque tracking error. Experimental results illustrate the effectiveness of the developed controller and the time-varying nature of the delayed response.

Comparing the force ripple during asynchronous and conventional stimulation

Introduction: Asynchronous stimulation has been shown to reduce fatigue during electrical stimulation; however, it may also exhibit a force ripple. We quantified the ripple during asynchronous and conventional single‐channel transcutaneous stimulation across a range of stimulation frequencies. Methods: The ripple was measured during 5 asynchronous stimulation protocols, 2 conventional stimulation protocols, and 3 volitional contractions in 12 healthy individuals. Results: Conventional 40 Hz and asynchronous 16 Hz stimulation were found to induce contractions that were as smooth as volitional contractions. Asynchronous 8, 10, and 12 Hz stimulation induced contractions with significant ripple. Conclusions: Lower stimulation frequencies can reduce fatigue; however, they may also lead to increased ripple. Future efforts should study the relationship between force ripple and the smoothness of the evoked movements in addition to the relationship between stimulation frequency and NMES‐induced fatigue to elucidate an optimal stimulation frequency for asynchronous stimulation. Muscle Nerve 50: 549–555, 2014

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 riders 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 Parkinsons 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.

Cadence control of stationary cycling induced by switched functional electrical stimulation control

Cycling induced by functional electrical stimulation (FES) is an effective means for exercise and rehabilitation of individuals suffering from neurological disorders such as stroke, spinal cord injury, and cerebral palsy. To achieve FES-cycling, potential fields are alternately applied across muscle groups in the lower extremities. Alternating stimulation of various muscle groups according to the crank position makes the FES-cycling system a switched system with autonomous state-dependent switching. This paper examines FES-cycling from a switched systems analysis perspective. Specifically, a switched sliding mode controller is developed to yield approximate tracking of a desired cadence despite an uncertain, nonlinear cycle-rider model. Cadence tracking is proven via a common Lyapunov-like function and experimental results are provided to demonstrate the performance of the switched controller.