Partition between volitional and induced forces in electrically augmented dynamic isometric muscle contractions

Optimization of Seating Position and Stimulation Pattern in Functional Electrical Stimulation Cycling: Simulation Study

Two significant challenges facing functional electrical stimulation (FES) cycling are the low power output and early onset of muscle fatigue, mainly due to the non-physiological and superficial recruitment of motor units and weakness of the antagonistic muscles. Thus optimization of the cycling biomechanical properties and stimulation pattern to achieve maximum output power with minimum applied electrical stimulus is of great importance. To find the optimal seating position and stimulation pattern, the previous works either ignored the muscle's force-velocity and force-length properties or employed complicated muscle models which was a massive barrier to clinical experiments. In this work, an easy-to-use and precise muscle model in conjunction with Jacobian-based torque transfer functions were adopted to determine the optimal seating position, trunk angle, crank arm length, and stimulation intervals. Furthermore, the impact of muscle force-velocity factor in finding the optimal seating position and stimulation intervals was investigated. The simulation models showed the trivial effect of the force-velocity factor on the resulting optimal seating position of six healthy simulated subjects. This method can enhance the FES-cycling performance and shorten the time-consuming process of muscle model identification for optimization purposes.

Human-FES Cooperative Control for Wrist Movement: A Preliminary Study

Functional electrical stimulation (FES) sometimes applies to patients with partial paralysis, so human voluntary control and FES control both exist. Our study aims to build a cooperative controller to achieve human-FES cooperation. This cooperative controller is formed by a classical FES controller and an impedance controller. The FES controller consists of a back propagation (BP) neural network-based feedforward controller and a PID-based feedback controller. The function of impedance controller is to convert volitional force/torque, which is estimated from a three-stage filter based on EMG, into additional angle. The additional angle can reduce the FES intensity in our cooperative controller, comparing to that in classical FES controller. Some assessment experiments are designed to test the performance of the cooperative controller.

Mechanical Impedance and Its Relations to Motor Control, Limb Dynamics, and Motion Biomechanics

The concept of mechanical impedance represents the interactive relationship between deformation kinematics and the resulting dynamics in human joints or limbs. A major component of impedance, stiffness, is defined as the ratio between the force change to the displacement change and is strongly related to muscle activation. The set of impedance components, including effective mass, inertia, damping, and stiffness, is important in determining the performance of the many tasks assigned to the limbs and in counteracting undesired effects of applied loads and disturbances. Specifically for the upper limb, impedance enables controlling manual tasks and reaching motions. In the lower limb, impedance is responsible for the transmission and attenuation of impact forces in tasks of repulsive loadings. This review presents an updated account of the works on mechanical impedance and its relations with motor control, limb dynamics, and motion biomechanics. Basic questions related to the linearity and nonlinearity of impedance and to the factors that affect mechanical impedance are treated with relevance to upper and lower limb functions, joint performance, trunk stability, and seating under dynamic conditions. Methods for the derivation of mechanical impedance, both those for within the system and material-structural approaches, are reviewed. For system approaches, special attention is given to methods aimed at revealing the correct and sufficient degree of nonlinearity of impedance. This is particularly relevant in the design of spring-based artificial legs and robotic arms. Finally, due to the intricate relation between impedance and muscle activity, methods for the explicit expression of impedance of contractile tissue are reviewed.

Contributions to muscle force and EMG by combined neural excitation and electrical stimulation

OBJECTIVE

Stimulation of muscle for research or clinical interventions is often superimposed on ongoing physiological activity without a quantitative understanding of the impact of the stimulation on the net muscle activity and the physiological response. Experimental studies show that total force during stimulation is less than the sum of the isolated voluntary and stimulated forces, but the occlusion mechanism is not understood.

APPROACH

We develop a model of efferent motor activity elicited by superimposing stimulation during a physiologically activated contraction. The model combines action potential interactions due to collision block, source resetting, and refractory periods with previously published models of physiological motor unit recruitment, rate modulation, force production, and EMG generation in human first dorsal interosseous muscle to investigate the mechanisms and effectiveness of stimulation on the net muscle force and EMG.

MAIN RESULTS

Stimulation during a physiological contraction demonstrates partial occlusion of force and the neural component of the EMG, due to action potential interactions in motor units activated by both sources. Depending on neural and stimulation firing rates as well as on force-frequency properties, individual motor unit forces can be greater, smaller, or unchanged by the stimulation. In contrast, voluntary motor unit EMG potentials in simultaneously stimulated motor units show progressive occlusion with increasing stimulus rate. The simulations predict that occlusion would be decreased by a reverse stimulation recruitment order.

SIGNIFICANCE

The results are consistent with and provide a mechanistic interpretation of previously published experimental evidence of force occlusion. The models also predict two effects that have not been reported previously--voluntary EMG occlusion and the advantages of a proximal stimulation site. This study provides a basis for the rational design of both future experiments and clinical neuroprosthetic interventions involving either motor or sensory stimulation.

Alteration of neural action potential patterns by axonal stimulation: the importance of stimulus location

OBJECTIVE

Stimulation of peripheral nerves is often superimposed on ongoing motor and sensory activity in the same axons, without a quantitative model of the net action potential train at the axon endpoint.

APPROACH

We develop a model of action potential patterns elicited by superimposing constant frequency axonal stimulation on the action potentials arriving from a physiologically activated neural source. The model includes interactions due to collision block, resetting of the neural impulse generator, and the refractory period of the axon at the point of stimulation.

MAIN RESULTS

Both the mean endpoint firing rate and the probability distribution of the action potential firing periods depend strongly on the relative firing rates of the two sources and the intersite conduction time between them. When the stimulus rate exceeds the neural rate, neural action potentials do not reach the endpoint and the rate of endpoint action potentials is the same as the stimulus rate, regardless of the intersite conduction time. However, when the stimulus rate is less than the neural rate, and the intersite conduction time is short, the two rates partially sum. Increases in stimulus rate produce non-monotonic increases in endpoint rate and continuously increasing block of neurally generated action potentials. Rate summation is reduced and more neural action potentials are blocked as the intersite conduction time increases. At long intersite conduction times, the endpoint rate simplifies to being the maximum of either the neural or the stimulus rate.

SIGNIFICANCE

This study highlights the potential of increasing the endpoint action potential rate and preserving neural information transmission by low rate stimulation with short intersite conduction times. Intersite conduction times can be decreased with proximal stimulation sites for muscles and distal stimulation sites for sensory endings. The model provides a basis for optimizing experiments and designing neuroprosthetic interventions involving motor or sensory stimulation.

Methods for Dynamic Characterization of the Major Muscles Activating the Lower Limb Joints in Cycling Motion

The functional activation, through electrical stimulation, of the lower limb consisting of several deficient muscles requires well-patterned and coordinated activation of these muscles. This study presents a method for characterizing the parameters of the major muscle groups controlling the ankle and knee joints in cycling motion, the latter having particular significance in the rehabilitation of locomotion. To lower mechanical indeterminacy in the joints the system is reduced by grouping the muscles acting in synergism. The joint torques were calculated by inverse dynamics methods from cycling motion data, including kinematics and foot/pedal reaction loads (forces, moments). The mechanical indeterminacy was resolved by applying optimization criteria and the individual muscle torques were parceled-out from the joint torques. System identification of the individual muscles, part of which being bi-articular, in this non-isometric condition was performed from the relationship between the evaluated force and the measured EMG of each the muscles, using both first and second order linear transfer functions. Feasibility of the presented method was demonstrated through the computation of the coefficients of the muscles involved and validating the results on the experimental data obtained from one subject.

Neural adaptations to electrical stimulation strength training

This review provides evidence for the hypothesis that electrostimulation strength training (EST) increases the force of a maximal voluntary contraction (MVC) through neural adaptations in healthy skeletal muscle. Although electrical stimulation and voluntary effort activate muscle differently, there is substantial evidence to suggest that EST modifies the excitability of specific neural paths and such adaptations contribute to the increases in MVC force. Similar to strength training with voluntary contractions, EST increases MVC force after only a few sessions with some changes in muscle biochemistry but without overt muscle hypertrophy. There is some mixed evidence for spinal neural adaptations in the form of an increase in the amplitude of the interpolated twitch and in the amplitude of the volitional wave, with less evidence for changes in spinal excitability. Cross-sectional and exercise studies also suggest that the barrage of sensory and nociceptive inputs acts at the cortical level and can modify the motor cortical output and interhemispheric paths. The data suggest that neural adaptations mediate initial increases in MVC force after short-term EST.

A phenomenological model that predicts forces generated when electrical stimulation is superimposed on submaximal volitional contractions

Superimposition of electrical stimulation during voluntary contractions is used to produce functional movements in individuals with central nervous system impairment, to evaluate the ability to activate a muscle, to characterize the nature of fatigue, and to improve muscle strength during postsurgical rehabilitation. Currently, the manner in which voluntary contractions and electrically elicited forces summate is not well understood. The objective of the present study is to develop a model that predicts the forces obtained when electrical stimulation is superimposed on a volitional contraction. Quadriceps femoris muscles of 12 able-bodied subjects were tested. Our results showed that the total force produced when electrical stimulation was superimposed during a volitional contraction could be modeled by the equation T=V+S[(MaxForce-V)/MaxForce]N, where T is the total force produced, V is the force in response to volitional contraction alone, S is the force response to the electrical stimulation alone, MaxForce is the maximum force-generating ability of the muscle, and N is a parameter that we posit depends on the differences in the motor unit recruitment order and firing rates between volitional and electrically elicited contractions. In addition, our results showed that the model predicted accurately (intraclass correlation coefficient>or=0.97) the total force in response to a wide range of stimulation intensities and frequencies superimposed on a wide range of volitional contraction levels. Thus the model will be helpful to clinicians and scientists to predict the amount of stimulation needed to produce the targeted force levels in individuals with partial paralysis.

Effects of electromyostimulation versus voluntary isometric training on elbow flexor muscle strength

The purpose of this study was to determine whether 7 weeks of standardized (same number and duration of repetitions, sets and rest strictly identical) electromyostimulation training of the elbow flexor muscles would induce strength gains equivalent to those of voluntary isometric training in isometric, eccentric and concentric contractions. Twenty-five males were randomly assigned to an electromyostimulated group (EMS, n=9), a voluntary isometric group (VOL, n=8), or a control group (CON, n=8). Maximal voluntary isometric, eccentric and concentric strength, electromyographic (EMG) activity of the biceps and triceps brachii muscles, elbow flexor muscle activation (twitch interpolation technique) and contractile properties were assessed before and after the training period. The main findings were that the isometric torque gains of EMS were greater than those of VOL after the training period (P<0.01) and that the eccentric and concentric torque gains were equivalent. In both groups, we observed that the mechanical twitch (Pt) was increased (P<0.05) and that torque improvements were not mediated by neural adaptations. Considering the respective intensities of the training programs (i.e., submaximal contractions for EMS versus maximal for VOL), it can be concluded that electromyostimulation training would be more efficient than voluntary isometric training to improve both isometric and dynamic strength.

Enhancement of muscle activity by electrical stimulation in cerebral palsy: a case-control study

The objectives of this study were to compare the effects of low-intensity electrical stimulation of the quadriceps muscle in children with cerebral palsy in the following 2 modes: reconditioning by long-term training of the muscle versus real-time assist to the muscle during motion. To evaluate the force enhancement in the assist mode, we developed a method to dissociate the volitional and the induced components from the total electromyographic signal. The study group, including 5 children with cerebral palsy (mean age, 3.3 years; 0.4 SD), underwent 2 testing sessions: 1 before and 1 after 3-month training by electrical stimulation. Each session included 2 series of trials: 1 with electrical stimulation, as an orthotic assist, and 1 without electrical stimulation. The tests included flexion-extension movements of the knee at a self-selected pace. The results showed that, compared to before training, there was a significant increase in the average motion velocity and a decrease in motion jerk and in knee torque after training in both the electrical stimulation- assisted and -unassisted modes. Of special interest was the significant decrease in quadriceps-hamstrings co-contraction following training by electrical stimulation but not during electrical stimulation-assisted motion. The results obtained for the group with cerebral palsy were statistically different from those of the control group, but this difference decreased after long-term training by electrical stimulation. It was concluded that, in children with cerebral palsy, electrical stimulation is more beneficial in long-term training than when used as a real-time motion assist. Although muscle strength is not affected, more centrally controlled attributes such as co-contraction are improved.

Evaluation of methods for extraction of the volitional EMG in dynamic hybrid muscle activation

Background

Hybrid muscle activation is a modality used for muscle force enhancement, in which muscle contraction is generated from two different excitation sources: volitional and external, by means of electrical stimulation (ES). Under hybrid activation, the overall EMG signal is the combination of the volitional and ES-induced components. In this study, we developed a computational scheme to extract the volitional EMG envelope from the overall dynamic EMG signal, to serve as an input signal for control purposes, and for evaluation of muscle forces.

Methods

A "synthetic" database was created from in-vivo experiments on the Tibialis Anterior of the right foot to emulate hybrid EMG signals, including the volitional and induced components. The database was used to evaluate the results obtained from six signal processing schemes, including seven different modules for filtration, rectification and ES component removal. The schemes differed from each other by their module combinations, as follows: blocking window only, comb filter only, blocking window and comb filter, blocking window and peak envelope, comb filter and peak envelope and, finally, blocking window, comb filter and peak envelope.

Results and conclusion

The results showed that the scheme including all the modules led to an excellent approximation of the volitional EMG envelope, as extracted from the hybrid signal, and underlined the importance of the artifact blocking window module in the process.

The results of this work have direct implications on the development of hybrid muscle activation rehabilitation systems for the enhancement of weakened muscles.