Constant-frequency Trains Research Articles

Event-Related Beta EEG Changes Induced by Various Neuromuscular Electrical Stimulation: A Pilot Study

Previous results demonstrated that neuromuscular electrical stimulation (NMES) with various configurations could induce different activity at both the central and peripheral levels. Although NMES generating different peripheral movements have been studied, it is still unclear whether the difference in NMES-induced cortical activity is due to movement- or stimulation- related differences. Because NMES-induced cortical activity impacts motor function recovery, it is essential to know when NMES with various configurations evoke the same movement, whether the induced cortical activity is still different. Four NMES configurations: 1) Eight-let Frequency Trains, 2) Doublet frequency trains (DFT), 3) Constant-frequency trains with narrow-pulse, and 4) wide-pulse, were delivered to the right biceps brachii muscle in nine healthy young adults. We adjusted the intensities of these NMES to evoke the same elbow flexion and compared the cortical activities over sensorimotor regions. Our results showed that the four NMES patterns induced different beta-band Event-Related Desynchronization (ERD), with the DFT providing the strongest ERD value given the same NMES-induced elbow flexion (p < 0.05). This difference is possibly due to NMES with different configuration activated in the amount of afferent proprioceptive fibers. Our pilot study suggests that the NMES-induced beta-band ERD may be an additional factor to consider when selecting the NMES configuration for a better motor function recovery.

The inclusion of interstimulus interval variability does not mitigate electrically-evoked fatigue of the knee extensors.

Transcutaneous electrical stimulation (TES) is used to activate muscles when volitional capacity is impaired but potential benefits are limited by rapid force loss (fatigue). Most TES fatigue protocols employ constant-frequency trains, with stimuli at a fixed interstimulus interval (ISI); however, a brief ISI between the first two pulses (variable-frequency train, VFT) to maximize the catchlike property of muscle can attenuate fatigue development. The purpose of this study was to investigate if a VFT that simulates intrinsic variability of voluntary motor unit discharge rates would also mitigate fatigue, owing to the sensitivity of muscle to acute activation history. On two visits, 24 healthy adults (25.3 ± 3.7years; 12 females) received 3min of intermittent TES to the quadriceps of the dominant leg. Trains of eight pulses at 10Hz were delivered with a constant (100ms) or variable ISI (80-120ms). Contractile impulse, rate of force development (RFD), and rate of relaxation (RFR) were determined for each tetanus RESULTS: During fatigue and recovery, contractile impulse did not differ between protocols (p ≥ 0.796) and sexes (p ≥ 0.493), with values of 77 ± 17% control at task end and 125 ± 19% control 2min later. RFD and RFR also showed no effect of the protocol (p ≥ 0.310) or participant sex (p ≥ 0.119). Both measures slowed (38 ± 23% and 33 ± 22%, respectively) but dissociated during recovery as RFD remained 16 ± 18% below control at 5min, whereas RFR recovered to control by 30s (101 ± 22%). Contrary to expectations, the VFT protocol did not attenuate fatigue development, which suggests no benefit to mimicking the inherent variability of motor unit discharge rates.

Muscle Fatigue in Response to Electrical Stimulation Pattern and Frequency in Spinal Cord Injury.

Functional electrical stimulation (FES) is widely used to induce functional movements for paralyzed muscles. However, rapid muscle fatigue during FES-induced muscle contractions limits FES clinical efficacy. To investigate muscle fatigue response across stimulation patterns and frequencies during FES in able-bodied individuals and in those with spinal cord injury (SCI). Four stimulation protocols combining 20 and 40 Hz average frequency with either constant frequency trains (CFTs) or with doublet frequency trains (DFTs) were applied to the quadriceps of seven adults with SCI and eight able-bodied participants. A FES-row training laboratory. Seven individuals with SCI (one female; age range, 25 ± 6 years) and eight age-matched able-bodied participants (one female). None. Fatigue was defined as the number of contractions until force decreased by 20% from the target level of 25% maximal contraction force. The number of contractions and the stimulation current used during the four stimulation protocols were compared. There was a significant effect of frequency, as well as interaction between group and stimulation pattern (P < .05). In both groups, 20-Hz trains increased the number of contractions to fatigue compared to 40-Hz trains. However, the responses to the pattern of stimulation differed. In the able-bodied participants, CFT increased the number of contractions to fatigue compared to DFT, whereas in those with SCI, DFT increased the number of contractions to fatigue. In fact, DFT resulted in similar number of contractions to fatigue in both populations. These results indicate that DFT at 20 Hz may be a better stimulation protocol to delay fatigue onset in the SCI population than the other three protocols. In addition, this work implies that results from able-bodied persons may not be directly applicable to those with SCI.

Contractile benefits of doublet-initiated low-frequency stimulation in rat extensor digitorum longus muscle exposed to high extracellular [K+] or fatiguing contractions.

During dynamic contractions, high-frequency muscle activation is needed to achieve optimal power. This must be balanced against an increased excitation-induced accumulation of extracellular K+, which can reduce excitability and ultimately may prevent adequate responses to high-frequency activation. Mean activation frequencies in vivo are often low (subtetanic), but activation patterns contain bursts of high (supratetanic) frequencies known as doublets, which enhance dynamic contraction in rested muscles at normal extracellular K+ concentration ([K+]o). Here, we examine how dynamic contractions in fast-twitch fibers stimulated by high frequency/doublets are affected during exposure to 11 mM [K+]o and during fatigue. Dynamic contractions were elicited by electrical stimulation in isolated rat extensor digitorum longus muscles incubated at 4 or 11 mM K+. When stimulation frequency was maintained constant, an increase from 150 to 300 Hz enhanced maximal power (Pmax), maximal velocity (Vmax), and rate of force development (RFD) at 4 mM K+ but only Vmax at 11 mM K+. With the use of subtetanic frequency trains (50 Hz) with or without an initiating doublet (300 Hz), the addition of a doublet increased maximal force, Pmax, Vmax, and RFD at both 4 and 11 mM K+. Furthermore, a work-matched fatiguing protocol was performed comparing a doublet-initiated subtetanic train (DT) of 60 Hz with a constant-frequency train (CFT) of 71 Hz during 100 dynamic contractions. We found that DT produced higher power, velocity, and RFD than CFT throughout the fatiguing protocol. The results indicate that doublets enhance dynamic contraction in fast-twitch muscles stimulated at subtetanic frequency during both normal and fatiguing conditions.

Effect of Electrical Stimulation Frequency and Pattern on Muscle Fatigue in Spinal Cord Injured Individuals

Spinal cord injury (SCI) leads to an extensive skeletal muscle atrophy in the regions below the level of injury. Moreover, the paralyzed skeletal muscles shift towards predominantly Type II (fast‐twitch and highly fatigable) fibers. Functional electrical stimulation (FES) has been used to produce purposeful and controlled muscle contractions to restore functional movements, but rapid muscle fatigue remains a problem. Both pattern and frequency of stimulation can influence muscle fatigue rate, but optimization of stimulation parameters has rarely been investigated for those with SCI. FES has traditionally used constant‐frequency trains (CFT; brief stimulation pulses separated by regular interpulse intervals). However, it has been suggested that doublet‐frequency trains (DFT; close‐spaced pulse trains separated by longer intervals) can result in lesser fatigue. Also, stimulation frequency may determine muscle fatigue, with high‐frequency trains resulting in lesser fatigue compared to low‐frequency trains. However, most studies comparing muscle fatigue across stimulation parameters have been only examined in able‐bodied individuals and it is unknown whether paralyzed muscles respond similarly. There are a few studies suggesting that low‐frequency trains produce less fatigue than high‐frequency trains in those with SCI, in direct contrast to the able‐bodied. Therefore, we examined differences in muscle fatigue across and within able‐bodied individuals (N=8) and those with SCI (N=6) during CFT and DFT stimulation patterns at high‐ (40 Hz) and low‐ (20 Hz) frequencies. Fatigue was defined as a sustained reduction in force of &gt;20% at a constant stimulation intensity. Differences were considered significant at p&lt;0.05 and values are mean ± SEM. For both groups, low‐frequency trains delayed fatigue onset compared to high‐frequency trains, on average by 30 ± 3 %. In the able‐bodied, CFT resulted in slower fatigue onset (#contractions to fatigue CFT vs DFT: 20 Hz 39 ± 4 vs. 34 ± 4, 40 Hz 26 ± 4 vs. 23 ± 3). In those with SCI, DFT resulted in slower fatigue onset (#contractions to fatigue CFT vs DFT: 20 Hz 26 ± 2 vs. 35 ± 4, 40 Hz 20 ± 2 vs. 25 ± 2). In fact, DFT resulted in similar #contractions to fatigue in both populations, whereas CFT induced muscle fatigue significantly faster in those with SCI. These results may indicate that DFT preferentially recruit fast‐twitch fibers, leading to similar fatigue in both able‐bodied and those with SCI. DFT has been previously shown to limit fatigue potentially by increasing calcium release from the sarcoplasmic reticulum and/or by increasing muscle stiffness. These data indicate that DFT trains appear to be preferable for fatigue reduction in the SCI population.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Dynamic optimization of stimulation frequency to reduce isometric muscle fatigue using a modified Hill-Huxley model.

Optimal frequency modulation during functional electrical stimulation (FES) may minimize or delay the onset of FES-induced muscle fatigue. An offline dynamic optimization method, constrained to a modified Hill-Huxley model, was used to determine the minimum number of pulses that would maintain a constant desired isometric contraction force. Six able-bodied participants were recruited for the experiments, and their quadriceps muscles were stimulated while they sat on a leg extension machine. The force-time (F-T) integrals and peak forces after the pulse train was delivered were found to be statistically significantly greater than the force-time integrals and peak forces obtained after a constant frequency train was delivered. Experimental results indicated that the optimized pulse trains induced lower levels of muscle fatigue compared with constant frequency pulse trains. This could have a potential advantage over current FES methods that often choose a constant frequency stimulation train. Muscle Nerve 57: 634-641, 2018.

Effects of Constant and Doublet Frequency Electrical Stimulation Patterns on Force Production of Knee Extensor Muscles.

This study compared knee extensors’ neuromuscular fatigue in response to two 30-minute stimulation patterns: constant frequency train (CFT) and doublet frequency train (DFT). Fifteen men underwent two separate sessions corresponding to each pattern. Measurements included torque evoked by each contraction and maximal voluntary contractions (MVC) measured before and immediately after the stimulation sessions. In addition, activation level and torque evoked during doublets (Pd) and tetanic contractions at 80-Hz (P80) and 20-Hz (P20) were determined in six subjects. Results indicated greater mean torque during the DFT stimulation session as compared with CFT. But, no difference was obtained between the two stimulation patterns for MVC and evoked torque decreases. Measurements conducted in the subgroup depicted a significant reduction of Pd, P20 and P80. Statistical analyses also revealed bigger P20 immediate reductions after CFT than after DFT. We concluded that DFT could be a useful stimulation pattern to produce and maintain greater force with quite similar fatigue than CFT.

Effects of Electrical Stimulation Pattern on Quadriceps Fatigue Development in Individuals with Spinal Cord Injury

Functional electrical stimulation (FES) has often been used to facilitate exercise in individuals with spinal cord injury (SCI). Unfortunately, rapid fatigue induced by constant frequency trains (CFT) limits the effectiveness of FES in those with SCI. Variable frequency trains (VFT) have therefore been suggested as useful strategies to offset the rapid fatigue during electrical stimulation. However, most results concern very short duration contractions with limited functional applications. PURPOSE: To test force and fatigue in response to VFT and CFT patterns in training-like conditions (2-s concentric contractions). METHODS: Seven SCI individuals underwent two testing sessions separated by at least 24 hours. During each testing session, muscle torque was measured during concentric leg extension (30°/s) under two sequential electrical stimulation train patterns presented in a randomized order. For the CFT pattern, this corresponded to 2 s of contraction generated by continuous 40 Hz stimulation with a pulse width of 450 μs; for the VFT pattern, this corresponded to 2 s of contraction generated by an 80 Hz doublet followed by continuous 20 Hz stimulation with a pulse width of 450 μs. Stimulation was applied every 5 s until the target torque (fixed at 50% of the maximal evoked force) was no longer produced. RESULTS: For an equivalent torque decrease (VFT: -24.0±0.6 vs CFT: -27.7±3.2 %), more contractions were performed when evoked with VFT as compared with CFT (VFT: 14.3±1.7 vs CFT: 9.3±2.5 contractions, P<0.05). In addition, the total work produced over sessions was significantly greater when VFT pattern was applied (1418.1 ± 115.7 Nm.s) as compared with CFT (879.8 ± 112.4 Nm.s). CONCLUSION: These results showed that paralyzed human skeletal muscle is more fatigue resistant to repetitive dynamic contractions evoked by a VFT stimulation pattern as compared with a CFT pattern. These data are of particular interest for training and rehabilitation after an SCI since they suggest that using VFT, people would be able to perform longer training sessions, potentially resulting in greater improvements.

The Catchlike Property in Skeletal Muscle: Influence of Contraction and Muscle‐Type on Augmented Contractile Function

In slow and fast motor units of skeletal muscle, discharge rates are modulated to match and preserve target force levels. Although relatively constant patterns in firing rates are most common, two or more closely-spaced pulses at the onset of activation may appear in both human and mammalian skeletal muscles. The outcome of this aberration in motor unit firing pattern has been shown to increase both the rate and extent of force development; an effect referred to as the catchlike property (CLP). The purpose of this study was to explore the contraction (i.e. isometric vs. concentric) and muscle-type (fast vs. slow twitch) dependence of the CLP in extensor digitorum longus (EDL) and soleus muscles of the mouse (n=10 for all data). The addition of a single pulse 10ms at the onset of an otherwise constant frequency train (CFT) was utilized to augment force and work output of the contractile event, referred to as the catchlike-inducing train (CLT). When compared to the CFT, the peak force during the CLT of soleus muscles was increased to 1.15±0.03 and 1.39±0.03 during isometric (ISO) and concentric (CON) contractions, respectively. In EDL muscles, however, forces were increased to 1.47±0.05 and 1.20±0.04 for ISO and CON contractions; demonstrating a muscle and contraction-type dependence (p <0.05). Similarly, the work done and/or force-time integral increased to 1.34±0.03 and 1.21±0.03 (soleus), and 1.43±0.05 and 1.18±0.02 (EDL), for ISO and CON contractions, respectively. However, when normalized for the number of pulses within each contraction, no net increase in work was evident. Therefore, the catch-like property may serve primarily as a mechanism for augmenting initial force levels rather than as a modulator of contractile efficiency within skeletal muscle.

Effects of electrical stimulation pattern on quadriceps isometric force and fatigue in individuals with spinal cord injury.

Variable frequency trains (VFT) or train combinations have been suggested as useful strategies to offset the rapid fatigue induced by constant frequency trains (CFT) during electrical stimulation. However, most studies have been of short duration with limited functional application in those with spinal cord injury (SCI). We therefore tested force and fatigue in response to VFT, CFT, and combined patterns in strength training-like conditions (6-s contractions). Ten SCI individuals underwent either CFT or VFT patterns until target torque was no longer produced and then switched immediately to the other pattern. Target torque was reached more times when VFT was used first (VFT: 6.7 ± 0.8 vs. CFT: 3.5 ± 0.2 contractions, P < 0.05) and when it was followed by the CFT pattern (VFT-CFT: 10.3 ± 1.2 vs. 6.9 ± 1.2 contractions, P < 0.05). These findings suggest that for the same initial forces the VFT pattern is less fatiguing than CFT and that when combining train types, VFT should be used first.

Optimization of a Stimulation Train based on a Predictive Model of Muscle Force and Fatigue

Optimizing stimulation frequency based on a mathematical model that can predict skeletal muscle force and fatigue may improve the effectiveness of functional electrical stimulation systems. Potentially, optimal stimulation patterns can maximize muscle force production while also delaying the onset of muscle fatigue. In this paper, dynamic optimization was used to generate an optimal, frequency varying pulse train that maintains a desired isometric knee extension without unnecessarily fatiguing the muscle. The optimization method employed a predictive mathematical model of muscle force and fatigue. Knee extension experiments were conducted on an able-bodied participant to identify the model parameters. To test the effectiveness of the optimized train to delay muscle fatigue, a second knee extension experiment was conducted to compare the performance of the optimized stimulation train and a 50Hz constant frequency train. The peak force and the force time integral of the optimized stimulation train were found to be higher than the 50Hz constant frequency train. These preliminary results show that optimizing stimulation patterns, based on a subject specific predictive mathematical model, may potentially delay the onset of muscle fatigue while obtaining desired force profiles.

Effects of electrical stimulation pattern on quadriceps isometric force and fatigue in individuals with spinal cord injury

Introduction Variable frequency trains (VFT) or train combinations have been suggested as useful strategies to offset the rapid fatigue induced by constant frequency trains (CFT) during electrical stimulation. However, most studies have been of short duration with limited functional application in those with spinal cord injury (SCI). We therefore tested force and fatigue in response to VFT, CFT, and combined patterns in strength training-like conditions (6-s contractions). Methods Ten SCI individuals underwent either CFT or VFT patterns until target torque was no longer produced and then switched immediately to the other pattern. Results Target torque was reached more times when VFT was used first (VFT: 6.7 ± 0.8 vs. CFT: 3.5 ± 0.2 contractions, P < 0.05) and when it was followed by the CFT pattern (VFT-CFT: 10.3 ± 1.2 vs. CFT-VFT: 6.9 ± 1.2 contractions, P < 0.05). Conclusions These findings suggest that for the same initial forces the VFT pattern is less fatiguing than CFT and that when combining train types, VFT should be used first. Muscle Nerve 52: 260–264, 2015

Effects of electrical stimulation pattern on quadriceps force production and fatigue

Mixed stimulation programs (MIX) that switch from constant frequency trains (CFT) to variable frequency trains have been proposed to offset the rapid fatigue induced by CFT during electrical stimulation. However, this has never been confirmed with long stimulation patterns, such as those used to evoke functional contractions. The purpose of this study was to test the hypothesis that MIX programs were less fatiguing than CFTs in strength training-like conditions (6-s contractions, 30-min). Thirteen healthy subjects underwent 2 sessions corresponding to MIX and CFT programs. Measurements included maximal voluntary isometric torque and torque evoked by each contraction. There were greater decreases of voluntary and evoked torque (P < 0.05) after CFT than MIX, and mean torque was 13 ± 1% higher during the MIX session (P < 0.05). These findings confirm that combining train types might be a useful strategy to offset rapid fatigue during electrical stimulation sessions with long-duration contractions.

Novel Patterns of Functional Electrical Stimulation Have an Immediate Effect on Dorsiflexor Muscle Function During Gait for People Poststroke

Foot drop is a common gait impairment after stroke. Functional electrical stimulation (FES) of the ankle dorsiflexor muscles during the swing phase of gait can help correct foot drop. Compared with constant-frequency trains (CFTs), which typically are used during FES, novel stimulation patterns called variable-frequency trains (VFTs) have been shown to enhance isometric and nonisometric muscle performance. However, VFTs have never been used for FES during gait. The purpose of this study was to compare knee and ankle kinematics during the swing phase of gait when FES was delivered to the ankle dorsiflexor muscles using VFTs versus CFTs. A repeated-measures design was used in this study. Thirteen individuals with hemiparesis following stroke (9 men, 4 women; age=46-72 years) participated in the study. Participants completed 20- to 40-second bouts of walking at their self-selected walking speeds. Three walking conditions were compared: walking without FES, walking with dorsiflexor muscle FES using CFTs, and walking with dorsiflexor FES using VFTs. Functional electrical stimulation using both CFTs and VFTs improved ankle dorsiflexion angles during the swing phase of gait compared with walking without FES (X+/-SE=-2.9 degrees +/- 1.2 degrees). Greater ankle dorsiflexion in the swing phase was generated during walking with FES using VFTs (X+/-SE=2.1 degrees +/- 1.5 degrees) versus CFTs (X+/-SE=0.3+/-1.3 degrees). Surprisingly, dorsiflexor FES resulted in reduced knee flexion during the swing phase and reduced ankle plantar flexion at toe-off. The findings suggest that novel FES systems capable of delivering VFTs during gait can produce enhanced correction of foot drop compared with traditional FES systems that deliver CFTs. The results also suggest that the timing of delivery of FES during gait is critical and merits further investigation.

The Effect of Using Variable Frequency Trains During Functional Electrical Stimulation Cycling

Objectives. This paper describes an experimental investigation of variable frequency stimulation patterns as a means of increasing torque production and, hence, performance in cycling induced by functional electrical stimulation. Materials and Methods. Experiments were conducted on six able-bodied subjects stimulating both quadriceps during isokinetic trials. Constant-frequency trains (CFT) with 50-msec interpulse intervals and four catchlike-inducing trains (CIT) were tested. The CITs had an initial, brief, high-frequency burst of two pulses at the onset of or within a subtetanic low-frequency stimulation train. Each stimulation train consisted of the same number of pulses. The active torques produced by each train were compared. Parametric main effect ANOVA tests were performed on the active torque-time integral (TTI), on the active torque peaks and on the time needed to reach those peaks (T2P). Results. The electrical stimulation of the quadriceps produced active torques with mean peak values in the range of 1.6-3.5 Nm and a standard error below 0.2 Nm. CITs produced a significant increase of TTI and torque peaks compared with CFTs in all the experimental conditions. In particular, during the postfatigue trials, the CITs with the doublet placed in the middle of the train produced TTIs and torque peaks about 61% and 28% larger than the CFT pattern, respectively. In addition, the CITs showed the lowest reduction of the performance between prefatigue and postfatigue conditions. Conclusions. The use of CITs improves the functional electrical stimulation cycling performance compared with CFT stimulation. This application might have a relevant clinical importance for individuals with stroke where the residual sensation is still present and thus the maximization of the performance without an excessive increase of the stimulation intensity is advisable. Therefore, exercise intensity can be increased yielding a better muscle strength and endurance that may be beneficially for later gait training in individuals with stroke.

Functional electrical stimulation in neurological disorders

Functional electrical stimulation (FES) refers to electrical stimulation of muscles in order to improve the impaired motor function. This is achieved by activating skeletal muscles with constant frequency trains of stimulations. This method has been found useful in various neurological disorders like hemiplegia, foot drop and paraplegia including spinal cord injuries. The first half of this review focuses on the broad clinical applications of functional electrical stimulation, its mechanism of action and the complications of this mode of therapy. Advanced Parkinson's disease (PD) is characterized by marked slowing of gait and frequent freezing episodes. Medical and surgical treatments are often ineffective in managing freezing episodes. The second half of this review discusses briefly the gait abnormalities in PD and the available treatment options. The possible role of FES in improving gait in parkinsonism and the importance of future research in this direction are highlighted.

Mathematical model that predicts the force–intensity and force–frequency relationships after spinal cord injuries

We have previously developed and tested a muscle model that predicts the effect of stimulation frequency on muscle force responses. The aim of this study was to enhance our isometric mathematical model to predict muscle forces in response to stimulation trains with a wide range of frequencies and intensities for the quadriceps femoris muscles of individuals with spinal cord injuries. Isometric forces were obtained experimentally from 10 individuals with spinal cord injuries (time after injury, 1.5-8 years) and then compared to forces predicted by the model. Our model predicted accurately the force-time integrals (FTI) and peak forces (PF) for stimulation trains of a wide range of frequencies (12.5-80 HZ) and intensities (150-600-mus pulse duration), and two different stimulation patterns (constant-frequency trains and doublet-frequency trains). The accurate predictions of our model indicate that our model, which now incorporates the effects of stimulation frequency, intensity, and pattern on muscle forces, can be used to design optimal customized stimulation strategies for spinal cord-injured patients.

Effects of activation pattern on nonisometric human skeletal muscle performance

During volitional muscle activation, motor units often fire with varying discharge patterns that include brief, high-frequency bursts of activity. These variations in the activation rate allow the central nervous system to precisely control the forces produced by the muscle. The present study explores how varying the instantaneous frequency of stimulation pulses within a train affects nonisometric muscle performance. The peak excursion produced in response to each stimulation train was considered as the primary measure of muscle performance. The results showed that at each frequency tested between 10 and 50 Hz, variable-frequency trains that took advantage of the catchlike property of skeletal muscle produced greater excursions than constant-frequency trains. In addition, variable-frequency trains that could achieve targeted trajectories with fewer pulses than constant-frequency trains were identified. These findings suggest that similar to voluntary muscle activation patterns, varying the instantaneous frequency within a train of pulses can be used to improve muscle performance during functional electrical stimulation.

Effect of electrical stimulation pattern on the force responses of paralyzed human quadriceps muscles

Most studies examining the effect of electrical stimulation pattern on the force response of muscle have been done in able-bodied persons. The purpose of this study was to examine the electrically elicited force responses of the paralyzed quadriceps femoris muscles of persons with spinal cord injuries (SCI) to see whether stimulation patterns that increase the force response in non-paralyzed muscle will do so in paralyzed muscle. Thirteen subjects ranging in age from 11 to 24 years old with motor-complete SCI were studied. Isometric muscle performance was tested using 6-pulse constant-frequency trains (CFTs), variable-frequency trains (VFTs), and doublet-frequency trains (DFTs) delivered at mean frequencies of 10, 20, 33, 50, and 100 HZ. In the non-fatigued and fatigued condition, the VFT and DFT peak forces were greater than the CFT peak forces at 10 HZ. In addition, in the fatigued condition the 20-HZ VFT peak forces were greater than the CFT peak forces, and there was a trend for the DFT peak forces to be greater than the CFT peak forces. In the non-fatigued condition, the 33-HZ and 50-HZ DFT force-time integrals were greater than both the CFT and VFT force-time integrals. In the fatigued condition, there was no significant effect of train-type on the force-time integrals. These results differ from those previously reported from studies using able-bodied persons and indicate that findings from studies of the electrically elicited force responses of the muscles of able-bodied persons do not apply to the paralyzed muscles of persons with SCI.

Switching stimulation patterns improves performance of paralyzed human quadriceps muscle

The clinical efficacy of functional electrical stimulation (FES) is limited by the rapid onset of fatigue. Functional electrical stimulation applications typically stimulate skeletal muscles with constant-frequency trains (CFTs). Our laboratory has identified trains that we call doublet-frequency trains (DFTs) and that produce greater forces than CFTs, but more fatigue during repetitive activation than comparable CFTs. The purpose of this study was to see whether a series of CFTs followed by DFTs would reach a targeted isometric peak force more times than either train type alone during repetitive isometric activation of the paralyzed quadriceps muscles of subjects with spinal cord injuries (SCI). The combination of CFTs followed by DFTs reached the targeted isometric force 14% more often than the CFTs alone and 18% more often than the DFTs alone. These findings confirm that switching train types may be a useful strategy to offset the rapid fatigue of the functionally important quadriceps muscle that persons with SCI experience when using FES.