Stochastically modulated inter-pulse intervals to increase the efficiency of functional electrical stimulation cycling

Introduction

Functional electrical stimulation cycling has various health benefits, but the mechanical power output and efficiency are very low compared to volitional muscle activation. Stimulation with variable frequency showed significantly higher power output values in experiments with a knee dynamometer. The aim of the present work was to compare stochastic modulation of inter-pulse interval to constant inter-pulse interval stimulation during functional electrical stimulation cycling.

Methods

Seventeen able-bodied subjects participated (n = 17). Quadriceps and hamstring muscle groups were stimulated with two activation patterns: P1-constant frequency, P2-stochastic inter-pulse interval. Power output was measured on functional electrical stimulation ergometer.

Results

Overall, mean power output with the stochastically modulated pattern P2 was lower than with P1 (12.57 ± 3.74 W vs. 11.44 ± 3.81 W, P1 vs. P2, p = 0.022), but no significant differences during the first 30 s and the last 30 s were observed.

Conclusions

This study showed that stimulation strategies that use randomized modulation of inter-pulse intervals can negatively affect power output generation during functional electrical stimulation cycling. To minimise voluntary contractions, power measurement and assessment should be focused on the periods where only the quadriceps are stimulated.

Changes in Post-Stroke Gait Biomechanics Induced by One Session of Gait Training

The objective of this study was to determine whether one session of targeted locomotor training can induce measurable improvements in the post-stroke gait impairments. Thirteen individuals with chronic post-stroke hemiparesis participated in one locomotor training session combining fast treadmill training and functional electrical stimulation (FES) of ankle dorsi- and plantar-flexor muscles. Three dimensional gait analysis was performed to assess within-session changes (after versus before training) in gait biomechanics at the subject's self-selected speed without FES. Our results showed that one session of locomotor training resulted in significant improvements in peak anterior ground reaction force (AGRF) and AGRF integral for the paretic leg. Additionally, individual subject data showed that a majority of study participants demonstrated improvements in the primary outcome variables following the training session. This study demonstrates, for the first time, that a single session of intense, targeted post-stroke locomotor retraining can induce significant improvements in post-stroke gait biomechanics. We posit that the within-session changes induced by a single exposure to gait training can be used to predict whether an individual is responsive to a particular gait intervention, and aid with the development of individualized gait retraining strategies. Future studies are needed to determine whether these single-session improvements in biomechanics are accompanied by short-term changes in corticospinal excitability, and whether single-session responses can serve as predictors for the longer-term effects of the intervention with other targeted gait interventions.

Relationship between stimulation train characteristics and dynamic human skeletal muscle performance

AIM

The purpose of the present study was to investigate the effect of activation frequency on dynamic human muscle performance for a range of train durations and number of pulses during free limb movement.

METHODS

The quadriceps femoris muscles of 10 subjects were activated with stimulation trains with different activation frequency, train durations and number of pulses. The peak excursion produced in response to each train was the dependent measure of muscle performance.

RESULTS

The excursion-frequency (for a 300-ms train duration) and excursion-train duration (for trains with frequencies of 10, 30 or 59 Hz) relationships could each be fit with a two-parameter exponential equation (R(2) values > 0.97). Because the number of pulses in a stimulation train is a function of both train duration and frequency, the excursion produced as a function of the number of pulses was characterized by a three-parameter exponential equation that represented this combined relationship. The relationship between the measured and predicted excursions in response to a wide range of stimulation trains had a R(2) = 0.96. In addition, one-way repeated measures analyses of variance (anovas) showed that the frequency at which the maximum excursion was produced increased with an increase in the number of pulses in the trains tested.

CONCLUSION

These results show the importance of train duration and the number of pulses contained within a train on the relationship between activation frequency and human skeletal muscle performance.

Challenging the role of pH in skeletal muscle fatigue

Muscle fatigue is frequently defined as a temporary loss in force- or torque-generating ability because of recent, repetitive muscle contraction (1). The development of this temporary loss of force is a complex process and results from the failure of a number of processes, including motor unit recruitment and firing rate, chemical transmission across the neuromuscular junction, propagation of the action potential along the muscle membrane and T tubules, Ca2+ release from the sarcoplasmic reticulum (SR), Ca2+ binding to troponin C, and cross-bridge cycling (for detailed reviews, see Bigland-Ritchie and Woods(1), McLester(2), and Favero(3)). Muscle fatigue may limit the time a person can stand, the distance a person can ambulate, or the number of stairs a person can ascend or descend. In practical terms, however, we cannot know what actually leads to a decline in function for a given patient. For a phenomenon that may have profound clinical implications, muscle fatigue often receives inadequate attention in physiology textbooks, many of which contain a page or less of information on the entire topic (4-8). In addition, many textbooks report that muscle fatigue is mainly the result of a decrease in pH within the muscle cell due to a rise in hydrogen ion concentration ([H+]) resulting from anaerobic metabolism and the accumulation of lactic acid (6-8). Recent literature, however, contradicts this assertion (9-10). The purpose of this update, therefore, is to provide a brief review of the role of pH in the development of muscle fatigue.

Comparison of fatigue produced by various electrical stimulation trains

Previous work has shown that variable-frequency trains (VFTs) that use an initial doublet to take advantage of the catch-like property of muscle produce more force in fatigued muscle than constant-frequency trains (CFTs); however, it is unclear whether repetitive activation with VFTs is more or less fatiguing than repetitive activation with CFTs. The purpose of this research was to investigate the forces and fatigue produced by various stimulation trains during repetitive isometric muscle contractions. Two force measurements, peak force and force-time integral, were used to measure the performance of the human quadriceps muscle. Three fatiguing protocols, each consisting exclusively of either CFTs, trains with an initial doublet (VFTs), or trains with doublets separated by longer intervals [doublet-frequency trains (DFTs)], were tested. In addition, force responses to each of the three train types were tested before and immediately following each fatiguing protocol. Regardless of the fatiguing protocol, the doublet-frequency testing trains produced the greatest peak forces and force-time integrals before and immediately following the fatiguing protocols. Repetitive activation with exclusively DFTs produced greater attenuation of the testing trains than repetitive activation with CFTs or VFTs. These results suggest that clinical applications of electrical stimulation to activate skeletal muscle may need to contain a combination of train types to optimize performance.

A novel stimulation pattern improves performance during repetitive dynamic contractions

The purpose of this study was to determine the effect of three different stimulation patterns on repetitive knee movements. Each subject's quadriceps femoris was stimulated with: (1) a constant-frequency train (CFT) with an interpulse interval (IPI) of 50 ms; (2) a variable-frequency train (VFT)-similar to the CFT, except with an initial doublet with an IPI of 5 ms; and (3) a doublet-frequency train (DFT) with multiple doublets (doublet IPI 5 ms) separated by 50 ms, while the muscle was resisted by a load equal to 10% of the muscle's maximum voluntary isometric contraction. The muscle was stimulated while the knee moved through a 50 degrees arc of motion (90 degrees to 40 degrees of flexion). Testing was stopped when the subject failed to reach the target three consecutive times. Results showed that DFTs reached the target (mean +/- SD) 36.4 +/- 14.4 times, followed by VFTs (25.4 +/- 17.9) and CFTs (17.4 +/- 11.9). The DFT was the best pattern for producing shortening contractions. The results suggest that DFTs may have significant benefits during clinical functional electrical stimulation.

A predictive model of fatigue in human skeletal muscles

Fatigue is a major limitation to the clinical application of functional electrical stimulation. The activation pattern used during electrical stimulation affects force and fatigue. Identifying the activation pattern that produces the greatest force and least fatigue for each patient is, therefore, of great importance. Mathematical models that predict muscle forces and fatigue produced by a wide range of stimulation patterns would facilitate the search for optimal patterns. Previously, we developed a mathematical isometric force model that successfully identified the stimulation patterns that produced the greatest forces from healthy subjects under nonfatigue and fatigue conditions. The present study introduces a four-parameter fatigue model, coupled with the force model that predicts the fatigue induced by different stimulation patterns on different days during isometric contractions. This fatigue model accounted for 90% of the variability in forces produced by different fatigue tests. The predicted forces at the end of fatigue testing differed from those observed by only 9%. This model demonstrates the potential for predicting muscle fatigue in response to a wide range of stimulation patterns.

Activation of human quadriceps femoris muscle during dynamic contractions: effects of load on fatigue

Muscle fatigue is both multifactorial and task dependent. Electrical stimulation may assist individuals with paralysis to perform functional activities [functional electrical stimulation (FES), e.g., standing or walking], but muscle fatigue is a limiting factor. One method of optimizing force is to use stimulation patterns that exploit the catchlike property of skeletal muscle [catchlike-inducing trains (CITs)]. Although nonisometric (dynamic) contractions are important parts of both normal physiological activation of skeletal muscles and FES, no previous studies have attempted to identify the effect that the load being lifted by a muscle has on the fatigue produced. This study examined the effects of load on fatigue during dynamic contractions and the augmentation produced by CITs as a function of load. Knee extension in healthy subjects was electrically elicited against three different loads. The highest load produced the least excursion, work, and average power, but it produced the greatest fatigue. CIT augmentation was greatest at the highest load and increased with fatigue. Because CITs were effective during shortening contractions for a variety of loads, they may be of benefit during FES applications.

Effects of activation frequency on dynamic performance of human fresh and fatigued muscles

The force-frequency relationship for an individual muscle depends on the fatigue state, the length at which it is activated, and the muscle's activation history. The relationship among stimulation frequency and dynamic (nonisometric) muscle performance measurements (e.g., excursion, work, peak power, and average power) has not been reported. The purpose of this study was to identify the relationship between stimulation frequency and dynamic performance measurements for fresh and fatigued muscles. Constant-frequency and catchlike-inducing trains (CFT and CIT, respectively) were tested. When fresh, interpulse intervals of 40-50 ms [20-25 pulses/s (pps)] produced maximum performance for CFTs. For CITs, maximum performance occurred at interpulse intervals of 50-60 ms ( approximately 16-20 pps). Generally, CFTs produced slightly greater performance than did CITs. When fatigued, however, CITs produced greater performance than did CFTs. Maximum performance for CFTs occurred at interpulse intervals of 20-40 ms (25-50 pps) and at 30-50 ms (20-33 pps) for CITs. Enhancement of performance by CITs when fatigued may be due to less susceptibility to impairments in excitation-contraction coupling and greater ability to maintain rates of rise of force than CFTs.

Development of a mathematical model that predicts optimal muscle activation patterns by using brief trains

Because muscles must be repetitively activated during functional electrical stimulation, it is desirable to identify the stimulation pattern that produces the most force. Previous experimental work has shown that the optimal pattern contains an initial high-frequency burst of pulses (i.e., an initial doublet or triplet) followed by a low, constant-frequency portion. Pattern optimization is particularly challenging, because a muscle's contractile characteristics and, therefore, the optimal pattern change under different physiological conditions and are different for each person. This work describes the continued development and testing of a mathematical model that predicts isometric forces from fresh and fatigued muscles in response to brief trains of electrical pulses. By use of this model and an optimization algorithm, stimulation patterns that produced maximum forces from each subject were identified.

Catchlike-inducing train activation of human muscle during isotonic contractions: burst modulation

Stimulation trains that exploit the catchlike property [catchlike-inducing trains (CITs)] produce greater forces and rates of rise of force than do constant-frequency trains (CFTs) during isometric contractions and isovelocity movements. This study examined the effect of CITs during isotonic contractions in healthy subjects. Knee extension was electrically elicited against a load of 10% of maximum voluntary isometric contraction. The stimulation intensity was set to produce 20% of maximum voluntary isometric contraction. The muscle was tested before and after fatigue with a 6-pulse CFT and 6-pulse CITs that contained an initial doublet, triplet, or quadruplet. For prefatigue responses, the greatest isotonic performance was produced by CITs with initial doublets. When the muscles were fatigued, triplet CITs were best. CITs produce greater excursion, work, peak power, and average power than do CFTs, because CITs produced more rapid rates of rise of force. Faster rates of rise of force enabled the preload on the muscle to be exceeded earlier during the stimulation train.

Effects of length on the catchlike property of human quadriceps femoris muscle

BACKGROUND AND PURPOSE

Recent reports have suggested that electrical stimulation trains that take advantage of the catchlike property of skeletal muscle can produce higher forces from skeletal muscle than traditionally used constant-frequency trains. This study investigated the effects of catchlike-inducing trains on human quadriceps femoris muscles while the kneejoint was held at 15 degrees of flexion.

SUBJECTS AND METHODS

Subjects (N=12) were tested with constant-frequency trains that had interpulse intervals ranging from 10 to 160 milliseconds and comparable catchlike-inducing trains. Data were collected during the control condition (1 train every 10 seconds) and during repetitive contractions (1 train per second).

RESULTS

During control and repetitive activation conditions, catchlike-inducing trains produced approximately 5% to 110% greater peak forces than comparable constant-frequency trains, depending on the frequencies being compared. Total forces produced (ie, force-time integrals) were increased up to 59% and 49% during the control and repetitive activation conditions, respectively.

CONCLUSION AND DISCUSSION

These results support earlier findings that catchlike-inducing trains may be advantageous in functional electrical stimulation applications.

Variable-frequency trains offset low-frequency fatigue in human skeletal muscle

Variable-frequency trains that exploit the catchlike property of skeletal muscle can augment force production in fatigued skeletal muscle. The present study is the first to examine the effect of such trains during recovery. The quadriceps femoris muscles of 12 healthy individuals were fatigued using six-pulse, 14.3-Hz trains delivered at a rate of 1/s for 3 min. The force-generating ability of the muscle was tested with several constant-frequency trains (8.3-100 Hz) and a variable-frequency train before and after fatigue and at 2, approximately 13, and approximately 38 min of recovery. The variable-frequency train produced significant augmentation of force versus the best constant-frequency train (12.5 Hz) in acute fatigue and during recovery. The fatiguing protocol also induced low-frequency fatigue (LFF); the time courses of the degree of LFF and the amount of variable-frequency train force augmentation were inversely related (r = 0.629; F = 38.024; P </= 0.001), suggesting a common mechanism between the two phenomena. These results suggest that clinical use of variable-frequency trains (e.g., functional electrical stimulation) will enable the muscle to generate more force during acute fatigue and offset, at least partially, the long-term effects of LFF.

Effects of activation frequency and force on low-frequency fatigue in human skeletal muscle

No comparison of the amount of low-frequency fatigue (LFF) produced by different activation frequencies exists, although frequencies ranging from 10 to 100 Hz have been used to induce LFF. The quadriceps femoris of 11 healthy subjects were tested in 5 separate sessions. In each session, the force-generating ability of the muscle was tested before and after fatigue and at 2, approximately 13, and approximately 38 min of recovery. Brief (6-pulse), constant-frequency trains of 9.1, 14.3, 33.3, and 100 Hz and a 6-pulse, variable-frequency train with a mean frequency of 14.3 Hz were delivered at 1 train/s to induce fatigue. Immediately postfatigue, there was a significant effect of fatiguing protocol frequency. Muscles exhibited greater LFF after stimulation with the 9.1-, 14.3-, and variable-frequency trains. These three trains also produced the greatest mean force-time integrals during the fatigue test. At 2, approximately 13, and approximately 38 min of recovery, however, the LFF produced was independent of the fatiguing protocol frequency. The findings are consistent with theories suggesting two independent mechanisms behind LFF and may help identify the optimal activation pattern when functional electrical stimulation is used.

Two-step, predictive, isometric force model tested on data from human and rat muscles

Functional electrical stimulation can assist paralyzed individuals to perform functional movements, but muscle fatigue is a major limitation to its practical use. An accurate and predictive mathematical model can facilitate the design of stimulation patterns that optimize aspects of the force transient while minimizing fatigue. Solution nonuniqueness, a major shortcoming in previous work, was overcome with a simpler model. The model was tested on data collected during isometric contractions of rat gastrocnemius muscles and human quadriceps femoris muscles under various physiological conditions. For each condition tested, parameter values were identified using the force response to one or two stimulation trains. The parameterized model was then used to predict forces in response to other stimulation patterns. The predicted forces closely matched the measured forces. The model was not sensitive to initial parameter estimates, demonstrating solution uniqueness. By predicting the force that develops in response to an arbitrary pattern of stimulation, we envision the present model helping identify optimal stimulation patterns for activation of skeletal muscle during functional electrical stimulation.

Effects of activation pattern on human skeletal muscle fatigue

Variable-frequency stimulation trains (VFTs) that take advantage of the catchlike property of skeletal muscle have been shown to augment the force production of fatigued muscles compared with constant-frequency trains (CFTs). The present study is the first to report the force augmentation produced by VFTs after fatiguing the muscle with VFTs versus fatiguing the muscle with CFTs. Data were obtained from the human quadriceps femoris muscles of 12 healthy subjects. Each subject participated in three experimental sessions. Each session fatigued the muscle with one of three protocols: CFTs with 70-ms interpulse intervals (CFT70); CFTs with 55.5-ms interpulse intervals (CFT55.5); or VFTs. Following each fatiguing protocol the muscles were tested with all three stimulation patterns (i.e., CFT55.5, CFT70, and VFT). At the end of the fatiguing protocol the VFT produced force-time integrals and peak forces approximately 18% and 32% greater than the CFT70, respectively. The testing trains showed that the VFT produced approximately 25-35% greater force-time integrals than either CFT and approximately 35-47% greater peak forces than the CFT70. For each testing train, approximately 10-15% greater force-time integrals were seen when the muscles were fatigued with the CFTs than when fatigued with the VFTs. These results support suggestions that VFTs may be useful during clinical applications of electrical stimulation.

New look at force-frequency relationship of human skeletal muscle: effects of fatigue

A muscle does not have a unique force-frequency relationship; rather, it is dynamic and depends on the activation history of muscle. The purpose of this study was to investigate the force-frequency relationship of nonfatigued and fatigued skeletal muscle with the use of both catchlike-inducing trains (CITs) that exploited the catchlike property of skeletal muscle and constant-frequency trains (CFTs). Quadriceps femoris muscles were studied during isometric contractions in twelve healthy subjects (5 females, 7 males). Both the peak force and force-time integrals produced in response to each stimulation train were analyzed. Compared with nonfatigued muscles, higher frequencies of activation were needed to produce comparable normalized peak forces when the muscles were fatigued (i.e., a "rightward" shift in the force-frequency relationship) for both the CFTs and the CITs. When using the normalized force-time integral to measure muscle performance, the CFTs required slightly higher frequencies to produce comparable normalized forces from fatigued muscles, but the CITs did not. Furthermore, when the muscles were fatigued, the CITs produced greater peak forces and force-time integrals than all comparable CFTs with frequencies </=20 pps. In general, the lower the frequency the greater the augmentation produced by the CITs. In addition, the CIT that elicited the greatest force-time integral produced a 25% greater force-time integral than the best CFT. Because the CITs augmented forces across a wide range of physiological relevant activation rates, these results may have important clinical implications when using electrical stimulation to aid patients with paralysis. The results of this study contribute to our understanding of the relationship between the activation pattern of a muscle and the force output produced.

Reduction of the fatigue-induced force decline in human skeletal muscle by optimized stimulation trains

OBJECTIVE

To identify the stimulation pattern that optimizes the force-time integral produced during isometric contractions of fatigued human skeletal muscle.

DESIGN

Twelve healthy subjects with no history of lower extremity orthopedic problems voluntarily participated.

RESULTS

The primary findings were that (1) the optimized trains showed augmentation only from fatigued muscles and (2) a simple stimulation pattern, containing one brief (5msec) initial interpulse interval, produced the greatest force-time integrals and rates of rise of force. With muscle fatigue, the rate of rise of force of the constant-frequency train slowed, whereas the rate of rise of force of the optimized trains remained unchanged. This difference in the rate of rise of force may explain why the optimized trains, which take advantage of the catchlike property of skeletal muscle, are able to augment forces from fatigued muscles when compared with the constant-frequency train.

CONCLUSIONS

These results may have important clinical implications when using brief trains of electric stimulation to aid patients in performing functional movements and contribute to our understanding of the relationship between the activation pattern of a muscle and the force output produced.

Variable-frequency train stimulation of canine latissimus dorsi muscle during shortening contractions

In cardiomyoplasty, the latissimus dorsi muscle (LDM) is wrapped around the heart ventricles and electrically activated with a constant-frequency train (CFT). This study tested the hypotheses that increased mechanical performance from the LDM could be achieved by activating the muscle with variable-frequency trains (VFTs) of shorter duration or containing fewer stimulus pulses than the CFT now used. The mechanical performance of the canine LDM (n = 7) during shortening contractions was measured while the muscle was stimulated with 5- and 6-pulse CFTs (of duration 132 and 165 ms, respectively) and 5- and 6-pulse VFTs (of duration 104 and 143 ms, respectively) that were designed to take advantage of the catchlike property of skeletal muscle. Measurements were made from fresh and fatigued muscles. For the fresh muscles, the VFTs elicited significantly greater peak power than did the 6-pulse CFT. When the muscles were fatigued, VFT stimulation significantly improved both the peak and mean power produced compared with stimulation by CFTs. These results show that stimulation of the LDM with shorter duration VFTs is potentially useful for application in cardiomyoplasty.

A mathematical model that predicts skeletal muscle force

This study demonstrates the validity of a mathematical model that predicts the force generated by rat skeletal muscles during brief subtetanic and tetanic isometric contractions. The model consists of three coupled differential equations (ODE's). The first two equations represent the calcium dynamics and the third equation represents force dynamics. The model parameters were identified from brief trains of regularly spaces pulses [constant-frequency trains (CFT's)] that produce subtetanic muscle responses. Using these parameters, the model was able to predict isometric forces from other stimulation patterns. For the gastrocnemius muscles predictions were made for responses to CFT's with interpulse intervals (IPI's) ranging from 10 to 50 ms and variable-frequency trains (VFT's), where the initial IPI = 10 ms and the remaining IPI's were identical to those used for the CFT's. For the soleus muscles predictions were made for 10-100-ms CFT's. The shape of the predicted responses closely match the experimental data. Comparisons between experimental and modeled force-time integrals, peak forces, and time-to-peak also suggest excellent agreement between the model and the experiment data. Many physiological parameters predicted by the model agree with values obtained independently by others. In conclusion, the model accurately predicts isometric forces generated by rat gastrocnemius and soleus muscles produced by brief stimulation trains.

Effects of asynchronous stimulation on the human quadriceps femoris muscle

OBJECTIVE

Examine the effects of asynchronously activating motor units of the human quadriceps femoris muscle during electrical stimulation.

METHODS

Two stimulation channels were used to activate the muscle. One pair of electrodes was placed over the vastus medialis (Channel 1) and another pair was placed over the vastus lateralis (Channel 2). All contractions were isometric. The stimulation frequency was 10 Hz for each channel and the effect of varying the delay between the two channels was tested.

RESULTS

Synchronous activation of both channels produced the greatest forces and stimulating the muscle 180 degrees out of phase (ie, with a 50-msec delay) produced the least forces.

CONCLUSIONS

Activating motor units asynchronously under certain conditions will be less efficient than synchronous activation. This study, therefore, helps to define the boundary conditions when asynchronous stimulation may be helpful.

Catchlike property of human muscle during isovelocity movements

This study examined the catchlike property of skeletal muscle during eccentric and concentric isovelocity contractions of fresh and fatigued quadriceps femoris muscles of 10 healthy subjects. During concentric contractions of fresh muscles, stimulation trains that elicited a catchlike response (CITs) produced greater force outputs and rates of rise force than comparable constant-frequency trains. These enhancements became more pronounced during fatigue. CITs were less effective in enhancing forces during eccentric contractions but did improve the rates of rise of force. Overall, the CIT that produced the greatest augmentation had a 5-ms initial interpulse interval. Proposed mechanisms for the catchlike property involve enhanced muscle stiffness for more efficient transmission of tension and increased calcium release. These results suggest that stimulation trains that take advantage of the catchlike property of skeletal muscle may be helpful during clinical applications where neuromuscular electrical stimulation is used to restore function in patients with damaged central nervous systems.

Effects of stimulation intensity on the physiological responses of human motor units

Quadriceps femoris muscles were studied in 50 healthy subjects to determine the physiological responses of the motor units recruited at different force levels during transcutaneous electrical stimulation. During one set of experiments force-frequency relationships were compared at stimulation intensities that produced tetanic contraction of 20%, 50%, or 80% of the maximum voluntary isometric contraction (MVC). No differences in the normalized force-frequency relationship were observed between the 20% and 50% of MVC conditions and only a slight shift to the left was observed at 80% of MVC. The other set of experiments measured the responses to electrically elicited fatigue tests using frequencies of 20, 40, or 60 pps and, at each frequency, intensities that produced 20% or 50% of MVC. Fatigue was greater for the 50% than 20% MVC force conditions. Within each force level fatigue increased with increasing frequency. However, though the differences in the level of recruitment needed to produce the two forces varied for each frequency, the differences in the amount of fatigue produced at each force did not vary between the three stimulation frequencies. This suggests that the fatigue characteristics of the recruited motor units were similar at all intensities tested. We posit, therefore, that the physiological recruitment order during transcutaneous electrical stimulation is less orderly than previously suggested.

Variable-frequency stimulation patterns for the optimization of force during muscle fatigue. Muscle wisdom and the catch-like property

Muscle wisdom is the process whereby the activation rates of motor units are modulated by the central nervous system to optimize the force during sustained voluntary contractions. During maximal voluntary contractions the activation rates decline as the muscle fatigues. No similar decline has been observed during submaximal contractions. Subsequent chapters explore the potential mechanisms for muscle wisdom. In this chapter, a historical background on the development of ideas on muscle wisdom is first presented. Next, artificial wisdom, the procedure used to optimize force during an electrically imposed tetanus by progressively reducing the stimulation frequency as the muscle fatigues, is discussed. Finally, recent studies are described in which fatigue was delayed and reduced by the use of variable-frequency stimulus trains that elicit the catch-like property of muscle.

Muscle fatigue: clinical implications for fatigue assessment and neuromuscular electrical stimulation

Muscle fatigue can be defined as a decrease in the force-generating ability of a muscle that resulted from recent activity. Recent studies of muscle fatigue are reviewed that are relevant to two areas of interest to physical therapists: clinical assessment of muscle fatigue and neuromuscular electrical stimulation. Volitional and electrical tests have been used to quantify muscle fatigue. Several variations on each type of test are discussed, as are the possible sites in which fatigue might occur. The rate of fatigue during the therapeutic application of electrical stimulation of skeletal muscle is much greater than that seen during volitional contractions. Factors contributing to this phenomenon are examined. The unique requirements affecting how stimulus variables can be manipulated to minimize muscle fatigue in three specific therapeutic uses of neuromuscular electrical stimulation are addressed.

Fatigability of human quadriceps femoris muscle following anterior cruciate ligament reconstruction

The responses of quadriceps femoris muscles to an electrically elicited fatigue test were recorded from both lower extremities of 18 patients who had recently undergone unilateral, anterior cruciate ligament reconstruction. The fatigue test consisted of 40 pps, 13-pulse electrical trains that were repeated once per second for 3 min. The intensity of stimulation was set for each extremity to produce 20% of the maximum voluntary isometric contraction of the uninvolved muscle. The uninvolved quadriceps femoris muscle showed a significantly greater rate of decline in force over the first minute than the involved muscle (0.803%.s-1 for uninvolved muscle vs 0.620%.s-1 for involved muscle). Similarly, the average forces produced over the last minute were significantly lower for the uninvolved than the involved quadriceps femoris muscle (uninvolved = 42.6%, involved = 50.4% of their original forces). These surprising results showed that the involved quadriceps femoris muscles were more endurant than the uninvolved muscles. It is suggested that the increases in endurance of the involved muscle may have been due, in part, to greater recruitment of Type I fibers with electrical stimulation or selective Type II fiber atrophy in the involved muscle.

Force response of rat soleus muscle to variable-frequency train stimulation

1. The purpose of this study was to study the effects of a high-frequency burst of pulses at the onset of a subtetanic train of pulses on the force output of the rat soleus muscle. 2. The soleus muscle was studied in eight rats deeply anesthetized with urethan. The effects of two-, three-, or four-pulse bursts at the onset of subtetanic trains containing a total of 12 pulses were studied in detail. 3. The results showed that two-pulse bursts at the onset of the train produced approximately 20% augmentation in average force and nearly a 50% reduction in the time required to reach a targeted level of force, compared with a comparable 12-pulse subtetanic constant-frequency train; three- or four- pulse bursts produced progressively less additional improvement. In contrast, the two-pulse bursts produced approximately 13% increase in the force-time integral (Area), the three-pulse burst did not significantly further increase the Area gain, and the use of four-pulse bursts markedly decreased the gain in Area. 4. For all three bursts, the observed force augmentation rapidly declined over the 12-pulse trains. Extrapolation beyond the actual data suggested that the force augmentation should last for between approximately 16 and 19 interpulse intervals. 5. To describe the characteristics of the contractile response of the muscle that explains or predicts the amount of force augmentation seen, we made three measurements of the response to the burst of pulses: 1) the peak force produced by the initial burst of pulses (PeakBURST), 2) the force at the time of arrival of the pulse that followed the burst (CatchBURST), and 3) the rise in force produced by the pulse that followed the burst (PotBURST). Of these three measurements, the CatchBURST was the best predictor of the force augmentation seen. 6. The present results showed 1) the importance of the stimulation pattern on the force output of skeletal muscle; 2) that the force-frequency relationship is multivalued, with force depending on both the stimulation history and stimulation frequency; and 3) that a relatively simple discharge strategy, where each train of pulses begins with one or two brief interpulse interval durations, will produce the maximum force from the muscle and result in a predictable force-frequency relationship.

Changes in the force-frequency relationship of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue

The purpose of this study was to identify the changes in the force-frequency relationship (FFR) of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue. Twenty nondisabled subjects each participated in one experimental session to test the effects of electrically induced fatigue on the FFR; 10 of these subjects participated in a second session in which voluntarily induced fatigue was produced. Fatigue was induced by having subjects perform repeated, 8-second, isometric contractions followed by 12-second rests until 50% of the initial force was produced. Markedly decreased forces were seen at all frequencies tested following fatigue. Low frequency fatigue was observed following both fatiguing protocols. The frequencies needed to produce near-maximum forces did not shift with fatigue. These results suggest that the most appropriate stimulation frequency to use when activating skeletal muscle depends on both the percentage of tetanic force desired and the fatigue state of the muscle. This study also provides the clinician with data on the FFR of healthy human quadriceps femoris muscle prior to fatigue. [Binder-Macleod SA, McDermond LR. Changes in the force-frequency relationship of the human quadriceps femoris muscle following electrically and voluntarily induced fatigue.

Use of a catchlike property of human skeletal muscle to reduce fatigue

This study compared the force output produced by variable frequency, short-duration trains (VFTs) of electrical pulses with the forces produced by constant frequency, short-duration trains (CFTs). Human quadriceps femoris muscle was stimulated with a 300-msec train of pulses once every second for 180 seconds. Each subject (n = 12) participated in 4 randomly assigned experimental sessions. During 3 sessions, a CFT of 80, 40 or 20 pps was used. During a fourth session, a VFT, which consisted of all 3 of the above frequencies, was used. The force at 100 msec, average force of each contraction and peak force were calculated for every 30th contraction. By the 90th contraction, the force at 100 msec and the average force were significantly greater for the VFT than for each CFT. Thus, the VFT, by using a catchlike property, may provide significant advantages over any CFT when using electrical stimulation for functional electrical stimulation.

Preservation of force output through progressive reduction of stimulation frequency in human quadriceps femoris muscle

The purpose of this study was to determine the effects of a reduction in the pulse frequency on the fatigue rate of human quadriceps femoris muscle during intermittent (8-second) contractions. Twelve healthy subjects each participated in two experimental sessions. Thirty cycles (cycle time: 8 seconds "on"/12 seconds "off") were applied during each session. During one session, a frequency of 60 pulses per second (pps) was used for all trains. During the other session, the subjects were stimulated with 60 pps for the first train. The stimulating frequency of each train was then progressively reduced, in 5-pps steps, for contractions 2, 3, 5, 8, 12, and 20. By the fifth contraction, the differences in average force produced by the 60-pps trains and the reduced-frequency trains were significant. The difference between the two conditions increased, with the variable-frequency protocol producing 46% more force than the constant-frequency protocol during the last contraction. These results showed that, compared with a constant pulse frequency, reducing the pulse frequency during a fatiguing contraction can markedly decrease the rate of force fatigue of skeletal muscle. This finding suggests that a variable-frequency protocol, similar to the one used in this may prove to be a more effective pattern of stimulation for activation of skeletal muscle than the traditionally used constant-frequency protocol.

Force output of cat motor units stimulated with trains of linearly varying frequency

1. A relation between stimulation frequency and muscle force is usually determined with stimulus trains of constant frequency and described as a single-valued sigmoid curve. This relationship fails to explain a number of features of rate coding. 2. Single motor units were isolated in medial gastrocnemius or soleus muscles of cats deeply anesthetized with pentobarbital sodium. Motor units were classified as fast or slow. Each unit was stimulated with a train whose frequency varied linearly from less than 3 pulses per second (pps) to 20% above the unit's fusion frequency and back to about 3 pps with a period of 5 s. 3. All motor units showed a marked hysteresis during frequency-varying stimulation. A greater force was produced when frequency was decreasing than when it was increasing. The force output of each unit remained nearly maximal as stimulus frequency declined from its maximum to about one-half of the unit's fusion frequency; force rapidly declined with further decreases in frequency. The force-frequency relation could change with each trial as frequency increased but was highly reproducible when frequency decreased. This suggested a strategy by which central nervous system (CNS) control could maximize the force at any discharge rate and produce a predictable force-frequency relation. 4. Posttetanic potentiation, motor unit slowing, and a preload which causes a motor unit to operate on the negatively sloping portion of the length-tension curve may each contribute to the observed hysteresis under certain circumstances. None can explain why hysteresis was consistently seen in all motor units. A time-dependent rate of tension development and decay together with a catchlike property can account for all of the properties of hysteresis and appeared to be the primary cause of hysteresis in fully potentiated motor units.

Use of the Krusen Limb Load Monitor to quantify temporal and loading measurements of gait

The purposes of this investigation were to determine whether the temporal and force measurements from the Krusen Limb Load Monitor produced clinically reliable data and to begin identifying the factors that determine the monitor's reliability. Temporal and loading measurements were made from the output of the Krusen Limb Load Monitor and compared to values obtained from a calibrated force platform. Such comparisons were made for 30 steps taken by two subjects on three separate occasions and from the same two subjects plus a third subject for 100 consecutive steps. For most measures, mean values from the limb-load monitor were significantly different from those recorded from the force platform. From a clinical perceptive, however, the range of measures was narrow for the 95 percent confidence level of the observed differences for the temporal components of stance between the limb-load monitor and force platform, with the narrowest range of measures related to the appropriateness of "fit" of the limb-load monitor force plate within the shoe. The loading components of stance showed a relatively wide 95 percent confidence interval that appeared unrelated to fit. Thus, given a "good fitting" force plate insert, the therapist can make clinically meaningful measurements of the temporal components of the stance phase of gait using the limb-load monitor.