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

Neuromuscular electrical stimulation after cardiovascular surgery mitigates muscle weakness in older individuals with diabetes

BACKGROUND

Cardiovascular surgery leads to postsurgical muscle weakness, probably because of muscle proteolysis and peripheral nerve dysfunction, which are augmented by aging and diabetes mellitus.

OBJECTIVE

We examined the effect of neuromuscular electrical stimulation (NMES) on postsurgical muscle weakness in older individuals with diabetes mellitus.

METHODS

We conducted a multicentre, randomized, controlled trial, and screened consecutive patients with diabetes who underwent cardiovascular surgery for eligibility (age ≥ 65 years). Those included were randomly assigned to the NMES or the sham group. The primary outcome was the percent change in isometric knee extension strength (%ΔIKES) from preoperative to postoperative day 7. Secondary outcomes were the percent change in usual (%ΔUWS), maximum walking speed (%ΔMWS), and grip strength (%ΔGS). A statistician who was blinded to group allocation used intention-to-treat analysis (student t test).

RESULTS

Of 1151 participants screened for eligibility, 180 (NMES, n = 90; sham, n = 90) were included in the primary analysis. %ΔIKES was significantly lower in the NMES than sham group (NMES: mean -2%, 95% confidence interval [CI] -6 to 1; sham: -13%, 95% CI -17 to -9, p < 0.001). Among the secondary outcomes, %ΔMWS was significantly lower and %ΔUWS and %ΔGS were lower, although not significantly, in the NMES than sham group.

CONCLUSIONS

A short course of NMES (< 1 week) mitigated postsurgical muscle weakness and functional decline in older persons with diabetes mellitus. NMES could be recommended as a part of postsurgical rehabilitation in older people with diabetes mellitus, especially those with a low functional reserve.

Myofibers

Muscle tissue is a highly specialized type of tissue, made up of cells that have as their fundamental properties excitability and contractility. The cellular elements that make up this type of tissue are called muscle fibers, or myofibers, because of the elongated shape they have. Contractility is due to the presence of myofibrils in the muscle fiber cytoplasm, as large cellular assemblies. Also, myofibers are responsible for the force that the muscle generates which represents a countless aspect of human life. Movements due to muscles are based on the ability of muscle fibers to use the chemical energy procured in metabolic processes, to shorten and then to return to the original dimensions. We describe in detail the levels of organization for the myofiber, and we correlate the structural aspects with the functional ones, beginning with neuromuscular transmission down to the biochemical reactions achieved in the sarcoplasmic reticulum by the release of Ca²⁺ and the cycling of crossbridges. Furthermore, we are reviewing the types of muscle contractions and the fiber-type classification.

Intramuscular stimulation of tibialis anterior in human subjects: the effects of discharge variability on force production and fatigue

Continuous intramuscular stimulation of tibialis anterior (TA) was used to test the hypothesis that irregular trains of stimuli can increase force production and offset the magnitude of fatigue when compared with a continuous train of regular stimuli at an identical mean frequency (19 or 24 Hz). To achieve this, tungsten microelectrodes were inserted into the muscle belly into the motor point of the tibialis anterior muscle of able-bodied individuals (aged 19-50) and stimulated at current intensities ranging from 5 to 7 mA. The motor point was stimulated with a continuous train of regular stimulation at either 19 or 24 Hz (n = 11) or until the force declined below 25% of the peak force at the onset of stimulation. For the first seven subjects, no fatigue was exhibited, and thus, we simply compared the forces generated by the regular and irregular segments of the continuous train (120 sec for each segment). For four additional subjects, we delivered a higher frequency train (24 Hz) that elicited some fatigue. Once the force had declined below 25% of the initial peak force (which took between 140 and 210 sec), the continuous irregular train was integrated. Interestingly, for those subjects who exhibited muscular fatigue, force always began to rise again once the irregularity was incorporated into the continuous regular train of stimulation at the identical mean frequency (24 Hz). We conclude that incorporating irregularity into continuous trains of stimuli offers a significant advantage to the human neuromuscular system during both fatigued and nonfatigued states and could offer benefits to therapies such as functional electrical stimulation (FES).

Microstimulation of single human motor axons in the toe extensors: force production during long-lasting trains of irregular and regular stimuli

Human motoneurones are known to discharge with a physiological variability of ~25% during voluntary contractions. Using microstimulation of single human motor axons, we have previously shown that delivering brief trains (10 pulses) of irregular stimuli, which incorporate discharge variability, generates greater contractile responses than trains of regular stimuli with identical mean frequency but zero variability. We tested the hypothesis that longer irregular (physiological) trains would produce greater contractile responses than regular (nonphysiological) trains of the same mean frequency (18 Hz) and duration (45 sec). Tungsten microelectrodes were inserted into the common peroneal nerve of human subjects, and single motor axons supplying the toe extensors (n = 14) were isolated. Irregular trains of stimuli showed greater contractile responses over identical mean frequencies in both fatigue-resistant and fatigable motor units, but because the forces were higher the rate of decline was higher. Nevertheless, forces produced by the irregular trains were significantly higher than those produced by the regular trains. We conclude that discharge irregularity augments force production during long as well as short trains of stimulation.

Musculoskeletal Effects of 2 Functional Electrical Stimulation Cycling Paradigms Conducted at Different Cadences for People With Spinal Cord Injury: A Pilot Study

OBJECTIVE

To compare the musculoskeletal effects of low cadence cycling with functional electrical stimulation (FES) with high cadence FES cycling for people with spinal cord injury (SCI).

DESIGN

Randomized pre-post design.

SETTING

Outpatient rehabilitation clinic.

PARTICIPANTS

Participants (N=17; 14 men, 3 women; age range, 22-67y) with C4-T6 motor complete chronic SCI were randomized to low cadence cycling (n=9) or high cadence cycling (n=8).

INTERVENTIONS

Low cadence cycling at 20 revolutions per minute (RPM) and high cadence cycling at 50 RPM 3 times per week for 6 months. Cycling torque (resistance per pedal rotation) increased if targeted cycling cadence was maintained.

MAIN OUTCOME MEASURES

Dual-energy x-ray absorptiometry was used to assess distal femur areal bone mineral density, magnetic resonance imaging was used to assess to assess trabecular bone microarchitecture and cortical bone macroarchitecture and thigh muscle volume, and biochemical markers were used to assess bone turnover. It was hypothesized that subjects using low cadence cycling would cycle with greater torque and therefore show greater musculoskeletal improvements than subjects using high cadence cycling.

RESULTS

A total of 15 participants completed the study. Low cadence cycling obtained a maximal average torque of 2.9±2.8Nm, and high cadence cycling obtained a maximal average torque of 0.8±0.2Nm. Low cadence cycling showed greater decreases in bone-specific alkaline phosphatase, indicating less bone formation (15.5% decrease for low cadence cycling, 10.7% increase for high cadence cycling). N-telopeptide decreased 34% following low cadence cycling, indicating decreased resorption. Both groups increased muscle volume (low cadence cycling by 19%, high cadence cycling by 10%). Low cadence cycling resulted in a nonsignificant 7% increase in apparent trabecular number (P=.08) and 6% decrease in apparent trabecular separation (P=.08) in the distal femur, whereas high cadence cycling resulted in a nonsignificant (P>.3) 2% decrease and 3% increase, respectively.

CONCLUSIONS

This study suggests that the greater torque achieved with low cadence cycling may result in improved bone health because of decreased bone turnover and improved trabecular bone microarchitecture. Longer-term outcome studies are warranted to identify the effect on fracture risk.

Comparison of contractile responses of single human motor units in the toe extensors during unloaded and loaded isotonic and isometric conditions

Much of the repertoire of muscle function performed in everyday life involves isotonic dynamic movements, either with or without an additional load, yet most studies of single motor units measure isometric forces. To assess the effects of muscle load on the contractile response, we measured the contractile properties of single motor units supplying the toe extensors, assessed by intraneural microstimulation of single human motor axons, in isotonic, loaded isotonic, and isometric conditions. Tungsten microelectrodes were inserted into the common peroneal nerve, and single motor axons (n = 10) supplying the long toe extensors were electrically stimulated through the microelectrode. Displacement was measured from the distal phalanx of the toe with either an angular displacement transducer for the unloaded (i.e., no additional load) and loaded (addition of a 4-g mass) isotonic conditions or a force transducer for the isometric conditions. Mean twitch profiles were measured at 1 Hz for all conditions: rise time, fall time, and duration were shortest for the unloaded isotonic conditions and longest for the isometric conditions. Peak displacements were lower in the loaded than unloaded isotonic conditions, and the half-maximal response in the loaded condition was achieved at lower frequencies than in the unloaded isotonic condition. We have shown that the contractile responses of single motor units supplying the human toe extensors are influenced by how they are measured: twitches are much slower when measured in loaded than unloaded isotonic conditions and slowest when measured in isometric conditions.

Feasibility of neuromuscular electrical stimulation immediately after cardiovascular surgery

OBJECTIVE

To determine the safety and feasibility of neuromuscular electrical stimulation (NMES) from postoperative days (PODs) 1 to 5 after cardiovascular surgery.

DESIGN

Pre-post interventional study.

SETTING

Surgical intensive care unit and thoracic surgical ward of a university hospital.

PARTICIPANTS

Consecutive patients (N=144) who underwent cardiovascular surgery were included. Patients with peripheral arterial disease, psychiatric disease, neuromuscular disease, and dementia were excluded. Patients with severe chronic renal failure and those who required prolonged mechanical ventilation after surgery were also excluded because of the possibility of affecting the outcome of a future controlled study.

INTERVENTIONS

NMES to the lower extremities was implemented from PODs 1 to 5.

MAIN OUTCOME MEASURES

Feasibility outcomes included compliance, the number of the patients who had changes in systolic blood pressure (BP) >20 mmHg or an increase in heart rate >20 beats/min during NMES, and the incidence of temporary pacemaker malfunction or postoperative cardiac arrhythmias.

RESULTS

Sixty-eight of 105 eligible patients participated in this study. Sixty-one (89.7%) of them completed NMES sessions. We found no patients who had excessive changes in systolic blood pressure, increased heart rate, or pacemaker malfunction during NMES. Incidence of atrial fibrillation during the study period was 26.9% (7/26) for coronary artery bypass surgery, 18.2% (4/22) for valvular surgery, and 20.0% (4/20) for combined or aortic surgery. No sustained ventricular arrhythmia or ventricular fibrillation was observed.

CONCLUSIONS

The results of this study demonstrate that NMES can be safely implemented even in patients immediately after cardiovascular surgery.

Comparison of the contractile responses to irregular and regular trains of stimuli during microstimulation of single human motor axons

During voluntary contractions, human motoneurons discharge with a physiological variability of ∼20%. However, studies that have measured the contractile responses to microstimulation of single motor axons have used regular trains of stimuli with no variability. We tested the hypothesis that irregular (physiological) trains of stimuli produce greater contractile responses than regular (nonphysiological) trains of identical mean frequency but zero variability. High-impedance tungsten microelectrodes were inserted into the common peroneal nerve and guided into fascicles supplying a toe extensor muscle. Selective microstimulation was achieved for 14 single motor axons. Contractile responses were measured via an angular displacement transducer over the relevant toe. After the responses to regular trains of 10 stimuli extending from 2 to 100 Hz were recorded, irregular trains of 10 stimuli, based on the interspike intervals recorded from single motor units during voluntary contractions, were delivered. Finally, the stimulation sequences were repeated following a 2-min period of continuous stimulation at 10 Hz to induce muscle fatigue. Regular trains of stimuli generated a sigmoidal increase in displacement with frequency, whereas irregular trains, emulating the firing of volitionally driven motoneurons, displayed significantly greater responses over the same frequency range (8-24 Hz). This was maintained even in the presence of fatigue. We conclude that physiological discharge variability, which incorporates short and long interspike intervals, offers an advantage to the neuromuscular system by allowing motor units to operate on a higher level of the contraction-frequency curve and taking advantage of catch-like properties in skeletal muscle.

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.

Variable stimulation patterns in younger and older thenar muscle

Neuromuscular electrical stimulation (NMES) is typically used with older adults receiving rehabilitation therapies, but little is known about the stimulation patterns that maximize force output and minimize fatigue in this population. The purpose of this study was to apply variable patterns of stimulation to the thenar muscles of the hand in younger and older adults to determine if force production and neuromuscular fatigue effects were similar. Three submaximal stimulation patterns were administered: A 20Hz constant frequency pattern, a pattern that increased from 20 to 40Hz, and a pattern that incorporated two closely spaced (5ms) doublet pulses. The doublet stimulation produced significantly higher average forces and force-time integrals (FTIs) than the constant frequency and increasing frequency patterns in both age groups. Additionally, older adults showed less fatigue than the younger group during isometric contractions performed after the fatiguing stimulation patterns. These results suggest that variable pulse NMES patterns enhance force production in the hand in both younger and older individuals better than constant frequency patterns, which are typically used in clinical applications. Also, greater fatigue resistance to electrical stimulation protocols may exist in the older population; this is critical information for the design and application of NMES rehabilitation regimens used with older adults.

Catchlike property in human adductor pollicis muscle

The "catchlike" property is defined as the dramatic force increase in skeletal muscles when a single pulse is added at the onset of a sub-tetanic low-frequency stimulation train. This property has been observed in single motor units, whole animal and human muscles. It is an inherent property of muscle fibres and is not related to an increase in motor unit recruitment. Despite an abundance of observations, its origin remains unclear. The aim of this study was to induce the catchlike property in human adductor pollicis and identify its possible origin. Thumb adduction forces were measured using ulnar nerve electrical stimulation at 10Hz for reference trains (RTs) with one extra pulse 8ms after the first stimulation pulse for the experimental trains (ETs). Tests were performed at two muscle length and three stimulation levels and muscle stiffness and potentiation were quantified for all test conditions. The ETs showed higher forces and greater rates of force increase than the RTs. In addition, force increase was more pronounced at short compared to long muscle length, but no differences were found in force increase for the three stimulation levels. Furthermore, potentiation and stiffness were similar across all experimental conditions. Together, these results suggest that the increase in force associated with the catchlike property is neither caused by an increased proportion of attached cross-bridges nor potentiation of the muscle, but appears to be muscle length dependent and present in both slow and fast motor units.

Electrically induced contraction levels of the quadriceps femoris muscles in healthy men: the effects of three patterns of burst-modulated alternating current and volitional muscle fatigue

OBJECTIVE

The purpose of this study was to compare electrically induced contraction levels produced by three patterns of alternating current in fatigued and nonfatigued skeletal muscles.

DESIGN

Eighteen male volunteers without health conditions, with a mean (SD) age of 24.9 (3.4) yrs were randomly exposed to a fatiguing volitional isometric quadriceps contraction and one of three patterns of 2.5-KHz alternating current; two were modulated at 50 bursts per second (10% burst duty cycle with five cycles per burst and 90% burst duty cycle with 45 cycles per burst), and one pattern was modulated at 100 bursts per second (10% burst duty cycle with 2.5 cycles per burst). The electrically induced contraction levels produced by the three patterns of electrical stimulation were compared before and after the fatiguing contraction.

RESULTS

The 10% burst duty cycles produced 42.9% (95% confidence interval, 29.1%-56.7%) and 32.1% (95% confidence interval, 18.2%-45.9%) more muscle force (P < 0.001) than did the 90% burst duty cycle pattern. There was no significant interaction effect (P = 0.392) of electrical stimulation patterns and fatigue on the electrically induced contraction levels.

CONCLUSIONS

The lower burst duty cycle (10%) patterns of electrical stimulation produced stronger muscle contractions. Furthermore, the stimulation patterns had no influence on the difference in muscle force before and after the fatiguing quadriceps contraction. Consequently, for clinical applications in which high forces are desired, the patterns using the 10% burst duty cycle may be helpful.

Novel patterns of functional electrical stimulation have an immediate effect on dorsiflexor muscle function during gait for people poststroke

BACKGROUND

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.

OBJECTIVE

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.

DESIGN

A repeated-measures design was used in this study.

PARTICIPANTS

Thirteen individuals with hemiparesis following stroke (9 men, 4 women; age=46-72 years) participated in the study.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Variable stimulation patterns for poststroke hemiplegia

Neuromuscular electrical stimulation can improve motor function in those affected by paralysis, but its use is limited by a high rate of muscular fatigue. Variable stimulation patterns have been examined in young adults with and without spinal cord injury, but much less investigation has been devoted to studying the effects of variable stimulation patterns administered to older adults or those paralyzed by stroke. Significant changes occur in the neuromuscular system with age that may affect the response to variable stimulation patterns. We administered three, 3-min intermittent stimulation patterns to the median nerves of 10 individuals with hemiplegia from stroke and 10 age-matched able-bodied adults: (1) constant 20 HZ, (2) a pattern that began at 20 HZ and progressively increased to 40 HZ in the latter half of the task, and (3) a 20-HZ pattern that switched to a 20-HZ doublet pattern after 90 s. In the able-bodied group the doublet pattern produced significantly higher force time integrals (FTI) (1409.72 +/- 3.15 N s) than the 20-40-HZ pattern (1067.46 +/- 1.15 N s) or the 20-HZ pattern (831 +/- 1.87 N s). In the poststroke individuals the doublet pattern also produced the highest FTI (724.04 +/- 2.02 N s), and there was no significant difference between the 20-40-HZ (636.42 +/- 1.65 N s) and 20-HZ (583.64 +/- 3.02 N s) patterns. These results indicate that protocols that incorporate doublets in the later stages of fatigue are effective in older adults and in older adults with paralysis from stroke.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Predicting muscle forces of individuals with hemiparesis following stroke

Background

Functional electrical stimulation (FES) has been used to improve function in individuals with hemiparesis following stroke. An ideal functional electrical stimulation (FES) system needs an accurate mathematical model capable of designing subject and task-specific stimulation patterns. Such a model was previously developed in our laboratory and shown to predict the isometric forces produced by the quadriceps femoris muscles of able-bodied individuals and individuals with spinal cord injury in response to a wide range of clinically relevant stimulation frequencies and patterns. The aim of this study was to test our isometric muscle force model on the quadriceps femoris, ankle dorsiflexor, and ankle plantar-flexor muscles of individuals with post-stroke hemiparesis.

Methods

Subjects were seated on a force dynamometer and isometric forces were measured in response to a range of stimulation frequencies (10 to 80-Hz) and 3 different patterns. Subject-specific model parameter values were obtained by fitting the measured force responses from 2 stimulation trains. The model parameters thus obtained were then used to obtain predicted forces for a range of frequencies and patterns. Predicted and measured forces were compared using intra-class correlation coefficients, r² values, and model error relative to the physiological error (variability of measured forces).

Results

Results showed excellent agreement between measured and predicted force-time responses (r² >0.80), peak forces (ICCs>0.84), and force-time integrals (ICCs>0.82) for the quadriceps, dorsiflexor, and plantar-fexor muscles. The model error was within or below the +95% confidence interval of the physiological error for >88% comparisons between measured and predicted forces.

Conclusion

Our results show that the model has potential to be incorporated as a feed-forward controller for predicting subject-specific stimulation patterns during FES.

Slow temporal filtering may largely explain the transformation of stick insect (Carausius morosus) extensor motor neuron activity into muscle movement

Understanding how nervous systems generate behavior requires understanding how muscles transform neural input into movement. The stick insect extensor tibiae muscle is an excellent system in which to study this issue because extensor motor neuron activity is highly variable during single leg walking and extensor muscles driven with this activity produce highly variable movements. We showed earlier that spike number, not frequency, codes for extensor amplitude during contraction rises, which implies the muscle acts as a slow filter on the time scale of burst interspike intervals (5-10 ms). We examine here muscle response to spiking variation over entire bursts, a time scale of hundreds of milliseconds, and directly measure muscle time constants. Muscle time constants differ during contraction and relaxation, and contraction time constants, although variable, are always extremely slow (200-700 ms). Models using these data show that extremely slow temporal filtering alone can explain much of the observed transform properties. This work also revealed an unexpected (to us) ability of slow filtering to transform steadily declining inputs into constant amplitude outputs. Examination of the effects of time constant variability on model output showed that variation within an SD primarily altered output amplitude, but variation across the entire range also altered contraction shape. These substantial changes suggest that understanding the basis of this variation is central to predicting extensor activity and that the animal could theoretically vary muscle time constant to match extensor response to changing behavioral need.

Different motor neuron spike patterns produce contractions with very similar rises in graded slow muscles

Graded muscles produce small twitches in response to individual motor neuron spikes. During the early part of their contractions, contraction amplitude in many such muscles depends primarily on the number of spikes the muscle has received, not the frequency or pattern with which they were delivered. Stick insect (Carausius morosus) extensor muscles are graded and thus would likely show spike-number dependency early in their contractions. Tonic stimulations of the extensor motor nerve showed that the response of the muscles differed from the simplest form of spike-number dependency. However, these differences actually increased the spike-number range over which spike-number dependency was present. When the motor nerve was stimulated with patterns mimicking the motor neuron activity present during walking, amplitude during contraction rises also depended much more on spike number than on spike frequency. A consequence of spike-number dependency is that brief changes in spike frequency do not alter contraction slope and we show here that extensor motor neuron bursts with different spike patterns give rise to contractions with very similar contraction rises. We also examined in detail the early portions of a large number of extensor motor neuron bursts recorded during single-leg walking and show that these portions of the bursts do not appear to have any common spike pattern. Although alternative explanations are possible, the simplest interpretation of these data is that extensor motor neuron firing during leg swing is not tightly controlled.

Mathematical model that predicts lower leg motion in response to electrical stimulation

Electrical stimulation of skeletal muscles of patients with upper motor neuron lesions can be used to restore functional movements such as standing or walking. Mathematical muscle models can assist in designing stimulation patterns that will enable patients to perform particular tasks more efficiently. In this study we extend our previous model to allow us to predict changes in knee joint angle in response to electrical stimulation of the human quadriceps femoris muscle. The model was tested both with and without inertial loads placed around the ankle joints of healthy subjects. Results showed that the model predicted the knee extensions with a RMS angle error that was generally <or=8 degrees. The coefficients of determination between the measured and predicted data showed the model accounted for approximately 71%, approximately 94%, approximately 73%, and approximately 89% of the variances in the experimental maximum excursion, time to maximum excursion, maximum shortening velocity, and time to maximum shortening velocity, respectively. This study showed that our general non-isometric model predicted the lower limb motion in response to a range of stimulation frequencies and patterns, and external loads. This model can be implemented in an algorithm for controlling the position of the lower leg during the swing phase of gait during functional electrical stimulation.

Strategies That Improve Paralyzed Human Quadriceps Femoris Muscle Performance During Repetitive, Nonisometric Contractions

OBJECTIVE

To determine the effect of combining different stimulation frequencies on the ability of paralyzed human quadriceps muscle to produce a 50 degrees knee excursion repetitively when starting at 90 degrees of flexion.

DESIGN

Repeated-measures design.

SETTING

Clinical research laboratory.

PARTICIPANTS

Complete data were collected from 9 subjects aged 11 to 25 years (mean +/- standard deviation, 17.1+/-4.5y) with spinal cord injury (SCI).

INTERVENTION

Three protocols were each tested during separate sessions: 20-Hz trains of pulses followed by 66-Hz trains (C20+66), 33-Hz trains followed by 66-Hz trains (C33+66), and 66-Hz trains alone (C66). For each frequency, stimulation was repeated until the knee failed to produce a 50 degrees excursion. This approach allowed us to evaluate the response to stimulation with 20-, 33-, and 66-Hz and combinations of 20- and 66-Hz and 33- and 66-Hz trains.

MAIN OUTCOME MEASURE

Number of successful contractions.

RESULTS

The C20 and C33 did not differ (mean, 41.0+/-12.6 excursions and 42.0+/-12.3 excursions, respectively), and each produced more excursions than the C66 protocol. The C20+66 and C33+66 protocols produced 51.4+/-15.0 and 44.9+/-13.6 excursions, respectively, and the C20+66 was the best protocol overall (all P<or=.05).

CONCLUSIONS

This study showed that stimulation strategies that start with low frequencies and switch to higher frequencies as the muscle fatigues could improve the ability of functional electric stimulation applications to perform repetitive, nonisometric contractions in subjects with SCI.

Doublets and low-frequency fatigue in potentiated human muscle

AIM

To test the hypothesis that doublets compensate for low-frequency fatigue. Doublets increase force output from muscles stimulated at low frequencies. Low-frequency fatigue is a decline in the force elicited by low-frequency stimulation.

METHODS

Human flexor carpi radialis muscles were stimulated with 20 Hz trains with and without an initial doublet and with and without low-frequency fatigue and the resulting force response measured.

RESULTS

An initial doublet caused an increase in the maximum rate of force rise of 179.6 +/- 27.9% in rested and 242.9 +/- 37.7% in muscles with low-frequency fatigue, and a substantial enhancement in force in the first three inter-pulse intervals after the extra pulse. The magnitude and time course of the early doublet enhancement were very similar regardless of low-frequency fatigue, consistent with current theories regarding the mechanisms of the doublet effect and of low-frequency fatigue. By the end of the 1 s stimulus train, force enhancement was insignificant in rested muscles and was small and subject-dependent in muscles with low-frequency fatigue (17.3 +/- 8.1% of force without a doublet), reducing the force deficit by 25.2 +/- 5.5%.

CONCLUSIONS

The time course of doublet force enhancement implies that an initial doublet may effectively compensate for the deficit in rate of force rise in muscles with low-frequency fatigue, but may not compensate for force deficits beyond the first few inter-pulse intervals.

Catchlike property of skeletal muscle: Recent findings and clinical implications

The catchlike property of skeletal muscle is the force augmentation produced by the inclusion of an initial, brief, high-frequency burst of two to four pulses at the start of a subtetanic low-frequency stimulation train. Catchlike-inducing trains take advantage of the catchlike property of skeletal muscle and augment muscle performance compared with constant-frequency trains, especially in the fatigued state. Literature spanning more than 30 years has provided comprehensive information about the catchlike property of skeletal muscle. The pattern of the catchlike-inducing train that maximizes muscle performance is fairly similar across different muscles of different species and under various stimulation conditions. This review summarizes the mechanisms of the catchlike property, factors affecting force augmentation, techniques used to identify patterns of catchlike-inducing trains that maximize muscle performance, and potential clinical applications to provide a historical and current perspective of our understanding of the catchlike property.

Force Maintenance With Submaximal Fatiguing Contractions

Whereas many definitions of fatigue include externally measurable decrements in force or performance, fatigue can be present with no change in the external output of the muscle. The maintenance of submaximal forces can be considered a compromise between neuromuscular force enhancement and competing inhibitory influences. An example of a muscle facilitatory process includes postactivation potentiation that results in an increased sensitivity to Ca++. The neuromuscular system copes with metabolic disruption and subsequent loss of force by recruiting additional motor units and increasing the firing frequency. If the contraction persists, firing frequency may decrease so as to optimize the stimulus rate with the prolonged duration of the muscle fibre action potential (muscle wisdom). The insertion of additional neural impulses into the train of stimuli can result in force potentiation (catch-like properties). Furthermore, there is evidence of neural potentiation and a dissociation of muscle activity with submaximal fatigue. Conversely, inhibition may be derived supraspinally or at the spinal level. While there may be some evidence of intrinsic motoneuronal fatigue, inhibitory afferent influences from chemical, tensile, pressure, and other factors play an important role in the competing influences on force output. Key words: postactivation potentiation, recruitment, rate coding, inhibition, catch-like properties

Metabolic costs of force generation for constant-frequency and catchlike-inducing electrical stimulation in human tibialis anterior muscle

Metabolic costs of force generation were compared for constant-frequency and catchlike-inducing electrical stimulation. Repetitive catchlike-inducing trains consisted of 2 interpulse intervals (IPIs) at 12.5 ms, 1 IPI at 25 ms, and 5 IPIs at 50 ms. Constant-frequency trains consisted of 8 IPIs at 37.5 ms. One train was delivered to the peroneal nerve every 2.5 s for 36 times under ischemic conditions. Anaerobic adenosine triphosphate (ATP) turnover was determined using 31-phosphorus magnetic resonance spectroscopy (P-MRS) of the human tibialis anterior muscle. Compared with constant-frequency trains, catchlike-inducing trains produced a faster force generation and were more effective in maintaining the force--time integral as well as peak force. However, ATP costs of force generation were similar for the catchlike-inducing and constant-frequency stimulation (6.7 plus/minus 1.1 and 6.6 plus/minus 1.0 micromol ATP/kg wet weight/Ncenter dots, respectively, P = 0.601). This suggests that the positive effects of catchlike-inducing stimulation on force maintenance are mediated by potentiated Ca(2+) release from the sarcoplasmic reticulum rather than by lower metabolic costs of muscle force generation. Our findings also suggest that catchlike-inducing stimulation produces larger forces in fatigued muscle than constant-frequency trains and thus may be beneficial for muscle training or rehabilitation when muscle loading needs to be maintained in repetitive contractions.

Effects of stimulation frequencies and patterns on performance of repetitive, nonisometric tasks

The purpose of this paper was to determine the effects of stimulation pattern and frequency on repetitive human knee movements. Quadriceps femoris muscles were stimulated against a load equal to 10% of each subject's maximum voluntary isometric force. The main variable of interest was the number of repetitions in which the leg reached a target angle of 40 degrees of knee extension. Sixteen different trains were tested, including 1) six constant-frequency trains with frequencies ranging from 9 to 100 Hz, 2) five variable-frequency trains with an initial 5-ms triplet and mean frequencies ranging from 11 to 35 Hz, and 3) five doublet-frequency trains, which used doublets (2 pulses with a 5-ms interpulse interval) to replace single pulses, with mean frequencies of 17-57 Hz. Testing was stopped when the subject failed to reach the target angle for three consecutive activations. Results showed that no single pattern was best for all subjects. The 33- and 100-Hz constant-frequency trains, 35-Hz variable-frequency trains, and 27- and 36-Hz doublet frequency trains each met the target the most times for some subjects. The results showed that, under our testing conditions, higher frequency trains were better suited for producing repetitive knee movements than lower frequency trains.

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.

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.