Induction of a fatigue-resistant phenotype in rabbit fast muscle by small daily amounts of stimulation

The Effect of Lower Limb Combined Neuromuscular Electrical Stimulation on Skeletal Muscle Signaling for Glucose Utilization, Myofiber Distribution, and Metabolic Function after Spinal Cord Injury

Maintaining healthy myofiber type and metabolic function early after spinal cord injury (SCI) may prevent chronic metabolic disorders. This study compares the effects of a 2-5 week combined (aerobic + resistance) neuromuscular electrical stimulation (Comb-NMES) regimen versus a sham control treatment on muscle protein signaling for glucose uptake, myofiber type distribution, and metabolic function. Twenty participants (31 ± 9 years of age) with an SCI (C4-L1, AIS level A-C) within 14 days of the SCI were randomly assigned to control (N = 8) or Comb-NMES (N = 12). Sessions were given three times per week. Fasting blood samples and vastus lateralis muscle biopsies were collected 24-48 h before or after the last session. Western blots were performed to quantify proteins, immunohistochemical analyses determined muscle myofiber distribution, and enzymatic assays were performed to measure serum glucose, insulin, and lipids. Our main findings include a decrease in fasting glucose (p < 0.05) and LDL-C (p < 0.05) levels, an upregulation of CamKII and Hexokinase (p < 0.05), and an increase in type I (+9%) and a decrease in type IIx (-36%) myofiber distribution in response to Comb-NMES. Our findings suggest that maintaining healthy myofiber type and metabolic function may be achieved via early utilization of Comb-NMES.

Long-Term Effects of Botulinum Toxin Complex Type A Injection on Mechano- and Metabo-Sensitive Afferent Fibers Originating from Gastrocnemius Muscle

The aim of the present study was to investigate long term effects of motor denervation by botulinum toxin complex type A (BoNT/A) from Clostridium Botulinum, on the afferent fibers originating from the gastrocnemius muscle of rats. Animals were divided in 2 experimental groups: 1) untreated animals acting as control and 2) treated animals in which the toxin was injected in the left muscle, the latter being itself divided into 3 subgroups according to their locomotor recovery with the help of a test based on footprint measurements of walking rats: i) no recovery (B0), ii) 50% recovery (B50) and iii) full recovery (B100). Then, muscle properties, metabosensitive afferent fiber responses to potassium chloride (KCl) and lactic acid injections and Electrically-Induced Fatigue (EIF), and mechanosensitive responses to tendon vibrations were measured. At the end of the experiment, rats were killed and the toxin injected muscles were weighted. After toxin injection, we observed a complete paralysis associated to a loss of force to muscle stimulation and a significant muscle atrophy, and a return to baseline when the animals recover. The response to fatigue was only decreased in the B0 group. The responses to KCl injections were only altered in the B100 groups while responses to lactic acid were altered in the 3 injected groups. Finally, our results indicated that neurotoxin altered the biphasic pattern of response of the mechanosensitive fiber to tendon vibrations in the B0 and B50 groups. These results indicated that neurotoxin injection induces muscle afferent activity alterations that persist and even worsen when the muscle has recovered his motor activity.

Impact of valves in a biomechanical heart model assisting failing hearts

Experimental valveless muscular blood pumps (biomechanical hearts) in goats can pump more than 1 L.min(-1), but due to a high pendulum volume, no significant flow contribution to the circulation is gained. Thus valved and valveless biomechanical hearts were compared for efficacy. Heart failure was induced in 5 adult Bore goats by repeated intracoronary embolization. A valved and balloon-equipped pumping chamber was integrated into the descending aorta, simulating standard biomechanical circulatory support. The valveless biomechanical heart supported a failing heart with a baseline cardiac output of 2,670 +/- 710 mL.min(-1) by contributing additional flow of 113 +/- 37 mL.min(-1). The biomechanical heart model incorporating an outlet valve offered an additional 304 +/- 126 mL.min(-1), and the use of 2 valves significantly enhanced pulmonary blood flow by 1,235 +/- 526 mL.min(-1). The use of 2 valves in biomechanical hearts seems to be essential to achieve adequate circulatory support. Double-valved biomechanical hearts driven by an appropriate skeletal muscle ventricle may contribute to the therapy of heart failure.

Adaptive change in electrically stimulated muscle: a framework for the design of clinical protocols

Adult mammalian skeletal muscles have a remarkable capacity for adapting to increased use. Although this behavior is familiar from the changes brought about by endurance exercise, it is seen to a much greater extent in the response to long-term neuromuscular stimulation. The associated phenomena include a markedly increased resistance to fatigue, and this is the key to several clinical applications. However, a more rational basis is needed for designing regimes of stimulation that are conducive to an optimal outcome. In this review I examine relevant factors, such as the amount, frequency, and duty cycle of stimulation, the influence of force generation, and the animal model. From these considerations a framework emerges for the design of protocols that yield an overall functional profile appropriate to the application. Three contrasting examples illustrate the issues that need to be addressed clinically.

Real-time polymerase chain reaction to follow the response of muscle to training

The adaptive response of muscle to changes in activity or loading can take many weeks. Changes in the levels of RNA within a muscle fiber can give an early indication of the nature of the response of that fiber to changes in activity or loading. We have designed a new primer set for quantitative polymerase chain reaction (PCR) that will allow us to follow these early transcriptional changes in rat muscle, and have shown that analysis can be performed by standard techniques on as little as 5 mg of muscle, an amount that can be obtained by needle biopsy.

Adaptive conditioning of skeletal muscle in a large animal model (Sus domesticus)

Recognition of the adaptive capacity of mammalian skeletal muscle has opened the way to a number of clinical applications. For most of these, the fast, fatigue-susceptible fibres need to be transformed stably to fast, fatigue-resistant fibres that express the 2A myosin heavy chain isoform. The thresholds for activity-induced change are size-dependent, so although the requisite patterns of electrical stimulation are known for the rabbit, in humans these same patterns would produce type 1 fibre characteristics, with an undesirable loss of contractile speed and power. We have used histochemistry, immunohistochemistry and electrophoretic separations to evaluate a possible conditioning regime in a large animal model. Stimulation of the porcine latissimus dorsi muscle with a phasic 30-Hz pattern for up to 41 days converted all type 2X and 2A/2X fibres to 2A with only a small increase in the type 1 population, from 17% to 22%. Stimulation for longer periods increased the proportion of type 1 fibres to 52%. Based on this model, stimulation regimes designed to achieve a stable 2A phenotype in humans should deliver fewer stimulating impulses, possibly by a factor of 2, than the pattern assessed here. Any such pattern needs to be tested for at least 8 weeks.

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine: differential responses of rat soleus and extensor digitorum longus

The K+ channel blocker 3,4-diaminopyrindine (DAP) increases diaphragm force, use of which could potentially improve muscle performance during functional neuromuscular stimulation. To determine the extent of hindlimb muscle force augmentation, and delineate whether DAP effects vary in muscles comprised of mainly slow versus fast fibers, rat soleus, extensor digitorum longus (EDL) and diaphragm muscle samples were studied in vitro. DAP increased force of all three muscles, but at high concentrations the force increases were transient and were followed by declines in force below baseline. The maximum DAP-induced twitch force increase was smaller for soleus (38 +/-7%) than both EDL (94+/-12%) (P < 0.05) and diaphragm (93+/-13%) (P < 0.01). During fatigue-inducing 20 Hz stimulation (tested at an intermediate DAP concentration), force of soleus muscle remained significantly elevated by DAP for the entire testing period, force of DAP-treated EDL muscle rapidly declined to values in untreated muscle, and force of DAP-treated diaphragm had an intermediate force-time profile. Muscles varied in extent to which isometric contractile kinetics were altered by DAP. Thus, the K+ channel blocker DAP improves contractile performance of limb muscles, but the profile of improvement is distinct between the soleus and EDL muscles.

Short-term electrical stimulation alters tongue muscle fibre type composition

OBJECTIVE

To examine whether short-term exogenous activation of a tongue muscle induced a phenotypic shift from a fast to a slow fibre-type, and thus assess a potential therapeutic avenue to protect against obstructive sleep apnoea (OSA).

METHODS

New Zealand White rabbit genioglossus (GG) muscle, characteristically a fast muscle, was continuously stimulated at a frequency attributed to slow muscle (10Hz, 3V DC pulses) using an implanted micro-circuit for 7 days. Changes in muscle fibre types and aerobic capacity were assessed between stimulated and un-stimulated (control) groups using immunohistochemistry and electrophoresis for myosin heavy chain (MHC) and assayed for citrate synthase.

RESULTS

Compared to the un-stimulated control group, stimulated GG muscles had more (approximately 13%) type I MHC (slow-twitch) content; a proportional decrease in type II MHC (fast-twitch) isoform also occurred in the stimulated GG muscle (P<0.05). Electrophoresis analysis on whole muscle and single fibre MHC showed an increased type I expression in the stimulated GG muscle (P<0.01). A commensurate rise in citrate synthase activity, indicating a change in aerobic capacity, was also observed in the stimulated GG muscles.

CONCLUSION

Together, these results demonstrate a successful alteration in tongue muscle characteristics using exogenous electrical stimulation and perhaps a potential therapeutic application for OSA.

Chronic electrostimulation after nerve repair by self-anastomosis: effects on the size, the mechanical, histochemical and biochemical muscle properties

This study tests the effects of chronic electrostimulation on denervated/reinnervated skeletal muscle in producing an optimal restoration of size and mechanical and histochemical properties. We compared tibialis anterior muscles in four groups of rats: in unoperated control (C) and 10 weeks following nerve lesion with suture (LS) in the absence of electrostimulation and in the presence of muscle stimulation with either a monophasic rectangular current (LSEm) or a biphasic modulated current (LSEb). The main results were (1) muscle atrophy was reduced in LSEm (-26%) while it was absent in LSEb groups (-8%); (2) the peak twitch amplitude decreased in LS and LSEm but not in LSEb groups, whereas the contraction time was shorter; (3) muscle reinnervation was associated with the emergence of type IIC fibers and proportions of types I, IIA and IIB fibers recovered in the superficial portion of LSEb muscles; (4) the ratio of oxidative to glycolytic activities decreased in the three groups with nerve injury and repair; however, this decrease was more accentuated in LSEm groups. We conclude that muscle electrostimulation following denervation and reinnervation tends to restore size and functional and histochemical properties during reinnervation better than is seen in unstimulated muscle.

The effect of random modulation of functional electrical stimulation parameters on muscle fatigue

Muscle contractions induced by functional electrical stimulation (FES) tend to result in rapid muscle fatigue, which greatly limits activities such as FES-assisted standing and walking. It was hypothesized that muscle fatigue caused by FES could be reduced by randomly modulating parameters of the electrical stimulus. Seven paraplegic subjects participated in this study. While subjects were seated, FES was applied to quadriceps and tibialis anterior muscles bilaterally using surface electrodes. The isometric force was measured, and the time for the force to drop by 3 dB (fatigue time) and the normalized force-time integral (FTI) were determined. Four different modes of FES were applied in random order: constant stimulation, randomized frequency (mean 40 Hz), randomized current amplitude, and randomized pulsewidth (mean 250 micros). In randomized trials, stimulation parameters were stochastically modulated every 100 ms in a range of +/-15% using a uniform probability distribution. There was no significant difference between the fatigue time measurements for the four modes of stimulation. There was also no significant difference in the FTI measurements. Therefore, our particular method of stochastic modulation of the stimulation parameters, which involved moderate (15%) variations updated every 100 ms and centered around 40 Hz, appeared to have no effect on muscle fatigue. There was a strong correlation between maximum force measurements and stimulation order, which was not apparent in the fatigue time or FTI measurements. It was concluded that a 10-min rest period between stimulation trials was insufficient to allow full recovery of muscle strength.

Reducing muscle fatigue due to functional electrical stimulation using random modulation of stimulation parameters

A major limitation of many functional electrical stimulation (FES) applications is that muscles tend to fatigue very rapidly. It was hypothesized that FES-induced muscle fatigue could be reduced by randomly modulating the pulse frequency, amplitude, and pulse width in a range of +/-15%. Seven subjects with spinal-cord injuries participated in this study. FES was applied to quadriceps and tibialis anterior muscles using surface electrodes. Isometric force was measured, and the time for the force to drop by 3 dB (fatigue time) was compared between trials. Four different modes of FES were applied in random order: constant stimulation, randomized frequency, randomized amplitude, and randomized pulse width. There was no significant difference between the fatigue-time measurements for the four modes of stimulation (P=0.329). Therefore, random modulation appeared to have no effect. Based on an observed correlation between maximum force measurements and trial order, we concluded that having 10-min rest periods between trials was insufficient.

Two-way communication for programming and measurement in a miniature implantable stimulator

Implantable stimulators are needed for chronic electrical stimulation of nerves and muscles in experimental studies. The device described exploits the versatility of current microcontrollers for stimulation and communication in a miniature implant. Their standard outputs can provide the required selectable constant-current sources. In this device, pre-programmed stimulation paradigms were selected by transcutaneous light pulses. The potential of a programmable integrated circuit (PIC) was thus exploited. Implantable devices must be biocompatible. A novel encapsulation method that require no specialised equipment and that used two classical encapsulants, silicone and Teflon was developed. It was tested for implantation periods of up to four weeks. A novel way to estimate electrode impedance in awake animals is also presented. It was thus possible to follow the evolution of the nerve-electrode interface and, if necessary, to adjust the stimulation parameters. In practice, the electrode voltage at the end of a known constant-current pulse was measured by the PIC. The binary coded value was then indicated to the user as a series of muscle twitches that represented the binary value of the impedance measurement. This neurostimulator has been successfully tested in vitro and in vivo. Thresholds and impedance values were chronically monitored following implantation of a self-sizing spiral cuff electrode. Impedance variations in the first weeks could reflect morphological changes usually observed after the implantation of such electrodes.

Combination of variable frequency train stimulation and K+ channel blockade to augment skeletal muscle force

Several innovative approaches are being used to optimize the input-output relationship of muscle, including nonlinear stimulation paradigms and altering muscle membrane ion channel conductances. We tested the hypothesis that the combination of the K+ channel blocker, 3,4-diaminopyridine (DAP), and variable frequency train (VFT) stimulation improves muscle force to a greater extent than either modality alone. Studies were done in vitro on rat diaphragm muscle and contractions were quantified with respect to peak force, mean force, and force area. DAP increased all three force parameters by >50% during conventional 10-20-Hz stimulation, whereas VFT stimulation improved contractile performance for peak force only. When combined, DAP and VFT stimulation augmented peak force to a significantly greater extent than either modality alone. However, this came at a cost of a moderate decline in force area relative to DAP alone, although mean force was preserved. These force increases were generally well-maintained over the course of short-term repetitive stimulation. Thus, VFT stimulation and K+ channel blockade interact in a complex manner to modulate skeletal muscle force. The utility of the combined intervention for functional electrical stimulation may be greatest for mechanical tasks requiring high force levels early during the contraction.

Neuromuscular rehabilitation by treadmill running or electrical stimulation after peripheral nerve injury and repair

Numerous studies have been devoted to the regeneration of the motor pathway toward a denervated muscle after nerve injury. However, the regeneration of sensory muscle endings after repair by self-anastomosis are little studied. In previous electrophysiological studies, our laboratory showed that the functional characteristics of tibialis anterior muscle afferents are differentially affected after injury and repair of the peroneal nerve with and without chronic electrostimulation. The present study focuses on the axonal regeneration of mechano- (fibers I and II) and metabosensitive (fibers III and IV) muscle afferents by evaluating the recovery of their response to different test agents after nerve injury and repair by self-anastomosis during 10 wk of treadmill running (LSR). Data were compared with control animals (C), animals with nerve lesion and suture (LS), and animals with lesion, suture, and chronic muscle rehabilitation by electrostimulation (LSE) with a biphasic current modulated in pulse duration and frequency, eliciting a pattern mimicking the activity delivered by the nerve to the muscle. Compared with the C group, results indicated that 1) muscle weight was smaller in LS and LSR groups, 2) the fatigue index was greater in the LS group and smaller in the LSE group, 3) metabosensibility remained altered in the LS and LSE groups, and 4) mechanosensitivity presented a large increase of the activation pattern in the LS and LSE groups. Our data indicated that chronic muscle electrostimulation partially favors the recovery of muscle properties (i.e., muscle weight and twitch response were close to the C group) and that rehabilitation by treadmill running also efficiently induced a better functional muscle afferent recovery (i.e., the discharge pattern was similar to the C group). The effectiveness of the chronic electromyostimulation and the treadmill exercise on afferent recovery is discussed with regard to parameters listed above.