Effects of fast and slow patterns of tonic long-term stimulation on contractile properties of fast muscle in the cat

Peripheral Nerve Regeneration and Muscle Reinnervation

Injured peripheral nerves but not central nerves have the capacity to regenerate and reinnervate their target organs. After the two most severe peripheral nerve injuries of six types, crush and transection injuries, nerve fibers distal to the injury site undergo Wallerian degeneration. The denervated Schwann cells (SCs) proliferate, elongate and line the endoneurial tubes to guide and support regenerating axons. The axons emerge from the stump of the viable nerve attached to the neuronal soma. The SCs downregulate myelin-associated genes and concurrently, upregulate growth-associated genes that include neurotrophic factors as do the injured neurons. However, the gene expression is transient and progressively fails to support axon regeneration within the SC-containing endoneurial tubes. Moreover, despite some preference of regenerating motor and sensory axons to “find” their appropriate pathways, the axons fail to enter their original endoneurial tubes and to reinnervate original target organs, obstacles to functional recovery that confront nerve surgeons. Several surgical manipulations in clinical use, including nerve and tendon transfers, the potential for brief low-frequency electrical stimulation proximal to nerve repair, and local FK506 application to accelerate axon outgrowth, are encouraging as is the continuing research to elucidate the molecular basis of nerve regeneration.

Medial gastrocnemius muscles fatigue but do not atrophy in paralyzed cat hindlimb after long-term spinal cord hemisection and unilateral deafferentation

This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.

Reconstruction of Anal Function by Transposed Gracilis Muscle with Electral Stimulation: Rabbit Model

For the reconstruction of anal function for fecally incontinent patients, it could be practicable to transpose the gracilis muscle around the anal canal, with electrical stimulation to maintain contraction. It is necessary to keep continuous tonus, so tetanic contraction or “summation” would be essential for fecal continence, with a stimulation which permits prolonged contraction. Transposition of the gracilis muscle around the rectum was performed in thirteen Japanese white male rabbits. The muscles of the conditioning group (n=8) were stimulated at 10 Hz for 6 weeks before the procedure. By stimulation at 15 Hz, a low frequency to permit prolonged contraction, the neoanal pressure increased maximally to 134.2 ± 55.6 cmH2O (mean ± s.d.) in the conditioning group, and to 115.0 ± 37.1 cmH2O in the non-conditionin group (n=5) (N.S.). But, the basal pressure with stimulation rose 82.3 ± 12.4% (mean ± s.d.) of the increase in the conditioning group, while that of the non-conditioning group remained at resting pressure (p<0.001). The conditioning made it possible for the rabbit's gracilis muscle to create anal pressure with a sufficient rise in the basal pressure at a frequency permitting prolonged contraction.

The adaptive response of skeletal muscle: What is the evidence?

Adult skeletal muscle is capable of adapting its properties in response to changing functional demands. This now sounds like a statement of the obvious, and many people assume it has always been this way. A mere 40 years ago, however, the picture was entirely different. In this Review and personal memoir, I outline the scientific context in which the theory was generated, the objections to it from entrenched opinion, and the way those objections were progressively met. The material should be of some historical interest, but, more importantly, it collects together the full range of evidence on which the current paradigm is based. Muscle Nerve 57: 531-541, 2018.

Comparison of morphological changes of muscle fibers in response to dynamic electrical muscle contraction and dynamic hydraulic stimulation in a rat hindlimb disuse model

This study attempted to compare the muscle fiber morphological responses to dynamic electrical muscle stimulation (DEMS) and dynamic hydraulic stimulation (DHS) in rats under hindlimb suspension (HLS). DEMS at 1 Hz, 50 Hz and 100 Hz for 10 min/day, 5 days/week were introduced to the animals' right quadriceps. Static and 2 Hz DHS were introduced to the right tibiae of other animal groups on a "10 min on - 5 min off - 10 min on" loading regime for 5 days/week. In the end of the 4-week experiments, histological changes in the corresponding soleus, gastrocnemius and quadriceps of the stimulated sites were examined. Compared to age-matched, HLS led to muscle atrophy and strongly reduced muscle wet weights and averaged cross-sectional fiber areas. Among the tested DEMS frequencies, the averaged cross-sectional quadriceps fiber area in the 50 Hz group was 29 % larger than the 100 Hz group. In contrast, difference in the muscle fiber response to the static and 2 Hz DHS was not observed in either soleus or gastrocnemius. Muscle fiber morphological responses to the active DEMS was in a load frequency dependent manner under disuse condition. Relatively passive compressions, either via static or 2Hz DHS, were unable to induce any difference in the muscle fiber responses under functional disuse.

Funktionelle Elektrostimulation Paraplegischer Patienten

Functional Electrical Stimulation on Paraplegic Patients. We report on clinical and physiological effects of 8 months Functional Electrical Stimulation (FES) of quadriceps femoris muscle on 16 paraplegic patients. Each patient had muscle biopsies, CT-muscle diameter measurements, knee extension strength testing carried out before and after 8 months FES training. Skin perfusion was documented through infrared telethermography and xenon clearance, muscle perfusion was recorded through thallium scintigraphy. After 8 months FES training baseline skin perfusion showed 86 % increase, muscle perfusion was augmented by 87 %. Muscle fiber diameters showed an average increase of 59 % after 8 months FES training. Muscles in patients with spastic paresis as well as in patients with denervation showed an increase in aerob and anaerob muscle enzymes up to the normal range. Even without axonal neurotropic substances FES was able to demonstrate fiberhypertrophy, enzyme adaptation and intracellular structural benefits in denervated muscles. The increment in muscle area as visible on CT-scans of quadriceps femoris was 30 % in spastic paraplegia and 10 % in denervated patients respectively. FES induced changes were less in areas not directly underneath the surface electrodes. We strongly recommend the use of Kern's current for FES in denervated muscles to induce tetanic muscle contractions as we formed a very critical opinion of conventional exponential current. In patients with conus-cauda-lesions FES must be integrated into modern rehabilitation to prevent extreme muscle degeneration and decubital ulcers. Using FES we are able to improve metabolism and induce positive trophic changes in our patients lower extremities. In spastic paraplegics the functions "rising and walking" achieved through FES are much better training than FES ergometers. Larger muscle masses are activated and an increased heart rate is measured, therefore the impact on cardiovascular fitness and metabolism is much greater. This effectively addresses and prevents all problems which result from inactivity in paraplegic patients.

The Response of Denervated Muscle to Long-Term Stimulation (1985, Revisited here in 2014)

In 1985, at a meeting in Abano, I presented results showing that direct stimulation of skeletal muscles with appropriate stimulus patterns prevents the effects of denervation on non-junctional properties of muscle fibers. Hence, it appeared unnecessary to postulate that unknown nerve-derived trophic factors control such properties, as posited by the (anterograde) neurotrophic hypothesis. Here I discuss this conclusion in the light of what we know today, particularly with respect to the many lines of evidence that were then taken to support the trophic hypothesis, but which today have alternative interpretations consistent with control by evoked impulse activity. Despite much effort, no one has yet identified any nerve-derived factor consistent with the neurotrophic hypothesis. Reports favoring the existence of neurotrophic factors were numerous before 2000. Now they have essentially disappeared from the literature, including original research papers, textbooks and handbooks, suggesting that the hypothesis is no longer arguable. Thus, the results that I presented in our paper in 1985 seem to have held up rather well.

The Response of Denervated Muscle to Long-Term Electrical Stimulation

Adapted from: Lømo T, Westgaard RH, Hennig R, Gundersen K. The response of denervated muscle to long-term electrical stimulation, In: Carraro U, Angelini C, eds. Proceedings of the First Abano Terme Meeting on Rehabilitation, 1985 August 28-30, Abano Terme, Padova, Italy, An International Symposium, Satellite Meeting of the XIII World Congress of Neurology, Hamburg 1985. Cleup Padova 1985. pp 81–90.

Reduced hexokinase II impairs muscle function 2 wk after ischemia-reperfusion through increased cell necrosis and fibrosis

We previously demonstrated that hexokinase (HK) II plays a key role in the pathophysiology of ischemia-reperfusion (I/R) injury of the heart (Smeele et al. Circ Res 108: 1165-1169, 2011; Wu et al. Circ Res 108: 60-69, 2011). However, it is unknown whether HKII also plays a key role in I/R injury and healing thereafter in skeletal muscle, and if so, through which mechanisms. We used male wild-type (WT) and heterozygous HKII knockout mice (HKII(+/-)) and performed in vivo unilateral skeletal muscle I/R, executed by 90 min hindlimb occlusion using orthodontic rubber bands followed by 1 h, 1 day, or 14 days reperfusion. The contralateral (CON) limb was used as internal control. No difference was observed in muscle glycogen turnover between genotypes at 1 h reperfusion. At 1 day reperfusion, the model resulted in 36% initial cell necrosis in WT gastrocnemius medialis (GM) muscle that was doubled (76% cell necrosis) in the HKII(+/-) mice. I/R-induced apoptosis (29%) was similar between genotypes. HKII reduction eliminated I/R-induced mitochondrial Bax translocation and oxidative stress at 1 day reperfusion. At 14 days recovery, the tetanic force deficit of the reperfused GM (relative to control GM) was 35% for WT, which was doubled (70%) in HKII(+/-) mice, mirroring the initial damage observed for these muscles. I/R increased muscle fatigue resistance equally in GM of both genotypes. The number of regenerating fibers in WT muscle (17%) was also approximately doubled in HKII(+/-) I/R muscle (44%), thus again mirroring the increased cell death in HKII(+/-) mice at day 1 and suggesting that HKII does not significantly affect muscle regeneration capacity. Reduced HKII was also associated with doubling of I/R-induced fibrosis. In conclusion, reduced muscle HKII protein content results in impaired muscle functionality during recovery from I/R. The impaired recovery seems to be mainly a result of a greater susceptibility of HKII(+/-) mice to the initial I/R-induced necrosis (not apoptosis), and not a HKII-related deficiency in muscle regeneration.

Partial hexokinase II knockout results in acute ischemia-reperfusion damage in skeletal muscle of male, but not female, mice

Cellular studies have demonstrated a protective role of mitochondrial hexokinase against oxidative insults. It is unknown whether HK protective effects translate to the in vivo condition. In the present study, we hypothesize that HK affects acute ischemia–reperfusion injury in skeletal muscle of the intact animal. Male and female heterozygote knockout HKII (HK^+/-), heterozygote overexpressed HKII (HK^tg), and their wild-type (WT) C57Bl/6 littermates mice were examined. In anesthetized animals, the left gastrocnemius medialis (GM) muscle was connected to a force transducer and continuously stimulated (1-Hz twitches) during 60 min ischemia and 90 min reperfusion. Cell survival (%LDH) was defined by the amount of cytosolic lactate dehydrogenase (LDH) activity still present in the reperfused GM relative to the contralateral (non-ischemic) GM. Mitochondrial HK activity was 72.6 ± 7.5, 15.7 ± 1.7, and 8.8 ± 0.9 mU/mg protein in male mice, and 72.7 ± 3.7, 11.2 ± 1.4, and 5.9 ± 1.1 mU/mg in female mice for HK^tg, WT, and HK^+/-, respectively. Tetanic force recovery amounted to 33 ± 7% for male and 17 ± 4% for female mice and was similar for HK^tg, WT, and HK^+/-. However, cell survival was decreased (p = 0.014) in male HK^+/- (82 ± 4%LDH) as compared with WT (98 ± 5%LDH) and HK^tg (97 ± 4%LDH). No effects of HKII on cell survival was observed in female mice (92 ± 2% LDH). In conclusion, in this mild model of acute in vivo ischemia–reperfusion injury, a partial knockout of HKII was associated with increased cell death in male mice. The data suggest for the first time that HKII mediates skeletal muscle ischemia–reperfusion injury in the intact male animal.

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.

Effects of chronic electrical stimulation on paralyzed expiratory muscles

Following spinal cord injury, the expiratory muscles develop significant disuse atrophy characterized by reductions in their weight, fiber cross-sectional area, and force-generating capacity. We determined the extent to which these physiological alterations can be prevented with electrical stimulation. Because a critical function of the expiratory muscles is cough generation, an important goal was the maintenance of maximal force production. In a cat model of spinal cord injury, short periods of high-frequency lower thoracic electrical spinal cord stimulation (SCS) at the T(10) level (50 Hz, 15 min, twice/day, 5 days/wk) were initiated 2 wk following spinalization and continued for a 6-mo period. Airway pressure (P)-generating capacity was determined by SCS. Five acute, spinalized animals served as controls. Compared with controls, initial P fell from 43.9 +/- 1.0 to 41.8 +/- 0.7 cmH(2)O (not significant) in the chronic animals. There were small reductions in the weight of the external oblique, internal oblique, transverses abdominis, internal intercostal, and rectus abdominis muscles (not significant for each). There were no significant changes in the population of fast muscle fibers. Because prior studies (Kowalski KE, Romaniuk JR, DiMarco AF. J Appl Physiol 102: 1422-1428, 2007) have demonstrated significant atrophy following spinalization in this model, these results indicate that expiratory muscle atrophy can be prevented by the application of short periods of daily high-frequency stimulation. Because the frequency of stimulation is similar to the expected pattern of clinical use for cough generation, the daily application of electrical stimulation could potentially serve the dual purpose of maintenance of expiratory muscle function and airway clearance.

Development of tonic firing behavior in rat soleus muscle

Tonic firing behavior in soleus muscle of unrestrained rats aged 7 to >or=100 days was studied by chronic single-motor-unit and gross-electromyographic (EMG) recordings. Median motor-unit firing frequency at 10 days was 19-26 Hz and did not change appreciably after this time, whereas interval-to-interval firing variability was reduced with age. Two units with median frequencies 40 and 59 Hz were encountered in one 7-day-old rat. Integrated rectified gross EMG developed from being phasic only to predominantly tonic during the second and third postnatal week. From the end of the third week, rather short tonic periods with irregular amplitude were replaced by longer lasting constant-amplitude periods. Quantitatively, median duration of gross-EMG activity episodes more than doubled, while 90th-percentile values for episode duration increased 19-fold, from 7.4 s at 7 days to 140 s in adults. The main part of this increase took place after 22 days. Previous work in adult rats has indicated that descending monoaminergic innervation is essential for maintained tonic motoneuron activity, which probably is caused by depolarizing plateau potentials. Such innervation of the lumbar spinal cord matures gradually to an adult pattern and density approximately 3-4 wk after birth. The present results, describing a concurrent considerable development of tonic firing behavior, support and extend these findings.

Studies of acetylcholine receptor subunit gene expression: chromatin structural changes during myogenesis

Myogenesis proceeds stepwise from pluripotential stem cell to differentiated myotube. The precise number of transitions that occur along the developmental pathway remains to be determined. We examined the myogenic pathway as modelled by mouse mesodermal stem cell and muscle cell lines for stage-specific alterations in the chromatin structure of the acetylcholine receptor delta and gamma subunit genes. We reasoned that such an analysis would allow us to observe either the primary events in the activation of these muscle-specific genes or processes secondary to the binding of muscle-specific regulatory proteins. We probed chromatin structure with DNase I (deoxyribonuclease I) and precisely mapped to the 5' ends of the delta and gamma genes DNase I hypersensitive (DH) sites whose induction is unique to each myogenic stage. Putative mesodermal stem cells have the simplest pattern of DH sites with no sites near the 5' ends of the delta and gamma genes, whereas differentiated myotubes express the most complex pattern; the myoblast pattern is intermediate and of two types. In muscle cell lines where differentiation must be induced the myoblasts have a simple pattern (one more site than stem cells); in muscle lines where differentiation is spontaneous the myoblasts express a complex pattern of DH sites (one less site than myotubes). Inducible myoblasts seem to be arrested in an earlier step in the myogenic pathway than spontaneously differentiating myoblasts. Thus, myogenic activation of acetylcholine receptor subunit genes appears to be a stepwise process that can be detected by chromatin structural changes specific to four distinct stages of muscle development: stem cell, early myoblast, late myoblast, and differentiated myotube.

Plasticity from muscle to brain

Recognition that the entire central nervous system (CNS) is highly plastic, and that it changes continually throughout life, is a relatively new development. Until very recently, neuroscience has been dominated by the belief that the nervous system is hardwired and changes at only a few selected sites and by only a few mechanisms. Thus, it is particularly remarkable that Sir John Eccles, almost from the start of his long career nearly 80 years ago, focused repeatedly and productively on plasticity of many different kinds and in many different locations. He began with muscles, exploring their developmental plasticity and the functional effects of the level of motor unit activity and of cross-reinnervation. He moved into the spinal cord to study the effects of axotomy on motoneuron properties and the immediate and persistent functional effects of repetitive afferent stimulation. In work that combined these two areas, Eccles explored the influences of motoneurons and their muscle fibers on one another. He studied extensively simple spinal reflexes, especially stretch reflexes, exploring plasticity in these reflex pathways during development and in response to experimental manipulations of activity and innervation. In subsequent decades, Eccles focused on plasticity at central synapses in hippocampus, cerebellum, and neocortex. His endeavors extended from the plasticity associated with CNS lesions to the mechanisms responsible for the most complex and as yet mysterious products of neuronal plasticity, the substrates underlying learning and memory. At multiple levels, Eccles' work anticipated and helped shape present-day hypotheses and experiments. He provided novel observations that introduced new problems, and he produced insights that continue to be the foundation of ongoing basic and clinical research. This article reviews Eccles' experimental and theoretical contributions and their relationships to current endeavors and concepts. It emphasizes aspects of his contributions that are less well known at present and yet are directly relevant to contemporary issues.

Functional and histological effects of intravaginal electrical stimulation on the pelvic muscles: a study in the rat

We studied functional and histological effects of electrical stimulation (ES) on pelvic muscles of the rat. With intravaginal electrodes, the musculus pubococcygeus and musculus iliococcygeus in the awake animal were stimulated three times 6 min per day with 5 min of rest in between, 5 days per week, 7 consecutive weeks with a biphasic rectangular symmetrical current of 25 Hz, 400-mus pulse duration, on/off time of 5/10 and with an amplitude of 2-4 mA. A "sham group" received the same handling but no stimulation. Contraction measured with intra-rectal pressure during stimulation increased more in the stimulated than in the sham group, but did not reach statistical significance probably due to low power. The 2A fast fibres increased with 14% in the musculus iliococcygeus and with 6% in the musculus pubococcygeus. Type 1 slow fibres did not change. Increased capillary density was found after stimulation. Repeated intravaginal ES has mainly an influence on the fast fibres in the pelvic muscles. To influence slow fibres, another stimulation program or current parameters would seem necessary.

Management of infant brachial plexus injuries

Management of brachial plexus injuries is geared toward normalization of limb function, primarily through optimization of nerve regeneration and mechanical increase in elbow flexion and shoulder stabilization. Changes in the skeletal muscles and the osteous structures of the upper extremity are ongoing throughout the course of treatment, mandating continual assessment and aggressive rehabilitation. In patients who present too late for microsurgical intervention, irreversible changes take place in skeletal muscles, highlighting the importance of early referral. However, secondary procedures have been shown to be beneficial in older patients and in those whose primary procedures failed. Further advances in bionics and stem cell therapy may help replace the dynamic functional deficits of obstetric brachial plexus palsy.

The resilience of the size principle in the organization of motor unit properties in normal and reinnervated adult skeletal muscles

Henneman's size principle relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement. After nerve injury and surgical repair, the relationship between motoneuron size and the number and size of the muscle fibers that the motoneuron reinnervates is initially lost but returns with time, irrespective of whether the muscles are self- or cross-reinnervated by the regenerated axons. Although the return of the size relationships was initially attributed to the recovery of the cross-sectional area of the reinnervated muscle fibers and their force per fiber, direct enumeration of the innervation ratio and the number of muscle fibers per motoneuron demonstrated that a size-dependent branching of axons accounts for the size relationships in normal muscle, as suggested by Henneman and his colleagues. This same size-dependent branching accounts for the rematching of motoneuron size and muscle unit size in reinnervated muscles. Experiments were carried out to determine whether the daily amount of neuromuscular activation of motor units accounts for the size-dependent organization and reorganization of motor unit properties. The normal size-dependent matching of motoneurons and their muscle units with respect to the numbers of muscle fibers per motoneuron was unaltered by synchronous activation of all of the motor units with the same daily activity. Hence, the restored size relationships and rematching of motoneuron and muscle unit properties after nerve injuries and muscle reinnervation sustain the normal gradation of muscle force during movement by size-ordered recruitment of motor units and the process of rate coding of action potentials. Dynamic modulation of size of muscle fibers and their contractile speed and endurance by neuromuscular activity allows for neuromuscular adaptation in the context of the sustained organization of the neuromuscular system according to the size principle.Key words: motor unit size, motor unit recruitment, innervation ratio, reinnervation.

Prevention of muscle disuse atrophy by low-frequency electrical stimulation in rats

When muscles lose neural drive, they atrophy rapidly. Neuromuscular electrical stimulation (NMS) has been used in attempts to prevent or reverse the atrophy, but optimal stimulation programs and parameters are not well defined. In this study, we investigated the effects of four different stimulation patterns on disuse atrophy produced in the tibialis anterior, lateral gastrocnemius, and soleus muscles of rats paralyzed with tetrodotoxin for seven days. Stimulation paradigms differed from one another by their stimulation frequency (2 or 10 pulses/s) and by their stimulation period (2 or 10 h a day). Results showed that stimulation with 2 pulses/s, paradigms were more effective at preventing disuse muscle atrophy than higher-frequency stimulation. The most marked difference was in the slow soleus muscle, which had only 10% mean atrophy when stimulated at 2 pulses/s for 10 h, compared to 26% atrophy when stimulated at 10 pulses/s for either 2 or 10 h and 32% atrophy in unstimulated, paralyzed controls. The level of atrophic change was not correlated with the levels of serum creatine kinase, used as an index of muscle damage. Results suggest that remediation of disuse atrophy may be accomplished using unphysiologically low rates of motor-unit activation despite the relatively low force produced by such unfused contractions. This may have significant implications for the design of therapies for muscle paralysis consequent to upper-motoneuron lesions.

The effect of chronic skeletal muscle stimulation on capillary growth in the rat: are sensory nerve fibres involved?

Indirect chronic electrical stimulation of skeletal muscle activates not only efferent but also afferent nerve fibres. To investigate effects specific to this on capillary growth, one of the earliest changes, cell proliferation and capillary ultrastructure were studied in ankle flexors of rats with and without deafferentation of the stimulated side. Two weeks after preganglionic section of dorsal roots L4-L6, the peroneal nerve was stimulated (10 Hz, 8 h day(-1)) for 2 or 7 days. Proliferating nuclei labelled by bromodeoxyuridine or proliferating cell nuclear antigen staining were colocalized to alkaline phosphatase-stained capillaries (Lc) or other interstitial nuclei (Li) in frozen sections of extensor digitorum longus. Capillary fine structure was examined in extensor hallucis proprius by transmission electron microscopy. The stimulation-induced increase in capillary and interstitial proliferation (Lc 9.9 +/- 1.9 %, Li 8.8 +/- 2.1 % vs. Lc 2.6 +/- 0.4 %, Li 1.9 +/- 0.3 % in controls, P < 0.05) was depressed at 2 days by dorsal root section (Lc 4.8 +/- 0.7 %, Li 3.2 +/- 0.9 %, P < 0.05), an effect likely to be mainly on fibroblasts; no depression was seen at 7 days. Dorsal root section reduced stimulation-induced capillary endothelial swelling at both time points. In contralateral muscles of intact rats, stimulation increased interstitial cell proliferation and capillary swelling, both effects being eliminated by dorsal root section. Capillary growth induced by stimulation (24 % increase in capillary : fibre ratio at 7 days) was unaffected by deafferentation. The reduction in capillary ultrastructural changes and interstitial proliferation in both stimulated and contralateral muscles implies that stimulation of afferent fibres leads directly to release of humoral factors and/or activation via dorsal roots of fibres that release humoral substances. Contralateral muscles are an inadequate control for the effects of chronic stimulation in the intact animal.

Dynamics of stimulation-induced muscle adaptation: insights from varying the duty cycle

We sought to gain insight into the dynamics of the signalling process that initiates adaptive change in mammalian skeletal muscles in response to chronic neuromuscular stimulation. Programmable miniature stimulators were implanted into rabbits and used to impose one of the following patterns on the dorsiflexors of one ankle: 10 Hz delivered in equal on/off periods of 30 s, 30 min, or 12 h (all equivalent in terms of aggregate impulse activity to continuous 5 Hz). Two further groups received continuous stimulation at 5 Hz or 10 Hz. In every case the stimulation pattern was maintained continuously for 6 weeks. Tibialis anterior muscles stimulated intermittently with equal on/off periods of 30 s, 30 min and 12 h had contractile characteristics that were significantly slower than the contralateral, unstimulated muscles but did not differ from those of muscles stimulated continuously at 5 Hz. Muscles stimulated continuously at 10 Hz were significantly slower than either contralateral muscles or muscles stimulated with any of the other patterns. Corresponding changes were seen in myosin heavy chain isoform composition. The fatigue index, defined as the fraction of tension remaining after 5 min of a standard fatigue test, was 0.4 for muscles in the contralateral group but equal to or greater than 0.85 for muscles of all the stimulated groups. These results were interpreted with the help of a simple model of the growth and decay of a putative signalling substance based on first order kinetics. The model suggests a rate constant for the accumulation of the signalling substance that is greater than 30 h(-1), and a rate constant for its removal that is greater than 50 h(-1).

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

We have shown that fatigue resistance can be induced in rabbit tibialis anterior (TA) muscles without excessive power loss by continuous stimulation at low frequencies, such as 5 Hz, and that the same result is obtained by delivering a 10-Hz pattern in equal on/off periods. Here we ask whether the same phenotype could be produced with daily amounts of stimulation that would be more appropriate for clinical use. We stimulated rabbit TA muscles for 6 wk, alternating fixed 30-min on periods of stimulation at 10 Hz with off periods of different duration. All patterns transformed fast-glycolytic fibers into fast-oxidative fibers. The muscles had fatigue-resistant properties but retained a higher contractile speed and power production than muscles transformed completely to the slow-oxidative type. We conclude that in the rabbit as little as one 30-min period of stimulation in 24 h can result in a substantial increase in the resistance of the muscle to fatigue.

Lung volume reduction surgery does not improve diaphragmatic contractile properties or atrophy in hamsters with elastase-induced emphysema

It is claimed that lung volume reduction surgery (LVRS) improves inspiratory muscle function. As diaphragm structure and function are not directly appraisable in patients, we studied the effects of LVRS on the diaphragm in vitro contractile properties and morphology in hamsters with elastase-induced emphysema. Four months after intratracheal instillation of elastase (40 U/100 g), hamsters underwent either bilateral LVRS (LVRS, n = 11) or a sham operation (SHAM, n = 8). Four animals died during the perioperative period in LVRS (n = 7). Hamsters instilled with saline served as control (CTL, n = 8). Animals were studied at the age of 9 mo. LVRS was associated with a significant 25% decrease in functional residual capacity compared to SHAM (p < 0.05). Compared with CTL, LVRS and SHAM showed a significant 18% and 14% reduction in diaphragm mass, respectively (p = 0.02). LVRS had a significantly decreased twitch tension compared to CTL and SHAM (p < 0.01). Both LVRS and SHAM showed increased resistance to muscle fatigue compared with CTL. The histochemical analysis revealed a significant shift from type IIx/b toward type IIa fibers in LVRS and SHAM compared with CTL. In conclusion, emphysema is associated with functional adaptations but LVRS does not appear to beneficially alter the diaphragm contractile and morphological characteristics in hamsters with elastase-induced emphysema.

Neuromuscular adaptation to microgravity environment

Morphological and/or functional char-acteristics of skeletal muscles have a greater adaptability in response to changes in environmental stimuli. For example, an atrophy associated with a shift of fiber characteristics toward fast-twitch type is a common adaptation of antigravity muscle to a microgravity environment. Neuromuscular responses and possible mechanisms of both neural and muscular adaptations to a microgravity environment are discussed in this article. Responses of morphological, metabolic, and contractile properties, as well as fiber phenotype, of muscles are briefly reviewed. Discussion is further extended to the patterns of electromyogram and tension development of muscle, responses of postural stability and locomotion, and/or motoneurons in order to study the mechanism for muscular adaptation to microgravity.

Spontaneous electromyographic activity in adult rat soleus muscle

Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16-25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20-30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.

The final common pathway in postural control--developmental perspective

A brief review is given concerning postural specialisations among mammalian muscle fibres and motor units. Most skeletal muscles contain a mixture of fibres with different characteristics, and their slow-twitch (S) units are well-known to possess properties suitable for postural tasks: they are highly fatigue-resistant, well equipped for oxidative metabolism, and their slowness makes them energetically cheap in (semi-)isometric contractions. These features are adequately employed in motor behaviour owing to characteristics of the associated motoneurones. In adult mammals, the way in which a muscle is used can influence its proportion of S units. This adjustment occurs within a restricted 'adaptive range' which differs between muscles and animal species, presumably being preset at an early age. In the course of early foetal development, part of the slow vs. fast differentiation of muscle fibre properties can take place independently of innervation. Once innervation has taken place, however, motoneurones influence the differentiation in various ways. On the whole, a well coordinated timing seems to exist between the early differentiation of central motor mechanisms and of the peripheral machinery, largely causing the neuromuscular system to be/become ready for use when the brain needs it.

Partial transformation from fast to slow muscle fibers induced by deafferentation of capsaicin-sensitive muscle afferents

Mechanical and histochemical characteristics of the lateral gastrocnemius (LG) muscle of the rat were examined 21 days after capsaicin injection into the LG muscle. The capsaicin caused a decrease in generation rate of twitch and tetanic tension and an increase in fatigue resistance of LG muscle. The histochemical muscle fiber profile evaluated by myosin adenosine triphosphatase and reduced nicotinamide adenine dinucleotide tetrazolium reductase methods showed an increase of type I and IIC fibers and a decrease of the type IIB in whole muscle, and a decrease of the IIA, IIX fibers in the red part accompanied by their increase in the white part. Therefore the capsaicin treatment, which selectively eliminated fibers belonging to the III and IV groups of muscle afferents, induced muscle fiber transformation from fast contracting fatiguing fibers to slowly contracting nonfatiguing ones.

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. I. Muscle and motor unit properties

This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance, and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We synchronously activated all MUs in the MG muscle with the same activity (20 Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24-h/day stimulation and for monitoring progressive changes in contractile force, endurance, and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low-frequency stimulation slowed muscle contractions and made them weaker, and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly, whereas the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance, and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indexes and twitch time to peak force. After chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the slow (type S) MUs of the normal MG. In no case was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20 Hz/50% duty cycle was imposed on paralyzed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Influence of electrical stimulation of the tibialis anterior muscle in paraplegic subjects. 1. Contractile properties

In adult paraplegic subjects one tibialis anterior muscle received daily electrical stimulation for 4 weeks at twice motor threshold to determine changes of its contractile properties (this paper) and its morphological and histochemical profiles (following paper). Force, speed of contraction and fatigue resistance were assessed by percutaneous electrical stimulation of the muscle with torque measured about the ankle. Comparative contractile tests were performed on 51 normal adult subjects and new parameters for force measurement proposed, particularly where maximum voluntary contraction cannot be obtained. In paraplegic subjects before stimulation the tibialis anterior muscle showed evidence of disuse with decreased force, faster contraction and relaxation, and reduced fatigue resistance. The effects of two stimulus patterns were compared: 10 Hz, and 10 Hz with 100 Hz bursts. After stimulation contraction was slower, fatigue resistance increased and there was a tendency for force to increase. No differences occurred using the different stimulus patterns. Four weeks later fatigue resistance was partially maintained, while speed of contraction reverted to pre-stimulus levels.

Conversion of the rabbit gracilis muscle for transposition as a neoanal sphincter by electrical stimulation

To re-establish anal function in fecally incontinent patients it may be feasible to transpose the gracilis muscle around the anal canal, using electrical stimulation to trigger contraction. However, because the fast-twitching gracilis muscle is incapable of prolonged contraction without fatigue, it is necessary to convert it to a slow-twitching, fatigue-resistant muscle. We demonstrated this conversion by longterm electrical stimulation at low frequencies using a rabbit model. The nerve to the gracilis muscle was continuously stimulated at 2 Hz, 5 Hz, and 10 Hz for 2, 4, or 6 weeks. In the 6-week conditioning group, the percentage of type I fibers, identified by ATPase staining, increased as the conditioning frequency became higher, but the twitch contraction speed reduced with conditioning at a frequency of more than 5 Hz. The fatigue resistance improved by conditioning at 10 Hz, and conversion occurred in 6 weeks. Thus, we concluded that conditioning at 10 Hz for 6 weeks can convert rabbit gracilis muscle to a slow-twitching, fatigue-resistant muscle suitable for use as a neoanal sphincter.

Effects of muscle length, frequency of stimulation, and fatigue on the isometric tension in canine latissimus dorsi

An experimental protocol was designed to study the mechanical response of the canine latissimus dorsi muscle stimulated to contract isometrically. Active and passive tensions were measured with the muscle's initial length varying within 10% of its physiologic length in situ. The force-frequency relationship was obtained at frequencies of stimulation of 1, 10, 15, 30, and 60 Hz. Muscle fatigability was assessed during 3 min of successive contractions. Tests were performed in pedicled muscles of anesthetized mongrel dogs (n = 10). Force-length characteristics were found to affect evoked tension markedly. Maximal active tension was generated near the muscle length in situ. Specific isometric tension measured with unit pulse stimulation was 2 +/- 0.4 N/cm2, and the time to peak twitch was 92.8 +/- 2.67 ms. With 60 Hz of stimulation frequency, the tension was 9.31 +/- 0.32 N/cm2, and the time to peak tension was 216.05 +/- 16.28 ms. After the fatigue test, the tension generated decreased to 62.5% of its initial value, and this decline was paralleled by the rate of tension development and tension relaxation.

The use of therapeutic electrical stimulation in the treatment of hemiplegic cerebral palsy

The effect of electrical stimulation of the anterior tibial muscles of children with hemiplegic cerebral palsy was studied. 10 children received electrical stimulation, applied by their parents daily for an hour for 35 days; they were compared with 10 matched controls. Active and passive ranges of movement of the ankle, and knee and ankle motion during walking were measured before and after therapy using electrogoniometers. The results showed a significant increase in passive range of movement among children receiving electrical stimulation. Gait analysis of knee and ankle motion showed little change.

Power production and working capacity of rabbit tibialis anterior muscles after chronic electrical stimulation at 10 Hz

1. The muscles of the distal anterior compartment of the left hindlimb of rabbits were subjected to continuous indirect electrical stimulation at 10 Hz for periods of up to 12 weeks by means of an implantable stimulator. 2. The maximum shortening velocity (Vmax) and the velocity for maximum power production in single contractions (Vopt) were reduced to 42% and 32% of control values respectively after 12 weeks of stimulation. The rate of change of these parameters was greatest between the second and sixth week of stimulation. These changes, it is suggested, reflect the documented time course of the replacement of fast with slow isoforms of myosin. 3. The reductions in force production and speed of the stimulated muscles combined to produce a marked, progressive decline in the maximum power produced in single contractions. After 8 weeks of stimulation, the maximum power output had fallen to less than 10% of the control value. 4. The fatigue resistance of the stimulated and control muscles was tested over several hours of cyclical shortening contractions designed to elicit an initial power output of 10 W kg-1 with the muscles set to contract at Vopt. This level of work output represented about 1.6% (control) and 25% (12-week-stimulated) of the absolute maximum power output achieved during single contractions. 5. Despite the large reduction in the maximum power output of single contractions, the stimulated muscles showed less than 10% reduction in their power output during the fatigue tests over periods of up to 7 h. The control muscles showed a 70% reduction over the same period. There was no difference in the fatigue resistance under this protocol between muscles stimulated for 2 weeks and those stimulated for longer periods. Transformation of myosin isoforms, which is known to occur later than 2 weeks after the start of stimulation, is not necessary for the induction of this degree of fatigue resistance.

Nerve stimulation and denervation induce differential patterns of immediate early gene mRNA expression in skeletal muscle

Many properties of skeletal muscle cells are closely regulated by motor nerves. Neuromuscular synaptic transmission (including the 'activity' it triggers) mediates many of these effects, while denervation results in a different spectrum of muscle cell changes. However, little is known about the early regulatory events that occur in mature muscle cells in response to muscle activity or denervation. We have examined the effects of motor nerve stimulation and denervation on the expression of 4 immediate early genes (IEGs)--c-jun, junB, zif268, and nur77--in mature mouse gastrocnemius muscle. Electrical stimulation of the sciatic nerve in a pattern of brisk intermittent exercise induced a marked rise in zif268 and c-jun mRNA levels within 45 min, a minimal rise in junB, and no change in nur77 mRNA levels. By contrast, surgical denervation resulted in a marked increase of c-jun, a slight rise in junB, and no change in nur77 or zif268 mRNA levels. These findings show that neural stimulation and denervation lead to differential patterns of IEG expression. The selectivity of these patterns suggests that differential IEG expression may play an important role in regulating the specific phenotypic changes in skeletal muscles that result from denervation, innervation, and various patterns of stimulation.

Neuromuscular electrical stimulation and voluntary exercise

Neuromuscular electrical stimulation (NMES) has been in practice since the eighteenth century for the treatment of paralysed patients and the prevention and/or restoration of muscle function after injuries, before patients are capable of voluntary exercise training. More recently NMES has been used as a modality of strengthening in healthy subjects and highly trained athletes, but it is not clear whether NMES is a substitute for, or a complement to, voluntary exercise training. Moreover the discussion of the mechanisms which underly the specific effects of NMES appears rather complex at least in part because of the disparity in training protocols, electrical stimulation regimens and testing procedures that are used in the various studies. It appears from this review of the literature that in physical therapy, NMES effectively retards muscle wasting during denervation or immobilisation and optimises recovery of muscle strength during rehabilitation. It is also effective in athletes with injured, painful limbs, since NMES contributes to a shortened rehabilitation time and aids a safe return to competition. In healthy muscles, NMES appears to be a complement to voluntary training because it specifically induces the activity of large motor units which are more difficult to activate during voluntary contraction. However, there is a consensus that the force increases induced by NMES are similar to, but not greater than, those induced by voluntary training. The rationale for the complementarity between NMES and voluntary exercise is that in voluntary contractions motor units are recruited in order, from smaller fatigue resistant (type I) units to larger quickly fatiguable (type II) units, whereas in NMES the sequence appears to be reversed. As a training modality NMES is, in nonextreme situations such as muscle denervation, not a substitute for, but a complement of, voluntary exercise of disused and healthy muscles.

Different effects of physical training on the morphology of motor nerve terminals in the rat extensor digitorum longus and soleus muscles

The morphology of nerve terminals in the rat extensor digitorum longus and soleus muscles was studied with light microscopy in 13-week-old male animals after 6 weeks of treadmill running and compared with data from untrained controls. The terminals were stained with methylene blue. Physical training tended to increase the area and length of the nerve terminals in relation to the corresponding muscle fiber diameter, and to reduce the density of nerve terminal varicosities, but significant differences between the trained group and the control group were obtained only in the extensor digitorum longus muscle. The different degrees of effect on the nerve terminals in the two muscles may be due to different abilities to respond to the training, but may also be due to differences in work load caused by the training. The effect of training on extensor digitorum longus junctions may reflect some transformation from fast to slow morphological characteristics.

A comparison of fibrillation in denervated skeletal muscle of the anaesthetized rat and guinea-pig

We have used intracellular recording in vivo to study fibrillation (spontaneous repetitive membrane activity) in extensor digitorum longus (fast twitch) and soleus (slow twitch) muscles of the anaesthetized rat and guinea-pig denervated for periods of about 10 to 60 days. The proportion of fibres fibrillating in the guinea-pig soleus was greater than 50% in most animals up to the longest period of denervation (65 days). Fibrillation was rarely found in rat soleus after three weeks of denervation. Its incidence in the extensor digitorum longus muscles of both species was intermediate. The mean frequency of fibrillation was higher in guinea-pig extensor digitorum longus (16 Hz) and soleus (8 Hz) than in the rat extensor digitorum longus (3 Hz) and soleus (2 Hz). The resting membrane potentials of the denervated muscles were less than normal and correlated inversely with the frequency of fibrillation but not with the incidence of fibrillation: in rat soleus, many fewer fibers were fibrillating at a given membrane potential than in the other three muscles. The incidence of fibrillation was compared with previously reported tensions of the four denervated muscles and was found to have the same rank order. We suggest that fibrillation may reduce atrophy (and hence tension loss) of denervated muscle, which may have implications for artificial stimulation. Fibrillation frequency was directly related to changes in twitch speed of the four muscles after denervation.

What is the ideal pulse frequency for skeletal muscle stimulation after cardiomyoplasty?

Routinely the latissimus dorsi (LD) muscle is stimulated with bursts of pulses at 30 pulses/sec after cardiomyoplasty to assist the failing heart. At a lower pulse frequency the contractile force decreases and at higher frequencies the energy demand of the pacemaker increases rapidly. We investigated the effect of the stimulus frequency variation on contractile force in untrained LD muscles and in muscles after 12 weeks of continuous cyclic electrical stimulation. In six dogs, two electrodes (Medtronic SP5528) were implanted in the left LD muscle together with an Itrel muscle stimulator. The LD muscle was stimulated with 30 pulses/sec in bursts to deliver initially 30 and after 10 weeks 80 contractions per minute. Both before and after training of the LD muscle maximum force was observed by stimulating with a frequency of 36 to 130 pulses/sec in a burst. However, training induced a shift in the steep part of the force-frequency relation to lower frequencies. In particular, at 15 pulses/sec 60% of the maximal force was obtained in contrast to 40% before training. A fatigue test of 8 minutes duration was performed specified by 100 bursts/min and a burst duration of 0.25 sec at pulse frequencies of 30, 36, 50, and 85 pulses/sec. The contractile force decreased significantly during the course of the test at all frequencies. However, the force obtained with 30 pulses/sec stimulation was lower during the initial phase and approximately 10% higher at the end of the fatigue test as compared to 36, 50, and 85 pulses/sec stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

External recording of twitch time course in cat ankle muscles

In chronic experiments concerning the activity-dependent plasticity of muscle properties, a simple and noninvasive method was used for monitoring changes of twitch speed in conscious adult cats. The animals had been provided with implanted electrodes for nerve stimulation, and a hand-held force transducer was pressed against the fully extended ankle joint while single test pulses were delivered to the common peroneal nerve. In the present report, this technique for the recording of ankle twitches is subjected to critical analysis and evaluation. The measurements were highly reproducible with respect to contraction time (time-to-peak) but less so for half-relaxation time and twitch amplitude; other methods should be used for the long-term monitoring of contractile force. The total force (torque) of the ankle twitch was mainly produced by tibialis anterior (about 45%), peroneus longus (PerL; 27%) and extensor digitorum longus (23%). The ankle twitch produced by PerL alone had about the same contraction time as that of all the muscles together. Among muscles that had become changed as a result of long-term electrical stimulation there was, in general, a good correspondence between the contraction times from simple external recordings of ankle twitches and those separately measured for PerL under general anesthesia (force transducer then directly connected to PerL tendon).

Effects of long-impulse electrical stimulation on atrophy and fibre type composition of chronically denervated fast rabbit muscle

The efficacy of electrical stimulation on a chronically denervated muscle depends on stimulus parameters, which have an important influence on the development of atrophy. Stimulus frequency and/or total activity are particularly responsible for the development of some histological, biochemical and contractile features. The present study in 18 rabbits deals with a recently developed electrical stimulus, which had proved effective in maintaining muscle force following denervation. This current has (1) unusual long bidirectional rectangular impulses (20 ms) and (2) a frequency of 25 Hz, which is between the frequencies of fast- and slow-firing motor units. Electrical stimulation began 28 (in one animal 53) days after total motor and sensory denervation of the right hindleg, and was continued until the end of the experiment, up to 205 days. To mimic a therapeutic regimen, which should be agreeable to patients, daily treatment times were kept to a minimum (2 x 6 min), and surface electrodes were used. Morphometric evaluation of the fast flexor digitorum sublimis muscle showed that such electrical stimulation was able to preserve fibre diameter at a level of 72-86% of the initial values for several months, while unstimulated fibres showed the usual atrophy with a decrease of diameters below 40% of normal. The stimulation induced a "hybrid" fibre type with properties of a slow muscle (rich in mitochondria in NADH-dependent tetrazolium reductase staining and electron microscopy) as well as of a fast-twitch muscle (fibre type IIb in myofibrillar ATPase stainings).