Fibre sizes and histochemical staining characteristics in normal and chronically stimulated fast muscle of cat

Long-term phase angle muscle imaging under electrical muscle stimulation (EMS) by phase angle electrical impedance tomography

Objectives. Phase angle muscle imaging has been proposed by phase angle electrical impedance tomography (ΦEIT) under electrical muscle stimulation (EMS) for long-term monitoring of muscle quality improvement, especially focusing on calf muscles.Approach. In the experiments, twenty-four subjects are randomly assigned either to three groups: control group (CG,n= 8), low voltage intensity of EMS training group (LG,n= 8), and optimal voltage intensity of EMS training group (OG,n= 8).Main results. From the experimental results, phase angle distribution imagesФare cleared reconstructed by ФEIT as four muscle compartments over five weeks experiments, which are called theM1muscle compartments composed of gastrocnemius muscle,M2muscle compartments composed of soleus muscle,M3muscle compartments composed of tibialis-posterior muscle, flexor digitorum longus muscle, and flexor pollicis longus muscle, andM4muscle compartment composed of the tibialis anterior muscle, extensor digitorum longus muscle, and peroneus longus muscle.Фis inversely correlated with age, namely theФdecreases with increasing age. A paired samplest-test was conducted to elucidate the statistical significance of spatial-mean phase angle in all domain <Ф>Ωand in each muscle compartment <Ф>Mwith reference to the conventional phase angle Ф by bioelectrical impedance analysis, muscle grey-scaleGmuscleby ultrasound, and maximal dynamic strengthSMaxby one-repetition maximum test.Significance. From thet-test results, <Ф>Ωhave good correlation with Ф andSMax. In the OG, <ФW5>Ω,ФW5, and (SMax)W5were significantly higher than in the first week (n= 8,p< 0.05). A significant increase in the phase angle of bothM1andM4muscle compartments is observed after five weeks in LG and OG groups. Only the OG group shows a significant increase in the phase angle ofM2muscle compartment after five weeks. However, no significant changes in the spatial-mean phase angle ofM3compartment are observed in each group. In conclusion, ФEIT satisfactorily monitors the response of each compartment in calf muscle to long-term EMS training.

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.

In Vivo Electrical Stimulation for the Assessment of Skeletal Muscle Contractile Function in Murine Models

Skeletal muscle electrical stimulation is commonly used for clinical purposes, assisting recovery, preservation, or even improvement of muscle mass and function in healthy and pathological conditions. Additionally, it is a useful research tool for evaluation of skeletal muscle contractile function. It may be applied in vitro, using cell culture or isolated fibers/muscles, and in vivo, using human subjects or animal models (neuromuscular electrical stimulation - NMES). This chapter focuses on the electrical stimulation of the sciatic nerve as a research method for evaluation of the contractile properties of murine hind limb muscles. Variations of this protocol allow for the assessment of muscle force, fatigue resistance, contraction and relaxation times, and can be used as a model of contraction-induced muscle injury, reactive oxygen species production, and muscle adaptation to contractile activity.

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.

Using distraction forces to drive an autodistractor during limb lengthening

Distraction osteogenesis can result in high forces developing in the limb. To determine and control the distraction forces (DF), a motorized distractor driven by feedback from DF was developed and used to lengthen the tibiae of 6 sheep undergoing distraction osteogenesis. The forces were measured continually, and, in 4 of the sheep, a force threshold was set, above which an increase in rate was initiated. The rate kept increasing to a set limit if forces remained above the threshold; otherwise, the rate was decreased. Radiographs were acquired biweekly, and muscle samples were analyzed from both the operated and contralateral limbs upon termination of the experiment. Results demonstrated a drop in DF associated with increased lengthening rate, attributed to separation of the callus as indicated by radiography. Histological evidence of muscle damage generally correlated with higher DF levels. There was a significant decrease in muscle fiber diameter in lengthened relative to contralateral limbs. Collectively, the results demonstrated the use of a force-driven distraction system and support the need for considering force data in regulating distraction rates to achieve optimal clinical outcomes.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Cellular prion protein promotes regeneration of adult muscle tissue

It is now well established that the conversion of the cellular prion protein, PrP(C), into its anomalous conformer, PrP(Sc), is central to the onset of prion disease. However, both the mechanism of prion-related neurodegeneration and the physiologic role of PrP(C) are still unknown. The use of animal and cell models has suggested a number of putative functions for the protein, including cell signaling, adhesion, proliferation, and differentiation. Given that skeletal muscles express significant amounts of PrP(C) and have been related to PrP(C) pathophysiology, in the present study, we used skeletal muscles to analyze whether the protein plays a role in adult morphogenesis. We employed an in vivo paradigm that allowed us to compare the regeneration of acutely damaged hind-limb tibialis anterior muscles of mice expressing, or not expressing, PrP(C). Using morphometric and biochemical parameters, we provide compelling evidence that the absence of PrP(C) significantly slows the regeneration process compared to wild-type muscles by attenuating the stress-activated p38 pathway, and the consequent exit from the cell cycle, of myogenic precursor cells. Demonstrating the specificity of this finding, restoring PrP(C) expression completely rescued the muscle phenotype evidenced in the absence of PrP(C).

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.

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.

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.

Proximo-distal organization and fibre type regionalization in rat hindlimb muscles

Five muscles of the rat's lower hindlimb were compared with regard to their histochemical fibre type distribution at seven different proximo-distal levels. The muscles were: extensor digitorum longus (ED), flexor digitorum and hallucis longus (FD), gastrocnemius medialis (GM), peroneus longus (PE) and tibialis anterior (TA). In all the five muscles, the relative density of the 'slow' type I fibres showed a striking and similar decrease from proximal toward more distal levels. In addition, the type I fibres were concentrated within smaller and more eccentrically placed regions at distal than at more proximal levels. As a background for the further analysis of these lengthwise aspects of type I fibre regionalization, architectural features of the muscles were determined. Pinnation angles and the position of major tendons and tendon sheets were assessed in fresh specimens. Muscle fibre lengths were measured for single fibres dissected from different regions of macerated muscles. In all cases, fibre length was much shorter than muscle length (mean fraction ranging from 21 to 55%), implying that the proximo-distal changes in histochemical fibre properties were indeed explainable as being due to gradual lengthwise changes in fibre type populations. The similarity of these lengthwise changes across the muscles was in contrast to their differences in other aspects of functional organization, such as the average density of type I fibres and architectural features determining their relative capacities for shortening and force generation. The possible functional role of the proximal accumulation of type I fibres was discussed in relation to circulation and thermal balance; besides, the proximo-distal regionalization of type I fibres might (partly) reflect processes that had been associated with early stages of muscle differentiation. Furthermore, the results underline that, when determining the fibre type composition of rat hindlimb muscles, identifying the proximo-distal level of sampling is a matter of great importance.

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.

Nonlinear tension summation of different combinations of motor units in the anesthetized cat peroneus longus muscle

The purpose of this study was to examine the linearity of summation of the forces produced by the stimulation of different combinations of type identified motor units (MUs) in the cat peroneus longus muscle (PL) under isometric conditions. The muscle was fixed at its twitch optimal length, and the tension produced by the single MU was recorded during 24- and 72-Hz stimulation. The summation analysis was first carried out for MUs belonging to the same functional group, and then different combinations of fast fatigable (FF) MUs were added to the nonfatigable slow (S) and fatigue resistant (FR) group. The tension resulting from the combined stimulation of increasing numbers of MUs (measured tension) was evaluated and compared with the linearly predicted value, calculated by adding algebraically the tension produced by the individual MUs assembled in the combination (calculated tension). Tension summation displayed deviations from linearity. S and FR MUs mainly showed marked more than linear summation; FF MUs yielded either more or less than linear summation; and, when the FF units were recruited after the S and FR MUs, less than linear summation always occurred. The magnitude of the nonlinear summation appeared stimulus frequency dependent for the fatigable FF and FI group. The relationship between measured tension and calculated tension for each MU combination was examined, and linear regression lines were fitted to each set of data. The high correlation coefficients and the different slope values for the different MU-type combinations suggested that the nonlinear summation was MU-type specific. The mechanisms of nonlinear summations are discussed by considering the consequences of internal shortening and thus the mechanical interactions among MUs and shifts in muscle fiber length to a more or less advantageous portion of single MU length-tension curves.

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.

Determination of muscle contractile properties: the importance of the nerve

Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.

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.

Innervation ratio and motor unit force in large muscles: a study of chronically stimulated cat medial gastrocnemius

1. The present study uses chronic low frequency stimulation of cat medial gastrocnemius (MG) muscle to investigate the relative contribution of innervation ratio to the wide range of motor unit force in large mammalian muscles by reducing the normal variation in muscle fibre cross-sectional area and specific force. 2. Isometric force recordings from isolated and physiologically characterized motor units were made 42-240 days after stimulation. Innervation ratio, fibre area and fibre type (I, II A, II B) were determined in one glycogen-depleted motor unit per muscle. 3. After 42 days of stimulation, all motor units were non-fatigable and were classified as either slow (S) or fast-fatigue resistant (FR). Despite the absence of fast-fatigable (FF) motor units, all three muscle fibre types were present, as identified according to their myofibrillar ATPase reactivity. After 143 days, all motor units and muscle fibres were classified as type S and type I, respectively. 4. A rapid decline in muscle and motor unit force to 30% of normal values after 42 days of chronic stimulation was accounted for by a reduction in muscle fibre area. Fibre areas did not change further with longer periods of stimulation but type II fibres were converted to type I. All stimulated muscle fibres were the size of normal type I fibres; the size of the fibres within single motor units covered the full range of the muscle fibre population. 5. In long-term stimulated muscles (> 100 days) when all muscle fibres were type I and all motor units type S, only differences in innervation ratio could account for the remaining range in motor unit force. Estimates of this range from the minimum and maximum values recorded and from values of tetanic force between the 5th and 95th percentiles indicate that the range in innervation ratio in the MG muscles is at least 15-fold and may be as large as 38-fold. Enumerations of glycogen-depleted muscle fibres from single motor units were consistent with this explanation. 6. The findings provide evidence that there is a wide range of innervation ratios in large muscles, which can account for the large range in motor unit forces in the muscles. Since motor unit force and innervation ratio vary with motoneurone size, these studies provide further support that the size of the peripheral field of innervation of motoneurones is related to their size.

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.

Fatigue of chronically overused motor units in prior polio patients

This study was undertaken to investigate the mechanisms underlying fatigue of chronically overused motor units (MUs). The force of the tibialis anterior muscle (TA) and the firing properties of single MUs were studied during prolonged maximum voluntary effort in 10 prior polio patients selected such that daily living required all residual TA power. Almost all TA fibers were hypertrophic type I. Activities of intermyofibrillar succinate dehydrogenase (SDH) and calcium-stimulated myofibrillar adenosine triphosphatase (ATPase) were measured in single TA fibers from a representative patient. Neither insufficient motoneuron activation nor peripheral blocking of the electrical impulse played a major role in the loss of force during prolonged contraction or for slow recovery after contraction. The ratio of SDH to calcium-stimulated ATPase, representing the relation between energy resynthesis and energy utilization, was significantly (P < 0.001) lower in prior polio patients (0.230 +/- 0.096) compared to control (0.515 +/- 0.097) type I fibers.

Relations among motor unit types, generated forces and muscle length in single motor units of anaesthetized cat peroneus longus muscle

The active length-tension curves of identified single motor units (MUs) belonging to peroneus longus muscle (PL) of anaesthetized adult cats were obtained by eliciting isometric single twitches and tetani. The recorded responses were evaluated by measuring the peak tension amplitude and the tension-time area at muscle lengths extending throughout the physiological length range of the muscle (mean 5.5 mm, standard deviation +/- 0.8). The muscle lengths at which each tested MU developed its maximal twitch (Ltw) and tetanic (Lte) tensions were determined and compared with the muscle length (Lo) at which the stimulation of all the alpha-axons, innervating PL and contained in L7 ventral root, developed their maximal twitch tension. The mean of single MU Ltw values was at Lo +1.08 +/- 1.1 mm. Slow MUs showed the longest values of Ltw (Lo +1.6 +/- 1.0 mm). Single MUs stimulated at tetanic frequencies presented their Lte at values shorter than Lo (Lo - 2.8 +/- 1.7 mm). Slow MUs had the shortest Lte (Lo - 3.4 +/- 1.5 mm). For all the units Lte was shorter than Ltw. Ltw and Lte were, respectively, negatively and positively correlated with the developed tension. Optimal length values also appeared to be related to the MU types. The possibility is discussed that the muscle and tendon compliances and the high non-linearities to the applied forces are the main factors which can determine the differences among Lo, Ltw and Lte values. The relationships between MU type and optimal length values are suggested to be, at least partly, an epiphenomenon due to the different contraction strengths of the various MU types.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Contractile properties and temperature sensitivity of the extraocular muscles, the levator and superior rectus, of the rabbit

1. Contractile and fatigue-resistance characteristics, temperature sensitivity (10-37 degrees C) of contraction, and histochemical fibre types were determined for two of the extraocular muscles, the superior rectus and levator palpebrae superioris (levator), of the rabbit. 2. The levator displayed similar contractile characteristics (time to peak, half-relaxation time of twitch response, and twitch-tetanus force ratio) to mammalian fast-twitch limb muscle at room temperature (20 degrees C). However, normalized twitch and tetanic force levels were significantly less than those found in limb muscle. The superior rectus displayed the characteristics of even faster contraction than the levator at 20 degrees C, but generated lower maximum force levels than the levator. 3. The twitch response of the superior rectus showed a biphasic relaxation phase. This response was not due to non-twitch (tonic) fibres present in the superior rectus as it was unaffected by propranolol application during muscle stimulation. 4. The superior rectus and levator displayed significantly less fatigue in the tetanic force response than fast-twitch limb muscles did in response to a fatiguing electrical stimulation protocol. The levator was significantly more fatigue resistant than the superior rectus. 5. The force responses of both extraocular muscles displayed a similar dependence on temperature (10-37 degrees C) to limb skeletal muscles. 6. The superior rectus and levator exhibited a high proportion of fast-twitch muscle fibres (type II) as shown by myosin ATPase staining. Succinate dehydrogenase activity indicated that these muscles showed a high oxidative capacity, with a staining intensity typical of type I or type II A fibres of limb muscles. 7. The results emphasize the morphological and functional complexity of mammalian extraocular muscles. The combination of very fast contractile properties with high oxidative capacity make these muscles well suited to their role in eye/eyelid movement.

Comparison of muscle cell fiber types and oxidative capacity in gracilis, rectus femoris, and triceps brachii muscles in the ferret (Mustela putorius furo) and the domestic dog (Canis familiaris)

Muscle cell fiber types in gracilis, rectus femoris, and long head of triceps brachii muscles of ferrets and dogs were identified on serial sections stained for myosin ATPase after preincubation at pH values of 9.8, 4.6, and 4.3 and for NADH-tetrazolium reductase (NADH-TR) activity. Although fiber types I and II were identified, the ATPase stain did not demonstrate classic type IIA/IIB fiber differences in either species. However, two type II fiber subtypes could be distinguished in the ferret because they differed slightly in staining intensity with ATPase at pH 4.3 and markedly with NADH-TR. One ferret type II fiber (designated II dark or IID) was smaller, slightly darker on ATPase, more oxidative on NADH-TR, and comprised more muscle volume than the other type II fiber (designated II light IIL). The IID fibers of ferret may represent the IID/X fibers of other authors. Both ferret type II fiber subtypes stained darker at pH 4.3 than canine II fibers. The NADH-TR staining indicated high oxidative activity in canine and ferret type I fibers. In contrast, type II fibers in the dog and IIL fibers in the ferret were moderately oxidative. Canine type IIC fibers were intermediate between type I and type II, whereas in the ferret, type IIC fibers were highly oxidative, as were type IID fibers. Ferret muscles are more oxidative than canine muscles according to NADH-TR staining. Also, ferret muscles possess 40-100% higher citrate synthase activity as compared to canine muscles.

The effects of endurance exercise on dystrophic mdx mice. I. Contractile and histochemical properties of intact muscles

Genetically normal (C57BL/10) and dystrophic (mdx) mice underwent a 15 week endurance swimming programme (2 hours per day, 5 days per week) where animals were weighted (5% body weight) during most sessions. No significant changes were seen in the contractile properties or morphology of muscles from control mice following the exercise protocol. In contrast, the soleus muscles of exercised mdx mice displayed higher normalized tensions, and the extensor digitorum longus (EDL) exhibited longer half-relaxation times compared with sedentary mdx mice. Both the EDL and soleus muscles of mdx mice exhibited increased resistance to fatigue after endurance exercise. Sedentary mdx mice exhibited increased proportions of type I (slow oxidative) fibres in the soleus and type IIA (fast, oxidative glycolytic) fibres in the EDL compared with animals of the normal strain. In both the EDL and soleus muscles of mdx mice an even greater proportion of type I fibres was apparent following the training programme. The endurance exercise was beneficial to the mdx mice, enhancing the regeneration of function of their muscles by increasing the proportion of oxidative fibres and reducing muscle fatiguability.

Follow-up of anal dynamic graciloplasty for fecal continence

The feasibility of anal dynamic graciloplasty (transposition of the gracilis muscle and subsequent implantation of a stimulation device) to restore continence, was assessed in a case-control study of 26 patients with severe fecal incontinence. It was shown that anal dynamic graciloplasty is capable of achieving the sphincter tone of healthy persons, as stimulated graciloplasty increased anal pressure from 46 mmHg without stimulation to 65 mmHg with stimulation (mean increase 19 mmHg; 95% confidence interval 13, 25; n = 25; p < 0.01). Time to retain a 250-ml phosphate enema increased from 52 seconds before to 204 seconds after 8 weeks of electrical stimulation (mean increase 151 seconds; 95% confidence interval 61, 241; n = 25; p < 0.01). Complete fecal continence was achieved in 17 patients (65%); two of these patients developed a wound infection, but one of the two realized continence without stimulation and the other patient became continent after reimplantation. Three other patients improved after anal dynamic graciloplasty, but infection necessitated removal of the stimulation device. One patient developed a fistula. Failures were encountered in five patients. Although our long-term follow-up results suggest a learning curve, it is concluded that electrical stimulation improves the results of conventional graciloplasty and avoids construction of a colostomy.

Effects of age on physiological, immunohistochemical and biochemical properties of fast-twitch single motor units in the rat

1. Physiological, enzyme-histochemical, biochemical and morphometrical properties of fast-twitch single motor units were compared between young (3-6 months) and old rats (20-24 months) using the glycogen depletion technique. Monoclonal antibodies (mAbs) were used to identify the myosin heavy chain (MHC) composition in the muscle fibres of the motor unit (motor unit fibres) in order to facilitate correlative physiological, histochemical, biochemical and morphometrical studies. 2. Earlier observations on effects of age on contractile properties of fast-twitch motor units were confirmed and extended. That is, the duration of the isometric twitch, and the twitch and tetanus forces, were increased. Further, motor unit fibres were rearranged, occupying a larger territory and displaying an increased innervation ratio in old age, indicating a denervation-reinnervation process. 3. Motor units with muscle fibres expressing the novel IIX myosin heavy chain (MHC) were observed in both young and old animals, and they constituted the predominant motor unity type identified in the old animals. In contrast to the type IIX MHC motor units in the young animals, the type IIX MHC units in old age often contained muscle fibres which expressed either the type IIA or type IIB MHC, although type IIX MHC fibres were in the majority (so called 'IIX' MHC motor units), but motor units containing all these three fibre types were never observed. There were also single fibres co-expressing IIX and IIB MHCs in old age. 4. In the young animals the IIX MHC motor units had a higher (P less than 0.001) resistance to fatigue (fatigue ratio 0.45 +/- 0.11) than the type IIB MHC units (0.03 +/- 0.05), a succinate dehydrogenase (SDH) activity (0.62 +/- .007) intermediate (P less than 0.001) between those of type IIA muscle fibres classified according to myofibrillar ATPase activity after acid pre-incubation, i.e. type IIA ATPase, (0.84 +/- 0.13) and type IIB MHC motor unit fibres (0.20 +/- 0.04), and cross-sectional fibre areas (1650 +/- 320 microns 2) which were similar to those of type IIA ATPase muscle fibres (1460 +/- 150 microns 2) but smaller (P less than 0.001) than type IIB MHC motor unit fibres (4650 +/- 1180 microns 2).(ABSTRACT TRUNCATED AT 400 WORDS)

MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type

Determinations of fatigue ratio, twitch and tetanus tension, and contraction and half-relaxation times of the isometric twitch were made in 21 single fast-twitch motor units from the rat tibialis anterior muscle. Single motor units were functionally isolated by microdissection of the ventral root, and the glycogen depletion technique was used to demonstrate the muscle fibers in the unit. Morphological and immuno- and enzyme-histochemical methods were applied to serial muscle cross sections to characterize the muscle fibers in the unit. Three of the units had muscle fibers of the IIa type according to staining both for myofibrillar adenosinetriphosphatase after acid preincubation and with the use of monoclonal antibodies specific for myosin heavy chains (MHCs), i.e., the IIa-MHC isoform. The other 18 units were of the IIb type according to enzyme-histochemistry, but immunohistochemistry showed that in six of these units the muscle fibers exhibited the novel type IIx-MHC isoform and in the other 12 units the IIb-MHC isoform. It was found that the IIx motor units have contraction and half-relaxation times similar to those of types IIa and IIb units but have morphological, physiological, and biochemical properties that distinguish them from the latter two types.

Human skeletal muscle: phasic type of electrical stimulation increases its contractile speed

Skeletal muscles, exposed to a prolonged period of specific functional demands, respond adaptively. Electrical stimulation, when employed as a technique for subjecting selected muscles to altered use, enables precise entrainment of the pattern of functional activity. In this investigation, the vastus lateralis muscle in a group of volunteers was stimulated. The stimulation program typical of a phasic type of activity (high frequency, high current amplitude, short pulse duration) intermittently subjected the stimulated muscles to brief periods of intense activity, followed by relatively long pauses. The activation-relaxation time ratio chosen was 1 to 13. It was determined to prevent the muscles from fatiguing. The effects of the chronic stimulation program were established by measurements of the time course of contraction and relaxation and fatigue of the vastus lateralis muscle. Chronic phasic electrical stimulation increased the speed of muscle contraction by 15% while the fatigue characteristics remained unchanged.

Effects of intermittent high frequency electrical stimulation on denervated EDL muscle of rabbit

This study was performed to determine whether electrical stimulation can retard denervation-induced changes. The denervated extensor digitorum longus of the rabbit, a fast-twitch muscle, was stimulated at a rate mimicking its motoneuron firing pattern. The 100-Hz stimulation given intermittently subjected the muscle to a low mean total daily frequency of 1.6 Hz. Four weeks of stimulation resulted in no effect upon the denervated stimulated muscle. This stimulation protocol, therefore, is unable to substitute for the lost neuronal influence of the nerve. The muscle contralateral to the stimulated side showed physiological changes making it unsuitable to serve as a control.

Correlation between shortening velocity, force-velocity relation and histochemical fibre-type composition in rat muscles

Isometric and isotonic contractions of three muscles in the rat hind leg (soleus, extensor digitorum longus (EDL) and peroneus longus (PL] were recorded in situ at 35 degrees C and with nerve stimulation. Additionally, the histochemical muscle fibre-type composition of the three muscles was determined by the method of Guth and Samaha (1970). The data obtained from soleus and EDL muscles were similar to those reported in previous studies. On the basis of twitch contraction time, rate of rise of tetanic tension and maximum shortening velocity, the contraction speed of EDL was 2-3 times higher than in soleus. In the PL muscle, the twitch contraction time, rate of tension rise and shortening velocity were 17 ms, 30 Po/s and 12 muscle fibre lengths/s, respectively; the data showed that the contraction speed of PL muscle was intermediate between that of the soleus and EDL muscles. In the case of soleus, more than 75% of the cross-sectional area was occupied by type 1 (slow) fibres; in both EDL and PL muscles more than 90% of the area was occupied by type 2 (fast fibres). However, the two fast muscles (EDL and PL) had different proportions of type 2B fibres; the area occupied by the type 2B fibre complement was less than 5% in PL, whereas it was around 70% in EDL muscle. The differences in shortening velocity and force-velocity relation among the three muscles could be explained on the basis of their respective muscle fibre-type compositions.

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).

Distribution of activity within the cat's peroneus longus muscle when activated in different ways via the central nervous system

We have studied whether a mechanically homogenous muscle (all units giving joint-torques in the same direction) would display different distributions of activity in response to different types of activation via the central nervous system. In adult cats anaesthetized with pentobarbitone, long-lasting contractions were evoked in m. peroneus longus (PerL), a mixed ankle dorsiflexor muscle. The contractions were elicited by continuous repetitive stimulation of: (1) the superficial peroneal nerve (NP, flexion reflex); and (2) the contralateral motor cortex (MC). During these contractions, isometric force was monitored and fine-wire electromyographic recordings (EMG) were simultaneously obtained from anterior and posterior portions of the PerL. The relative degree of activation in anterior vs posterior muscle portions was quantified by measuring, at various force levels: (1) the average spike amplitude (Spike); and (2) the total number of spikes per unit time (Count; 0.5-s measurement periods). For both types of stimulation (MC and NP), the results indicated that activation was more effective for posterior than for anterior muscle portions: the Spike-measurements were typically higher in posterior than in anterior PerL and, for MC-elicited contractions, this was true for the Count-measurements as well. With respect to both types of EMG-quantification (Spike and Count), the 'posterior bias' of activity was significantly more pronounced for the cortically evoked contractions than for those elicited via stimulation of the peroneal nerve. As it is known that anterior and posterior PerL portions tend to be innervated by rostral and caudal motoneurones, respectively, these findings indicate that different inputs to the PerL motoneurone pool may differ significantly with respect to the intraspinal spatial distribution of synaptic effects among the motoneurones. The results were discussed in relation to the organization of task-related recruitment schemes.

Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise. 2. Muscle enzymes and fatigue characteristics

The fatigue characteristics and the activities of oxidative and glycolytic enzymes were determined in tetrodotoxin (TTX)-induced disuse muscles and in muscles of animals recovering from TTX-induced disuse (TTX-rec). In addition, the effects of additional daily exercise (grid-climbing and swimming) on the fatigue and metabolic profiles of muscles from TTX-rec and control animals were investigated. The activities of citrate synthase (CS), phosphofructokinase (PFK), and alpha-glycerophosphate dehydrogenase (alpha-GPD) were depressed following 28 days of inactivity produced by the chronic neural application of TTX. The response of these muscles to a pattern of stimulation that has been used to classify fast-twitch motor units according to their fatigability (6) (330 ms, 40 Hz, l/s, 4 min) was not affected to any great extent by inactivity, except for a loss in the ability to summate or maintain forces during each 330-ms burst, as fatigue developed. After 28 days of recovery, the concentration of CS had returned to normal, whereas the concentrations of PFK and alpha-GPD remained depressed. TTX-rec muscles, on the other hand, appeared more resistant to fatigue than control muscles, based on several indices of muscle fatigue. Control and TTX muscles responded similarly to daily training. Swimming but not climbing increased the activity of CS and the fatigue resistance of the muscle. Neither exercise influenced the activity of PFK and alpha-GPD. Although the activity of CS was influenced by the level of neuromuscular usage, the former did not appear to play a dominant role in determining the fatigue resistance of the muscle, emphasizing the need to consider other factors as primary determinants of muscle fatigue.

Contractile properties and fatiguability of the human adductor pollicis and first dorsal interosseus: a comparison of the effects of two chronic stimulation patterns

We have developed a method for simultaneously measuring the isometric force of the adductor pollicis (AP) and first dorsal interosseus (FDI) muscles of the hand. Both muscles were found to have very similar force-frequency relationships, relaxation rates and fatigue indices. The effect of 2 patterns of chronic electrical stimulation on the force production, contractile properties and fatiguability of the AP and FDI were compared in 2 groups of 5 subjects. One pattern consisted of uniform 10 Hz stimulation and the other a random pattern containing a few high frequency bursts against an essentially low frequency background. Both patterns were found to increase the fatigue resistance of both muscles by approximately 30% and caused a smaller and less consistent shift in the force-frequency relationship such that more force was generated at low frequencies of stimulation. There was a decrease in both the maximum voluntary and tetanic force after stimulation with the uniform 10 Hz but not the non-uniform patterns. The maintenance of force with the latter pattern may be due to the presence of the high frequency bursts.

Differential effects of 10-Hz and 50 Hz-stimulation of the tibialis anterior on the ipsilateral, unstimulated soleus muscle

Twelve rabbits were cast-immobilized for 4 weeks during which either 10- or 50-Hz stimulation was applied transcutaneously to the anterior compartment muscles. After the treatment period, tibialis anterior and soleus muscle contractile and histochemical properties were measured. Tibialis anterior stimulation at either 10 or 50 Hz had significantly different effects on the ipsilateral, unstimulated soleus muscles. Whereas soleus muscles of both groups demonstrated significant atrophy relative to nonstimulated, nonimmobilized soleus muscles, the soleus muscles from the 50-Hz group demonstrated significantly less atrophy than did the soleus muscles from the 10-Hz group as indicated by significantly greater muscle mass, maximum tetanic tension, and fast fiber area. The results indicate that muscle stimulation may have beneficial effects on ipsilateral muscles that are passively stretched secondary to stimulation. In addition, passive tension, not just muscle activation, appears to have an important role in regulating muscle size.