Fiber Type Proportions Research Articles

Fibre Composition of Human Intrinsic Tongue Muscles

The muscle fibre composition of three human intrinsic tongue muscles, the longitudinalis, verticalis and transversus, was investigated in four anterior to posterior regions of the tongue using morphological and enzyme- and immunohistochemical techniques. All three muscles typically contained type I, IIA and IM/IIC fibres. Type I fibres expressed slow myosin heavy chain (MyHC), type II fibres fast MyHC, mainly fast A MyHC, whereas type IM/IIC coexpressed slow and fast MyHCs. Type II fibres were in the majority (60%), but regional differences in proportion and diameter of fibre types were obvious. The anterior region of the tongue contained a predominance of relatively small type II fibres (71%), in contrast to the posterior region which instead showed a majority of larger type I and type IM/IIC fibres (66%). In general, the fibre diameter was larger in the posterior region. This muscle fibre composition of the tongue differs from those of limb, orofacial and masticatory muscles, probably reflecting genotypic as well as phenotypic functional specialization in oral function. The predominance of type II fibres and the regional differences in fibre composition, together with intricate muscle structure, suggest generally fast and flexible actions in positioning and shaping the tongue, during vital tasks such as mastication, swallowing, respiration and speech.

Clenbuterol induces expression of multiple myosin heavy chain isoforms in rat soleus fibres.

Clenbuterol, a beta2-agonist, administration results in hypertrophy of fast fibres and an increase in the fast myosin heavy chain (MHC) composition of both fast and slow muscles. The present study was designed to determine the phenotypic response at the single fibre level. Clenbuterol was added to the drinking water (30 mg L(-1)) of adult male Wistar rats for 4 weeks. Single fibres from the soleus muscle of control (10 rats; 555 fibres) and clenbuterol-treated (10 rats; 577 fibres) were dissected and their MHC isoform composition was determined using sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis. Body, heart, and soleus weights were 9, 24, and 27% higher in clenbuterol-treated than control rats. The mean cross-sectional areas of fast and slow/fast hybrid fibres were approximately 64 and approximately 74% larger in the clenbuterol-treated than control rats, whereas the size of the slow fibres were similar in the two groups. Fibres from control soleus showed three MHC patterns: pure type I (84%), pure type IIa (4%), and type I + IIa (12%) MHC. Some fibres from clenbuterol-treated soleus showed a de novo expression of type IIx MHC resulting in the following fibre type proportions: pure type I (62%), pure type IIa (2%), type I + IIa (26%), type I + IIa + IIx (6%), and type IIa + IIx (1%). In those fibres containing multiple MHCs, there was a shift towards the faster MHC isoforms after clenbuterol treatment. These data indicate that clenbuterol results in muscle fibre hypertrophy, stimulates a de novo expression of type IIx MHC and increases the percentage of fibres containing multiple MHC isoforms in the rat soleus muscle. These phenotypic changes at the single fibre level are consistent with a clenbuterol-related shift in the functional properties of the soleus towards those observed in a faster muscle.

Spatial distribution of fiber types within skeletal muscle fascicles from Standardbred horses.

Skeletal muscle fascicles from superficial and deep portions of semitendinosus (ST) and gluteus medius (GM) muscles from Standardbred trotters were analyzed with regard to muscle fiber type proportion (types I, IIA, and IIB) and spatial distribution. Muscle fibers within a fascicle were divided into four layers (L(1-4)) from the fascicle periphery toward the center. The observed proportions of fiber types among layers were found to be statistically significantly different from a random distribution of fiber types. Type IIB fibers predominated in the peripheral layer, type I fibers prevailed in the layer underneath, and proportions close to the mean of the whole fascicles were observed in the central layer. This pattern of spatial distribution of fiber types within the layers of the fascicles was observed at all four muscle sampling sites. The functional significance of the common pattern is unknown, but possible functional roles are discussed.

Anatomical and electrophysiological analysis of the trigeminal nerve in a teleost fish, Oncorhynchusmykiss

The trigeminal nerve in the rainbow trout, Oncorhynchus mykiss, was examined for the presence of A-delta and C fibres. Sections of the three branches of the trigeminal nerve were found to comprise a range of fibre types including A-delta and C fibres. The size range of the cell bodies of the trigeminal ganglion reflected the fibre range since they correlated with the size range of axons in the nerve branches. Electrophysiological recordings of evoked activity from the ganglion confirmed the presence of these fibre types and the proportion of these mirrored the proportion of fibre types in the anatomical analyses. A-beta fibres were most common followed by A-delta fibres, then A-alpha fibres with C fibres being the fewest fibre type found. In higher vertebrates, A-delta and C fibres in the trigeminal nerve convey both somatosensory and nociceptive information to the brain. The evolutionary significance of these results is discussed as well as the potential for nociceptive capability in a lower vertebrate.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Fibre type composition of soleus and extensor digitorum longus muscles in normal female inbred Lewis rats

We have analysed the fibre type composition of soleus and extensor digitorum longus (EDL) muscles of normal female 4-6-month-old inbred Lewis rats. This rat strain is used in our ongoing study of the effects of thyroid hormone on myosin heavy chain (MyHC) isoform expression. On the basis of the mATPase reaction, soleus muscles contained 96.1 +/- 2.9% of type 1 fibres supplemented by 2A fibres. EDL muscles contained type 1 (5.5 +/- 1.0%), type 2A (18.8 +/- 1.7%) and type 2B (75.7 +/- 2.2%) fibres. Immunohistochemical analysis and SDS gel electrophoresis confirmed that most fibres in the soleus muscle expressed the type 1 (slow) MyHC isoform and that only a small proportion of fibres expressed the fast 2a MyHC isoform. Immunohistochemical analysis and SDS gel electrophoresis demonstrated that almost half of the 2B fibres of EDL muscles expressed the 2x/d MyHC isoform. In both muscle types, many fibres expressed more than one MyHC isoform. The content of slow fibres in the soleus muscle of female inbred Lewis rats was slightly higher than that reported for Wistar rats, but was considerably higher than that of Sprague-Dawley rats, whereas substantial differences were not found in the proportion of slow and fast fibre types in EDL muscles in these strains.

Comparison of immunochemical and histochemical analysis of fibre type distribution in ovine skeletal muscles

The characterisation of muscle fibres has become increasingly important as the proportion of slow and fast fibre types are known to influence the biochemical and physiological properties of muscle during postmortem tenderisation (Ouali and Talmant, 1990). Current histochemical methods are labour intensive, time consuming and hazardous, requiring rapid freezing of samples in isopentane cooled in liquid nitrogen. The purpose of this study was to investigate an alternative immunochemical approach for identifying fibre types by examining the expression of slow myosin heavy chain (MHC-s) and fast myosin heavy chain (MHC-f) and comparing the data with classical histochemical techniques. Five different ovine skeletal muscles with known differences in fibre types distribution were studied.

Morphometry of Macaca mulatta forelimb. II. Fiber‐type composition in shoulder and elbow muscles

The present study examined the fiber-type proportions of 22 muscles spanning the shoulder and/or elbow joints of three Macaca mulatta. Fibers were classified as one of three types: fast-glycolytic (FG), fast-oxidative-glycolytic (FOG), or slow-oxidative (SO). In most muscles, the FG fibers predominated, but proportions ranged from 25-67% in different muscles. SO fibers were less abundant except in a few deep, small muscles where they comprised as much as 56% of the fibers. Cross-sectional area (CSA) of the three fiber types was measured in six different muscles. FG fibers tended to be the largest, whereas SO fibers were the smallest. While fiber-type size was not always consistent between muscles, the relative size of FG fibers was generally larger than FOG and SO fibers within the same muscle. When fiber CSA was taken into consideration, FG fibers were found to comprise over 50% of the muscle's CSA in almost all muscles.

Neuromuscular compartments and fiber-type regionalization in the human inferior pharyngeal constrictor muscle.

The inferior pharyngeal constrictor (IPC) muscle functions during swallowing, respiration, and vocalization. The most-caudal portion of the IPC is believed to be part of the functional upper esophageal sphincter (UES). We hypothesized that the caudal fibers of the human IPC may have enzyme-histochemical characteristics similar to those of the cricopharyngeus muscle, a major component of the UES. In this study, human IPC muscles obtained from autopsy were studied using Sihler's stain to examine innervation patterns, and using myofibrillar ATPase, NADH tetrazolium reductase (NADH-TR), and succinic dehydrogenase (SDH) techniques to investigate the distribution and oxidative capacity of the slow- (type I) and fast- (type II) twitch fibers in the muscle. The results showed that the human IPC consists of at least two neuromuscular compartments (NMCs): rostral and caudal. Each of the NMCs was innervated by a separate nerve branch derived from the pharyngeal branch of the vagus nerve. The rostral NMC is faster (39% type I, 61% type II) than the caudal NMC (70% type I, 30% type II). In addition, two histochemically-delineated fiber layers were identified in the human IPC: a slow inner layer (SIL) with predominantly type I fibers (66%), and a fast outer layer (FOL) with predominantly type II fibers (62%) (P < 0.01). However, the dimensions of both fiber layers and proportions of the muscle fiber types varied with the NMCs. Specifically, the ratio of the thickness of the SIL to FOL was approximately 2:1 for the caudal NMC and approximately 1:2 for the rostral NMC, respectively. In the SIL the type I fibers accounted for 84% for the caudal NMC and 69% and 44% for the lower and upper portions of the rostral NMC. In contrast, the type II fibers in the FOL accounted for 46% for the caudal NMC and 67% and 74% for the lower and upper portions of the rostral NMC, respectively (P < 0.01). The caudal NMC of the IPC shared histochemical characteristics with the cricopharyngeus muscle, in that it contained predominantly slow oxidative fibers. Overall, the caudal NMC and the SIL in the IPC had high NADH-TR and SDH activities. However, different patterns of oxidative enzyme activity were identified in both type I and type II fibers. This study provided histochemical evidence for the concept that the caudal NMC within the IPC contributes to the functional UES. In addition, the two histochemically-defined fiber layers in the IPC may be a specialized adaptation in humans to enable different upper-airway functions during respiration, swallowing, and speech.

Changes in a rat facial muscle after facial nerve injury and repair.

This study describes changes in a rat facial muscle innervated by the mandibular and buccal facial nerve branches 4 months after nerve injury and repair. The following groups were studied: (A) normal controls; (B) spontaneous reinnervation by collateral or terminal sprouting; (C) reinnervation after surgical repair of the mandibular branch; and (D) chronic denervation. The normal muscle contained 1200 exclusively fast fibers, mainly myosin heavy chain (MyHC) IIB fibers. In group B, fiber number and fiber type proportions were normal. In group C, fiber number was subnormal. Diameters and proportions of MyHC IIA and hybrid fibers were above normal. The proportion of MyHC IIB fibers was subnormal. Immediate and delayed repair gave similar results with respect to the parameters examined. Group D rats underwent severe atrophic and degenerative changes. Hybrid fibers prevailed. These data suggest that spontaneous regeneration of the rat facial nerve is superior to regeneration after surgical repair and that immediacy does not give better results than moderate delay with respect to surgical repair. Long delays are shown to be detrimental.

Fibre type proportion and fibre size in trapezius muscle biopsies from cleaners with and without myalgia and its correlation with ragged red fibres, cytochrome-c-oxidase-negative fibres, biomechanical output, perception of fatigue, and surface electromyography during repetitive forward flexions.

In the literature enlarged/increased cross-sectional area (CSA) of type I muscle fibres has been reported as a morphological mark of work-related localised myalgia in the descending part of the trapezius muscle of women. These studies did not use enough subjects or lacked an adequate control group. The recording of surface electromyograms (EMG) is central to the research field of work-related myalgia. However, the influence of intrinsic muscle properties such as the effect of muscle fibre distribution upon surface EMG has to be better understood in order properly to evaluate this method as a possible diagnostic and preventive tool. This study had two aims. Firstly, it investigated the muscle fibre distribution and CSA in work-related myalgia in trapezius muscles. Secondly, the multivariate relationships among muscle morphology and histochemistry [ragged-red (RR) fibres, and cytochrome-c-oxidase-negative-fibre changes] EMG, perceived fatigue, and biomechanical output of shoulder flexions were analysed. The raw data have been presented in an earlier study. The participants in this study were 25 female cleaners with work-related myalgia of the trapezius muscle and 25 female cleaners not experiencing work-related myalgia of this muscle. The control group comprised 21 healthy female teachers who had not been exposed to highly repetitive work or static muscle work. Smaller CSA of type II fibres were found in cleaners compared to teachers. In this study the CSA of type I fibres of the trapezius muscles associated with myalgia were no greater than in muscles without myalgia. The prevalence of RR fibres together with age, fibre type proportions, CSA and working as a cleaner correlated with the ability to relax as recorded electromyographically. The relative mean frequency of the EMG of the trapezius muscle correlated with the prevalence of RR fibres, but it did not correlate with the proportions and CSA of different fibre types. Low biomechanical outputs and low signal amplitude increases of the EMG during the test were associated with high proportions of type -IIB fibres. The smaller CSA of type II fibres in cleaners might have reflected a different muscle activation pattern due to different occupational demands in cleaners than in teachers. Morphological or histopathological variables can influence the three EMG variables investigated.

CAPILLARIZATION OF THE ANKLE DORSIFLEXOR MUSCLES IN YOUNG, HEALTHY, RECREATIONALLY ACTIVE MEN AND WOMEN

The purpose of this study was to compare ankle dorsiflexor muscle capillarization between young (20 to 31 years), healthy, recreationally active men (M, n = 15) and women (W, n = 15) using global and individual muscle fiber based capillary indices. Muscle fiber types and areas were determined using myosin ATPase staining. Immunohistochemical double staining for lectin and collagen was performed for capillary visualization. T-tests were done to determine differences between M and W for the global indices of capillarization. Repeated measures analyses of variance were used to determine the effects of gender and fiber type on individual muscle fiber capillarization indices. There were no significant differences between the M and W for capillary density (CD). Fiber density (FD) was greater in the W (p < 0.001), therefore the capillary to fiber density ratio (CD/FD) was lower in the W (p < 0.001). The M and W did not differ in their fiber type proportions, which was on average about 74% type 1. However, the M did have larger fibers (area or perimeter) than the W (p < 0.01), with type 2 fibers being proportionally larger than type 1 (p < 0.01). The area (FA/C) or perimeter (FP/C) supplied by each capillary was greater for type 2 fibers (p < 0.01), but there was no significant gender effect, although there was a trend (p = 0.07) for a gender effect for perimeter supplied per capillary. In conclusion, even though fiber sizes were different between M and W, there appear to be no major gender effects on muscle capillarization.Table

Altered Primary Myogenesis in NFATC3−/− Mice Leads to Decreased Muscle Size in the Adult

Signal transduction pathways involving calcineurin and its downstream effector NFAT have been implicated in regulating myogenesis. Several isoforms of NFAT exist that may differentially contribute to regulating skeletal muscle physiology. The purpose of this study was to determine the role of the NFATC3 isoform in skeletal muscle development. Adult mice lacking NFATC3 have reduced muscle mass compared to control mice. The smaller size of the muscles is not due to atrophy or blunted myofiber growth, but rather to a reduced number of myofibers. This reduction in myofiber number is not limited to a specific fiber type nor are the proportions of fiber types altered. The lower fiber number found in the adult NFATC3−/− mice is a consequence of impaired muscle development during embryogenesis. Immunohistochemical studies of E15 EDL muscles indicate that the total number of primary myofibers is decreased in NFATC3−/− embryos. At E17.5 no further decrease in primary myofiber number occurs; the size and organization of the myofibers are unaltered, and secondary myogenesis proceeds normally, suggesting a role for NFATC3 during early events in primary myogenesis. These results suggest a heretofore unknown role for the transcription factor NFAT in early skeletal muscle development.

Differential expression of myosin heavy-chain mRNA in muscles of mastication during functional advancement of the mandible in pigs

Surgical and orthodontic treatment of retrognathia aims to improve orofacial function by adaptation and training of muscle capacity, which is connected with a change in muscle fibre-type proportions. The aim here was to analyse the proportion of myosin-heavy chain (MyHC) gene expression in type I (slow twitch/ST) and type IIb (fast twitch/FT) fibres during sagittal advancement of the mandible by reverse transcriptase-polymerase chain reaction (RT-PCR). The experiments were carried out on 10-week-old pigs (six test animals, six controls) over a 28-day period. Six pigs were fitted with acrylic bite blocks for sagittal advancement of the mandible. Tissue was taken from seven different regions of the masseter, temporal, medial pterygoid, and geniohyoid muscles. The 84 samples were used for histological fibre differentiation with ATPase staining and for isolation of total RNA. To measure the two MyHC isoforms, RT-PCR (in a single tube reaction with MyHC I, MyHC IIb, and GAPDH primers) was used. A significant increase was registered in the percentage of ST fibres and in mRNA from MyHC I in the anterior region of the masseter and in the posterior region of the temporal muscle of the treated animals. The proportion of ST fibres to FT fibres was increased by up to 12% after functional advancement of the mandible. The histological findings corresponded with the data for fibre mRNA generated by RT-PCR.

Oxidative state and histological changes in muscles of mastication after conditioning training

Abstract Stress due to endurance training of striated muscles leads to adaptive changes in the distribution of muscle fiber types (i.e. ratio of type I and type II fibers). Moreover, severe training leads to tissue hypoxia and oxidative stress in muscles. In the current study, we examined the relationship between histological changes and oxidative state in muscles of mastication during the acute adaptation phase to a sustained muscle load. Six domestic pigs received build-ups on the molar teeth in order to induce a sustained load of the muscles of mastication for a duration of four weeks. Afterwards the masseter (M1, M2, M3), medial pterygoid (PM), temporal (TP1, TP2), and geniohyoid muscles (GH) were removed and the fiber type distribution was determined by enzyme histochemistry. Additionally, the tissue content of glutathione and lipid peroxidation (LPO) products were measured. The above treatment led to muscle fiber transformation of type II into type I (M1, M2, TP2, PM) and a decrease of the GSH content (M1, M2 and TP2). The changes in the GSH/GSSG ratio were in accordance with the changes in proportions of muscle fiber types, with the lowest GSH/GSSG ratios in the most stressed muscles of the treated animals. No significant changes in LPO products were found. The decrease of the GSH/GSSG ratio in the most stressed muscles indicates an increased intracellular oxidative stress, which may be caused by tissue hypoxia during the chronic phase of muscle adaptation.

Functional and histological characteristics of skeletal muscle and the effects of leptin in the genetically obese (ob/ob) mouse.

Skeletal muscle mass in genetically obese (ob/ob) mice displays a reduced mass compared with their normal lean counterpart mice. However, the functional capacity of the available skeletal muscle mass in these animals has not yet been determined. To investigate the properties of skeletal muscle in ob/ob mice and determine the effects of leptin administration on skeletal muscle in these mice. Following 4 weeks of i.p. leptin administration (or control treatment) anaesthetized ob/ob and lean mice had their extensor digitorum longus and soleus muscles removed, and standard measures of isometric contractile properties and fatigability were performed. Histochemistry was used to determine fibre type proportions and individual fibre areas of all muscles. Leptin had no effect on the morphology or function of ob/ob skeletal muscle despite reducing body mass in ob/ob mice. Force production was unaltered in obese mice. However, a significant prolongation of contraction and relaxation times were evident. Obese skeletal muscle was also more fatigue resistant. Fibre proportions displayed a more slow type profile in ob/ob skeletal muscle, and in conjunction with previous work a reduced ability to hypertrophy. Skeletal muscle from obese mice is morphologically and functionally different from lean mouse skeletal muscle. Obese muscle is very similar to skeletal muscle from aged mice, and the specific contractile properties examined appear to be determined by the fibre make-up of these muscles.

Muscle fibre types and size distribution in sub‐antarctic notothenioid fishes

The presumptive tonic muscles fibres of Cottoperca gobio, Champsocephalus esox, Harpagifer bispinis, Eleginops maclovinus, Patagontothen tessellata, P. cornucola and Paranotothenia magellanica stained weakly or were unstained for glycogen, lipid, succinic dehydrogenase (SDHase) and myosin ATPase (mATPase) activity. Slow, intermediate and fast twitch muscle fibres, distinguished on the basis of the pH stability of their mATPases, showed intense, moderate and low staining activity for SDHase, respectively. Slow fibres were the major component of the pectoral fin adductor profundis muscle. The proportion of different muscle fibre types varied from the proximal to distal end of the muscle, but showed relatively little variation between species. The myotomes contained a lateral superficial strip of red muscle composed of presumptive tonic, slow twitch and intermediate fibres, thickening to a major wedge at the horizontal septum. All species also had characteristic secondary dorsal and ventral wedges of red muscle. The relative abundance and localization of muscle fibre types in the red muscle varied between species and with body size in the protandric hermaphrodite E. maclovinus. The frequency distribution of diameters for fast twitch muscle fibres, the major component of deep white muscle, was determined in fish of a range of body sizes. The absence of fibres &lt;20 μm diameter was used as a criterion for the cessation of muscle fibre recruitment. Fibre recruitment had stopped in P. tessellata of 13·8 cm LT and E. maclovinus of 32·8 cm LT, equivalent to 49 and 36·5% of their recorded maximum sizes respectively. As a result in 20‐cm P. tessellata, the maximum fibre diameter was 300 μm and 36% of fibres were in excess of 200 μm. The unusually large maximum fibre diameter, the general arrangement of the red muscle layer and the extreme pH lability of the mATPase of fast twitch fibres are all common characters of the sub‐Antarctic and Antarctic Notothenioids, including Cottoperca gobio, the suggested sister group to the Notothenidae.

Influences of IGF-I gene disruption on the cellular profile of the diaphragm.

The impact of a targeted disruption of the Igf1 gene, encoding the insulin-like growth factor I (IGF-I), on diaphragm (DIA) cellularity was studied in 2-mo-old homozygous mutant [IGF-I(-/-)] mice and their wild-type [WT; i.e., IGF-I(+/+)] littermates. DIA fiber types were classified histochemically. DIA fiber cross-sectional areas (CSA) were determined from digitized muscle sections, and fiber succinate dehydrogenase (SDH) activity was determined histochemically using a microdensitometric procedure. An acidic ATPase reaction was used to visualize capillaries. Myosin heavy chain (MyHC) isoforms were identified by SDS-PAGE, and their proportions were determined by scanning densitometry. The body weight of IGF-I(-/-) animals was 32% that of WT littermates. DIA fiber type proportions were unchanged between the groups. The CSAs of types I, IIa, and IIx DIA fibers of IGF-I(-/-) mutants were 63, 68, and 65%, respectively, those of WT animals (P < 0.001). The DIA thickness and the number of fibers spanning its entire thickness were reduced by 36 and 25%, respectively, in IGF-I(-/-) mice (P < 0. 001). SDH activity was significantly increased in all three types of DIA fibers of IGF-I(-/-) mutants (P < 0.05). The number of capillaries per fiber was reduced approximately 30% in IGF-I(-/-) animals, whereas the capillary density was preserved. The proportions of MyHC isoforms were similar between the groups. Muscle hypoplasia likely reflects the importance of IGF-I on cell proliferation, differentiation, and apoptosis (alone or in combination) during development, although reduced cell size highlights the importance of IGF-I on rate and/or maintenance of DIA fiber growth in the postnatal state. Reduced capillarity may result from both direct and indirect influences on angiogenesis. Improved oxidative capacity likely reflects DIA compensatory mechanisms in IGF-I(-/-) mutants.

The influences of muscle fibre proportions and areas upon EMG during maximal dynamic knee extensions.

This study is an investigation of the relationship between muscle morphology and surface electromyographic (EMG) parameters [mean frequency of the power spectrum (MNF), signal amplitude (root mean square, RMS) and the signal amplitude ratio (SAR; i.e. the ratio between the RMS level during the passive part of the contraction cycle and the RMS level during the active part of the contraction cycle)] during 100 maximal dynamic knee extensions at 90 degrees. s(-1). Each contraction cycle comprised of 1 s of active knee extension and 1 s of passive knee flexion. The surface EMG was recorded from the vastus lateralis muscle. Twenty clinically healthy subjects participated in the study, and muscle biopsy samples of the vastus lateralis were obtained from 19 of those subjects. The relationships between muscle morphology and EMG were investigated at three stages of the test: initially, during the fatigue phase (initial 40 contractions), and at the endurance level (the final 50 contractions). Major findings on correlations are that SAR and MNF tended to correlate positively with the proportion of type 1 fibres, and RMS correlated positively with the proportion of type 2 muscle fibres. The muscle fibre areas showed little correlation with the EMG variables under investigation. The results of the present study showed that the three EMG variables of a dynamic endurance test that were investigated (RMS, MNF and SAR) were clearly correlated with the proportions of the different fibre types, but only to a small extent with fibre areas. These findings contradict some of the theoretical models of the EMG, especially for parameters in the frequency domain.

Skeletal muscle oxidative capacity, fiber type, and metabolites after lung transplantation.

Lung transplant (LTx) recipients have a low peak work rate, peak oxygen consumption (V O2peak), and early lactate threshold on incremental exercise. We hypothesized that LTx recipients have reduced oxidative function and altered fiber type proportion in peripheral skeletal muscle. Seven stable LTx recipients and seven age- and sex-matched control subjects were studied. Incremental exercise testing with arterialized venous sampling and a resting quadriceps femoris punch muscle biopsy were performed. Muscle specimens were analyzed for fiber type proportion, metabolites, oxidative and glycolytic enzyme activities, and mitochondrial ATP production rate (MAPR) using standard techniques. The results showed that mean V O2peak in LTx recipients was 52% of control subjects. Compared with the control subjects, LTx skeletal muscle exhibited: (1) a lower MAPR; (2) lower activity of the mitochondrial enzymes glutamate dehydrogenase (GDH), citrate synthase (CS), 2-oxogluterate dehydrogenase (OGDH), and 3-hydroxyacyl-CoA-dehydrogenase (HAD). There was no difference in the activities of anaerobic enzymes, except for higher phosphofructokinase activity; (3) a lower proportion of type I fibers; (4) a higher lactate and inosine monophosphate (IMP) content and a lower ATP content at rest indicating a high reliance on anaerobic metabolism. The reduced type I fiber proportion and severely reduced mitochondrial oxidative capacity may play an important role in exercise limitation after LTx.