The origin of tubular aggregates in human myopathies

Tubular aggregates are morphological abnormalities characterized by the accumulation of densely packed tubules in skeletal muscle fibres. To improve knowledge of tubular aggregates, the formation and role of which are still unclear, the present study reports the electron microscopic analysis and protein characterization of tubular aggregates in six patients with 'tubular aggregate myopathy'. Three of the six patients also presented with myasthenic features. A large panel of immunochemical markers located in the sarcoplasmic reticulum, T-tubules, mitochondria, and nucleus was used. Despite differences in clinical phenotype, the composition of tubular aggregates, which contained proteins normally segregated differently along the sarcoplasmic reticulum architecture, was similar in all patients. All of these proteins, calsequestrin, RyR, triadin, SERCAs, and sarcalumenin, are involved in calcium uptake, storage, and release. The dihydropyridine receptor, DHPR, specifically located in the T-tubule, was also present in tubular aggregates in all patients. COX-2 and COX-7 mitochondrial proteins were not found in tubular aggregates, despite being observed close to them in the muscle fibre. The nuclear membrane protein emerin was found in only one case. Electron microscopy revealed vesicular budding from nuclei, and the presence of SAR-1 GTPase protein in tubular aggregates shown by immunochemistry, in all patients, suggests that tubular aggregates could arise from endoplasmic reticulum exit sites. Taken together, these results cast new light on the composition and significance of tubular aggregates.     

Ultrastructural study of the alveolar-capillary membrane and the tubular endoplasmic reticulum in alveolar type I cells of the caprine lung

The ultrastructural features of alveolar type I cells of the goat lung were studied by using vascular perfusion and direct airway instillation of fixatives. The morphological features of smooth endo-plasmic reticulum (SER) were characterized by measuring the diameter of individual SER tubules which conformed in size and appearance to the tubular endoplasmic reticulum (TER) already described in various types of epithelium. The TER appeared as large tubular aggregates in a palisade arrangement; these aggregates ramified into various areas of the extended cytoplasm of alveolar type I cells. The TER also existed as a mixture of short cisternae and vesicles, and glyco-gen alpha particles were present in the non-perikar-yonic portion of the cell. The different forms of TER had varying relationships to the plasmalemma. The interchangable configurations seen in the structure of TER indicated the functional modalities of the cells and were comparable to similar structural modifications in electrolyte-secreting cells. The role of TER, microtubules, and large populations of endocytic vesicles in the alveolar type I cells in the goat lung is examined in the context of physiological eructation of rumenal gases and the absorption of electrolyte-rich fluids which escape into the lung at each eructation in ruminants.     

Resynthesis of glycogen in different muscle fibre types after prolonged exhaustive exercise in man

The rate of glycogen resynthesis was examined in different muscle fibre types after prolonged exhaustive exercise. Six subjects exercised to exhaustion at 75% of VO2 max, and muscle biopsies were taken after 0, 90 and 180 min of recovery. Glucose drinks (1.4, 0.7 and 0.7 g kg-1 body wt) were taken at time 0, 60 and 120 min. Photometric determination of periodic acid-Schiff stain intensity revealed a 65% faster rate of glycogen resynthesis in type IIA and IIAB as compared to type I fibres during the first 90 min. Thereafter no differences between the various fibre types were detected. No differences in the rate of glycogen resynthesis were observed between the subgroups of type II muscle fibres. These results suggest that there was a slower acceleration of glycogen resynthesis in type I compared to type II fibres. In all fibre types a positive relationship between rate of synthesis and glycogen concentration was observed. It is suggested that the size of the glycogen molecule and hence the number of available terminal glucosyl units is a major determinant of rate of resynthesis.     

Muscle fibre size and capillarity in Korean diving women

AIM: Effects of prolonged habitual cold-water immersion on fibre size and capillarity in vastus lateralis muscle were studied in human beings. The hypothesis tested in the present study was that cold acclimatized human skeletal muscle would have reduced muscle fibre size and higher capillarity, favouring the idea of efficacy of recruitment under cold environment. METHODS: Ten women breath-hold divers (BHDs) and 10 active women (controls CONs) participated in this study. Muscle biopsy was obtained from vastus lateralis and determined fibre type composition and capillary density. RESULTS: A major finding was that all BHDs revealed a markedly smaller cross-sectional area (CSA) in all fibre types than the CONs, or even than any other morphological data reported in previous investigations. Furthermore, mean CSA of type II fibre (range 1205-2766 microm2) was much smaller than type I fibre (2343-4327 microm2). The number of capillaries per fibre in different fibre types in the BHDs was higher than in the CONs (P < 0.001), and diffusional area was smaller in type II fibres than in type I fibres (P < 0.001). The BHDs and the CONs have similarity in the percentage of type I fibres, but type II fibre was predominant in both groups. Interestingly the proportion of type IIx fibre in the BHDs was higher (31%) than in the CONs (22%). No significant difference was found in the thigh circumference between the groups. CONCLUSION: The present study demonstrates that prolonged habitual cold-water immersion may induce a decrease in fibre size and an increase in capillarity in human skeletal muscle.     

Effect of exercise on concentrations of free amino acids in pools of type I and type II fibres in human muscle with reduced glycogen stores

A few animal studies have shown that some amino acid concentrations vary between different muscle fibre types. In the present study, amino acid concentrations were measured in separate pools of different fibre types in human skeletal muscle, with reduced glycogen stores, before and after sustained exercise. Five subjects exercised at a submaximal work rate for 60 min and then at a maximal rate for 20 min. Biopsy samples were taken from the vastus lateralis muscle before and after exercise; they were freeze-dried and individual fibres were dissected out. Fragments of these fibres were stained for myosin-adenosine triphosphatase (ATPase) and identified as type I or type II fibres. The concentrations of free amino acids were measured by high performance liquid chromatography (HPLC) in perchloric acid (PCA) extracts containing pools of either type of fibre. After exercise, glycogen was decreased in type I fibres (53%) and in four subjects also in type II fibres. The concentrations of most amino acids were similar in the two fibre types before exercise, but the glutamate, aspartate and arginine levels were 10% higher in type II than in type I fibres. After exercise, the glutamate concentration was decreased by 45% in both fibre types and the branched-chain amino acids (BCAA) were decreased in type II fibres (14%). Exercise caused an increase by 25-30% in tyrosine concentration in both type I and type II fibres. The results show that amino acids can be measured in pools of fibre fragments and suggest that amino acid metabolism play an important role in both type I and type II fibres during exercise.     

Differences in Fibre Type Composition Between Human Masseter and Biceps Muscles in Young and Adults Reveal Unique Masseter Fibre Type Growth Pattern

The human jaw system is different from those of other primates, carnivores, ruminants, and rodents in temporomandibular joint and muscle anatomy. In adults, jaw muscles also differ markedly from limb and trunk muscles in composition and distribution of fibre types. It can be assumed that age‐related changes between young age to adulthood in terms of craniofacial growth, teeth eruption, and improvement of jaw functions are paralleled by alterations also in composition and distribution of jaw muscle fibre types. To address this question, we have examined the fibre type composition of the human masseter, a jaw closing muscle, at young age. For comparison, the young biceps brachii was examined. The results were compared with previous data for adult masseter and biceps muscles. Young masseter and biceps were similar in that type I fibres outnumbered other fibre types and were of the same diameter. However, they differed in composition of other fibre types. Young masseter contained fibre types I, IM, IIC, IIAB, IIB, and scarce IIA, with regional differences, whereas young biceps showed types I, IIA, IIAB, and few IIB. Young masseter differed from young biceps also by smaller type II fibre diameter and by containing fetal MyHC. In addition, the masseter and biceps differed in age‐related changes of composition and distribution of fibre types between young age and adulthood. We conclude that the human masseter is specialized in fibre types already at young age and shows a unique fibre type growth pattern, in concordance with being a separate allotype of muscle. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.     

Twitch contractile adaptations are not dependent on the intensity of isometric exercise in the human triceps surae

Summary Ultrastructural and twitch contractile characteristics of the human triceps surae were determined in six healthy but very sedentary subjects before and after 16 weeks of isometric training at 30% maximal voluntary contraction (MVC). Following training, twitch contraction time was approximately 16% shorter, although no differences were observed in one-half relaxation time or peak twitch torque. Percent fibre type was not changed by training. The mean area of type I and type II fibres in the soleus increased by approximately 30% but only type II fibres showed an increase in area in the lateral gastrocnemius (30%). Despite such changes in fibre area the volume density of the sarcoplasmic reticulum-transverse tubular network averaged 3.2 ± 0.6% and 5.9 ± 0.9% in type I and type II fibres respectively, before and after training in the two heads of the gastrocnemius. The results indicate that contractile adaptations to isometric training at 30% MVC were limited to twitch contraction time and were not directly related to changes in percent fibre distribution or the volume of sarcoplasmic reticulum and transverse tubules in either type I or type II fibres. The data further demonstrate that substantial fibre hypertrophy is achieved by training with low-intensity contractions.     

Differences of morphometrical parameters in hind limb muscle fibres between ovarectomized and sexually intact female dogs.

Slow and fast twitch fibres of the Mm. tibialis cranialis, semitendinosus and sartorius of seven sexually intact and seven ovarectomized female beagles were histochemically and morphometrically analysed. Along with type I and type IIA fibres, another main type II fibre (IIS), which seems to be peculiar to the dog, was found in the Mm. semitendinosus and tibialis cranialis. Type I fibers comprised 26% and type II fibres 74% of all recorded muscle fibres in the M. tibialis cranialis, 29% (type I) and 71% (type II) in the M. semitendinosus and 51% (type I) and 49% (type II) in the M. sartorius, respectively. The average single profile area and the corresponding mean diameter of fibre types I and II in the investigated hind limb muscles were generally larger in ovarectomized than in sexually intact animals. This was more evident in type II than in type I fibres. However only the type II fibres of the M. tibialis cranialis and sartorius exhibited a statistically significant increase in diameter (p < 0.01 and p < 0.05, respectively). Accordingly, the mean density (number of fibres/mm2) of both fibre types in the hind limb muscles of spayed dogs was generally reduced. Again, this reduction attained statistical relevance in the type I and II fibres of the tibialis cranialis. In addition, the fibre densities of type I in the semitendinosus and type II in the sartorius muscles were also significantly reduced in ovarectomized dogs. In conclusion, ovarectomized beagles showed a generally increased mean diameter of the investigated type I and II hind limb muscle fibres and a concomitant decreased average fibre density of the respective types when compared to sexually intact animals.     

Regional differences in fibre type composition in the human temporalis muscle

Anatomical and electromyographic studies point to regional differences in function in the human temporalis muscle. During chewing and biting the anterior portions of the muscle are in general more intensively activated and they are capable of producing larger forces than the posterior portions. It was hypothetised that this heterogeneity in function is reflected in the fibre type composition of the muscle. The composition and surface area of different fibre types in various anteroposterior portions of the temporalis muscle were investigated in 7 cadavers employing immunohistochemistry with a panel of monoclonal antibodies against different isoforms of myosin heavy chain. Pure slow muscle fibres, type I, differed strongly in number across the muscle. In the most posterior portion of the muscle there were 24% type I fibres, in the intermediate portion 57%, and in the most anterior portion 46%. The mean fibre cross-sectional area (m-fcsa) of type I fibres was 1849 μm², which did not differ significantly across the muscle. The proportion of pure fast muscle fibres, type IIA and IIX, remained more or less constant throughout the muscle at 13% and 11% respectively; their m-fcsa was 1309 μm² and 1206 μm², respectively, which did not differ significantly throughout the muscle. Pure type IIB fibres were not found. The relative proportion of hybrid fibres was 31% and did not differ significantly among the muscle portions. Fibre types I+IIA and cardiac α+I+IIA were the most abundant hybrid fibre types. In addition, 5% of the type I fibres had an additional myosin isoform which has only recently been described by means of electrophoresis and was named Ia. In the present study they were denoted as hybrid type I+Ia muscle fibres. It is concluded that intramuscular differences in type I fibre distribution are in accordance with regional differences in muscle function.     

Morphology of diaphragm neuromuscular junctions on different fibre types

Summary We hypothesize that the morphology of the neuromuscular junction on different muscle fibre types varies, reflecting differences in activation history. In the rat diaphragm muscle, we used a three-colour fluorescent immunocytochemical technique to simultaneously visualize (1) innervating axons and presynaptic nerve terminals, (2) motor endplates and (3) myosin heavy chain isoform expression (muscle fibre type). Laser-scanning confocal microscopy was then used to optically section the triple-labelled muscle fibres, and create three-dimensional views of the neuromuscular junction. Type I fibres were innervated by the smallest axons, and type IIa, IIx and IIb fibres by progressively larger axons. Absolute planar areas of nerve terminals and endplates progressively increased from type I, IIa, IIx to IIb fibres. When normalized for fibre diameter planar areas of nerve terminals were largest on type I fibres, with no difference among type II fibres. The normalized planar area of endplates were larger for type I and IIb fibres, compared to type IIa and IIx fibres. The three-dimensional surface area of endplates was largest on type I fibres, with no differences across type II fibres. When normalized for fibre diameter, endplate surface areas increase progressively from type I, IIa, IIx to IIb fibres. The branches increased progressively from type I, IIa, IIx to IIb fibres. Conversely, individual branch length was longest on type I fibres, and shortest on type IIb fibres. The extent of overlap of pre- and postsynaptic elements of the neuromuscular junction decreased progressively on type I, IIa, IIx and IIb fibres. We conclude that these morphological differences at the neuromuscular function of different fibre types reflect differences in activation history and may underlie phenotypic differences in neuromuscular transmission.     

Histochemical and biochemical changes in human skeletal muscle with age in sedentary males, age 22--65 years.

Biopsies for histochemical and biochemical analyses were taken from the vastus lateralis muscle of 55 untrained, healthy male subjects from 22 to 65 years of age. Fibre type distribution changed towards a decrease in the percentage of type II fibres, both in type IIA and type IIB fibres, whereas type IIB/IIA fibre ratio and type IIC percentage did not change with increasing age. It was found that the type IIB/IIA fibre ratio was inversely related to type I fibres, i.e. subjects rich in type I fibres had a relatively smaller proportion of type IIB fibres. Fibre area determinations revealed a selective decrease in type II fibre area. Consequently, the type II/I fibre area ratio and relative type II fibre area decreased. No changes in the specific activities of Mg2+ stimulated ATPase and myokinase were observed, while the activity of lactate dehydrogenase (LDH) was higher in the youngest groups than in the oldest. LDH isozyme pattern shifted towards a decrease in percentage distribution of the muscle specific isozymes and a corresponding decrease in muscle specific activity while the activity of the heart specific isozymes did not change.     

Changes in muscle fibre type from adolescence to adulthood in women and men.

Age-related changes in muscle fibre characteristics have been presented in cross-sectional studies previously. The aim of the present study was to investigate longitudinally whether the muscle fibre type composition and muscle fibre area change from adolescence to adulthood. Fifty-five men and 28 women were studied at the age of 16 and again at the age of 27. Biopsies were taken from the vastus lateralis muscle and analysed for fibre types (I, IIA, IIB, IIC) and fibre areas. Different development of fibre type composition with increased age were seen in women and men: the type I percentage tended to increase in the women (51 +/- 9 to 55 +/- 12) and decrease significantly in the men (55 +/- 12 to 48 +/- 13). The fibre areas remained unchanged in both sexes. It is suggested that there is a sex-related fibre adaptation to increased age.     

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCβ), was relatively high and greater than has been previously reported in the jaw‐closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCβ‐containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.     

Age and sex affect human muscle fibre adaptations to heavy-resistance strength training

This study assessed age and sex effects on muscle fibre adaptations to heavy-resistance strength training (ST). Twenty-two young men and women (20-30 years old) and 18 older men and women (65-75 years old) completed 9 weeks of heavy-resistance knee extension exercises with the dominant leg 3 days week(-1); the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P<0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P<0.05) along with a decrease in type IIx fibres (P=0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27-39%; all P<0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.     

Fibre type related changes in the metabolic profile and fibre diameter of human vastus medialis muscle after anterior cruciate ligament rupture

Vastus medialis muscles of patients with chronic anterior instability of the knee after anterior cruciate ligament rupture were analysed to investigate changes in defined muscle fibres of the diseased leg in comparison to the healthy leg of the same patient. Metabolic and morphological parameters were obtained by cytophotometrical measurements of the activities of succinate dehydrogenase (a marker of oxidative metabolism) and glycerol-3-phosphate dehydrogenase (a marker of glycolytic metabolism) of slow-oxidative (SO), fast-oxidative glycolytic (FOG) and fast-glycolytic (FG) fibre types in serial sections and by measuring the minimal fibre diameters of type I (slow) and type II (fast) fibres. We found decreased glycolytic activity and a shift to more oxidative metabolism in each fibre type suggesting diminished fast force and shift to endurance force development. The latter was interpreted as a sign of active compensation for the knee instability. Significantly decreased minimal fibre diameters to 85.9% in type I fibres, and to 88.7% in type II fibres of the diseased muscle were measured, indicating the fibre atrophy. Our findings suggest that the atrophied muscle fibres of the affected vastus medialis muscle adapt to the altered conditions by changing their metabolic profile. Muscle fibres of different types were found to be affected similarly.     

Effects of age and life-time physical training on fibre composition of slow and fast skeletal muscle in rats

The effects of age and endurance training on muscle fibre characteristics were studied in a slow (m. soleus, MS) and in a fast (m. rectus femoris, MRF) skeletal muscle. Wistar rats at ages of 1, 2, 4, 10, and 24 months were used as experimental animals. The trained rats were put to run on a motor-driven treadmill 5 d/wk beginning from the age of 1 month. The body weights of the animals increased continuously throughout their lives. The muscle weights increased up to the age of 10 months, after which they tended to decrease. The trained adult rats had lower body weights as well as lower muscle weights than the untrained adult rats. The amount of the intramuscular lipid decreased with age, especially during the first months of life. The activity of isocitrate dehydrogenase (ICDH) decreased during the growth period in both muscles and remained more or less constant thereafter, whereas the activity of phosphofructokinase decreased with age only in MS. In MS, the trained animals tended to have higher ICDH activities than the untrained animals. The cross-sectional area of the different fibre types in both muscles increased up to the age of 10 months. The major fibre types, type I in MS and type IIB in MRF, were smaller for trained than untrained rats. The percentage number of the slower fibre types of both muscles — type I in MS and types I and IIA in MRF —increased with advancing age. The muscles of the trained animals contained higher percentages of the slower fibre types than those of the untrained rats. The present paper indirectly supports the opinion that not only the metabolic but also the contractile properties of different muscle cell types can be changed by age and long-term endurance-type physical training.     

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCβ), was relatively high and greater than has been previously reported in the jaw-closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCβ-containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.     

Histochemical and biochemical changes in human skeletal muscle with age in sedentary males,age 22–65 years

Biopsies for histochemical and biochemical analyses were taken from the vastus lateralis muscle of 55 untrained, healthy male subjects from 22 to 65 years of age. Fibre type distribution changed towards a decrease in the percentage of type II fibres, both in type IIA and type IIB fibres, whereas type IIB/IIA fibre ratio and type IIC percentage did not change with increasing age. It was found that the type IIB/flA fibre ratio was inversely related to type I fibres, i.e. subjects rich in type I fibres had a relatively smaller proportion of type IIB fibres. Fibre area determinations revealed a selective decrease in type II fibre area. Consequently, the type II/I fibre area ratio and relative type II fibre area decreased. No changes in the specific activities of Mg²⁺ stimulated ATPase and myokinase were observed, while the activity of lactate dehydrogenase (LDH) was higher in the youngest groups than in the oldest. LDH isozyme pattern shifted towards a decrease in percentage distribution of the muscle specific isozymes and a corresponding decrease in muscle specific activity while the activity of the heart specific isozymes did not change.     

Tubular aggregate myopathy: a case report

We report a first Korean case of presumably dominantly inherited primary tubular aggregate myopathy in a 19-yr-old man, who presented with slowly progressive proximal muscle stiffness and weakness. In hematoxylin and eosin stain, it showed subsarcolemmal, or central pale basophilic granular vacuoles, which stained red with modified Gomori's trichrome and intensive blue with nicotinamide adenonine dinucleotide-tetrazolium reductase, respectively. Ultrastructurally, aggregates of 60 nm-sized hexagonal tubules were found in both type 1 and type 2 fibers. We briefly review the pathologic findings of the previously reported cases of tubular aggregate myopathy and discuss the possible pathogenesis of this disease. We briefly discuss the possible pathogenesis of sarcoplasmic reticulum and review the ultrastructural characteristics.     

Relationship between myosin heavy chain IId isoform and fibre types in soleus muscle of the rat after hindlimb suspension

Summary The relationship between the myosin heavy chain (HC) IId isoform and histochemically defined fibre types was investigated in the rat soleus muscle after hindlimb suspension. After 4 weeks of suspension, right and left muscles were removed and fibre type composition and total fibre number were examined by histochemical myosin adenosine triphosphatase staining sections. Myosin HC isoforms were analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis. After the suspension, there was a significant decrease in the percentage of type I fibres and a concomitant increase in that of type IIa fibres. However, the total number of fibres was not affected by suspension. The synthesis of HC IId isoform, which was not found in the control, and the decrease in the ratio of slow type myosin heavy chain isoform (HC I) were observed after suspension. These results would may suggest that the change of fibre type composition was caused by a shift from type I to IIa fibres after suspension. Furthermore, it could be suggested that the synthesis of HC IId isoform occurred during the stage of type shift from type I to IIa fibres.