Muscle fibre type distribution,muscle cross-sectional area and maximal voluntary strength in humans

The relationship between maximum voluntary concentric strength, muscle fibre type distribution and muscle cross-sectional areas were examined in 23 subjects (7 female and 11 male phys. ed. students as well as 5 male bodybuilders). Maximal knee and elbow extension as well as elbow flexion torque at the angular velocities 30, 90 and 180 degrees per second was measured. Muscle biopsies were taken from vastus lateralis and m. triceps brachii. The muscle cross-sectional area of the thigh and upper arm was measured with computed tomography scanning. The maximal torque correlated strongly to the muscle cross-sectional area times an approximative measure on the lever arm (body height). Maximal tension developed per unit of muscle cross-sectional area did not correlate significantly with per cent type I fibre area and did not differ between the female and male students or bodybuilders. Neither did the relative decrease in torque with increasing contraction velocity show any significant relationship to the per cent type I fibre area. The total number of muscle fibres was estimated by dividing the muscle cross-sectional area with the mean fibre area of m. triceps brachii. The number of fibres did not seem to differ between the sexes.     

Repeatability of fibre type and enzyme activity measurements in human skeletal muscle

In order to assess the variability in repeated determination of human muscle fibre type distribution, fibre area and enzyme activity measurements, two biopsies were taken within 10 days in the same vastus lateralis for 12 females and 13 males, and in the right and in the left muscles for 25 other subjects (13 females and 12 males). Within muscle, intraclass reliability coefficients were 0.88, 0.82 and 0.56 for type I, IIa and IIb per cent fibres, respectively, and ranged from 0.74 to 0.82 for fibre areas and from 0.71 to 0.90 for enzyme markers of different metabolic pathways. Correlations between right and left muscle measurements were also high for fibre areas (from 0.85 to 0.91) and enzyme activities (from 0.71 to 0.87), except for phosphofructokinase (r = 0.63). In contrast, the right and left thigh muscle correlation reached 0.67, 0.40 and 0.64 for type I, IIa and IIb fibre distribution, respectively. Thus, the variation in muscle sampling and technical procedures reached about 15% of the total variation (i.e. total differences between subjects) for the proportion of fibre type I and IIa and about 20-25% for fibre areas and enzyme activities. On the other hand, the technical error for the proportion of fibre type I and IIa is about 6-7%. This implies that differences brought about by any experimental treatment on these skeletal muscle characteristics in human studies have to be of a relatively large magnitude before being detectable. On the other hand, fibre areas and enzyme activities measured in single needle biopsy sample, from one of the vastus lateralis muscles, are quite representative of the other vastus lateralis. Similarity in fibre type proportion between right and left vastus lateralis cannot be postulated, however, without investigating both muscles.     

Long-term resistance training improves force and unloaded shortening velocity of single muscle fibres of elderly women

Four elderly women (78 ± 4.3 years) were resistance trained (RT) for one year and needle biopsies of the vastus lateralis muscle were taken pre- and post-RT. A training intensity of 60% one-repetition maximum (1RM) was adopted for the entire duration of the study. Following RT, significant gain in isometric force of the quadriceps muscles was observed and MHC isoform distribution of vastus lateralis muscle showed a very significant shift from MHC-1 and MHC-2X towards MHC-2A, i.e. a bi-directional shift. A large population (n = 310) of individual skinned muscle fibres were dissected from biopsy samples and used for determination of cross-sectional area (CSA), specific force (Po/CSA) and unloaded shortening velocity (Vo) at 12°C and maximum activation. Fibres were classified on the basis of their MHC isoform content determined by SDS-PAGE in three pure fibre types (1, 2A and 2X) and two hybrid fibre types (1-2A and 2AX). Whereas the CSA of individual muscle fibres did not change, Po/CSA was significantly higher in both type 1 and type 2A fibres post training. Vo of type 1 fibres was significantly higher post-training, whereas no change in Vo was observed in type 2A fibres. The number of pure type 2X fibres was very low especially post-training and did not enable a statistically significant comparison. The data suggest that in elderly women moderate and prolonged RT: (1) can determine similar adaptations in MHC distribution of skeletal muscle to those expected in young subjects; (2) can improve function of muscle fibres.     

Glycogen depletion of human skeletal muscle fibers in response to high-frequency electrical stimulation.

The purpose of the present study was to evaluate the pattern of change in muscular glycogen content in response to high-frequency electrical stimulation (HFES). Muscle biopsies were taken from the vastus lateralis muscle of 7 healthy young men before, 15 min after, and 30 min after electrical stimulation delivered at a 50-Hz frequency (15 s on, 45 s off) at an intensity of 100 mA. The glycogen content of type I, IIA, and IIB muscle fibres was evaluated using microphotometry of periodic acid Schiff (PAS) stained fibres. After 15 min of electrical stimulation, the glycogen content in type I, IIA, and IIB muscle fibres significantly decreased from 113 +/- 10 (mean +/- SE) to 103 +/- 10 (p < or = 0.05), 129 +/- 9 to 102 +/- 12 (p < or = 0.01), and 118 +/- 8 to 90 +/- 13 (p < or = 0.01) arbitrary relative units, respectively. No further decrement in glycogen content was observed in all three fibre types following an additional 15 min of HFES. In addition, isometric force decreased by approximately 50%, from 125.9 +/- 20.0 N to 64.2 +/- 7.7 N (p < or = 0.01), during the first 15 contractions. No further decrease in isometric force was observed following an additional 15 contractions of HFES. These results reveal that significant reductions in isometric force of knee extensor muscles and glycogen content of all human skeletal muscle fibre types in vastus lateralis muscle are observable after 15 min of neuromuscular high-frequency transcutaneous electrical stimulation.     

Work-induced hypertrophy in exercised normal muscles of different ages and the reversibility of hypertrophy after cessation of exercise

1. Groups of male hamsters of different ages were subjected to a weight-lifting exercise regimen, and the biceps brachii, soleus and extensor digitorum longus (EDL) muscles examined for structural changes occurring in response to the increased workload. In addition, two groups of adult hamsters were left to recover from the exercise stimulus for 5 and 15 weeks respectively.2. All the exercised muscles exhibited muscle fibre hypertrophy, and the extent of the hypertrophy was greater in the younger animals. In all age groups, the biceps brachii and EDL showed more hypertrophy than did the soleus. There was no significant increase in fibre number after exercise.3. In both groups allowed to recover from the exercise stimulus, the fibre dimensions reverted back to those of the control muscles; this appeared to be complete after 15 weeks recovery.4. Electron microscopical studies of fibres from exercised and control biceps brachii revealed no significant changes with exercise in the proportions present of myofibrillar, mitochondrial and tubular components within muscle fibres of the same size.     

Changes in signalling molecule levels in 10-day hindlimb immobilized rat muscles

AIM: Hindlimb immobilization produces similar percentage decreases in muscle mass in the predominantly type I soleus and type II vastus lateralis muscles. Consequently we hypothesized that the percentage changes in potential regulatory molecules for atrophy would be similar in the two muscle fibre types. METHODS: Therefore, the purpose of the current study was to measure phosphorylated p38 MAPK and JNK, as well as the protein levels of p53, growth arrest and DNA damage-inducible 45 (GADD45), and full-length poly(ADP-ribose) polymerase (PARP) to determine whether their changes in expression in the soleus and vastus lateralis muscles were similar at 10th day of hindlimb immobilization in young rats. RESULTS: Unexpectedly, preferential increases in phosphorylation of p38 MAPK and JNK and in the protein levels of p53, GADD45, as well as decreases in full-length PARP occurred in the soleus muscle, while only p38 phosphorylation increased in the white portion of the vastus lateralis muscle at 10th day of hindlimb immobilization. CONCLUSION: Taken together, these results are interpreted to suggest that some of regulatory processes or kinetics in the atrophy of type I and II muscle fibres during limb immobilization may differ at the 10th day of limb immobilization.     

Variability in muscle fibre areas in whole human quadriceps muscle. How much and why?

To determine the variability in fibre areas in the human vastus lateralis muscle, cross-sections (15 microns) of whole autopsied muscles from eight young men have been prepared, and the cross-sectional area (CSA) of 375 type 1 and 375 type 2 fibres has been measured in five different regions throughout each muscle. The CSA of both fibre types varied significantly within all muscle cross-sections. Fibres in the deep parts of the muscle were larger than superficially. There was a significant correlation between the CSA of the two fibre types within each region: if a fibre of a given type was small, or large, the other fibre type was also small, or large. The CSA of type 2 fibres was larger than the CSA of type 1 fibres in 26 of the 40 regions: regions with type 1 fibres larger than type 2 fibres were mostly (71%) found deep in the muscle. The standard deviation of the CSA of type 1 fibres was significantly larger than for type 2 fibres in 35 of the 40 regions. In conclusion, the CSA of the different fibre types in the vastus lateralis of young men varies non-randomly. The pattern of variation, both throughout the muscle and in small sample regions, supports the general opinion that the functional demands placed on the fibre population are an important factor in the development of the fibre properties.     

Spatiotemporal surface EMG characteristics from rat triceps brachii muscle during treadmill locomotion indicate selective recruitment of functionally distinct muscle regions

Multichannel surface EMG recordings of a multiheaded skeletal muscle during cyclic locomotion combined with cineradiography were analysed in a chronic experiment. The resulting detailed two-dimensional activation pattern from the long and lateral triceps brachii heads of the rat during treadmill locomotion were combined with gait characteristics and fibre typing of the muscle. Shortly before ground contact of the forelimb, maximum muscle activity was found in the proximal part of the long head of the muscle. During the stance phase maximum activity was observed in the proximal part of the lateral head. The frequency dependent behaviour of cross-covariance functions over both muscle heads confirmed this selective shift in activation. In the lateral triceps brachii head of the investigated rats, exclusively type II fibres were found. In the long head the frequency of type I fibres was the highest in the deep muscle layers, proximally more than distally, whereas type II fibres were dominant in more superficial muscle layers. A combination of physiological and histological findings supports an anticipating mechanism whereby fine-tuning of the vertical foot down manoeuvre is mainly achieved by the (type I fibre dominated) proximal deep compartment of the biarticular long triceps brachii head and force generation is predominantly executed by the monoarticular lateral triceps brachii head.     

Effects of strength training with eccentric overload on muscle adaptation in male athletes

In classic concentric/eccentric exercise, the same absolute load is applied in concentric and eccentric actions, which infers a smaller relative eccentric load. We compared the effects of 6 weeks of classic concentric/eccentric quadriceps strength training (CON/ECC, 11 subjects) to eccentric overload training (CON/ECC+, 14 subjects) in athletes accustomed to regular strength training. The parameters determined included functional tests, quadriceps and fibre cross-sectional area (CSA), fibre type distribution by ATPase staining, localisation of myosin heavy chain (MHC) isoform mRNAs by situ hybridization and the steady-state levels of 48 marker mRNAs (RT-PCR) in vastus lateralis biopsies taken before and after training. Both training forms had anabolic effects with significant increases in quadriceps CSA, maximal strength, ribosomal RNA content and the levels of mRNAs involved in growth and regeneration. Only the CON/ECC+ training led to significantly increased height in a squat jump test. This was accompanied by significant increases in IIX fibre CSA, in the percentage of type IIA fibres expressing MHC IIx mRNA, in the level of mRNAs preferentially expressed in fast, glycolytic fibres, and in post-exercise capillary lactate. The enhanced eccentric load apparently led to a subtly faster gene expression pattern and induced a shift towards a faster muscle phenotype plus associated adaptations that make a muscle better suited for fast, explosive movements.     

Changes in signalling molecule levels in 10‐day hindlimb immobilized rat muscles

Aim: Hindlimb immobilization produces similar percentage decreases in muscle mass in the predominantly type I soleus and type II vastus lateralis muscles. Consequently we hypothesized that the percentage changes in potential regulatory molecules for atrophy would be similar in the two muscle fibre types. Methods: Therefore, the purpose of the current study was to measure phosphorylated p38 MAPK and JNK, as well as the protein levels of p53, growth arrest and DNA damage‐inducible 45 (GADD45), and full‐length poly(ADP‐ribose) polymerase (PARP) to determine whether their changes in expression in the soleus and vastus lateralis muscles were similar at 10th day of hindlimb immobilization in young rats. Results: Unexpectedly, preferential increases in phosphorylation of p38 MAPK and JNK and in the protein levels of p53, GADD45, as well as decreases in full‐length PARP occurred in the soleus muscle, while only p38 phosphorylation increased in the white portion of the vastus lateralis muscle at 10th day of hindlimb immobilization. Conclusion: Taken together, these results are interpreted to suggest that some of regulatory processes or kinetics in the atrophy of type I and II muscle fibres during limb immobilization may differ at the 10th day of limb immobilization.     

Muscle fibre number is a possible determinant of muscle fibre composition in rats

The purpose of the present study was to investigate whether the muscle fibre composition is related to the number of muscle fibres. To resolve this issue, we developed fast-twitch fibre dominant rats (FFDR) by selective breeding and compared the findings to those of control rats (CR) obtained by random breeding. Percentage of type I fibres of the deep portion of gastrocnemius (DG), soleus (SOL), vastus intermedius (VI), adductor longus (AL), and biceps brachii (BB) muscles in FFDR were lower than CR. Percentage of type IIB fibres in DG, VI and AL and percentage of type IIA fibres of SOL in FFDR were higher than CR. However, fibre composition of plantaris (PLAN), extensor digitorum longus (EDL), rectus abdominis (RA), diaphragm (DIA), and palmaris longus (PL) muscles in FFDR were identical with CR. Total fibre numbers on the cross-sectional area in SOL, PLAN, EDL, AL and PL were counted. Numbers of type I fibres of all those muscles in FFDR were not different from CR. Numbers of type IIA fibres of SOL and AL and of type IIB fibres of AL in FFDR were greater than CR, but there were no significant differences in the number of type IIA or type IIB fibres of PLAN, EDL or PL between the two groups. Based on these observations, it is suggested that there are pleiotropic and muscle-specific effects on muscle fibre composition. In addition, the number of type II fibres is a possible determinant of muscle fibre composition.     

Protein synthesis rates in human muscles: neither anatomical location nor fibre-type composition are major determinants

In many animals the rate of protein synthesis is higher in slow-twitch, oxidative than fast-twitch, glycolytic muscles. To discover if muscles in the human body also show such differences, we measured [¹³C]leucine incorporation into proteins of anatomically distinct muscles of markedly different fibre-type composition (vastus lateralis, triceps, soleus) after an overnight fast and during infusion of a mixed amino acid solution (75 mg amino acids kg⁻¹ h⁻¹) in nine healthy, young men. Type-1 fibres contributed 83 ± 4% (mean ± s.e.m. ) of total fibres in soleus, 59 ± 3% in vastus lateralis and 22 ± 2% in triceps. The basal myofibrillar and sarcoplasmic protein fractional synthetic rates (FSR, % h⁻¹) were 0.034 ± 0.001 and 0.064 ± 0.001 (soleus), 0.031 ± 0.001 and 0.060 ± 0.001 (vastus), and 0.027 ± 0.001 and 0.055 ± 0.001 (triceps). During amino acid infusion, myofibrillar protein FSR increased to 3-fold, and sarcoplasmic to 2-fold basal values ( P < 0.001). The differences between muscles, although significant statistically (triceps versus soleus and vastus lateralis, P < 0.05), were within ∼15%, biologically probably insignificant. The rates of collagen synthesis were not affected by amino acid infusion and varied by < 5% between muscles and experimental conditions.     

Recruitment of fibre types and quadriceps muscle portions during repeated, intense knee-extensor exercise in humans

To investigate recruitment of slow-twitch (ST) and fast-twitch (FT) muscle fibres, as well as the involvement of the various quadriceps femoris muscle portions during repeated, intense, one-legged knee-extensor exercise, 12 healthy male subjects performed two 3-min exercise bouts at ~110% maximum thigh O₂ consumption (EX1 and EX2) separated by 6 min rest. Single-fibre metabolites were determined in successive muscle biopsies obtained from the vastus lateralis muscle (n=6) and intra-muscular temperatures were continuously measured at six quadriceps muscle sites (n=6). Creatine phosphate (CP) had decreased (P<0.05) by 27, 73 and 88% in ST fibres and 25, 71 and 89% in FT fibres after 15 and 180 s of EX1 and after 180 s of EX2, respectively. CP was below resting mean–1 SD in 15, 46, 84 and 100% of the ST fibres and 9, 48, 85 and 100% of the FT fibres at rest, after 15 and 180 s of EX1 and after 180 s of EX2, respectively. A significant muscle temperature increase (T_m) occurred within 2–4 s at all quadriceps muscle sites. T_m varied less than 10% between sites during EX1, but was 23% higher (P<0.05) in the vastus lateralis than in the rectus femoris muscle during EX2. T_m in the vastus lateralis was 101 and 109% of the mean quadriceps value during EX1 and EX2, respectively. We conclude that both fibre types and all quadriceps muscle portions are recruited at the onset of intense knee-extensor exercise, that essentially all quadriceps muscle fibres are activated during repeated intense exercise and that metabolic measurements in the vastus lateralis muscle provide a good indication of the whole-quadriceps muscle metabolism during repeated, intense, one-legged knee-extensor exercise.     

Voluntary resistance wheel exercise during post-natal growth in rats enhances skeletal muscle satellite cell and myonuclear content at adulthood

Aim: To determine whether voluntary free wheel (FW) or resistance wheel (RW) exercise or reduced muscle activity would influence maturational increases in muscle mass and the number of satellite cells (SCs) and myonuclei (MN) accrued by adulthood. Methods: Hind limb muscles of male rats housed with, or without, FWs from 4 to 5, 7 or 10 weeks of age, and rats housed with RWs from 4 to 10 week of age, were evaluated. To assess the effect of reduced muscle activity, gastrocnemius muscles of 4‐week‐old rats were injected with botulinum toxin (Btx) and collected at 7 weeks of age. Muscle fibre size and the frequency of Pax7‐positive SCs and MN were determined in 7‐ and 10‐week‐old muscles via immunohistochemical methods. Results: Free wheel exercise enhanced muscle growth and the frequency of SCs in the medial gastrocnemius (MG) (threefold) and vastus lateralis (VL) (twofold) of rats at 10 week of age. Resistance wheel exercise increased the number of SCs and MN (22–30%), with more muscle fibre nuclei being associated with larger fibre size, in the soleus, MG and VL muscles. Btx impaired the normal increases in muscle fibre size and the accrual of MN but not SCs. Conclusion: A greater volume of exercise during maturational growth was important for enhancing SC numbers, whereas their conversion to MN required higher‐intensity exercise. The enhanced muscle fibre nuclear populations may influence the capacity of the muscle to adapt to exercise, injury or disuse in later adulthood.     

Changes of enzyme activities in the myocardium and skeletal muscle fibres of cardiomyopathic hamsters. A cytophotometrical study.

Cytophotometrical measurements of enzyme activities were performed in the myocardium and skeletal muscle fibres from normal and cardiomyopathic hamsters (BIO 8262) during ageing from 12-14 to 120-190 days. Myocardium as well as vastus lateralis muscles of cardiomyopathic hamsters showed changes in enzyme activities. The skeletal muscle fibres were typed into slow-oxidative, fast-oxidative glycolytic and fast-glycolytic to investigate fibre type-related changes in muscles of cardiomyopathic hamsters. The following myopathic changes were mainly found: Myofibrillic ATPase was depressed in the myocardium of both ventricles in all investigated age stages. The ATPase activity of the right ventricle was more decreased than that of the left one. Additionally, a metabolic shift was observed in myocardium and slow-oxidative muscle fibres at the onset of clinical symptoms, which appeared from day 150 to day 190. During the period from 42 up to 190 days of life an increase of oxidative (succinate dehydrogenase) activity was measured in the myocardium of both ventricles and in slow oxidative fibres of vastus lateralis muscle as a proximal muscle. At earlier ages, the fast fibres of myopathic vastus lateralis muscle showed higher glycolytic (glycerol-3-phosphate dehydrogenase) activity than those of normal muscles. However, at the age of 120-190 days the metabolic profile of fast fibres was normalized. In gastrocnemius muscle as a distal muscle no changes of enzyme activities were measured, suggesting the investigated hereditary myopathy effected proximal, but not distal muscles.     

Fatiguability and fibre composition of human skeletal muscle.

The fatiguability of the quadriceps muscle was investigated in 10 male subjects (25-40 yrs), with inter-individual differences in fibre composition of their vastus lateralis muscles (range 25-65% fast twitch, FT, muscle fibres). Fatiguability was assessed as the decline in maximal force (in % of initial values) with 50 repeated isokinetic knee-extensions at fast angular velocity (3.14 rad/s). Each contraction lasted 0.5 s and the rest periods were about 0.7 s. Every subject was tested on two occasions and the standard deviation for a single determination of fatiguability was 1.4%. The decline in force after 50 contractions was on the average about 45%. The individual values varied, however, and a linear correlation was present between fatiguability and % FT fibres (r = 0.86, p less than 0.01). It was concluded that development of fatigue in human skeletal muscle performing repeated fast dynamic contractions with maximal effort was most marked in muscles with a high proportion FT fibres. This finding was in conformity with earlier results from animal skeletal muscle preparations.     

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.     

Molecular biomarkers monitoring human skeletal muscle fibres and microvasculature following long-term bed rest with and without countermeasures

The cellular mechanisms of human skeletal muscle adaptation to disuse are largely unknown. The aim of this study was to determine the morphological and biochemical changes of the lower limb soleus and vastus lateralis muscles following 60 days of head-down tilt bed rest in women with and without exercise countermeasure using molecular biomarkers monitoring functional cell compartments. Muscle biopsies were taken before (pre) and after bed rest (post) from a bed rest-only and a bed rest exercise group (n = 8, each). NOS1 and NOS3/PECAM, markers of myofibre 'activity' and capillary density, and MuRF1 (E3 ubiquitin-ligase), a marker of proteolysis, were documented by confocal immunofluorescence and immunoblot analyses. Morphometrical parameters (myofibre cross-sectional area, type I/II distribution) were largely preserved in muscles from the exercise group with a robust trend for type II hypertrophy in vastus lateralis. In the bed rest-only group, the relative NOS1 immunostaining intensity was decreased at type I and II myofibre membranes, while the bed rest plus exercise group compensated for this loss particularly in soleus. In the microvascular network, NOS3 expression and the capillary-to-fibre ratio were both increased in the exercise group. Elevated MuRF1 immunosignals found in subgroups of atrophic myofibres probably reflected accelerated proteolysis. Immunoblots revealed overexpression of the MuRF1 protein in the soleus of the bed rest-only group (> 35% vs. pre). We conclude that exercise countermeasure during bed rest affected both NOS/NO signalling and proteolysis in female skeletal muscle. Maintenance of NO signalling mechanisms and normal protein turnover by exercise countermeasure may be crucial steps to attenuate human skeletal muscle atrophy and to maintain cell function following chronic disuse.