Arrangement of fiber types within fascicles of human vastus lateralis muscle

The pH heterogeneity in human calf muscle during neuromuscular electrical stimulation

PURPOSE

The aim of the study was to examine pH heterogeneity during fatigue induced by neuromuscular electrical stimulation (NMES) using phosphorus magnetic resonance spectroscopy (³¹ P-MRS). It is hypothesized that three pH components would occur in the ³¹ P-MRS during fatigue, representing three fiber types.

METHODS

The medial gastrocnemius of eight subjects was stimulated within a 3-Tesla whole body MRI scanner. The maximal force during stimulation (F_stim ) was examined by a pressure sensor. Phosphocreatine (PCr), adenosintriphosphate, inorganic phosphate (Pi), and the corresponding pH were estimated by a nonvolume-selective ³¹ P-MRS using a small loop coil at rest and during fatigue.

RESULTS

During fatigue, F_stim and PCr decreased to 27% and 33% of their initial levels, respectively. In all cases, the Pi peak increased when NMES was started and split into three different peaks. Based on the single Pi peaks during fatigue, an alkaline (6.76 ± 0.08), a medium (6.40 ± 0.06), and an acidic (6.09 ± 0.05) pH component were observed compared to the pH (7.02 ± 0.02) at rest.

CONCLUSION

It is suggested that NMES is able to induce pH heterogeneity in the medial gastrocnemius, and that the single Pi peaks represent the different muscle fiber types of the skeletal muscle. Magn Reson Med 77:2097-2106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Shape-Changing Photodegradable Hydrogels for Dynamic 3D Cell Culture

Inspired by natural examples of swelling-actuated self-folding, we utilize photodegradable hydrogels as dynamically tunable, shape-changing scaffolds for culturing cells. Poly(ethylene glycol) diacrylate-based thin films incorporating ortho-nitrobenzyl (o-NB) moieties are transformed from flat 2D sheets to folded 3D structures by exposure to 365 nm UV light. As the UV light is attenuated through the thickness of the gel, a cross-link density gradient is formed. This gradient gives rise to differential swelling and a bending moment, resulting in gel folding. By tuning the UV light dose and the molar ratio of photodegradable to nondegradable species, both the initial degree of folding and the relaxation of tubular structures can be accurately controlled. These self-folding photodegradable gels were further functionalized with a cell-adhesive RGD peptide for both seeding and encapsulation of C2C12 mouse myoblasts. Light-induced folding of RGD functionalized hydrogels from flat sheets to tubular structures was demonstrated 1 or 3 days after C2C12 seeding. The C2C12s remained adhered on the inner walls of folded tubes for up to 6 days after folding. The minimum measured diameter of a tubular structure containing C2C12s was 1 mm, which is similar to the size of muscle fascicles. Furthermore, the viability of encapsulated C2C12s was not adversely affected by the UV light-induced folding. This is the first account of a self-folding material system that allows 2D-3D shape change in the presence of both seeded and encapsulated cells at a user-directed time point of choice.

Software for muscle fibre type classification and analysis

Fibre type determination requires a large series of differently stained muscle sections. The manual identification of individual fibres through the series is tedious and time consuming. This paper presents a software that enables (i) adjusting the position of individual fibres through a series of differently stained sections (image registration) and identification of individual fibres through the series as well as (ii) muscle fibre classification and (iii) quantitative analysis. The data output of the system is the following: numerical and areal proportions of fibre types, fibre type size and optical density (grey level) of the final reaction product in every fibre. The muscle fibre type can be determined stepwise, based on one set of stained sections while further, newly stained sections can be added to the already defined muscle fibre profile. Several advantages of the presented software application in skeletal muscle research are presented. The system is semiquantitative, flexible, and user friendly.

Effects of intermittent hypoxic training on amino and fatty acid oxidative combustion in human permeabilized muscle fibers

The effects of concurrent hypoxic/endurance training on mitochondrial respiration in permeabilized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1-W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired Po(2) (Pi(O(2)))] or hypoxic conditions ( approximately 100 Torr Pi(O(2)), approximately 3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) increased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O(2) uptake (Vo(2 max)) remained unchanged: 58.1 +/- 0.8 vs. 61.0 +/- 1.2 ml.kg(-1).min(-1) and 58.5 +/- 0.7 vs. 58.3 +/- 0.6 ml.kg(-1).min(-1) for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 +/- 0.37 vs. 8.51 +/- 0.33 mumol O(2).min(-1).g dry weight(-1)) and significantly decreased for palmitate + malate (3.88 +/- 0.23 vs. 2.77 +/- 0.08 mumol O(2).min(-1).g dry weight(-1)) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in Vo(2 max), and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.

Superficial motor units are larger than deeper motor units in human vastus lateralis muscle

Previous studies have suggested that regionalization may occur for human motor units, whereby smaller motor units are located in deeper parts of the muscle and larger motor units are located in more superficial portions. We examined this possibility in the human vastus lateralis muscle using macro-EMG (electromyography) to estimate motor unit size. The sample consisted of nine individuals from whom 114 motor units were recorded at forces ranging between 5% and 60% MVC. Peak-to-peak macro-EMG amplitude was well correlated with macro area (Spearman rho = 0.96). There was a statistically significant inverse relationship between recording depth and macro peak-to-peak amplitude (rho = -0.402, p < 0.001). We conclude that there is a nonrandom distribution of motor units in human muscle, with larger motor units located in more superficial regions and smaller units located in deeper regions. Clinicians who monitor motor unit activity need to recognize that a representative sample of motor unit recordings should include motor units from both deeper and more superficial regions of 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.

Variability in human quadriceps muscles: quantitative study and review of clinical literature

Knowledge of variations in normal human thigh musculature is important for the interpretation of various clinical and biomedical investigations. In this study, cross-sections from whole thighs of 34 cadavers were analyzed qualitatively and morphometrically. Sections were cut from right and left limbs at three levels in the region often used for muscle biopsy. Measurements were made of limb circumferences and cross-sectional areas of the three vasti and of the femur, and of the extent of overlap and fusion between the vasti on the lateral aspect of the thigh. Limb circumference proved to be a good predictor of total cross-sectional area of the quadriceps muscle. Large individual variations were found for most measurements but especially for the amount of fusion between the muscle bellies of vastus lateralis and vastus intermedius. Significant differences were found for most variables between the three levels, but there were few differences between sides except for the cross-sectional area of vastus lateralis which was greater on the right at the most distal level. These variations in size and form of thigh musculature should be considered when interpreting data obtained from muscle biopsy, imaging, biomechanical, electromyographic, and functional studies of the human thigh. Review of the literature on variability of quadriceps muscle and its clinical relevance showed a diversity of approaches but insufficient definitive data suitable for interpreting functional outcomes in response to exercise or following surgery. Further work is suggested combining current anatomical datasets with advanced imaging techniques capable of determining muscle fiber orientation and fiber type composition, and including 3D reconstruction.

A model of EMG generation

Simulation models are unavoidable in experimental research when the point is to develop new processing algorithms to be applied on real signals in order to extract specific parameter values. Such algorithms have generally to be optimized by comparing true parameter values to those deduced from the algorithm. Only a simulation model can allow the user to access and control the actual process parameter values. This constraint is especially true when dealing with biomedical signals like surface electromyogram (SEMG). This work is an attempt to produce an efficient SEMG simulation model as a help for assessing algorithms related to SEMG features description. It takes into account the most important parameters which could influence these characteristics. This model includes all transformations from intracellular potential to surface recordings as well as a fast implementation of the extracellular potential computation. In addition, this model allows multiple graphically-programmable electrode-set configurations and SEMG simulation in both voluntary and elicited contractions.

A fractal characterization of the type II fiber distribution in the extensor digitorum longus and soleus muscles of the adult rat

A method is proposed for the quantitative characterization of fiber type spatial distribution by means of the (correlation) fractal dimension. The method is applied to type II fiber distributions of the soleus and extensor digitorum longus muscles of the adult rat. The results obtained suggest that these distributions have fractal properties with a strong tendency for spreading, more pronounced in soleus muscle. The density of muscle fibers or the age do not seem to alter the features of the distribution. Computer-generated random patterns have virtually the same fractal dimension as the extensor digitorum longus distributions but fail to approximate those of the soleus muscle. This fractal method could find application as an alternative in the quantitative assessment of the fiber type grouping.

A quantitative histochemical study of the spatial distribution of intrafascicular fibre types in the porcine masseter and soleus muscles

The distribution of intrafascicular type I and II muscle fibres together with proportions of edge- and centrally located type I and II fibres within whole fascicles were analysed by myosin ATP-ase histochemistry in 241 porcine masseter fascicles (six masseter muscles) and compared with result from 63 pig soleus fascicles (five soleus muscles). All fascicles were from 11 domestic pigs (1 yr old, 70-90 kg body weight, all female). The proportions of type I fibres (slow) and type II fibres (fast) on the edge of fascicles differed significantly from the proportions centrally. All the soleus fascicles had higher proportions of centrally located type I fibres. Only seven out of 241 (3%) masseter fascicles diverged in this respect and showed reversed intrafascicular fibre-type proportions with more edge-located type I fibres. Analysis of the fascicular distribution of type I and II fibres revealed that the porcine masseter had type II fibres as the predominant type. Between 68-87% of the total fibres were type II (p < 0.001). The intrafascicular content of type I fibres increased towards the deep part of the masseter. In four of five soleus muscles the type II fibre population was dominant (p < 0.01). However, one soleus revealed equal proportions of type I and II fibres.

Fibre size, atrophy, and hypertrophy factors in vastus lateralis muscle from 18- to 29-year-old men

In order to study the size of muscle fibres, cross-sections of autopsied vastus lateralis muscle from 8 healthy men, aged 18 to 29 years, who have died suddenly were prepared and analyzed. Data were obtained on cross-sectional area, on lesser diameter, and on atrophy and hypertrophy factors of type 1 and type 2 fibres, subdivided into 2a, 2b, and 2c fibres. The difference in mean fibre size between type 1 and 2 fibres was not significant, whereas the differences between type 1 and 2b, type 1 and 2a, and type 2a and 2b fibres were significant. In the whole material type 2a fibres were the largest and type 2b fibres the smallest. There were considerable differences between post-mortem subjects. Because of these differences and the variability of all fibre types in respect of size in a sample the normal ranges of fibre size were large. The normal ranges and a continuous scale of weights were used to determine the atrophy and hypertrophy factors in each sample, and the upper limits of these factors accepted as being normal. The estimates of the limits of normality of the area, diameter, atrophy and hypertrophy factors of type 1, 2, 2a and 2b fibres reflect the situation in the vastus lateralis muscle of healthy young men. These values might be useful in studying physiological and pathological conditions influencing the size of different fibre types.

Localisation of transforming growth factor beta 1 in developing muscles: implications for connective tissue and fiber type pattern formation

Skeletal muscles are highly ordered mixtures of cell types, with each muscle having its own characteristic pattern of fiber types, connective tissues, and vasculature. The precursors of the myogenic and connective elements of a muscle are initially intermixed and are proliferating and differentiating together in a manner that generates an ordered array of mature cells. The molecular basis of myogenesis is unknown, although in vitro studies have revealed numerous putative regulators. The results obtained from in vitro studies are not easily related to in vivo myogenesis because of a lack of information about the localisation of the putative regulators in developing muscles. The objective of this paper was therefore to describe the spatial and temporal distribution of transforming growth factor beta 1 (TGF-beta 1), a small peptide that affects cultured fibroblasts, myoblasts, and vascular endothelial cells. TGF-beta 1-immunoreactivity was associated with the epimysia, perimysia, and vasculature of the developing muscles. The expression of TGF-beta 1 within developing muscles had a distinct spatial and temporal pattern that correlated with the fate of adjacent myotubes. Myotubes which formed prior to the expression of TGF-beta 1 developed into slow fibers whereas those which formed adjacent to TGF-beta 1-containing connective tissue matured into fast fibers. The possibility that TGF-beta 1 is involved in the generation of the pattern of epi- and perimysia and/or fiber types is discussed.

Myosin heavy chain composition of single fibres and their origins and distribution in developing fascicles of sheep tibialis cranialis muscles

The myosin heavy chain (MHC) composition of single muscle fibres in developing sheep tibialis cranialis muscles was examined immunohistochemically with monoclonal antibodies to MHC isozymes. Data were collected with conventional microscopy and computerized image analysis from embryonic day (E) 76 to postnatal day (PN) 20, and from adult animals. At E76, 23% of the young myofibres stained for slow-twitch MHC. The number of these fibres considerably exceeded the number of primary and secondary myotubes. By E100, smaller fibres, negative for slow-twitch MHC, encircled each fibre from the initial population to form rosettes. A second population of small fibres appeared in the unoccupied spaces between rosettes. Small fibres, whether belonging to rosettes or not, did not initially express slow-twitch MHC, expressing mainly neonatal myosin instead. These small fibres then diverged into three separate groups. In the first group most fibres transiently expressed adult fast myosin (maximal at E110-E120), but in the adult expressed slow myosin. This transformation to the slow MHC phenotype commenced at E110, was nearing completion by 20 postnatal days, and was responsible for approximately 60% of the adult slow twitch fibre population. In the other two groups expression of adult fast MHC was maintained, and in the adult they accounted for 14% (IIa MHC) and 17% (IIb MHC) of the total fibre numbers. We conclude that muscle fibre formation in this large muscle involves at least three generations of myotube. Secondary myotubes are generated on a framework of primary myotubes and both populations differentiate into the young myofibres which we observed at E76 to form rosettes. Tertiary myotubes, in turn, appear in the spaces between rosettes and along the borders of fascicles, using the outer fibres of rosettes as scaffolds.

Modeling fiber type grouping by a binary Markov random field

A new approach to the quantification of fiber type grouping is presented, in which the distribution of histochemical type in a muscle cross section is regarded as a realization of a binary Markov random field (BMRF). Methods for the estimation of the parameters of this model are discussed. The first order BMRF, which is used in this article, contains 2 parameters: alpha and beta. The parameter beta is of prime importance, as it is an interaction parameter which governs the degree of type grouping. The value of this parameter is estimated for 9 muscle biopsies. The interpretation of the results is discussed.