Fiber Type Profile Research Articles

Effects of Anabolic Steroids and High-Intensity Aerobic Exercise on Skeletal Muscle of Transgenic Mice

In an attempt to shorten recovery time and improve performance, strength and endurance athletes occasionally turn to the illicit use of anabolic-androgenic steroids (AAS). This study evaluated the effects of AAS treatment on the muscle mass and phenotypic characteristics of transgenic mice subjected to a high-intensity, aerobic training program (5d/wk for 6 weeks). The transgenic mice (CETP+/-LDLr-/+) were engineered to exhibit a lipid profile closer to humans. Animals were divided into groups of sedentary (Sed) and/or training (Ex) mice (each treated orally with AAS or gum arabic/vehicle: Sed-C, Sed-M, ex-C, ex-M). The effects of AAS (mesterolone: M) on specific phenotypic adaptations (muscle wet weight, cross-sectional area, and fiber type composition) in three hindlimb muscles (soleus:SOL, tibialis anterior:TA and gastrocnemius:GAS) were assessed. In order to detect subtle changes in fiber type profile, the entire range of fiber types (I, IC, IIAC, IIA, IIAD, IID, IIDB, IIB) was delineated using mATPase histochemistry. Body weight gain occurred throughout the study for all groups. However, the body weight gain was significantly minimized with exercise. This effect was blunted with mesterolone treatment. Both AAS treatment (Sed-M) and high-intensity, aerobic training (ex-C) increased the wet weights of all three muscles and induced differential hypertrophy of pure and hybrid fibers. Combination of AAS and training (ex-M) resulted in enhanced hypertrophy. In the SOL, mesterolone treatment (Sed-M and ex-M) caused dramatic increases in the percentages of fiber types IC, IIAC, IIAD, IID, with concomitant decrease in IIA, but had minimal impact on fiber type percentages in the predominantly fast muscles. Overall, the AAS-induced differential adaptive changes amounted to significant fiber type transformations in the fast-to-slow direction in SOL. AAS treatment had a significant effect on muscle weights and fiber type composition in SOL, TA and GAS which was even maximized in animals subjected to metabolically high-intensity aerobic exercise.

Sternohyoid Flap for Facial Reanimation

Neuromuscular reanimation of the face provides the correct specific neural functional input and thereby prevents synkinesis. Unfortunately, this ideal situation is rarely encountered in the clinical setting. To assess the technical feasibility of and define the surgical procedure for harvesting the sternohyoid muscle as a novel free flap for use in facial reanimation indications. Fresh, postmortem, nonfixed cadavers were used to define the anatomy and perform the flap harvest procedures. Twenty-four flap harvests were performed. Angiography was performed on the pedicle of the harvested flaps to assess potential flap perfusion. Adenosine triphosphatase staining was performed on the muscle specimens to establish fiber type. The harvest technique, pedicle (arterial or venous), nerve length, and flap geometry parameters were characterized. The sternohyoid muscle was found to be reliably vascularized by the superior thyroid artery in all cases with an appropriate arterial and venous pedicle for vascular anastomosis. The mean arterial (5.5 cm) and venous (5.9 cm) pedicle lengths are comparable with gracilis flaps. The mean motor nerve length was 10.7 cm. The inclusion of the hyoid bone allows rigid fixation, and the muscle size, fiber type, and volume profiles all compare favorably to the gracilis flap for use in the indication of facial reanimation. Mock surgical procedures were performed to define inset parameters. This flap potentially allows single-stage cross-facial neurorrhaphies to be performed. This is the first article, to our knowledge, of the sternohyoid muscle as a potential donor site for free-tissue transfer. This muscle has a predictable vascular pedicle and neural innervation along with size and fiber type parameters that make it an ideal potential free flap for facial reanimation. NA.

Locomotor muscle profile of a deep (Kogia breviceps) versus shallow (Tursiops truncatus) diving cetacean

When a marine mammal dives, breathing and locomotion are mechanically uncoupled, and its locomotor muscle must power swimming when oxygen is limited. The morphology of that muscle provides insight into both its oxygen storage capacity and its rate of oxygen consumption. This study investigated the m. longissimus dorsi, an epaxial swimming muscle, in the long duration, deep-diving pygmy sperm whale (Kogia breviceps) and the short duration, shallow-diving Atlantic bottlenose dolphin (Tursiops truncatus). Muscle myoglobin content, fiber type profile (based upon myosin ATPase and succinate dehydrogenase assays), and fiber size were measured for five adult specimens of each species. In addition, a photometric analysis of sections stained for succinate dehydrogenase was used to create an index of mitochondrial density. The m. longissimus dorsi of K. breviceps displayed significantly a) higher myoglobin content, b) larger proportion of Type I (slow oxidative) fibers by area, c) larger mean fiber diameters, and d) lower indices of mitochondrial density than that of T. truncatus. Thus, this primary swimming muscle of K. breviceps has greater oxygen storage capacity, reduced ATP demand, and likely a reduced rate of oxygen consumption relative to that of T. truncatus. The locomotor muscle of K. breviceps appears able to ration its high onboard oxygen stores, a feature that may allow this species to conduct relatively long duration, deep dives aerobically.

Novel locomotor muscle design in extreme deep-diving whales

Most marine mammals are hypothesized to routinely dive within their aerobic dive limit (ADL). Mammals that regularly perform deep, long-duration dives have locomotor muscles with elevated myoglobin concentrations that are composed of predominantly large, slow-twitch (Type I) fibers with low mitochondrial volume densities (V(mt)). These features contribute to extending ADL by increasing oxygen stores and decreasing metabolic rate. Recent tagging studies, however, have challenged the view that two groups of extreme deep-diving cetaceans dive within their ADLs. Beaked whales (including Ziphius cavirostris and Mesoplodon densirostris) routinely perform the deepest and longest average dives of any air-breathing vertebrate, and short-finned pilot whales (Globicephala macrorhynchus) perform high-speed sprints at depth. We investigated the locomotor muscle morphology and estimated total body oxygen stores of several species within these two groups of cetaceans to determine whether they (1) shared muscle design features with other deep divers and (2) performed dives within their calculated ADLs. Muscle of both cetaceans displayed high myoglobin concentrations and large fibers, as predicted, but novel fiber profiles for diving mammals. Beaked whales possessed a sprinter's fiber-type profile, composed of ~80% fast-twitch (Type II) fibers with low V(mt). Approximately one-third of the muscle fibers of short-finned pilot whales were slow-twitch, oxidative, glycolytic fibers, a rare fiber type for any mammal. The muscle morphology of beaked whales likely decreases the energetic cost of diving, while that of short-finned pilot whales supports high activity events. Calculated ADLs indicate that, at low metabolic rates, both beaked and short-finned pilot whales carry sufficient onboard oxygen to aerobically support their dives.

Comparison of the motor discharge to the voluntary sphincters of continence in the rat

The rat external anal sphincter (EAS) and external urethral sphincter (EUS) are voluntary muscles of continence that can display similar synchronous electromyographic (EMG) activity patterns. However, the two sphincters are quite different in structure and function. The EUS is a fast twitch muscle and contains fibers expressing type 2B myosin. In contrast, the EAS exhibits slower kinetics and lacks type 2B fibers. This striking contrast in kinetics and fiber type profiles may be shaped by differences in the basal motor drive that each sphincter receives. A double EMG approach was used to obtain spontaneously active single motor unit action potentials from the EUS and EAS simultaneously and compare their basal discharge frequencies in urethane anaesthetized rats. The basal firing rates of motor units of the EUS and EAS were not significantly different (3.9 ± 0.9 Hz vs. 3.1 ± 1.6 Hz, respectively, n = 7 animals, P = 0.32, paired Student's t-test). However, auto-correlogram analysis showed that EUS is driven by neurons with faster instantaneous firing frequencies: 30.5 ± 2.4 Hz vs 14.3 ± 0.9 Hz in the EAS (P = 0.03, paired Student's t-test). The oscillator(s) driving the EUS operate(s) at a frequency twice that of the EAS. This may explain the presence of type 2B fibers in the EUS. In the inter-micturition periods no cross correlation was found in motor discharge to the sphincters suggesting that the two muscles do not share a common central drive to sustain the continent tonus of the two outlet tracts.

Immunohistochemical Fiber Type Profile in the Human Vocal Muscle

SUMMARY: The vocal muscle is a striated muscle with important functions in the emission of laryngeal sound and physiology of the voice. Therefore the knowledge of its constitution is the basis for the prevention and management of voice disorders. We used 10 samples from the middle third of vocal muscles obtained from autopsies of 6 male and 4 female subjects aged between 36 and 71 years. The samples were analyzed with BA-F8 monoclonal antibody to slow type I fibers, and antimyosin HC monoclonal antibody and antimyosin fast clone MY-32 antibody for types IIA, IIB, IIX, and neonatal fibers. We determined the distribution of the muscle fiber types and morphometric characteristics, evaluating the differences by sex and age group. The human vocal muscle presented a het erogeneous formation with a predominance of type II fibers at 51.99%, while type I fibers reached 48.01%; this difference was significant (p 0.05). In conclusion, the human vocal muscle the fibers were predominantly type II fast. The vocal muscle is an intrinsic larynx muscle disposed between the vocal process and the oblong pit of the arytenoid cartilage, and the inner face of the thyroid cartilage near the midline. This muscle is the most important element of the vocal folds due to its role in the shortening and relaxation of these folds. Histologically it is composed of striated skeletal muscle tissue, grouped in bundles that are arranged to determine the upper portions, and lower ave- rage muscle (Di Fiore, 2001; Latarjet & Ruiz Liard, 1997). The altered function of the muscles of the larynx is presented in various diseases of the voice, these changes have expression in the metabolism of muscle fibers in its organization and morphological appearance, as histochemical and immunohistochemical study of muscle fibers is a valuable tool for evaluation. The histochemical differentiation of muscle fibers supports the diagnosis of many myopathies (Hebling & Esquisatto, 2005). The preferred method for the differentiation of muscle fibers is based on the histochemical reaction for the identification of the enzyme nicotinamide dinucleotides tetrazolium reductase (NADH-

Changes in muscle strength and morphology after muscle unloading in Special Forces missions

The purpose of the present study was to determine the changes in maximal muscle strength, rapid force capacity, jumping performance and muscle morphology following a Special Forces military operation involving 8 days of muscle unloading. Nine male Special Forces soldiers were tested before (pre) and immediately after (post1) an 8-day simulated special support and reconnaissance (SSR) mission and after 3 h of active recovery (post2). Maximal muscle strength (MVC) and rate of force development (RFD) were measured along with maximal counter movement jump height (JH). Muscle biopsies were obtained from the vastus lateralis at pre and post1. Acute reductions were found in MVC (11%), JH (10%) and RFD (17-22%) after 8 days of muscle unloading (post1) (P≤0.05). Type IIX fiber type area% increased (P≤0.05) at post1 together with a tendency toward increased type IIX fiber type % (P=0.09) and decreased type I fiber type % (P=0.06), suggesting a transition toward a less fatigue-resistant fiber-type profile. In conclusion, short-term unloading during SSR missions led to marked reductions in mechanical muscle function and functional performance, which may be partly explained by the changes in muscle morphology. Future studies should identify intervention strategies to counter-act the observed impairments.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Fibre types in skeletal muscle: a personal account

Muscle performance is in part dictated by muscle fibre composition and a precise understanding of the genetic and acquired factors that determine the fibre type profile is important in sport science, but is also relevant to neuromuscular diseases and to metabolic diseases, such as type 2 diabetes. The dissection of the signalling pathways that determine or modulate the muscle fibre phenotype has thus potential clinical significance. In this brief review, I examine the evolution of the notion of muscle fibre types, discuss some aspects related to species differences, point at problems in the interpretation of transgenic and knockout models and show how in vivo transfection can be used to identify regulatory factors involved in fibre type diversification, focusing on the calcineurin-nuclear factor of activated T cells (NFAT) pathway.

Skeletal Muscle Signature of a Champion Sprint Runner

Human skeletal muscle contains three types of the main motor protein myosin heavy chain (MHC) and individual muscle fibers contain only one type of MHC (pure fiber types: I, IIa and IIx) or a mixture of MHC (hybrid fiber types: I/IIa and IIa/IIx). Each of these fiber types has a distinct phenotypic profile that contributes to human locomotion. The mixture of these fiber types within the muscle can have a profound impact on athletic performance. Our laboratory recently had the opportunity to examine the skeletal muscle profile of a World Record holder and World Champion sprint runner. PURPOSE: To examine single muscle fiber MHC composition (i.e. fiber type) and single muscle fiber contractile function (strength, speed and power) from this World Class sprinter. METHODS: A large number of isolated single muscle fibers from the vastus lateralis were analyzed for contractile function (n=53) and fiber type profile (n=329 fibers). RESULTS: Fiber type profile showed that 72% of the muscle contained fast motor proteins (MHC IIa and IIx). The novel finding was that the sprint champion's muscle contained 24% of the pure MHC IIx fiber type. Normal healthy human adult muscle contains less than 2% of the pure MHC IIx fiber type. Power output of the MHC IIx muscle fibers were the highest values recorded to date by a wide margin (35±4 W·l-1). Single fiber power output data from other athletic populations (swimmers and distance runners) and recreational athletes generally fall in the range of 5-10 W·l-1 and do not exceed 20 W·l-1. CONCLUSION: The unusually high abundance and high power output of the rare and powerful pure MHC IIx fiber type represent a unique muscle signature that provides a myocellular basis for the high level of athletic success achieved in sprinting events by this individual.

Multi-step immunofluorescent analysis of vitamin D receptor loci and myosin heavy chain isoforms in human skeletal muscle

Vitamin D receptors have been shown to be present in human skeletal muscle using different techniques. We developed a multi-staining immunofluorescent method to detect vitamin D receptor expression and co-localize it with myosin heavy chain isoform expression in skeletal muscle biopsies in older female subjects. Serial sections were cut from frozen samples obtained by needle biopsy of the vastus lateralis. Samples were probed with a primary vitamin D receptor monoclonal antibody and then re-probed with a type IIa myosin heavy chain isoform-specific antibody. Independent unfixed sections followed a similar protocol and were probed with type IIx and type I myosin heavy chain isoform-specific antibodies. Immunohistochemistry and fluorescent microscopy co-localized vitamin D receptor loci and myosin heavy chain isoforms in whole skeletal muscle sections. We quantified intranuclear vitamin D receptor staining patterns and number of individual muscle fiber subtypes within a muscle section. Immunohistochemical staining of the vitamin D receptor was confirmed by Western blot using the same monoclonal antibody. This multi-staining immunofluorescent technique allows for measurement of intranuclear vitamin D receptor expression in the context of the specific muscle fiber type profile in a single section. This method can thus be a useful approach to study potential relationships between muscle fiber subtypes and vitamin D receptor expression.

Immunohistochemical Analysis of Laryngeal Muscles in Normal Horses and Horses With Subclinical Recurrent Laryngeal Neuropathy

We used immunohistochemistry to examine myosin heavy-chain (MyHC)-based fiber-type profiles of the right and left cricoarytenoideus dorsalis (CAD) and arytenoideus transversus (TrA) muscles of six horses without laryngoscopic evidence of recurrent laryngeal neuropathy (RLN). Results showed that CAD and TrA muscles have the same slow, 2a, and 2x fibers as equine limb muscles, but not the faster contracting fibers expressing extraocular and 2B MyHCs found in laryngeal muscles of small mammals. Muscles from three horses showed fiber-type grouping bilaterally in the TrA muscles, but only in the left CAD. Fiber-type grouping suggests that denervation and reinnervation of fibers had occurred, and that these horses had subclinical RLN. There was a virtual elimination of 2x fibers in these muscles, accompanied by a significant increase in the percentage of 2a and slow fibers, and hypertrophy of these fiber types. The results suggest that multiple pathophysiological mechanisms are at work in early RLN, including selective denervation and reinnervation of 2x muscle fibers, corruption of neural impulse traffic that regulates 2x and slow muscle fiber types, and compensatory hypertrophy of remaining fibers. We conclude that horses afflicted with mild RLN are able to remain subclinical by compensatory hypertrophy of surviving muscle fibers.

The gross morphology and histochemistry of respiratory muscles in bottlenose dolphins, Tursiops truncatus

Most mammals possess stamina because their locomotor and respiratory (i.e., ventilatory) systems are mechanically coupled. These systems are decoupled, however, in bottlenose dolphins (Tursiops truncatus) as they swim on a breath hold. Locomotion and ventilation are coupled only during their brief surfacing event, when they respire explosively (up to 90% of total lung volume in approximately 0.3 s) (Ridgway et al. 1969 Science 166:1651-1654). The predominantly slow-twitch fiber profile of their diaphragm (Dearolf 2003 J Morphol 256:79-88) suggests that this muscle does not likely power their rapid ventilatory event. Based on Bramble's (1989 Amer Zool 29:171-186) biomechanical model of locomotor-respiratory coupling in galloping mammals, it was hypothesized that locomotor muscles function to power ventilation in bottlenose dolphins. It was further hypothesized that these muscles would be composed predominantly of fast-twitch fibers to facilitate the bottlenose dolphin's rapid ventilation. The gross morphology of craniocervical (scalenus, sternocephalicus, sternohyoid), thoracic (intercostals, transverse thoracis), and lumbopelvic (hypaxialis, rectus abdominis, abdominal obliques) muscles (n = 7) and the fiber-type profiles (n = 6) of selected muscles (scalenus, sternocephalicus, sternohyoid, rectus abdominis) of bottlenose dolphins were investigated. Physical manipulations of excised thoracic units were carried out to investigate potential actions of these muscles. Results suggest that the craniocervical muscles act to draw the sternum and associated ribs craniodorsally, which flares the ribs laterally, and increases the thoracic cavity volume required for inspiration. The lumbopelvic muscles act to draw the sternum and caudal ribs caudally, which decreases the volumes of the thoracic and abdominal cavities required for expiration. All muscles investigated were composed predominantly of fast-twitch fibers (range 61-88% by area) and appear histochemically poised for rapid contraction. These combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation.

Sex difference in peak oxygen uptake in prepubertal children

Prepubertal boys’ greater aerobic fitness (peak V ˙ ⁡ O 2 ) has been attributed to their larger lean body mass (LBM); this bestowing a greater heart size and consequent larger maximum cardiac output. No difference in peak arterio-venous (A-VO 2) difference is thought to exist. However other work indicates that boys’ aerobic fitness remains 5% higher even after controlling for differences in LBM. Consequently the purpose of this study was to investigate whether peak V ˙ ⁡ O 2 , heart size, peak cardiac output and peak A-VO 2 difference would be comparable between a group of boys and girls with a similar LBM. A group of 9 prepubertal boys and 9 prepubertal girls with a similar mean LBM (27.0 ± 1.4 boys vs. 27.0 ± 2.0 kg girls) were selected. Left ventricular mass (LVM) and end diastolic volume (LVEDV) were measured using cardiac magnetic resonance imaging. Peak V ˙ ⁡ O 2 was determined on a cycle ergometer following an incremental exercise protocol to exhaustion, and cardiac output was recorded using thoracic bioimpedance. Boys’ peak V ˙ ⁡ O 2 (1.41 ± 0.18 L min −1 vs. 1.23 ± 0.08 L min −1) and A-VO 2 difference (14.8 ± 2.1 mL 100 mL −1 vs. 12.6 ± 1.6 mL 100 mL −1) were significantly ( p < 0.05) higher than girls’ values, but there were no significant sex differences in peak cardiac output (10.0 ± 1.4 L min −1 vs. 9.9 ± 1.40 L min −1), LVM (97 ± 13 g vs. 93 ± 20 g) or LVEDV (77 ± 8 mL vs. 70 ± 13 mL). Central factors of heart size and peak cardiac output are proportional to the LBM of the individual and sex independent. Sex differences in peripheral factors such as muscle fibre type profile, may affect A-VO 2 difference and underlie prepubertal boys’ higher peak V ˙ O 2 .

Immunohistochemical Analysis of Myosin Heavy Chain Expression in Laryngeal Muscles of the Rabbit, Cat, and Baboon

We studied myosin heavy chain (MyHC) expression and fiber type distribution in laryngeal muscles in the rabbit, cat, and baboon using immunohistochemistry with highly MyHC-specific antibodies. Two types of variation in MyHC expression were found: between muscles of different function within species and within specific muscles between species. Within species, thyroarytenoid (Ta), an adductor, had faster MyHCs and fiber type profiles than the abductor, posterior cricoarytenoid (PCA), which expressed faster MyHCs than the vocal fold tensor, cricothyroid (CT). Between species, laryngeal muscles generally expressed faster MyHCs in small animals than in larger ones: extraocular (EO) MyHC was expressed in the Ta and PCA of the rabbit but not in the cat and baboon, whereas 2B MyHC was expressed in these muscles of the cat but not of the baboon. The CT expressed only MyHC isoforms and fiber types found in the limb muscles of the same species. These results are discussed in light of the hypothesis that the between-species variations in laryngeal muscle fiber types are evolutionary adaptations in response to changes in body mass and respiratory frequency. Within-species variations in fiber types ensure that protective closure of the glottis is always faster than movements regulating airflow during respiration.

Activity-Dependent Signaling Pathways Controlling Muscle Diversity and Plasticity

A variety of fiber types with different contractile and metabolic properties is present in mammalian skeletal muscle. The fiber-type profile is controlled by nerve activity via specific signaling pathways, whose identification may provide potential therapeutic targets for the prevention and treatment of metabolic and neuromuscular diseases.

Phenotype-Specific Effect of Protein Kinase A on Skeletal Muscle Induction of Hsp70 Following Exercise

Heat shock protein 70 (Hsp70) has been found to play cyto-protective roles in both cardiac and skeletal muscle. To this point, little attention has been paid to the signaling mechanisms involved in expression of Hsp70 in skeletal muscle. PURPOSE: To characterize the role of protein kinase A (PKA) in regulation of the Hsp70 response to endurance exercise in skeletal muscle. METHODS: Twenty-eight 12 week-old male Sprague-Dawley rats were divided into sham (5 % DMSO; n=14) and PKA-inhibited (0.36 mg/kg H-89 in 5% DMSO; n=14) groups and run on a motor-driven treadmill for 60 min at 30 m/min on a 2% grade. Exercised (EX) animals were subsequently killed at 30 min (Hsp70 mRNA evaluated by RT-PCR; n=7) or 24 h (western blot analysis of Hsp70; n=7) post-exercise and portions of the white vastus (WV) and soleus (Sol), representing muscles with different recruitment patterns and fiber-type profiles, were taken and evaluated compared to appropriate non-exercised controls (CON; n=7 per group). RESULTS: All data are expressed as means ± SE. Hsp70 protein (% of control standard) was increased (P<0.05) in sham-injected rats in WV (CON: 30.30 ± 4.14, EX: 113,1 ± 11.81) but not Sol (CON: 152.42 ± 23.62, EX: 134.81 ± 32.33) following exercise. PKA inhibition suppressed this response in WV (EX: 25.01 ± 6.26). However, Hsp70 mRNA (corrected to β-Actin mRNA), was significantly increased (P<0.05) in both muscles (CON: 5.69 ± 0.59, EX: 26.43 ± 10.55 for WV; CON: 3.22 ± 0.44, EX: 56.87 ± 14.37 for Sol) following exercise and this increase was unaffected by PKA inhibition. CONCLUSIONS: Exercise selectively increases the relative content of Hsp70 in a muscle-specific, PKA-dependent manner. In skeletal muscle, the exercise-induced increase in Hsp70 appears to be under both transcriptional and translational control and PKA appears to regulate this process at the level of translation. Supported by NSERC grant # 8170–00 and Ontario Graduate Scholarship funding

Does daily activity level determine muscle phenotype?

The activation level of a muscle is presumed to be a major determinant of many mechanical and phenotypic properties of its muscle fibers. However, the relationship between the daily activation levels of a muscle and these properties has not been well defined, largely because of the lack of accurate and sustained assessments of the spontaneous activity levels of the muscle. Therefore, we determined the daily activity levels of selected rat hindlimb muscles using intramuscular EMG recordings. To allow comparisons across muscles having varying activity levels and/or muscle fiber type compositions, we recorded EMG activity in a predominantly slow plantarflexor (soleus), a predominantly fast plantarflexor (medial gastrocnemius, MG), a predominantly fast ankle dorsiflexor (tibialis anterior, TA) and a predominantly fast knee extensor (vastus lateralis, VL) in six unanesthetized rats for periods of 24 h. EMG activity levels were correlated with the light:dark cycle, with peak activity levels occurring during the dark period. The soleus was the most active and the TA the least active muscle in all rats. Daily EMG durations were highest for soleus (11-15 h), intermediate for MG (5-9 h) and VL (3-14 h) and lowest for TA (2-3 h). Daily mean EMG amplitudes and integrated EMG levels in the soleus were two- to threefold higher than in the MG and VL and seven- to eightfold higher than in the TA. Despite the three- to fourfold difference in activation levels of the MG and VL vs the TA, all three predominantly fast muscles have been reported to have a similar, very low percentage of slow fibers. Comparing these relative EMG levels to the published fiber type profiles of these muscles yields a very poor relationship between daily activity level and fiber type composition in the same muscles across several species. Although it is clear that changing the levels of activity can modulate the expression of the myosin phenotype, these results indicate that factors other than activation must play critical roles in determining and maintaining normal phenotypic properties of skeletal muscle fibers.

Lack of myostatin in mice results in excessive muscle but impaired force generation, tubular aggregates and alterations in fibre type profile

Lack of myostatin in mice results in excessive muscle but impaired force generation, tubular aggregates and alterations in fibre type profile

Laryngeal muscle fibre types

The internal laryngeal muscles have evolved to subserve the highly specialized functions of airways protection, respiration, and phonation. Their contractile properties, histochemistry, biochemical properties, myosin heavy chain (MyHC) expression and their regulation by nerves and hormones are reviewed and compared with limb muscle fibres. Cricothyroid, the vocal cord tensor, is limb-like in MyHC composition and fibre type properties, while the vocal fold abductor and adductors are allotypically different, with capacity for expressing an isoform of MyHC that is kinetically faster than the fastest limb MyHC. In rats and rabbits the faster isoform is the extraocular (EO) MyHC, while in carnivores, it is the IIB MyHC. These adaptations enable the abductor and adductor muscles to remain always faster than the cricothyroid as the latter changes in speed during evolution to match changing metabolic and respiratory rates in relation to scaling with body mass. Such phylogenetic plasticity is vital to the airways protection and respiratory functions of these muscles. The posterior cricoarythenoid, the abductor muscle, is tonically driven during expiration, and consequently has a slower fibre type profile than the principal adductor, the thyroarythenoid. The human thyroarythenoid appears not to express EO or IIB MyHC significantly, but is unique in expressing the slow-tonic MyHC. The concepts of allotype and phylogenetic plasticity help to explain differences in fibre type between limb and laryngeal muscles and between homologous laryngeal muscles in different species. Laryngeal muscle fibres exhibit physiological plasticity as do limb muscles, being subject to neural and hormonal modulation.