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PubMed1996

Muscle fiber type correlates with innervation topography in the rat serratus anterior muscle.

Previous studies have reported that motoneurons from the sixth spinal nerve (C6) innervate the majority of muscle fibers in the rat serratus anterior (SA) muscle. The seventh spinal nerve (C7) innervates a limited number of SA fibers, increasing caudally. This topographic map is partially reestablished following denervation. In the present study, muscle fibers of the SA were stained with monoclonal antibodies for the muscle-specific fast myosin heavy chain (F-MHC) and slow myosin heavy chain (S-MHC) proteins. We found that the majority of fibers in the SA muscle stained for F-MHC antibody, and the percentage of muscle fibers staining for S-MHC antibody increased caudally. When newborn SA muscles were denervated and then reinnervated by the entire long thoracic (LT) nerve or only the C6 branch to the LT nerve, the reinnervated muscle had the normal proportion of muscle fibers expressing S-MHC protein. However, if the LT nerve was crushed and only C7 motoneurons allowed to reinnervate the SA muscle, a greater percentage of muscle fibers stained for S-MHC antibody than normal. We conclude that there is a correlation between muscle fiber type and innervation topography in the SA muscle of the rat.

Animals↗
PubMed2003

Muscle-derived stem cells: potential for muscle regeneration.

Duchenne muscular dystrophy (DMD) is a devastating X-linked muscle disease characterized by progressive muscle weakness caused by the lack of dystrophin expression at the sarcolemma of muscle fibers. Although various approaches to delivering dystrophin in dystrophic muscle have been investigated extensively (e.g., cell and gene therapy), there is still no treatment that alleviates the muscle weakness in this common inherited muscle disease. The transplantation of myoblasts can enable transient delivery of dystrophin and improve the strength of injected dystrophic muscle, but this approach has various limitations, including immune rejection, poor cellular survival rates, and the limited spread of the injected cells. The isolation of muscle cells that can overcome these limitations would enhance the success of myoblast transplantation significantly. The efficiency of cell transplantation might be improved through the use of stem cells, which display unique features, including (1) self-renewal with production of progeny, (2) appearance early in development and persistence throughout life, and (3) long-term proliferation and multipotency. For these reasons, the development of muscle stem cells for use in transplantation or gene transfer (ex vivo approach) as treatment for patients with muscle disorders has become more attractive in the past few years. In this paper, we review the current knowledge regarding the isolation and characterization of stem cells isolated from skeletal muscle by highlighting their biological features and their relationship to satellite cells as well as other populations of stem cells derived from other tissues. We also describe the remarkable ability of stem cells to regenerate skeletal muscle and their potential use to alleviate the muscle weakness associated with DMD.

Animals↗
PubMed1990

Ultrastructural and metabolic characteristics of single muscle fibres belonging to the same type in various muscles in rats.

Single muscle fibres from soleus (SOL) as well as extensor digitorum longus (EDL) muscles from Wistar male rats in relaxing solution were divided into three types by their histochemical features--slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), or fast-twitch glycolytic (FG) fibres. The relationship between ultrastructural profiles (mitochondrial volume, number, and Z-line width) and metabolic profiles (glycolytic and oxidative enzymes' activity) were analyzed using the same types of fibres dissected from different SOL and EDL muscles using stereological and biochemical techniques. The Z-line width is specialized in different fibre types. Fast-twitch (FG and FOG) fibres have narrow Z-line width compared to slow-twitch (SO) fibre in SOL and EDL muscles. A significant difference was observed between SOL muscle SO and FOG fibres and EDL muscle SO and FOG fibres. All glycolytic (lactate dehydrogenase (LDH), phosphofructokinase (PFK), pyruvate kinase (PK) and creatine kinase (CK) activities in FOG fibres from EDL muscles were significantly higher (p less than 0.01) than those dissected from SOL muscles. The oxidative enzyme (succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) activity in SO and FOG fibres dissected from SOL muscle were significantly higher (p less than 0.01) than those dissected from EDL muscles. Mitochondrial volume and number in SO fibres dissected from SOL muscle were significantly higher (volume, p less than 0.01, number, p less than 0.01) than those dissected from EDL muscles. A significant difference was not observed in mitochondrial volume of FOG fibres between SOL and EDL muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low-angle X-ray fibre diagrams from non-overlap muscle.

The regulation of striated muscle contraction involves changes in the interactions of troponin and tropomyosin with actin thin filaments. In resting muscle, myosin-binding sites on actin are thought to be blocked by the coiled-coil protein tropomyosin. During muscle activation, Ca2+ binding to troponin alters the tropomyosin position on actin, resulting in cyclic actin-myosin interactions that accompany muscle contraction. Evidence for this steric regulation by troponin-tropomyosin comes from X-ray data [Haselgrove, J.C., 1972. X-ray evidence for a conformational change in the actin-containing filaments of verterbrate striated muscle. Cold Spring Habor Symp. Quant. Biol. 37, 341-352; Huxley, H.E., 1972. Structural changes in actin and myosin-containing filaments during contraction. Cold Spring Habor Symp. Quant. Biol. 37, 361-376; Parry, D.A., Squire, J.M., 1973. Structural role of tropomyosin in muscle regulation: analysis of the X-ray diffraction patterns from relaxed and contracting muscles. J. Mol. Biol. 75, 33-55] and electron microscope (EM) data [Spudich, J.A., Huxley, H.E., Finch, J., 1972. Regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin-troponin complex with actin. J. Mol. Biol. 72, 619-632; O'Brien, E.J., Gillis, J.M., Couch, J., 1975. Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J. Mol. Biol. 99, 461-475; Lehman, W., Craig, R., Vibert, P., 1994. Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65-67] each with its own particular strengths and limitations. Here we bring together some of the latest information from EM analysis of single thin filaments from Pirani et al. [Pirani, A., Xu, C., Hatch, V., Craig, R., Tobacman, L.S., Lehman, W. (2005). Single particle analysis of relaxed and activated muscle thin filaments. J. Mol. Biol. 346, 761-772], with synchrotron X-ray data from non-overlapped muscle fibres to refine the models of the striated muscle thin filament. This was done by incorporating current atomic-resolution structures of actin, tropomyosin, troponin and myosin subfragment-1. Fitting these atomic coordinates to EM reconstructions, we present atomic models of the thin filament that are entirely consistent with a steric regulatory mechanism. Furthermore, fitting the atomic models against diffraction data from skinned muscle fibres, stretched to non-overlap to preclude crossbridge binding, produced very similar results, including a large Ca2+-induced shift in tropomyosin azimuthal location but little change in the actin structure or apparent alteration in troponin position.

Actin Cytoskeleton↗
PubMed1997

Increased intramuscular concentration of bradykinin increases the static fusimotor drive to muscle spindles in neck muscles of the cat.

The aim of the present study was to investigate if increased intramuscular concentrations of bradykinin (BK) in one muscle influence the activity in primary and secondary muscle spindle afferents (MSAs) originating from both ipsi- and contralateral muscles, via fusimotor reflexes. The ipsilateral trapezius (TR) and the splenius (SP) muscles were subjected to sinusoidal stretches and 2-3 MSAs were simultaneously recorded from these muscles. Responses of 29 MSAs (15 SP and 14 TR) were registered in five adult cats anaesthetised with alpha-chloralose. Intramuscular injections of 0.5 ml BK (6-86 micrograms/ml) were administered to both the ipsi- and contralateral SP and TR muscles. Similar doses of BK (5-10 micrograms) have been shown to induce muscle pain when injected into the temporal muscle in man. The responsiveness of the MSAs to the injections of BK was 86% and 87.5% from the contralateral TR and SP muscles, respectively. The effects were predominantly static onto the MSAs. The duration of the effects was on average 3.5-4 min, however some effects lasted for more than 15 min. The effects were always abolished after cutting the nerve to the injected muscle. The large majority of the spindle afferents were unresponsive to i.m. Tyrode injections (23 of 29). For the afferents that were responsive to injection of Tyrode, the effects were always considerably smaller and with shorter duration than those evoked by BK injections. Thus, increased intramuscular concentrations of BK may excite primary and secondary MSAs from ipsi- and contralateral muscles, via fusimotor reflexes evoked most probably by activity in chemosensitive muscle afferents. The results are discussed in relation to a recent hypothesis on pathophysiological mechanisms behind genesis, spread and perpetuation of muscle tension and pain in chronic musculoskeletal pain syndromes.

Animals↗
PubMed2002

Acetylcholine receptor density in human cricopharyngeal muscle and pharyngeal constrictor muscle.

BACKGROUND: Upper esophageal sphincter resting tone is reduced during partial neuromuscular block, whereas contraction of the pharyngeal constrictor muscle is only slightly affected. We hypothesized that this difference may arise from differential nicotinic acetylcholine receptor (nAChR) density, the density supposedly being lower in the more sensitive cricopharyngeal muscle than in the resistant pharyngeal constrictor muscle. The aim of this study was to determine the density of nAChR in the main component of the upper esophageal sphincter, the cricopharyngeal muscle, and in the pharyngeal constrictor muscle. METHOD: After approval by the institutional ethics committee and informed consent, muscle specimens were obtained from five patients undergoing surgery with laryngectomy for malignancies of the larynx or thyroid gland. None had received radiation therapy to the affected area. The nAChR from these tissue specimens were solubilized and incubated with 125I-alpha-bungarotoxin. The quantity of radioligand-receptor complex was measured by radioactive decay in a liquid scintillation counter. The receptor density was expressed as femtomoles per milligram of protein (fmol/mg protein). RESULTS: The nAChR density was determined to 6.8 (3.5) fmol/mg protein (mean (SD)) in the cricopharyngeal muscle and 5.6 (2.1) fmol/mg protein in the pharyngeal constrictor muscle (P = 0.22). Although we could not find any difference in mean nAChR density, contrary to our hypothesis, the density in four of the five patients was higher in the cricopharyngeal muscle than in the pharyngeal constrictor muscle. CONCLUSION: Our results indicate that the density of nicotinic acetylcholine receptors is similar in the cricopharyngeal muscle and in the pharyngeal constrictor muscle. Nicotinic acetylcholine receptor density, as determined by 125I-alpha-bungarotoxin assay, cannot explain the difference in response to neuromuscular blocking drugs between the investigated muscles.

Humans↗
PubMed2003

Effect of selection for growth rate on muscle damage during turkey breast muscle development.

Damage to the turkey pectoralis major muscle was studied in a randombred control line (RBC2), a subline (F) from the RBC2 line selected long-term for increased 16-wk BW only, and a commercial sire line (B) at 25 d of incubation and at 1, 4, 8, 16, and 20 wk posthatch. Pectoralis major muscle samples were obtained from three females and three males from each line in a manner to avoid contraction. After being fixed and sectioned, the muscle samples were stained with hematoxylin and eosin to view muscle fiber and muscle fiber bundle morphology. Beginning at 8 wk posthatch, differences in muscle fiber morphology were observed among the different lines. The RBC2 line throughout the duration of the study maintained well-organized muscle fibers and muscle fiber bundles with large capillary networks. In contrast, the growth-selected F line began to show muscle fiber degeneration at 8 wk posthatch, and limited capillary beds were observed as development proceeded. The B line had intermediate muscle morphology between the RBC2 and F lines, but by 20 wk posthatch significant muscle fiber degeneration was present with limited capillary supply. The degenerative muscle fiber changes were predominant in the growth-selected F-line, suggesting that growth selection for just BW may be associated with posthatch muscle damage.

Aging↗
PubMed2003

Denervated muscle fibers explain the deficit in specific force following reinnervation of the rat extensor digitorum longus muscle.

The authors tested the hypothesis that, after denervation and reinnervation of skeletal muscle, observed deficits in specific force can be completely attributed to the presence of denervated muscle fibers. The peroneal nerve innervating the extensor digitorum longus muscle in rats was sectioned and the distal stump was coapted to the proximal stump, allowing either a large number of motor axons (nonreduced, n = 12) or a drastically reduced number of axons access to the distal nerve stump (drastically reduced, n = 18). A control group of rats underwent exposure of the peroneal nerve, without transection, followed by wound closure (control, n = 9). Four months after the operation, the maximum tetanic isometric force (Fo) of the extensor digitorum longus muscle was measured in situ and the specific force (sFo) was calculated. Cross-sections of the muscles were labeled for neural cell adhesion molecule (NCAM) protein to distinguish between innervated and denervated muscle fibers. Compared with extensor digitorum longus muscles from rats in the control (295 +/- 11 kN/m2) and nonreduced (276 +/- 12 kN/m2) groups, sFo of the extensor digitorum longus muscles from animals in the drastically reduced group was decreased (227 +/- 15 kN/m2, p < 0.05). The percentage of denervated muscle fibers in the extensor digitorum longus muscles from animals in the drastically reduced group (18 +/- 3 percent) was significantly higher than in the control (3 +/- 1 percent) group, but not compared with the nonreduced (9 +/- 2 percent) group. After exclusion of the denervated fibers, sFo did not differ between extensor digitorum longus muscles from animals in the drastically reduced (270 +/- 20 kN/m2), nonreduced (301 +/- 13 kN/m2), or control (303 +/- 10 kN/m2) groups. The authors conclude that, under circumstances of denervation and rapid reinnervation, the decrease in sFo of muscle can be attributed to the presence of denervated muscle fibers.

Animals↗
PubMed2004

Dynamic reconstruction of eye closure by muscle transposition or functional muscle transplantation in facial palsy.

For patients with facial palsy, lagophthalmus is often a more serious problem than the inability to smile. Dynamic reconstruction of eye closure by muscle transposition or by free functional muscle transplantation offers a good solution for regaining near-normal eye protection without the need for implants. This is the first quantitative study of three-dimensional preoperative and postoperative lid movements in patients treated for facial paralysis. Between February of 1998 and April of 2002, 44 patients were treated for facial palsy, including reconstruction of eye closure. Temporalis muscle transposition to the eye was used in 34 cases, and a regionally differentiated part of a free gracilis muscle transplant after double cross-face nerve grafting was used in 10 cases. Patients' facial movements were documented by a three-dimensional video analysis system preoperatively and 6, 12, 18, and 24 months postoperatively. For this comparative study, only the data of patients with preoperative and 12-month postoperative measurements were included. In the 27 patients with a final result after temporalis muscle transposition for eye closure, the distance between the upper and lower eyelid points during eye closing (as for sleep) was reduced from 10.33 +/- 2.43 mm (mean +/- SD) preoperatively to 5.84 +/- 4.34 mm postoperatively on the paralyzed side, compared with 0.0 +/- 0.0 mm preoperatively and postoperatively on the contralateral healthy side. In the resting position, preoperative values for the paralyzed side changed from 15.11 +/- 1.92 mm preoperatively to 13.46 +/- 1.94 mm postoperatively, compared with 12.17 +/- 2.02 mm preoperatively and 12.05 +/- 1.95 mm postoperatively on the healthy side. In the nine patients with a final result after surgery using a part of the free gracilis muscle transplant reinnervated by a zygomatic branch of the contralateral healthy side through a cross-face nerve graft, eyelid closure changed from 10.21 +/- 2.72 mm to 1.68 +/- 1.35 mm, compared with 13.70 +/- 1.56 mm to 6.63 +/- 1.51 mm preoperatively. The average closure for the healthy side was from 11.20 +/- 3.11 mm to 0.0 +/- 0.0 mm preoperatively and from 12.70 +/- 1.95 mm to 0.0 +/- 0.0 mm postoperatively. In three cases, the resting tonus of the part of the gracilis muscle transplant around the eye had increased to an extent that muscle weakening became necessary. Temporalis muscle transposition and free functional muscle transplantation for reanimation of the eye and mouth at the same time are reliable methods for reconstructing eye closure, with clinically adequate results. Detailed analysis of the resulting facial movements led to an important improvement of the authors' operative techniques within the last few years. Thus, the number of secondary operative corrections could be significantly reduced. These qualitative and quantitative studies of the reconstructed lid movements by three-dimensional video analysis support the authors' clinical concept of temporalis muscle transposition being the first-choice method in adult patients with facial palsy. In children, free muscle transplantation is preferred for eye closure, so as not to interfere with the growth of the face by transposition of a masticatory muscle. In addition, a higher degree of central plasticity in children might be expected.

Adolescent↗
PubMed2002

Skeletal muscle IGF-binding protein-3 and -5 expressions are age, muscle, and load dependent.

The purpose of the current study was to examine IGFBP-3, -4, and -5 mRNA and protein expression levels as a function of muscle type, age, and regrowth from an immobilization-induced atrophy in Fischer 344 x Brown Norway rats. IGFBP-3 mRNA expression in the 4-mo-old animals was significantly higher in the red and white portions of the gastrocnemius muscle compared with the soleus muscle. However, there were no significant differences in IGFBP-3 mRNA expression among any of the muscle groups in the 30-mo-old animals. There were no significant differences in IGFBP-5 mRNA expression in any of the muscle groups, whereas in the 30-mo-old animals there was significantly less IGFBP-5 mRNA expression in the white gastrocnemius compared with the red gastrocnemius muscles. Although IGFBP-3 and -5 proteins were detected in the type I soleus muscle with Western blot analyses, no detection was observed in the type II red and white portions of the gastrocnemius muscle. Aging from adult (18 mo) to old animals (30 mo) was associated with decreases in IGFBP-3 mRNA and protein and IGFBP-5 protein only in the soleus muscle. After 10 days of recovery from 10 days of hindlimb immobilization, IGFBP-3 mRNA and protein increased in soleus muscles from young (4-mo) rats; however, only IGFBP-3 protein increased in the old (30-mo) rats. Whereas there were no changes in IGFBP-5 mRNA expression during recovery, IGFBP-5 protein in the 10-day-recovery soleus muscle did increase in the young, but not in the old, rats. Because one of the functions of IGFBPs is to modulate IGF-I action on muscle size and phenotype, it is hypothesized that IGFBP-3 and -5 proteins may have potential modulatory roles in type I fiber-dominated muscles, aging, and regrowth from atrophy.

Aging↗
PubMed1987

Skeletal muscle blood flow capacity: role of muscle pump in exercise hyperemia.

An appreciation for the potential of skeletal muscle vascular beds for blood flow (blood flow capacity) is required if one is to understand the limits of the cardiorespiratory system in exercise. To assess this potential, an index of blood flow capacity that can be objectively measured is required. One obvious index would be to measure maximal muscle blood flow (MBF). However, a unique value for maximal MBF cannot be measured, since once maximal vasodilation is attained MBF is a function of perfusion pressure. Another approach would be to measure maximal or peak vascular conductance. However, peak vascular conductance is different among skeletal muscles composed of different fiber types and is a function of perfusion pressure during peak vasodilation within muscle composed of a given fiber type. Also, muscle contraction can increase or decrease blood flow and/or the apparent peak vascular conductance depending on the experimental preparation and the type of muscle contraction. Blood flows and calculated values of conductance appear to be greater during rhythmic contractions (with the appropriate frequency and duration) than observed in resting muscle during what is called "maximal" vasodilation. Moreover, dynamic exercise in conscious subjects produces the greatest skeletal muscle blood flows. The purpose of this review is to consider the interaction of the determinants of muscle blood flow during locomotory exercise. Emphasis is directed toward the hypothesis that the "muscle pump" is an important determinant of perfusion of active skeletal muscle. It is concluded that, during normal dynamic exercise, MBF is determined by skeletal muscle vascular conductance, the perfusion pressure gradient, and the efficacy of the muscle pump.

Animals↗
PubMed1994

Muscle metaboreflex triggers parallel sympathetic activation in exercising and resting human skeletal muscle.

Activation of a metabolically generated reflex in exercising skeletal muscle (muscle metaboreflex) in humans is known to trigger increases in sympathetic nerve activity (SNA) to resting skeletal muscles. In seven healthy human subjects, to determine whether this reflex mechanism also increases SNA to the exercising muscles, we recorded muscle SNA with microelectrodes in the right peroneal nerve and in fascicles of the left peroneal nerve selectively innervating the exercising muscles of the left foot. Subjects performed static toe extension at 20% maximal voluntary contraction alone or in combination with foot ischemia. Only static toe extension at 20% MVC during ischemia activated the muscle metaboreflex. This paradigm caused increases in SNA to exercising muscle that paralleled those to the resting muscles: during the first minute of exercise SNA was unchanged, but during the second minute SNA increased from 29 +/- 2 to 38 +/- 2 bursts/min (P < 0.05) to the exercising muscles and from 30 +/- 3 to 40 +/- 2 bursts/min (P < 0.05) to the resting muscles. These bilateral increases in SNA were maintained when metaboreflex activation was sustained by postexercise foot ischemia. In conclusion, these data provide neurophysiological evidence that the muscle metaboreflex evokes parallel sympathetic activation in exercising and resting human skeletal muscle.

Adult↗
PubMed2006

The frequency of alternate muscle activity is associated with the attenuation in muscle fatigue.

Alternate muscle activity between synergist muscles has been demonstrated during low-level sustained contractions [< or =5% of maximal voluntary contraction (MVC) force]. To determine the functional significance of the alternate muscle activity, the association between the frequency of alternate muscle activity during a low-level sustained knee extension and the reduction in knee extension MVC force was studied. Forty-one healthy subjects performed a sustained knee extension at 2.5% MVC force for 1 h. Before and after the sustained knee extension, MVC force was measured. The surface electromyogram was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The frequency of alternate muscle activity for RF-VL, RF-VM, and VL-VM pairs was determined during the sustained contraction. The frequency of alternate muscle activity ranged from 4 to 11 times/h for RF-VL (7.0 +/- 2.0 times/h) and RF-VM (7.0 +/- 1.9 times/h) pairs, but it was only 0 to 2 times/h for the VL-VM pair (0.5 +/- 0.7 times/h). MVC force after the sustained contraction decreased by 14% (P < 0.01) from 573.6 +/- 145.2 N to 483.3 +/- 130.5 N. The amount of reduction in MVC force was negatively correlated with the frequency of alternate muscle activity for the RF-VL and RF-VM pairs (P < 0.001 and r = 0.65 for both) but not for the VL-VM pair. The results demonstrate that subjects with more frequent alternate muscle activity experience less muscle fatigue. We conclude that the alternate muscle activity between synergist muscles attenuates muscle fatigue.

Adaptation, Physiological↗
PubMed1985

Muscle-fiber compartmentalization in cat splenius muscles.

Splenius is a structurally complex muscle that is supplied by nerves from four cervical segments. We have examined the architectural organization of extrafusal fibers in splenius by microdissecting muscles stained "en bloc" with gold chloride. We also analyzed the distribution of motor units supplied from different cervical segments using the glycogen-depletion technique. Microdissections show that the fiber architecture of splenius is different in medial and lateral muscle parts. In medial splenius, muscle fascicles run uninterrupted from the muscle origin to insertion, while in lateral splenius, separate compartments of muscle fascicles are linked in series by two tendinous inscriptions. In muscle fascicles longer than 2.0 cm, many extrafusal fibers do not run the whole length of the fascicle. Instead, these fibers end intramuscularly by tapering to a fine strand that is closely adherent to adjacent extrafusal fibers. Less than 1% of extrafusal fibers in splenius exceeds 3.0 cm in length. Experiments using glycogen depletion showed that nerves from different spinal segments innervate different muscle regions. Nerves from C1 supply muscle fibers in a small rostromedial zone. Nerves from C2-C4 have motor domains that extend from the medial to the lateral muscle edge and are arranged in series. In lateral splenius, the motor domains are separated by tendinous inscriptions. In medial splenius, where no inscriptions are present, the motor domains of adjacent segmental nerves have overlapping boundaries due to the extensive interdigitation of motor units supplied from different segmental nerves. A single muscle fascicle in medial splenius is commonly composed of two sets of fibers supplied by adjacent segments that are distributed at opposite ends of the fascicle and interdigitate in midregions of the fascicle. The in-series compartmentalization of splenius has important implications for its motor control. To secure effective muscle shortening or tension development, central mechanisms may be required to coordinate the recruitment of in-series motor units. The presence of many short fibers with intramuscular insertions leads us to question how contractile forces are developed and transmitted in splenius and other mammalian muscles that are composed of short, in-series fibers.

Animals↗
PubMedDate not supplied

The muscle fiber composition of skeletal muscle as a predictor of athletic success. An overview.

Human skeletal muscle is composed of varying percentages of fiber types. This percent composition varies widely between muscles and among individuals. The fiber composition of some skeletal muscle could be construed as being advantageous to successful performance in selected athletic event. However, this relationship is not sufficiently close to warrant the conclusion that the fiber composition of the muscle per se is the determinant of the superior performance of elite athletes. Reasonably good evidence exists to support the position that the fiber composition of a muscle is the result of a genetic endowment. Although muscle fibers are mutable, present evidence is equivocal as to whether habitual participation in given type of physical activity is responsible for high percentages of a given fiber type being present in the muscles of some athletes. Although considerable knowledge has come from the study of muscle samples obtained from sedentary individuals, athletes of a wide range of performance capacity, and individuals before and after training, a considerable gap remains for a full understanding of how the characteristics of muscle are related to performance capacity. The observation that considerable variation exists in the percent distribution of the fibers within a muscle and that athletes with a wide range of fiber populations in their muscles can be successful in the same athletic event cautions against the routine application of the biopsy technique to estimate the fiber distribution of muscles and also to use such data as a routine screening procedure for predicting athletic success. The point, as was made in an earlier paper, that the biopsy technique for studying muscle is a research tool, will probably continue to be true for the near future.

Adenosine Triphosphate↗
PubMed1987

Neural determination of muscle fibre numbers in embryonic rat lumbrical muscles.

The generation and development of muscle cells in the IVth hindlimb lumbrical muscle of the rat was studied following total or partial denervation. Denervation was carried out by injection of beta-bungarotoxin (beta-BTX), a neurotoxin which binds to and destroys peripheral nerves. Primary myotubes were generated in denervated muscles and reached their normal stable number on embryonic day 17 (E17). This number was not maintained and denervated muscles examined on E19 or E21 contained many degenerating primary myotubes. Embryos injected with beta-bungarotoxin (beta-BTX) on E12 or E13 suffered a partial loss of motoneurones, resulting in a reduced number of axons in the L4 ventral root (the IVth lumbrical muscle is supplied by axons in L4, L5 and L6 ventral roots) and reduced numbers of nerve terminals in the intrinsic muscles of the hindfoot. Twitch tension measurements showed that all myotubes in partly innervated muscles examined on E21 contracted in response to nerve stimulation. Primary myotubes were formed and maintained at normal numbers in muscles with innervation reduced throughout development, but a diminished number of secondary myotubes formed by E21. The latter was correlated with a reduction in number of mononucleate cells within the muscles. If beta-BTX was injected on E18 to denervate muscles after primary myotube formation was complete, E21 embryo muscles contained degenerating primary myotubes. After injection to denervate muscles on E19, the day secondary myotubes begin to form, E21 muscles possessed normal numbers of primary myotubes. In both cases, secondary myotube formation had stopped about 1 day after the injection and the number of mononucleate cells was greatly reduced, indicating that cessation of secondary myotube generation was most probably due to exhaustion of the supply of competent myoblasts. We conclude that nerve terminals regulate the number of secondary myotubes by stimulating mitosis in a nerve-dependent population of myoblasts and that activation of these myoblasts requires the physical presence of nerve terminals as well as activation of contraction in primary myotubes.

Animals↗
PubMed2005

Troponin T expression in trout red muscle correlates with muscle activation.

Red or aerobic muscle from the anterior of rainbow trout Oncorhynchus mykiss activates (generates force) more quickly than that from the posterior. TnT is a component of the troponin complex that modulates muscle activation once Ca(2+) is bound. Since trout express at least two forms of TnT in their red muscle (S1 and S2), the differential expression of these two forms was predicted to explain variations in contractile properties. TnT isoforms from trout muscle were identified through hydroxy-apatite chromatography of purified myofibrillar proteins followed by SDS-PAGE. Western blots employing a mammalian anti-troponin T monoclonal antibody were used to identify TnT isoforms. The relative expression of the two isoforms of TnT was then examined at seven longitudinal positions from each of three fish using SDS-PAGE and densitometry on the silver-stained TnT bands. A significant shift in expression was detected from anterior to posterior in all three fish with TnT S1 becoming more dominant in the posterior red muscle. As predicted, a shift in TnT expression was associated with the decrease in activation rate along the length of the fish. This study was then extended to include a different species of salmonid, brook trout Salvelinus fontinalis, to explore the generality of TnT modulation of muscle activation. Muscle contractile properties were determined from anterior and posterior muscle, and relative expression of S1 and S2 was determined. Unlike rainbow trout, there is no consistent longitudinal pattern of muscle activation in brook trout: some fish have kinetically faster muscle in the anterior, some in the posterior. Similarly, there is no consistent pattern of TnT expression. Individual analysis of muscle activation and TnT expression in brook trout provides insight into the role of TnT in modulating muscle activation in slow fish muscle.

Animals↗
PubMed1999

How muscles deal with real-world loads: the influence of length trajectory on muscle performance.

The performance of skeletal muscles in vivo is determined by the feedback received when the muscle interacts with the external environment via various morphological structures. This interaction between the muscle and the 'real-world load' forces us to reconsider how muscles are adapted to suit their in vivo function. We must consider the co-evolution of the muscles and the morphological structures that 'create' the load in concert with the properties of the external environment. This complex set of interactions may limit muscle performance acutely and may also constrain the evolution of morphology and physiology. The performance of skeletal muscle is determined by the length trajectory during movement and the pattern of stimulation. Important features of the length trajectory include its amplitude, frequency, starting length and shape (velocity profile). Many of these parameters interact. For example, changing the velocity profile during shortening may change the optimum values of the other parameters. The length trajectory that maximizes performance depends on the task to be performed. During cyclical work, muscles benefit from using asymmetric cycles with longer shortening than lengthening phases. Modifying this 'sawtooth' cycle by increasing the velocity during shortening may further increase power by augmenting force output and speeding deactivation. In contrast, when accelerating an inertial load, as in jumping, the predicted 'optimal' velocity profile has two peak values, one early and one late in shortening. During level running at constant speed, muscles perform tasks other than producing work and power. Producing force to support the body weight is performed with nearly isometric contractions in some of the limb muscles of vertebrates. Muscles also play a key role in producing stability during running, and the intrinsic properties of the musculoskeletal system may be particularly important in stabilizing rapid running. Recently, muscles in running invertebrates and vertebrates have been described that routinely absorb large amounts of work during running. These muscles are hypothesized to play a key role in stability.

Animals↗