Muscle Fiber Type Distribution Research Articles

Glucose intolerance in the West African Diaspora: a skeletal muscle fibre type distribution hypothesis

In the United States, Black Americans are largely descendants of West African slaves; they have a higher relative proportion of obesity and experience a higher prevalence of diabetes than White Americans. However, obesity rates alone cannot explain the higher prevalence of type 2 diabetes. Type 2 diabetes is characterized by insulin resistance and beta-cell dysfunction. We hypothesize that the higher prevalence of type 2 diabetes in African Americans (as compared to White Americans) is facilitated by an inherited higher percentage of skeletal muscle fibre type II and a lower percentage of skeletal muscle fibre type I. Skeletal muscle fibre type II is less oxidative and more glycolytic than skeletal muscle fibre type I. Lower oxidative capacity is associated with lower fat oxidation and a higher disposal of lipids, which are stored as muscular adipose tissue in higher amounts in Black compared to White Americans. In physically active individuals, the influence of muscle fibre composition will not be as detrimental as in physically inactive individuals. This discrepancy is caused by the plasticity in the skeletal muscle fibre characteristics towards a higher activity of oxidative enzymes as a consequence of physical activity. We suggest that a higher percentage of skeletal muscle fibre type II combined with physical inactivity has an impact on insulin sensitivity and high prevalence of type 2 diabetes in Blacks of West African ancestry.

Skeletal muscle structural and energetic characteristics in subjects with sickle cell trait, α-thalassemia, or dual hemoglobinopathy

Previous studies have shown that subjects with sickle cell trait (SCT), alpha-thalassemia (alpha-t), and the dual hemoglobinopathy (SCT/alpha-t) manifest subtle, albeit significant, differences during exercise. To better understand such differences, we assessed skeletal muscle histomorphological and energetic characteristics in 10 control HbAA subjects (C), 5 subjects with alpha-t (alpha-t), 6 SCT carriers (SCT) and 9 SCT carriers with alpha-t (SCT/alpha-t). Subjects underwent a muscle biopsy and also performed an incremental maximal exercise and a time to exhaustion test. There were no observable differences in daily energy expenditure, maximal power output (Pmax), or time to exhaustion at 110% Pmax (Tex) among the groups. Blood lactate concentrations measured at the end of the Tex, muscle fiber type distribution, and mean phosphofructokinase (PFK), lactate dehydrogenase (LDH), beta-hydroxyacyl-CoA-dehydrogenase (HAD), and citrate synthase (CS) activities were all similar among the four groups. However, SCT was associated with a lower cytochrome-c oxidase (COx) activity in type IIa fibers (P<0.05), and similar trends were observed in fiber types I and IIx. Trends toward lower creatine kinase (CK) activity (P=0.0702) and higher surface area of type IIx fibers were observed in SCT (P=0.0925). In summary, these findings support most of the previous observations in SCT, such as 1) similar maximal power output and associated maximal oxygen consumption (VO2max) values and 2) lower exercise performances during prolonged submaximal exercise. Furthermore, performances during short supramaximal exercise were not different in SCT. Finally, the dual hemoglobinopathy condition does not seem to affect muscle characteristics.

The upper extremity of the professional tennis player: muscle volumes, fiber‐type distribution and muscle strength

The effects of professional tennis participation on dominant and non-dominant upper extremity muscle volumes, and on fiber types of triceps brachii (lateral head) and vastus lateralis muscles were assessed in 15 professional tennis players. Magnetic resonance imaging (MRI, n=8) examination and dual-energy x-ray absorptiometry (DXA, n=7) were used to assess muscle volumes and lean body mass. Muscle fiber-type distribution assessed by biopsy sampling was similar in both triceps brachii (2/3 were type 2 and 1/3 type 1 fibers). The VL was composed of 1/3 of type 2 and 2/3 of type 1 fibers. The dominant had 12-15% higher lean mass (DXA/MRI) than the non-dominant (P<0.05). Type 1, 2a and 2x muscle fibers of the dominant were hypertrophied compared with the non-dominant by 20%, 22% and 34% (all P<0.01), respectively. The deltoid, triceps brachii, arm flexors and forearm superficial flexor muscles of the dominant were hypertrophied (MRI) compared with the non-dominant by 11-15%. These muscles represented a similar fraction of the whole muscle volume in both upper extremities. Dominant muscle volume was correlated with 1RM on the one-arm cable triceps pushdown exercise (r=0.84, P<0.05). Peak power during vertical jump correlated with VL muscle fibers's cross-sectional area (r=0.82-0.95, P<0.05).

Distribution Pattern of Muscle Fiber Types in the Perivertebral Musculature of Two Different Sized Species of Mice

Many physiological parameters scale with body size. Regarding limb muscles, it has been shown that the demands for relatively faster muscles, less postural work, and greater heat production in small mammals are met by lower proportions of Type I and conversely higher proportions of Type II fibers. To investigate possible adaptations of the perivertebral musculature, we investigated the proportion, spatial distribution, and cross-sectional area (csa) of the different muscle fiber types in the laboratory and harvest mouse. Serial cross sections from the posterior thoracic to the lumbo-sacral region were prepared and Type I, IIA, and IIB fibers identified using enzymehistochemistry. The general distribution of Type I and IIB fibers, as well as the more or less equal distribution of IIA fibers, resembles the pattern found in other mammals. However, the overall proportion of Type I fibers was very low in the laboratory mouse and particularly low in the harvest mouse. Muscular adaptations to a small body size were met primarily by increased Type IIA fiber proportions. Thereby, not all muscles or muscle regions similarly reflected the expected scaling effects. However, our results clearly show that body size is a critical factor when fiber-type proportions are compared among different sized mammals.

Effect of Age and Sex on Histomorphometrical Characteristics of Two Muscles of Laticauda Lambs

The aim of the present experiment was to determine the effect of sex and age on histochemical and morphometric characteristics of muscle fibres (myocytes) in lambs born by single, twin, triplet and quadruplet birth. Thirty lambs were slaughtered at 60 days of age; thirty were weaned at 60 days and fed until 120 days with flakes (60%) and food supplements, and then slaughtered. Muscle tissues were obtained from two muscles, namely m. semitendinosus and m. longissimus dorsi of all lambs. For each fibre type, area perimeter and diameter (maximum and minimum) were measured and slow-twitch oxidative fibres, fast-twitch glycolytic fibres, fast-twitch oxidative-glycolytic fibres were histochemically differentiated. The muscles were stained for myosin ATPase, and succinic dehydrogenase. At 60 days, females had fibres larger than males, whereas the opposite was observed at 120 days. Besides, at 60 days, the lambs born by single birth had fibres larger than those born by multiple birth, whereas the opposite was observed at 120 days. Single lambs were heavier than twin lambs and multiple lambs. Fast-twitch glycolytic fibres had the largest size, followed by slow-twitch oxidative and fast-twitch oxidative glycolytic fibres. The dimensions of fibre types in m. longissimus dorsi were larger than in m. semitendinosus (P &lt; 0.001).These muscle fibre characteristics are thought to be important factors influencing meat quality, which is often related to metabolic and contractile properties as determined by the muscle fibre type distribution.

Skeletal muscle fiber‐type distribution and habitual physical activity in daily life

The capacity to perform physical activity largely depends on physical fitness. Muscle fiber-type distribution (Muscle(FTD)) is associated with physical fitness and may influence the capacity to perform physical activity. The purpose of this study was to determine whether habitual physical activity in daily life (PA(DL)) and Muscle(FTD) are related. Thirty-eight healthy non-athletes (31 women, 7 men) were recruited. PA(DL) was measured twice for 14 days using a tri-axial accelerometer for movement registration (Tracmor). From Tracmor output, the proportion of time subjects were physically active at low, moderate, and high intensities was determined (%Low, %Moderate, and %High, respectively). A total activity index (PA(index)) and sub-scores on work, leisure-time and sports were obtained using the Baecke questionnaire. Muscle(FTD) was determined using immuno-fluorescence against respective myosin heavy chain isoforms. No relationship was observed between PA(DL) and Muscle(FTD). %Low, %Moderate, and %High, as well as PA(index) and its sub-scores, were not related to Muscle(FTD) either. The time spent on sports was associated with the proportion of type I and II(X) fibers (P=0.06 and P<0.01, respectively). In conclusion, Muscle(FTD) probably cannot explain why some people are more prone to engaging in physical activities than others.

The Arm Of The Professional Tennis Player

Professional tennis players have a marked inter-arm symmetry due to 10-20% greater muscle mass in the dominant compared to the non-dominant arm. However, the effect of tennis on individual muscles has not been previously studied. PURPOSE: The aim of this study was to describe the effect of professional tennis participation on the individual muscle volumes of the dominant arm, and the changes elicited in the morphology of the triceps brachii muscle, using the non-dominant arm as a control. METHODS: Fifteen professional tennis players (22±4 years) were divided into two groups: one underwent muscle volumes examination by magnetic resonance imaging (n=8) in both arms, and the other group a body composition examination (DXA, n=7) and muscle biopsies from both triceps brachii (short head, n=7). RESULTS: Muscle fiber type distribution was similar in both arms. The dominant arm had 15% higher lean mass (DXA) than the non-dominant arm (P<0.05).The type 1, 2a and 2x muscle fibers of the dominant arm were hypertrophied compared to the non-dominant arm by 20 (4962 ± 452 and 4210 ± 460 μm2, P < 0.01), 22 (7700 ± 873 and 6311 ± 707 μm2, P < 0.01) and 34% (7058 ± 878 and 5225 ± 451 μm2, P < 0.01), respectively. Compared to the contralateral arm, the dominant arm had a 10% greater mean muscle volume (P < 0.001). All the muscle groups of the dominant arm were hypertrophied compared to the non-dominant arm: the deltoid muscle 13 % (P < 0.001), the arm flexors 9 % (P < 0.05) and the triceps brachii 13 % (P < 0.05). The volume of the superficial flexor muscles of the forearm was 12 % higher in the dominant compared to the contralateral forearm (P < 0.05). No significant differences were observed between the dominant and non-dominant forearm in deep flexors, mobile pad, forearm extensors and supinator muscle volumes (P = 0.62, P = 0.23, P = 0.98 and P = 0.59, respectively). CONCLUSIONS: Professional participation in tennis elicits an harmonic muscle hypertrophy in the dominant arm, where individual muscle maintain between them the same proportions observed in the contralateral arm. However, the forearm muscles show great variability between tennis players. Granted by (BFI2003-09638, BFU2006-13784, PI2005/177 and FEDER). The authors wish to thank Hospital San Roque Maspalomas (Grupo San Roque) and Sánchez-Casal Tennis Academy.

Effects of Ovariectomy on Rat Genioglossal Muscle Contractile Properties and Fiber-Type Distribution

To test the hypothesis that ovariectomy has no effects on contractile, histochemical, or biochemical properties of the rat genioglossus (GG). Eight-week-old female Sprague-Dawley rats were randomly assigned into three groups: normal group (Normal), sham-operated group (Sham), and ovariectomized group (OVX). Four weeks later, genioglossal electromyography activity (EMGgg) and contractile properties were measured, including relative integrated EMG (iEMG), maximal twitch tension, 70%-decay time, and fatigue index (FI). Then rats were sacrificed and paired GG were removed for further analysis. Adenosine-triphosphatase (ATPase) staining was performed to determine the percent fiber-type distribution and to identify cross-sectional area (CSA) of muscle fibers. Myosin heavy chain (MHC) phenotypes were determined by gel electrophoresis. Ovariectomy reduced EMG activity and contractile properties of the GG. Following ovariectomy, the CSA of type IIA and the proportion of MHCIIA decreased significantly. The MHC isoform composition of GG transferred from relative slow-twitch to fast-twitch isoform, following the order MHCIIB --> MHCIIX --> MHCIIA. Sham operation had no effect on any of the parameters. The hypothesis is rejected. The contractile properties of the GG are sensitive to ovariectomy. These changes were, at least in part, associated with changes in the amount and type of contractile protein expressed.

Skeletal Muscle Lacking the Extreme C-Terminal SH3 Domain of Nebulin Exhibits Heightened Vulnerability to Eccentric Contraction-Induced Injury

Nemaline myopathy is a congenital myopathy afflicting roughly 1 in 50,000 children. Nemaline myopathy is a disease of the thin filament, and mutations in the giant thin filament template nebulin contribute to its etiology. A clinical case report has demonstrated that loss of the extreme C-terminal Src homology 3 (SH3) domain of nebulin can cause nemaline myopathy. The nebulin SH3 domain is believed to anchor the thin filament to the Z-disk through its interaction with myopalladin. To further elucidate the physiological roles of the nebulin SH3 domain, the skeletal muscle phenotype of wild-type (nebulin+/+) mice was compared to that of mice homozygous for the I6611X mutation in the nebulin gene (nebulinI6611X/I6611X). The I6611X mutation introduces a premature truncation of the nebulin transcript and eliminates the SH3 domain from the nebulin protein. Contractile measurements revealed that baseline isometric stress production was identical in nebulinI6611X/I6611X and nebulin+/+ muscle (247±6 kPa vs. 253±6 kPa, respectively; P=0.50). However, nebulinI6611X/I6611X muscle exhibited a greater vulnerability to eccentric contraction-induced injury compared to nebulin+/+ muscle, where “injury” was defined as a decline in isometric stress production across a series of 10 eccentric contractions (39.3±0.8% vs. 29.1±1.6%, respectively; P<0.01). The corresponding decline in passive stiffness was identical in nebulinI6611X/I6611X and nebulin+/+ muscle (13.5±2.4% vs. 14.4±2.1%, respectively; P=0.79). Muscle fiber type distributions and cross-sectional areas were also identical in nebulinI6611X/I6611X and nebulin+/+ muscle. These data indicate that the nebulin SH3 domain is dispensable for isometric stress production in skeletal muscle but necessary for protecting muscle from injurious eccentric contractions. It is conceivable that heightened vulnerability to eccentric contraction-induced muscle injury, or to other types of biomechanical challenges, explains the pathology observed in children with nemaline myopathy.

Skeletal muscle in paramyotonia congenita: biochemistry, histochemistry and morphology.

In 12 patients with paramyotonia congenita, percutaneous needle biopsies from the brachial biceps muscle were performed. Muscle fibre area, distribution of muscle fibre types I, II-A and II-B and capillarization were not different from healthy controls. Signs of myopathy with central nuclei in the muscle cells were noted in 9 of the patients. 4 of these patients also had small areas with degeneration and, in one, vacuoles were observed. Quantitative determination of muscle glycogen, water and protein content were within normal range as were enzyme activities for hexokinase, lactate dehydrogenase, citrate synthetase and 3-hydroxy-acyl-CoA dehydrogenase.

Effects of Ovariectomy on Rat Genioglossal Muscle Contractile Properties and Fiber-Type Distribution

Effects of Ovariectomy on Rat Genioglossal Muscle Contractile Properties and Fiber-Type Distribution

Effect of pre-exercise carbohydrate ingestion on substrate consumption in persons with spinal cord injury

To investigate the influence of pre-exercise carbohydrate ingestion on fat and carbohydrate oxidation during prolonged arm cranking exercise in persons with spinal cord injury. Six male paraplegic subjects (PS, Ll-Th3, 46.3+/-6.6 years) and seven able-bodied subjects (AB, 43.1+/-4.6 years) were volunteered to participate in the present study. The subjects were required to consume a glucose solution (1 g kg(-1) body mass and 500 ml plain water; glucose experiment) or only plain water (water experiment) before the prolonged exercise. Then the subjects performed for 1-h arm cranking exercise at a moderate workload. In the water experiment, the carbohydrate oxidation slightly decreased and the fat oxidation increased continuously in AB. In contrast, the carbohydrate and fat oxidation of PS was constant during the exercise in the water experiment. In the glucose experiment, the fat oxidation did not rise and the carbohydrate oxidation was constant until the end of the exercise in PS and AB. PS oxidized more fat than AB in the glucose experiment (P<0.05), but no significant difference was found between PS and AB in the water experiment. Using a wheelchair in daily life regularly was regarded as an exercise training that disciplined PS indirectly and is considered to cause PS to have more percentage of type I fiber than AB in the anterior deltoid muscle. Thus, the distribution of muscle fiber type in anterior deltoid muscle might be one of the factors that impacted the fat oxidation of PS in glucose experiment.

High-energy phosphate metabolism in the exercising muscle of patients with peripheral arterial disease

Patients with peripheral arterial disease (PAD) suffer from impaired muscle function due to insufficient oxygen supply during exercise, mitochondrial damages, unfavourable muscle fibre type distribution and impaired exercise tolerance. These factors influence the symptoms as well as the quality of life in PAD patients and are closely connected to failures of high-energy phosphate metabolism. At onset of muscle exercise, the mitochondrial capacity cannot match the increased demand. The oxygen supply via blood flow must be increased. Meanwhile, anaerobic glycolysis and internal stores of oxygen like mixed venous blood and myoglobin as well as internal stores of high-energy phosphates like phosphocreatine (PCr) are adducted for the provision of additional adenosine-triphosphate (ATP), which is consumed by the ATPase at the myofibrils in order to fuel muscle contraction. Since the ATP production is insufficient, this phase (anaerobic phase) is characterized by a progressive decrease in PCr, which can be accurately measured by phosphorus 31 magnetic resonance spectroscopy (31p MRS). If the oxygen supply is improved, the mitochondrial capacity can match the increased metabolic demand. This phase is the aerobic phase, which is indicated by a steady-state of PCr hydrolysis. In PAD patients or experimental models of peripheral ischemia, the anaerobic phase is prolonged or does not pass into the aerobic phase resulting in exercise abortion. This review summarizes the results of 31p MRS studies investigating the high-energy phosphate metabolism during ischemic exercise in healthy humans and during ramp or incremental exercise in PAD patients.

Strenuous Exercise-Induced Alterations of Muscle Fiber Cross-Sectional Area and Fiber-Type Distribution in Steroid Myopathy Rats

To investigate the effect of exercise intensity on the muscle histopathology in steroid myopathy rats. Eight-week-old male Wistar rats (n=40) were divided into four groups: a control group (n=4), steroid-only group (n=12), moderate exercise + steroid group (n=12), and a high-intensity exercise + steroid group (n=12). Five weeks after triamcinolone injection, the soleus muscle (SOL) and extensor digitorum longus muscle (EDL) were removed and stained for adenosine triphosphatase (ATPase). The muscle fiber area and fiber type distribution of each fiber type were measured. In the high-intensity exercise group, the type I fibers in the SOL and the type IIb fibers in the EDL showed significant atrophy. In the fiber distribution of the SOL, type I fibers decreased in the steroid-only group and high-intensity exercise group as compared with the control, whereas there was a significant increase in the moderate exercise group vs. the steroid-only group. In the EDL, type I fibers were significantly greater in the moderate- and high-intensity exercise groups, whereas type IIb fibers significantly decreased in the moderate-exercise group as compared with the steroid-only group. In rats with steroid myopathy, high-intensity exercise caused changes such as muscle atrophy. The fiber type distribution of the SOL changed from type II fibers to type I fibers in the moderate exercise group. Intensive exercise, however, resulted in transformation from type I to type II in the fiber type distribution. For the EDL, no significant fiber type changes were observed with high-intensity exercise when compared with moderate exercise.

Muscle fiber type distribution in climbing Hawaiian gobioid fishes: Ontogeny and correlations with locomotor performance

Three species of Hawaiian amphidromous gobioid fishes are remarkable in their ability to climb waterfalls up to several hundred meters tall. Juvenile Lentipes concolor and Awaous guamensis climb using rapid bursts of axial undulation, whereas juvenile Sicyopterus stimpsoni climb using much slower movements, alternately attaching oral and pelvic sucking disks to the substrate during prolonged bouts of several cycles. Based on these differing climbing styles, we hypothesized that propulsive musculature in juvenile L. concolor and A. guamensis would be dominated by white muscle fibers, whereas S. stimpsoni would exhibit a greater proportion of red muscle fibers than other climbing species. We further predicted that, because adults of these species shift from climbing to burst swimming as their main locomotor behavior, muscle from adult fish of all three species would be dominated by white fibers. To test these hypotheses, we used ATPase assays to evaluate muscle fiber type distribution in Hawaiian climbing gobies for three anatomical regions (midbody, anal, and tail). Axial musculature was dominated by white muscle fibers in juveniles of all three species, but juvenile S. stimpsoni had a significantly greater proportion of red fibers than the other two species. Fiber type proportions of adult fishes did not differ significantly from those of juveniles. Thus, muscle fiber type proportions in juveniles appear to help accommodate differences in locomotor demands among these species, indicating that they overcome the common challenge of waterfall climbing through both diverse behaviors and physiological specializations.

ACTN3 (R577X) genotype is associated with fiber type distribution

alpha-Actinin-3 is a Z-disc structural protein found only in type II muscle fibers. The X allele of the R577X polymorphism in the ACTN3 gene results in a premature stop codon and alpha-actinin-3 deficiency in XX homozygotes. Associations between the R577X polymorphism and the muscle-power performance of elite athletes have been described earlier. About 45% of the fiber type proportions are determined by genetic factors. The ACTN3 variant could be one of the contributing genes in the heritability of fiber type distribution through its interaction with calcineurin. The aim of this study was to quantify the association between the polymorphism and muscle fiber type distribution and fast-velocity knee extension strength. Ninety healthy young men (18-29 y) were genotyped for ACTN3 R577X. Knee extensor strength was measured isometrically (45 degrees ) and at different dynamic velocities (100-300 degrees /s) on a programmable dynamometer. Twenty-two XX and twenty-two RR subjects underwent a biopsy of the right vastus lateralis muscle. Fiber type composition was determined by immunohistochemistry. Homozygotes for the R allele show significantly higher relative dynamic quadriceps torques at 300 degrees /s, compared with XX carriers (P < 0.05). Fiber type characteristics differed significantly between the two genotype groups. The percentage surface and number of type IIx fibers were greater in the RR than the XX genotype group (P < 0.05), and alpha-actinin-3 protein content is systematically higher in type IIx compared with type IIa fibers (staining intensity ratio IIx to IIa = 1.17). This study shows that the mechanism, by which the ACTN3 polymorphism has its effect on muscle power, might rely on a control function of fiber type proportions.

A New Muscle Architecture Model with Non-Uniform Distribution of Muscle Fiber Types

A New Muscle Architecture Model with Non-Uniform Distribution of Muscle Fiber Types

Comparison Of Calculated Estimates And Muscle Biopsy Of Fiber Type Distribution In Master Athletes

Muscle fiber type distribution may be an important factor in determining physical performance in master athletes. Muscle biopsy is an invasive procedure to determine fiber type distribution and presents many difficulties. Therefore, a non-invasive method using isokinetic dynamometry to estimate fiber type distribution would be advantageous. PURPOSE: To establish a relationship between calculated estimates of Type II muscle fiber proportions and the percentage of Type II fibers determined from muscle biopsies of the vastus lateralis (VL) in master athletes. METHODS: The participants included 12 master athletes (53.5±12.6 years; 70.9±11.3 kg), including 3 women and 9 men. Isokinetic dynamometery (Kin-Com 500H) was used to assess peak torque and power output at 180°/sec as well as a fatigue test which consisted of 50 consecutive contractions at 180°/sec to establish fatigue resistance of the knee extensors. Fat-free thigh mass was determined by dual-energy X-ray absorptiometry (Hologic QDR 1500). Muscle biopsies were taken of the VL on a separate day from the isokinetic testing to determine percentage of Type II fibers. Linear regression analysis was used to estimate the proportion of Type II fibers. RESULTS: The regression model included the following variables: T50 = relative torque after 50 contractions in the fatigue test; P180 = power at an angular velocity of 180°/s; FFTM= fat free thigh mass. Regression estimates of proportion of Type II fibers were not significant at the p<0.05 level (R = 0.377, R2 = 0.142, SEE = 15.21). CONCLUSION: Isokinetic dynamometry is not a valid tool for estimating muscle fiber type composition in Master athletes.

Biochemistry of Anterior, Medial, and Posterior Genioglossus Muscle in the Rat

The tongue plays a vital role in swallowing actions. However, tongue muscles have been understudied, and it is unclear if tongue muscles are homogeneous with respect to muscle fiber-type distribution. We examined myosin heavy chain (MHC) composition of anterior, medial, and posterior sections of the genioglossus muscle (GG) using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in ten adult (9 months old) male Fischer 344/Brown Norway hybrid rats. We found that Type IIx MHC predominated in the anterior, medial, and posterior regions of the GG muscle (p=0.002), followed by IIa, then IIb. The anterior GG contained a significantly greater (p=0.004) proportion of Type IIa than did the medial or posterior regions, while the posterior GG contained a significantly greater (p=0.002) proportion of Type IIb MHC than did the medial or anterior GG. Accordingly, we found variable expression of MHC isoforms across anterior, medial, and posterior portions of the GG muscle, with more fast-contracting isoforms found posteriorly. Because motor control of the tongue requires precise and rapid movements for bolus manipulation and airway protection, variable expression of MHC isoforms along the anteroposterior axis of the GG muscle may be required to efficiently achieve deglutition and maintenance of airway patency.

Fiber type and myosin heavy chain compositions of adult pretarsal orbicularis oculi muscle

Pretarsal orbicularis oculi muscle (POOM) is an important structure of eyelid movement in human. The aim of this study was to investigate fiber histomorphology and myosin heavy chain (MyHC) isoform composition of adult POOM, and to clarify their age-related changes. Eyelid specimens from 58 subjects (age range, 21 to 91 years) were collected during upper blepharoplasty procedures. Serial cross sections of POOM were ATPase-stained and examined under miscroscope. Quantitative measures of muscle fiber size and fiber type distribution were obtained in 35 subjects with adequate fiber cross sections. Relative MyHC isoform contents of POOM were retrieved by gel electrophoresis in all 58 subjects. Examination of the histochemical staining revealed an abundance of type II fiber ( >85%) in human POOM, with more type IIX than IIA fibers. Decreased mean area of all fibers and type IIA fibers were noted in the old group when compared to the young. As for MyHC analysis, the relative content of MyHC isoforms exhibited an order of IIX > IIA > I, and the relative MyHC IIA content showed a negative correlation with age. Comparing with previous studies of limb or masticatory muscles, adult POOM exhibits a unique fiber and MyHC composition, as well as a different aging pattern.