Myosin heavy chain isoforms in single fibres from m. vastus lateralis of soccer players: effects of strength-training

The myosin heavy chain (MHC) composition of single fibres (n= 2171) was analysed with an electrophoretic technique in biopsy material from m. vastus lateralis of two groups of soccer players before and after a 3-month period of either strength- (n= 8) or non-training (control) (n= 6). Traditional myofibrillar ATPase histochemistry demonstrated a decrease in type IIA fibres with strength-training (35.4 ± 2.1 vs. 26.7 ± 2.4% (P < 0.05)). This was not observed in the non-training group (25.7 ± 4.6 vs. 23.8 ± 1.7%). One-dimensional electrophoresis on muscle homogenates showed no significant change in the amount of MHC isoforms in either of the two groups. The MHC isoform I IB was undetectable in all but three samples. No changes in the proportions of fibres containing any of the MHC isoforms were observed. Fibres containing only MHC isoform I IB were found in very small numbers (only 11 out of 2171). Before the experimental period, between 6 and 10% histochemical type IIB fibres were found in both groups. This was identical with the proportion of fibres showing co-existence of MHC isoforms IIA and IIB, but in contrast to the very few fibres containing only MHC isoform IIB. This suggests that nearly all histochemical type IIB fibres of the soccer players display co-existence of both MHC isoform IIA and IIB. No major change in the muscle fibre area of the two groups was observed. The strength-training group increased mean power output in a short-term dynamic knee-extensor test after strength-training (119 ± 6.6 vs. 136.5 ± 4.7 W (P < 0.01)), whereas no change was observed in the non-training group (151.2 ± 4.5 vs. 148.4 ± 6.5 W).     

Regional differences in fiber characteristics in the rat temporalis muscle

The behavioral differences in muscle use are related to the fiber type composition of the muscles among other variables. The aim of this study was to examine the degree of heterogeneity in the fiber type composition in the rat temporalis muscle. The temporalis muscle was taken from 10‐week‐old Wistar strain male rats (n = 5). Fiber types were classified by immunohistochemical staining according to their myosin heavy chain content. The anterior temporalis revealed an obvious regional difference of the fiber type distribution, whereas the posterior temporalis was homogeneous. The deep anterior temporalis showed a predominant proportion of type IIA fibers and was the only muscle portion displaying slow type fibers (< 10%). The other two muscle portions, the superficial anterior and posterior temporalis, did not differ significantly from each other and contained mainly type IIB fibers. Moreover, the deep anterior temporalis was the only muscle portion showing slow type fibers (< 10%). In the deep portion, type IIX fibers revealed the largest cross‐sectional area (1943.1 ± 613.7 µm²), which was significantly (P < 0.01) larger than those of type IIA and I + IIA fibers. The cross‐sectional area of type IIB fibers was the largest in the remaining two muscle portions and was significantly (P < 0.01) larger than that of type IIX fibers. In conclusion, temporalis muscle in rats showed an obvious heterogeneity of fiber type composition and fiber cross‐sectional area, which suggests multiple functions of this muscle.     

Muscle fiber types of women after resistance training — Quantitative ultrastructure and enzyme activity

Muscle biopsies of the vastus lateralis muscle taken before and after 18 weeks of resistance training were compared by preparing frozen cross sections for electron microscopy and using adjacent sections for fiber typing by myosin ATPase activity. Quantitative ultrastructural changes were observed in histochemically-identified muscle fiber types of twelve young women who underwent the training. The percentage of type IIB fibers decreased and IIA fibers increased. The cross-sectional area of all major fiber types increased with training. The absolute volume of myofibrils, intermyofibrillar space, and mitochondria increased with training for most major fiber types (type I, IIA and IIAB), but the relative volume percentages were not significantly changed because of corresponding fiber hypertrophy. Mean mitochondrial size for types I and IIA and myofibril size for types IIC and IIB increased significantly with training. The capillary number per fiber and density did not change with training. Activity levels were measured for selected glycolytic and oxidative enzymes. Cytochrome oxidase and hexokinase increased significantly with training, while creatine kinase, citrate synthase, phosphofructokinase, glyceraldehyde phosphate dehydrogenase and hydroxyacyl CoA dehydrogenase enzymes were not significantly altered. The results suggest that this type of high-repetition resistance training causes the intracellular components of all fiber types to increase proportionally with an increase in fiber size. In addition, the enzyme analysis indicates the muscle as a whole may increase its oxidative phosphorylation capacity in conjunction with the decreased percentage of type IIB fibers. Present address: Department of Medicine, University of California at San Diego, School of Medicine, San Diego, CA, 92103, USA     

Skeletal muscle adaptations in elastic resistance-trained young men and women

Skeletal muscle adaptations (fiber-type composition, cross-sectional area, myosin heavy chain (MHC) content, and capillarity) were assessed in the vastus lateralis muscle of young men and women after 8 weeks of training with the Sportcord, an elastic resistance device. Ten men [mean (SD) age 20 (1.1) years] and 13 women [20 (1.2) years] performed two sets each to failure of single leg squats and leg extensions at approximately 50 repetitions x min(-1). Biopsy samples were taken from the right vastus lateralis muscle before and after training. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were classified using myofibrillar ATPase histochemistry. Training with the Sportcord caused a small, but significant, increase in one-repetition maximum using free weights and a large increase in repetitions to failure. In addition, elastic resistance training caused an increase in the percentage of fibers classified as type IIAB for both men and women, and a decrease in the percentage of type IIB fibers in the men. MHC analysis supported these findings (a significant increase in the percentage of MHCIIa for the men). The cross-sectional areas ofboth the type I and IIAB + IIB fibers increased after training for the men, whereas no area changes were found for the women. The capillary:fiber ratio and capillary contacts per fiber type increased significantly for the men, and similar trends were noted for the women. Capillary density did not change in either the men or the women. These data suggest minor changes in fiber type composition (IIB-->IIAB), fiber size, and capillarization following short-term training with elastic resistance. Although muscular changes did occur using the Sportcord, the extent of these changes was less than those reported previously for short-term resistance-training programs using free weights.     

Segmental Distribution of Myosin Heavy Chain Isoforms Within Single Muscle Fibers

Despite many studies looking at the distribution of myosin heavy chain (MHC) isoforms across a transverse section of muscle, knowledge of MHC distribution along the longitudinal axis of a single skeletal muscle fiber has been relatively overlooked. Immunocytochemistry was performed on serial sections of rat extensor digitorum longus (EDL) muscle to identify MHC types I, IIA, IIX, IIY, and IIB. Sixteen fascicles which contained a total of 362 fibers were randomly and systematically sampled from the three EDL muscles. All MHC type I and type II isoforms were expressed. Segmental expression occurred within a very limited segment. MHC isoform expression followed the accepted traditional order from I?IIA?IIX?IIB, however, in some samples expression of an isoform was circumvented from IIB to I or from I to IIB directly. Segmental distribution of MHC isoforms along a single muscle fiber may be because of the myonuclear domain. Anat Rec, 300:1636–1642, 2017. © 2017 Wiley Periodicals, Inc.     

Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of a group of male sprint athletes (n= 6) was analysed, before and after a three months period of intensive strength- and interval-training, using a sensitive gel electrophoretic technique. Significant improvements were observed after training in almost all of a series of performance tests. After training the sprinters revealed a decrease in fibres containing only MHC isoform I (52.0 ±3.0% vs. 41.2 ±4.7% (mean±SE) (P < 0.05)) and an increase in the amount of fibres containing only MHC isoform IIA (34.7 ±6.1% vs. 52.3 ±3.6% (P < 0.05)). Fibres showing co-existence of MHC isoforms IIA and IIB decreased with training (12.9 ± 5.0% vs. 5.1 ±3.1%, (P < 0.05)). Only one out of 1000 fibres analysed contained only MHC isoform IIB. In contrast, a higher amount of type IIB fibres (18.8 ± 3.6% vs. 10.5 ± 3.9%, (P < 0.05)) was observed with myofibrillar ATPase histochemistry. The majority of histochemically determined type IIB fibres of sprinters seems therefore to contain both MHC isoforms IIA and IIB. Sprint-training appears to induce an increased expression of MHC isoform IIA in skeletal muscles. This seems related to a bi-directional transformation from both MHC isoforms I and IIB towards MHC isoform IIA.     

Changes in myosin heavy chain profile of mature regenerated muscle with endurance training in rat

The objective of the present study was to examine the response of fast-twitch muscle to endurance training long after the muscle had regenerated from toxin injury. Seventeen male Wistar rats were randomly assigned to a sedentary (S, n = 10) or a trained group (T, n = 7). Endurance training by treadmill running (5 days week(-1), 30 m min(-1), 7% grade, 2 h day(-1) for 5 weeks) was initiated 5 weeks after myofibre degeneration was induced in the right extensor digitorum longus muscle (EDL) by two injections of 0.2 mL of the unfractionated venom from Naja nigricollis snake. Gel electrophoresis analyses showed that training alone resulted in a 140% increase in type IIX myosin heavy chain (MHC) (P < 0.01) and a slight decrease in type IIB MHC (-14% P < 0.05). Regeneration alone induced an increase in both type IIA and IIX MHC expression (103%, P < 0.05, and 131%, P < 0.01, respectively), and a concomitant decrease in the percentage of type IIB MHC (P < 0.05). The shift from type IIB toward type IIA MHC composition observed in regenerated muscles of T rats resulted not only from an additive, but from a cumulative effect of training and regeneration. Immunohistochemical analysis of MHC content in individual fibres showed similar changes. These data suggest that the impact of endurance training on fast-type MHCs was more marked in mature regenerated muscles than in regenerating ones, and provide evidence of the heightened plasticity of fully regenerated muscles to repeated exercise.     

Muscle hypertrophy and fast fiber type conversions in heavy resistance-trained women

Summary Twenty-four women completed a 20-week heavy-resistance weight training program for the lower extremity. Workouts were twice a week and consisted of warm-up exercises followed by three sets each of full squats, vertical leg presses, leg extensions, and leg curls. All exercises were performed to failure using 6–8 RM (repetition maximum). Weight training caused a significant increase in maximal isotonic strength (1 RM) for each exercise. After training, there was a decrease in body fat percentage (p<0.05), and an increase in lean body mass (p<0.05) with no overall change in thigh girth. Biopsies were obtained before and after training from the superficial portion of the vastus lateralis muscle. Sections were prepared for histological and histochemical examination. Six fiber types (1, IC, IIC, IIA, IIAB, and IIB) were distinguished following routine myofibrillar adenosine triphosphatase histochemistry. Areas were determined for fiber types 1, IIA, and IIAB + IIB. The heavy-resistance training resulted in significant hypertrophy of all three groups: I (15%), IIA (45%), and IIAB + IIB (57%). These data are similar to those in men and suggest considerable hypertrophy of all major fiber types is also possible in women if exercise intensity and duration are sufficient. In addition, the training resulted in a significant decrease in the percentage of IIB with a concomitant increase in IIA fibers, suggesting that strength training may lead to fiber conversions.     

Fiber differentiation of the human laryngeal muscles using the inhibition reactivation myofibrillar ATPase technique

Summary The aim of the present study was to further subdivide the type II fibers of the human thyroarytenoid and posterior cricoarytenoid muscles by means of a modified myosin ATPase reaction. In order to understand the functioning of these highly strained muscles better, it is important to know the respective percentage of fatigue-resistant type IIA fibers and fatigable type IIB fibers. The material comprised the larynges of seven laryngectomized males aged between 45 and 70 years and four laryngectomized females aged between 39 and 72 years. After having been frozen in nitrogen, 10-m-thick sections were cut from the laryngeal muscles in a cryostat. The pH-lability of the enzyme that can be utilized in a classical myosin ATPase reaction permits a differentiation between fiber types I, IIA and IIB. Evidently, this is not possible with every human muscle. The fiber types IIA and IIB of the thyroarytenoid and the posterior cricoarytenoid muscles could be clearly distinguished by means of the inhibition reactivation myofibrillar ATPase technique. Using this method, the myosin ATPase enzyme was initially inhibited by hydroxymer curibenzoate and subsequently reactivated by cysteine. Regarding the incidence of type I and IIA fibers, there was a statistically significant difference between the thyroarytenoid and the posterior cricoarytenoid muscles. The type IIA fiber content was statistically significantly higher in the arytenoid muscle than in the posterior cricoarytenoid muscle. The percentage of type IIB fibers was low, not only in the thyroarytenoid muscle and the posterior cricoarytenoid muscle but also in the other laryngeal muscles. The share of fiber types I, IIA and IIB in the thyroarytenoid muscles varied greatly from one patient to another. This was also true for the other laryngeal muscles. This aspect may be especially significant with regard to an individual's vocal character and vocal fatigability under stress.     

Histochemical and biochemical changes in human skeletal muscle with age in sedentary males,age 22–65 years

Biopsies for histochemical and biochemical analyses were taken from the vastus lateralis muscle of 55 untrained, healthy male subjects from 22 to 65 years of age. Fibre type distribution changed towards a decrease in the percentage of type II fibres, both in type IIA and type IIB fibres, whereas type IIB/IIA fibre ratio and type IIC percentage did not change with increasing age. It was found that the type IIB/flA fibre ratio was inversely related to type I fibres, i.e. subjects rich in type I fibres had a relatively smaller proportion of type IIB fibres. Fibre area determinations revealed a selective decrease in type II fibre area. Consequently, the type II/I fibre area ratio and relative type II fibre area decreased. No changes in the specific activities of Mg²⁺ stimulated ATPase and myokinase were observed, while the activity of lactate dehydrogenase (LDH) was higher in the youngest groups than in the oldest. LDH isozyme pattern shifted towards a decrease in percentage distribution of the muscle specific isozymes and a corresponding decrease in muscle specific activity while the activity of the heart specific isozymes did not change.     

Statistical analysis of fiber area in human skeletal muscle.

Previous research has indicated that 50 fiber measurements per individual for type I and II fibers would be sufficient to characterize the fiber areas. This study replicated the work of McCall et al. (1998) using the three major fiber types (I, IIA, and IIB) and sampling larger populations of fibers. Random blocks of fibers were also examined to investigate how well they correlated with the overall mean average fiber area. Using random blocks of 50 fibers provided an accurate reflection of the type IIB fibers (r = 0.96-0.98) but not for the type I (r = 0.85-0.94) or IIA fibers (r = 0.80-0.91). Type I fibers were consistently reflected by a random block of 150 fibers (r = 0.95-0.98) while type IIA fibers required random blocks of 200 fibers (r = 0.94-0.98), which appeared to provide an accurate reflection of the cross-sectional area. These results indicate that for a needle biopsy different numbers of fibers are needed depending on the fiber type to accurately characterize the mean fiber population.     

Immunohistochemistry of kangaroo rat hindlimb muscles

Kangaroo rats (Dipodomys spp.) use specialized bipedal hopping like that of kangaroos. In contrast to kangaroos that have elastic tendons capable of storing energy, kangaroo rats have inelastic tendons that are unable to store large amounts of energy. Thus, the musculature of the ankle joint provides the greatest power contribution to kangaroo rat hopping. Skeletal muscle can be characterized by several fiber types, including slow twitch (Type I) and fast twitch (Type II) fibers. Fast fibers are found in higher concentration in muscles that perform quick, dynamic movements, whereas slow fibers are found in higher proportion in muscles that perform slow, endurant movements. Using fiber type specific antibodies, we identified four pure (Types I, IIA, IIB, and IIX) and two hybrid (Types I/IIA and IIA/IIX) fiber types in six hindlimb muscles from three kangaroo rats (Dipodomys merriami) to investigate the relationship between fiber composition and hindlimb muscle function. Hindlimb muscles (except soleus) were dominated by Type IIB fibers, which were largest in cross-sectional area, and are known to be best suited for rapid and explosive movements. Oxidative Type IIA and Type IIX fibers were found at moderate concentrations and likely function in maintaining continual saltatory locomotion. Thus, kangaroo rats can use these two fiber type populations as “gears” for both endurant and explosive behaviors.     

Skeletal muscle fiber area alterations in two opposing modes of resistance-exercise training in the same individual

Summary The purpose of this study was to observe fiber area changes that might occur in the same subject from two opposing resistance-exercise training regimes isolating the quadriceps muscle group. Twelve college-age men divided into two groups participated in each of two 7.5-week regimens; one performed a muscular strength program (high-resistance, low-repetition) 4 days a week on a resistance-exercise apparatus, while the other performed a muscular endurance (low-resistance, high-repetition) program. After a 5.5-week hiatus, the groups changed regimens for the second 7.5 weeks. Closed-needle biopsies of the dominant vastus lateralis and isokinetic dynamometer evaluations were made before and at the end of each training period. The muscle samples were analyzed for area changes. In both groups the initial exercise stimulus, whether for strength or endurance, increased the area of fibers of all three major types (I, IIA, and IIB). Subjects doing strength exercises as their second treatment showed a further increase in the area of type I and IIB fibers, whereas those doing endurance exercises showed a decrease in all fiber types. From the first to the last biopsy all fiber areas were decreased (P<0.05) in the control-strength-endurance group and increased (P<0.05) in the control-endurance-strength group. These results suggested that endurance exercise preceding strength exercise in an isolated muscle group maximized fiber area adaptations to exercise stress. Consideration should thus be given in exercise and rehabilitation programs to the muscle cellular adaptations evidenced in different orders of training, particularly if muscular strength is considered important.     

Adaptation of rat extensor digitorum longus muscle to gamma irradiation and overload

The right extensor digitorum longus (EDL) muscle of growing male rats was overloaded by ablation of its synergist tibialis anterior (TA) muscle. Four weeks later, the overloaded muscle was heavier and contained larger type IIA, IIX and IIB fibres than either untreated contralateral muscle or control muscle from an untreated animal. The myonuclear-to-myoplasmic volume ratio was maintained in the overloaded muscle. Overloaded EDL muscle, previously subjected to a dose of irradiation sufficient to sterilise satellite cells, and EDL muscle which had been only irradiated, were significantly lighter and contained significantly smaller fibres than controls, though a significant amount of normal EDL muscle growth did occur following either treatment. The myonuclear-to-myoplasmic volume ratio of the irradiated muscles was smaller than in controls. Overloaded muscle, with or without prior irradiation, possessed a smaller proportion of fibres containing IIB myosin heavy chain (MHC) and a larger proportion of fibres containing IIA and IIX MHC; a significant percentage of these fibres coexpressed either type IIA and IIX MHC or type IIX and IIB MHC. Thus in the absence of satellite cell mitosis, muscles of young rats possess a limited capacity for normal growth but not for compensatory hypertrophy. Adaptations in MHC gene expression to chronic overload are completely independent of satellite cell activity.     

Effects of 14 days of microgravity on fast hindlimb and diaphragm muscles of the rat

The purpose of the present study was to determine the effects of 14 days of microgravity on specific rat fast-twitch muscles, and to compare these data with previous data from rat fast-twitch muscles exposed to microgravity for 10 days (Kraemer et al. 2000). Hindlimb muscles containing predominately fast fibers [extensor digitorum longus (EDL), superficial “white” (GSW) and deep “red” (GDR) gastrocnemius] and the diaphragm (DIA) were removed from flight and ground-based control animals and analyzed for: muscle mass, fiber type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content. Gravitational unloading for 14 days caused significant decreases in muscle mass (8–9%) and cross-sectional area of almost all fiber types (10–35%) from both EDL and gastrocnemius muscles. However, microgravity had little effect on fiber type composition in these muscles with significant changes occurring only in the EDL type IID fiber population (9.5% decrease). Similarly, relative MHC isoform content was only slightly altered by exposure to microgravity (increased content of MHCIIa in flight EDL). No changes in area, fiber type percentages, or MHC isoform content were detected in the DIA following the 14-day spaceflight. Similar to data gathered following a 10-day spaceflight (Kraemer et al. 2000), the 14-day flight did not appear to cause significant slow-to-fast (I → IIA) or fast-to-faster (IIA → IID → IIB) transformations in hindlimb muscles containing predominantly fast-twitch fibers. However, the longer period of gravitational unloading did result in additional loss in muscle fiber cross-sectional area with involvement of more major fiber types.     

Myosin heavy chain isoform transformation in single fibres from m. vastus lateralis in spinal cord injured individuals: Effects of long-term functional electrical stimulation (FES)

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of five spinal-cord-injured (SCI) individuals was analysed by Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) before, and after 6 and 12 months of functional electrical stimulation (FES)-training, administrated for 30 min three times per week. Prior to FES training 37.2% of the fibres contained only MHC HB, 21.2% only MHC IIA, and 40.7% co-expressed MHC IIA and MHC IIB. After 6 months of FES-training the number of fibres containing only MHC IIB was reduced to 2.6% (P < 0.05), the number of fibres containing only MHC IIA was increased to 44.3 (P < 0.05), and the number of fibres co-expressing MHC IIA and MHC HB was 50.9% (ns). After 12 months almost all fibres (91.2%,P < 0.05) contained only MHC IIA. The number of fibres containing only MHC IIB was 2.3 % and the fibres co-expressing MHC HA and HB had decreased to 4.6% (P < 0.05). The amount of fibres containing only MHC I never exceeded 0.5%. Likewise, the number of fibres co-expressing MHC I and MHC IIA was below 2% throughout the study period. In total, the MHC composition of 1596 single fibres was determined. This study shows that FES-training of paralysed human skeletal muscle administrated over a prolonged period of time, can lead to a marked switch in MHC expression from about equal amounts of MHC HA and MHC HB to an almost total dominance of MHC HA.     

How to calculate human muscle fibre areas in biopsy samples—methodological considerations

Cross-sectional muscle fibre areas (type I, IIA and IIB) were determined in duplicate biopsies from the left vastus lateralis (n=11) and in biopsies from right and left vastus lateralis (n=8).The SD for the difference in means between duplicate biopsies was 510 μm² for type I, 1020 μm² for type II A and 860 μm² for type II B.Expressed as coefficient of variation (CV) these SD constituted 10, 15 and 15%, respectively. The variation in fibre size within a sample was considerably less than the variation between samples on the assumption that at least 15–20 areas of each fibre type were measured per sample. No difference in mean fibre area for type I, IIA and IIB fibres was obtained between the right and left muscle. Several artefacts due to the sampling and preparing procedures are discussed and a method for determining muscle fibre areas in biopsy samples is suggested.     

Cell size and oxidative enzyme activity of rat biceps brachii and triceps brachii muscles

Fiber-type distributions, cross-sectional areas, and oxidative enzyme activities of type-identified fibers in the biceps brachii and triceps brachii muscles of 10-week-old male Wistar rats were determined and compared with those in the soleus and plantaris muscles. The soleus and plantaris muscles consisted of two (I and IIA) and three (I, IIA, and IIB) types of fibers, respectively. The deep regions of the biceps brachii and triceps brachii muscles consisted of three types of fibers, while the surface regions of those muscles consisted only of type IIB fibers. The cross-sectional areas of fibers in the deep and surface regions of the plantaris muscle and in the deep regions of the biceps brachii and triceps brachii muscles were in the rank order of type I = type IIA < type IIB, while the oxidative enzyme activities of fibers in the deep and surface regions of the plantaris muscle and in the deep region of the triceps brachii muscle were in the rank order of type IIB < type I = type IIA. These results indicate that fiber-type distributions, cross-sectional areas, and oxidative enzyme activities are muscle type- and region-specific. Therefore, the metabolic and functional significance of the biceps brachii and triceps brachii muscles, especially in the surface regions, where only type IIB fibers are located, in those muscles, appears to be determined by their fibers having larger cells and lower oxidative enzyme activity.     

Clenbuterol-induced fiber type transition in the soleus of adult rats

This study examined the effects of 6 weeks of treatment with theβ ₍₂₎-adrenoceptor agonist, clenbuterol, on the soleus muscle of adult female Sprague-Dawley rats. Animals (4 months old) were divided into two groups: clenbuterol treated (CL,n=7) (2 mg kg⁻¹ body mass injected subcutaneously every other day), and control (CON,n=7) (injected with isotonic saline). Post-treatment body weights were ≈ 5% greater in the CL group compared to CON (P<0.05). Polyacrylamide gel electrophoresis (SDS-PAGE) of soleus myofibrillar protein indicated a clenbuterol-induced decrease (P<0.05) in the relative percentage of type I myosin heavy chain (MHC) with a concomitant increase (P<0.05) in type IIdx MHC, while the proportion of type IIa MHC was unaffected. ATPase fiber typing revealed increases (P<0.05) in the proportion of type II fibers expressed both as a percentage of total fiber number and total cross-sectional area (CSA). Finally, mean type II fiber CSA was ≈25% greater (P<0.05) in the CL groups as compared to the CON group. These data indicate that clenbuterol treatment results in alterations in the MHC phenotype and an increased proportion of type II fiber CSA in the soleus of adult rats. These observations were due to an increase in the total number of type II fibers, as well as hypertrophy of these fibers. Thus, the relative increase in the number of histochemically determined type II fibers and the emergence of the normally unexpressed type IIdx MHC isoform in the soleus suggest a clenbuterol-induced transition of muscle fiber phenotype as well as selective hypertrophy of the type 11 fibers.