Mammalian skeletal muscle fiber type transitions.

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Histochemical, biochemical, and ultrastructural analyses of single human muscle fibers, with special reference to the C-fiber population.

A muscle biopsy from the vastus lateralis muscle of a strength-trained woman was found to contain an unusual fiber type composition and was analyzed by histochemical, biochemical, and ultrastructural techniques. Special attention was given to the C-fibers, which comprised over 15% of the total fiber number in the biopsy. The mATPase activity of the C-fibers remained stable to varying degrees over the pH range normally used for routine mATPase histochemistry. Although a continuum existed, the C-fibers were histochemically subdivided into three main fiber types: IC, IIC, and IIAC. The IC fibers were histochemically more similar to the Type I, the IIAC were more similar to the Type IIA, and the IIC were darkly stained throughout the pH range. Biochemical analysis revealed that all C-fibers coexpressed myosin heavy chains (MHC) I and IIa in variable ratios. The histochemical staining intensity correlated with the myosin heavy chain composition such that the Type IC fibers contained a greater ratio of MHCI/MHCIIa, the IIAC contained a greater ratio of MHCIIa/MHCI, and the Type IIC contained equal amounts of these two heavy chains. Ultrastructural data of the C-fiber population revealed an oxidative capacity between fiber Types I and IIA and suggested a range of mitochondrial volume percent from highest to lowest such that I greater than IC greater than IIC greater than IIA-C greater than IIA. Under physiological conditions, it appears that the IC fibers represent Type I fibers that additionally express some fast characteristics, whereas the Type IIAC are Type IIA fibers that additionally express some slow characteristics. Fibers expressing a 50:50 mixture of MHCI and MHCIIa (IIC fibers) were rarely found. It is not known whether C-fibers represent a distinct population between the fast- and slow-twitch fibers that is specifically adapted to a particular usage or whether they are transforming fibers in the process of going from fast to slow or slow to fast.

Human skeletal muscle fiber type adaptability to various workloads.

Muscle biopsy specimens were removed from the vastus lateralis muscles of three groups of human subjects: controls, weight lifters, and distance runners. The runners proved to be a unique group with respect to the variables measured (low body weight and percentage body fat, and high VO2 max). Additionally, a histochemical analysis of the biopsy specimens revealed that the runners had a significantly higher percentage of fiber types I and IIC than either the controls or the weight lifters. Using a cryostatic retrieval method, each of the fibers identified histochemically was then analyzed morphometrically using electron microscopy. The results of volume-percent mitochondria demonstrated a strong relationship between the ATPase activity and oxidative potential of the fiber types for all three groups such that the oxidative activity would be ranked I greater than IIA greater than IIB. Irrespective of fiber type, there were significant differences between the groups with regard to muscle-fiber mitochondrial (runners greater than lifters greater than controls) and lipid content (runners greater than controls greater than lifters). The lifters had a significantly greater content of mitochondria than the controls, which may suggest that inactivity rather than the lifting exercise contributes to a low volume-percent mitochondria and a high percentage of type IIB fibers.

Fiber type composition of the vastus lateralis muscle of young men and women.

This study presents data collected over the past 10 years on the muscle fiber type composition of the vastus lateralis muscle of young men and women. Biopsies were taken from the vastus lateralis muscle of 55 women (21.2+/-2.2 yr) and 95 men (21.5+/-2.4 yr) who had volunteered to participate in various research projects. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were classified using mATPase histochemistry, and cross-sectional area was measured for the major fiber types (I, IIA, and IIB). Myosin heavy chain (MHC) content was determined electrophoretically on all of the samples from the men and on 26 samples from the women. With the exception of fiber Type IC, no significant differences were found between men and women for muscle fiber type distribution. The vastus lateralis muscle of both the men and women contained approximately 41% I, 1% IC, 1% IIC, 31% IIA, 6% IIAB, and 20% IIB. However, the cross-sectional area of all three major fiber types was larger for the men compared to the women. In addition, the Type IIA fibers were the largest for the men, whereas the Type I fibers tended to be the largest for the women. Therefore, gender differences were found with regard to the area occupied by each specific fiber type: IIA>I>IIB for the men and I>IIA>IIB for the women. These data establish normative values for the mATPase-based fiber type distribution and sizes in untrained young men and women.

Myosin isoforms, muscle fiber types, and transitions.

Skeletal muscle is an extremely heterogeneous tissue composed of a variety of fast and slow fiber types and subtypes. Moreover, muscle fibers are versatile entities capable of adjusting their phenotypic properties in response to altered functional demands. Major differences between muscle fiber types relate to their myosin complement, i.e., isoforms of myosin light and heavy chains. Myosin heavy chain (MHC) isoforms appear to represent the most appropriate markers for fiber type delineation. On this basis, pure fiber types are characterized by the expression of a single MHC isoform, whereas hybrid fiber type express two or more MHC isoforms. Hybrid fibers bridge the gap between the pure fiber types. The fiber population of skeletal muscles, thus, encompasses a continuum of pure and hybrid fiber types. Under certain conditions, changes can be induced in MHC isoform expression heading in the direction of either fast-to-slow or slow-to-fast. Increased neuromuscular activity, mechanical loading, and hypothyroidism are conditions that induce fast-to-slow transitions, whereas reduced neuromuscular activity, mechanical unloading, and hyperthyroidism cause transitions in the slow-to-fast direction.

Effects of high-intensity resistance training on untrained older men. I. Strength, cardiovascular, and metabolic responses.

Most resistance training studies of older subjects have emphasized low-intensity, short-term training programs that have concentrated on strength measurements. The purpose of this study was, in addition to the determination of strength, to assess intramuscular and transport factors that may be associated with strength increments. Eighteen untrained men ages 60-75 years volunteered for the study; 9 were randomly placed in the resistance-training group (RT), and the other half served as untrained (UT) or control subjects. RT subjects performed a 16-week high-intensity (85-90% 1 repetition maximum (RT]) resistance training program (2 x/wk) consisting of 3 sets each to failure (6-8 repetitions based on 1 RM of 3 exercises): leg press (LP), half squat (HS), and leg extension (LE) with 1-2 minutes rest between sets. Pre- and post- training strength was measured for the 3 training exercises using a 1 RM protocol. Body fat was calculated using a 3-site skinfold method. Biopsies from the vastus lateralis m. were obtained for fiber type composition, cross-sectional area, and capillarization measurements. Exercise metabolism, electrocardiography, and arterial blood pressure were observed continuously during a progressive treadmill test, and resting echocardiographic data were recorded for all subjects. Pre- and post-training venous blood samples were analyzed for serum lipids. Resistance training caused significant changes in the following comparisons: % fat decreased in the RT group by almost 3%, strength improved for all exercises: LE = + 50.4%, LP = + 72.3%, HS = + 83.5%; type IIB fibers decreased and IIA fibers increased; cross-sectional areas of all fiber types (I, IIA, IIB) increased significantly, and capillary to fiber ratio increased but not significantly. No differences were noted for ECG and echocardiographic data. The RT group significantly improved treadmill performance and VO2max. Pre- and post-training serum lipids improved but not significantly. No significant changes occurred in any pre- to post-tests for the UT group. The results show that skeletal muscle in older, untrained men will respond with significant strength gains accompanied by considerable increases in fiber size and capillary density. Maximal working capacity, VO2max, and serum lipid profiles also benefited from high-intensity resistance training, but no changes were observed for HR max, or maximal responses of arterial blood pressure. Older men may not only tolerate very high intensity work loads but will exhibit intramuscular, cardiovascular, and metabolic changes similar to younger subjects.

Effects of high-intensity resistance training on untrained older men. II. Muscle fiber characteristics and nucleo-cytoplasmic relationships.

During growth and repair of skeletal muscle fibers, satellite cells become activated, undergo mitosis, and a daughter nucleus becomes incorporated into the muscle fiber to increase myonuclear numbers. An increase in myonuclei appears to be required for this postnatal growth. This study examined whether muscle fibers of elderly men can hypertrophy with strength training and, if so, whether they have the capacity to incorporate nuclei into the fibers. The sarcoplasmic area associated with each myonucleus was calculated in nine elderly men before and after 16 weeks of strength training, and compared to nine elderly control men. Muscle fiber type changes and myosin heavy chain composition were also compared. All major fiber types (I, IIA, IIB) became significantly larger after training, and a transition of type IIB fibers to IIA occurred with training. The area occupied by each fiber type correlated with myosin heavy chain percentage, and both of these changed similarly with strength training. The cytoplasm-to-myonucleus ratio increased, but not significantly (p = .07), with muscle fiber hypertrophy. Number of myonuclei per fiber and myonuclei per unit length of muscle fiber increased, but not significantly. Cross-sectional areas of the muscle fibers in untrained elderly men were much smaller than in untrained young men (when compared with our earlier studies). Training increased the sizes of the elderly muscle fibers to that of the untrained young men. This hypertrophy of muscle fibers by 30% with training resulted in no change in the cytoplasm-to-myonucleus ratio. This suggests that the myonuclear population continues to adapt to growth stimuli in the elderly muscles.

Influence of complete spinal cord injury on skeletal muscle within 6 mo of injury.

This study examined the influence of spinal cord injury (SCI) on affected skeletal muscle. The right vastus lateralis muscle was biopsied in 12 patients as soon as they were clinically stable (average 6 wk after SCI), and 11 and 24 wk after injury. Samples were also taken from nine able-bodied controls at two time points 18 wk apart. Surface electrical stimulation (ES) was applied to the left quadriceps femoris muscle to assess fatigue at these same time intervals. Biopsies were analyzed for fiber type percent and cross-sectional area (CSA), fiber type-specific succinic dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPDH) activities, and myosin heavy chain percent. Controls showed no change in any variable over time. Patients showed 27-56% atrophy (P = 0.000) of type I, IIa, and IIax+IIx fibers from 6 to 24 wk after injury, resulting in fiber CSA approximately one-third that of controls. Their fiber type specific SDH and GPDH activities increased (P

Skeletal muscle adaptations during early phase of heavy-resistance training in men and women.

An 8-wk progressive resistance training program for the lower extremity was performed twice a week to investigate the time course for skeletal muscle adaptations in men and women. Maximal dynamic strength was tested biweekly. Muscle biopsies were extracted at the beginning and every 2 wk of the study from resistance-trained and from nontrained (control) subjects. The muscle samples were analyzed for fiber type composition, cross-sectional area, and myosin heavy chain content. In addition, fasting blood samples were measured for resting serum levels of testosterone, cortisol, and growth hormone. With the exception of the leg press for women (after 2 wk of training) and leg extension for men (after 6 wk of training), absolute and relative maximal dynamic strength was significantly increased after 4 wk of training for all three exercises (squat, leg press, and leg extension) in both sexes. Resistance training also caused a significant decrease in the percentage of type IIb fibers after 2 wk in women and 4 wk in men, an increase in the resting levels of serum testosterone after 4 wk in men, and a decrease in cortisol after 6 wk in men. No significant changes occurred over time for any of the other measured parameters for either sex. These data suggest that skeletal muscle adaptations that may contribute to strength gains of the lower extremity are similar for men and women during the early phase of resistance training and, with the exception of changes in the fast fiber type composition, that they occur gradually.

Effects of sprint cycle training on human skeletal muscle.

Eleven men sprint trained two to three times per week for 6 wk to investigate possible exercise-induced slow-to-fast fiber type conversions. Six individuals served as controls. Both groups were tested at the beginning and end of the study to determine anaerobic performance and maximal oxygen consumption. In addition, pre- and postbiopsies were extracted from the vastus lateralis muscle and were analyzed for fiber type composition, cross-sectional area, and myosin heavy chain (MHC) content. No significant changes were found in anaerobic or aerobic performance variables for either group. Although a trend was found for a decrease in the percentage of type IIb fibers, high-intensity sprint cycle training caused no significant changes in the fiber type distribution or cross-sectional area. However, the training protocol did result in a significant decrease in MHC IIb with a concomitant increase in MHC IIa for the training men. These data appear to support previous investigations that have suggested exercise-induced adaptations within the fast fiber population (IIb-->IIa) after various types of training (endurance and strength).

Strength and skeletal muscle adaptations in heavy-resistance-trained women after detraining and retraining.

Six women who had participated in a previous 20-wk strength training study for the lower limb detrained for 30-32 wk and subsequently retrained for 6 wk. Seven untrained women also participated in the 6-wk "retraining" phase. In addition, four women from each group volunteered to continue training an additional 7 wk. The initial 20-wk training program caused an increase in maximal dynamic strength, hypertrophy of all three major fiber types, and a decrease in the percentage of type IIb fibers. Detraining had relatively little effect on fiber cross-sectional area but resulted in an increased percentage of type IIb fibers with a concomitant decrease in IIa fibers. Maximal dynamic strength decreased but not to pretraining levels. Retraining for 6 wk resulted in significant increases in the cross-sectional areas of both fast fiber types (IIa and IIab + IIb) compared with detraining values and a decrease in the percentage of type IIb fibers. The 7-wk extension accentuated these trends such that cross-sectional areas continued to increase (nonsignificant) and no IIb fibers could be found. Similar results were found for the nonpreviously trained women. These data suggest that rapid muscular adaptations occur as a result of strength training in previously trained as well as non-previously trained women. Some adaptations (fiber area and maximal dynamic strength) may be retained for long periods during detraining and may contribute to a rapid return to "competitive" form.

The effects of detraining on an elite power lifter. A case study.

Muscle biopsies were taken from the vastus lateralis muscle of an elite power lifter during training and following a 7-month detraining period. The effects of detraining were investigated by combining ultrastructure, histochemistry, and pertinent metabolic data. Muscle fibers were classified ultrastructurally as fast-twitch and slow-twitch. Fast-twitch fibers were histochemically subdivided into a fast-twitch glycolytic (FG) and a fast-twitch oxidative glycolytic (FOG) in order to compare oxidative capacities between biopsy 1 and biopsy 2. The high intensity level of strength training prior to biopsy 1 caused an apparent hypertrophy of all fiber types. Detraining and weight loss resulted in a reversal of the training effect toward "control" values and adjustments in the oxygen delivery system. Atrophy occurred in all of the fiber types and altered the fiber composition of the muscle. A shifting of fibers classified as FG to the more oxidative FOG fibers caused a significant increase in the percent distribution of oxidative fibers (slow-twitch + FOG). The hypothesis is presented that the mitochondrial content (both number and form) of the detrained fiber remains constant (for a strength trained muscle), but as the volume of the fiber decreases the mitochondrial volume percent increases.

Lipid depletion and repletion in skeletal muscle following a marathon.

Intramuscular lipid content was investigated in muscle biopsies from 10 well-trained endurance athletes before, immediately after, and 1, 3, 5, and 7 days after a marathon. Diets were controlled throughout the entire period of the study. Triglyceride content was ultrastructurally determined by the use of stereological methods. The volume percent lipid significantly decreased after the marathon and was lowest at 3 days post-marathon, rising slightly but still 35% lower than the pre-marathon value by 7 days post-marathon. Glycogen granules were abundant and tightly packed in the pre-marathon biopsies, scarce immediately post-marathon, and abundant, but less tightly packed, 7 days post-marathon. Post-marathon fluctuations in the volume percentages of mitochondria indicated possible fluid shifts within the muscle fibers: dehydration immediately post-marathon followed by rehydration with possible edema. Assuming the content of mitochondria remained constant throughout the recovery period, the ratio of the volume percentage of lipid to the volume percentage of mitochondria indicated that lipid content may have reached pre-marathon levels after 7 days post-marathon.

Muscle fiber necrosis associated with human marathon runners.

This study describes the events occurring in exercise-induced muscular necrosis. Biopsies of the gastrocnemius muscles of volunteer human marathon runners were extracted prior to and at intervals for 7 days following a marathon, and investigated ultrastructurally. Most of the preparations, including the pre-marathon samples, showed evidence of muscle fiber necrosis and inflammation. These preparations had many mitochondria, erythrocytes, leukocytes and other phagocytic cells within the extracellular and extravascular spaces. Less frequently observed were Z-line streaming and degeneration, contracture knots, disrupted sarcolemma, presence of erythrocytes within the muscle fibers, and empty basal lamina tubes in which the contents of the fibers and the sarcolemma had broken down to leave only the basal lamina outlining the former fiber. These abnormal conditions were most prevalent at 1 and 3 days after the marathon. These ultrastructural changes are compared and correlated with the reports of clinical manifestations of rhabdomyolysis and myoglobinuria. Because the abnormalities persist for the 7 day duration of these observations, and because many of these were observed in the pre-marathon biopsies, we conclude that both the intensive training for, and the marathon itself, induce inflammation and fiber necrosis which are manifested in the clinical symptoms for rhabdomyolysis and myoglobinuria. The inflammatory reaction that accompanies these activities may be a major factor in post-exercise soreness. The combined influences of training and necrosis are discussed in relation to muscle fiber type compositions of endurance athletes.

A morphometric analysis of human muscle fibers with relation to fiber types and adaptations to exercise.

Muscle biopsies were obtained from the vastus lateralis of 14 male subjects: 3 long distance runners, 2 world class power lifters and 9 active, although not highly trained, individuals used as controls. The fibers were investigated by electron microscopy and the mitochondrial volume percent, lipid volume percent and Z-line width were analyzed morphometrically. With the combined data a direct correlation was found between mitochondrial volume percent and lipid volume percent, lipid volume percent and Z-line width and mitochondrial volume percent and Z-line width. The muscle fibers were classified as slow-twitch oxidative (SO), fast-twitch-oxidative-glycolytic (FOG) and fast-twitch-glycolytic (FG) based on relationships found in the data and well established properties of muscle fiber types. Although no distinct patterns emerged, a good approximation of fiber type characteristics was obtained, and values for volume percent of central mitochondria, volume percent lipid and Z-line width are reported. The fibers classified as SO were characterized by having wide Z-lines, a high mitochondrial volume percent and high lipid volume percent. The fast-twitch fibers (fibers with narrow Z-lines) were separated into 2 groups, those with high mitochondrial volume percent (FOG) and those with low mitochondrial volume percent (FG). No distinction could be made between the fast-twitch subgroups with regard to Z-line width. The fibers from distance runners differed from those from controls by exhibiting a greater capacity for aerobic activity as evidenced by the increased volume percent of mitochondria and lipid in both slow- and fast-twitch fibers. The high strength, anaerobic activity of the world class power lifters was reflected by the low mitochondrial volume percent of many fast-twitch fibers (FG) and the decreased lipid stores in all fibers.

Performance and muscle fiber adaptations to creatine supplementation and heavy resistance training.

PURPOSE: The purpose of this study was to examine the effect of creatine supplementation in conjunction with resistance training on physiological adaptations including muscle fiber hypertrophy and muscle creatine accumulation. METHODS: Nineteen healthy resistance-trained men were matched and then randomly assigned in a double-blind fashion to either a creatine (N = 10) or placebo (N = 9) group. Periodized heavy resistance training was performed for 12 wk. Creatine or placebo capsules were consumed (25 g x d(-1)) for 1 wk followed by a maintenance dose (5 g x d(-1)) for the remainder of the training. RESULTS: After 12 wk, significant (P < or = 0.05) increases in body mass and fat-free mass were greater in creatine (6.3% and 6.3%, respectively) than placebo (3.6% and 3.1%, respectively) subjects. After 12 wk, increases in bench press and squat were greater in creatine (24% and 32%, respectively) than placebo (16% and 24%, respectively) subjects. Compared with placebo subjects, creatine subjects demonstrated significantly greater increases in Type I (35% vs 11%), IIA (36% vs 15%), and IIAB (35% vs 6%) muscle fiber cross-sectional areas. Muscle total creatine concentrations were unchanged in placebo subjects. Muscle creatine was significantly elevated after 1 wk in creatine subjects (22%), and values remained significantly greater than placebo subjects after 12 wk. Average volume lifted in the bench press during training was significantly greater in creatine subjects during weeks 5-8. No negative side effects to the supplementation were reported. CONCLUSION: Creatine supplementation enhanced fat-free mass, physical performance, and muscle morphology in response to heavy resistance training, presumably mediated via higher quality training sessions.

Correlation between myofibrillar ATPase activity and myosin heavy chain composition in single human muscle fibers.

Single human muscle fibers were analysed using a combination of histochemical and biochemical techniques. Routine myofibrillar adenosine triphosphatase (mATPase) histochemistry revealed a continuum of staining intensities between the fast fiber types IIA and IIB (type IIAB fibers) after preincubation at pH 4.6. Electrophoretic analysis of single, histochemically-identified fibers demonstrated a correlation between the staining intensity and the myosin heavy chain (MHC) composition. All fibers classified as type I contained exclusively MHCI and all type IIA fibers contained only MHCIIa. Type IIAB fibers displayed variable amounts of both MHCIIa and MHCIIb; the greater the staining intensity of these fibers after preincubation at pH 4.6, the greater the percentage of MHCIIb. Those fibers histochemically classified as type IIB contained either entirely MHCIIb or, in addition to MHCIIb, a small amount of MHCIIa. These data establish a correlation between the mATPase activity and MHC content in single human muscle fibers.

Human skeletal muscle fiber types: delineation, development, and distribution.

This brief review attempts to summarize a number of studies on the delineation, development, and distribution of human skeletal muscle fiber types. A total of seven fiber types can be identified in human limb and trunk musculature based on the pH stability/lability of myofibrillar adenosine triphosphatase (mATPase). For most human muscles, mATPase-based fiber types correlate with the myosin heavy chain (MHC) content. Thus, each histochemically identified fiber has a specific MHC profile. Although this categorization is useful, it must be realized that muscle fibers are highly adaptable and that innumerable fiber type transients exist. Also, some muscles contain specific MHC isoforms and/or combinations that do not permit routine mATPase-based fiber typing. Although the major populations of fast and slow are, for the most part, established shortly after birth, subtle alterations take place throughout life. These changes appear to relate to alterations in activity and/or hormonal levels, and perhaps later in life, total fiber number. Because large variations in fiber type distribution can be found within a muscle and between individuals, interpretation of data gathered from human muscle is often difficult.

Molecular basis of the phenotypic characteristics of mammalian muscle fibres.

Adult mammalian skeletal muscle fibres can be separated into two distinct groups, fast and slow. Within each group there is a continuum of metabolic enzyme activity levels. In addition there are fast and slow isoforms of various myofibrillar proteins such as myosin, tropomyosin and troponin. These proteins are multimeric and multiple isoforms of their subunits assemble to create a continuum of subtypes within each major group. Fibres which coexpress both fast and slow subunit isoforms have an increased number of possible isoform combinations such that an entire spectrum of fibre 'types' is found between the two extremes, fast and slow. Numerous myosin heavy chain and fast troponin T isoforms further increase the diversity of muscle fibres. Such cellular diversity helps to explain the dynamic nature of skeletal muscle. Each individual fibre is able to respond to various functional demands by appropriate changes in its phenotypic expression of specific proteins.