Contrasts in muscle and myofibers of elite male and female bodybuilders

Muscle cross-sectional area (CSA), fiber area, and fiber number were determined from the biceps brachii of eight elite male bodybuilders (MB) and five elite female bodybuilders (FB) who had similar training characteristics. Biceps CSA was obtained from computer tomographic scanning and corrected for noncontractile tissue. Biceps CSA was twofold greater in MB relative to FB and strongly correlated to lean body mass (R = 0.93). Biceps CSA expressed per kilogram lean body mass (LBM) or per centimeter body height (BH) was 35% greater in MB compared with FB. Most of the gender difference in muscle CSA was because of greater absolute mean fiber areas in MB (9,607 microns2) relative to FB (5,386 microns2); however, MB also had a significantly greater population of small type II fibers (less than 2,000 microns2) compared with FB. Type II fiber area/LBM averaged 1.6-fold greater in MB compared with FB; however, type I fiber area/LBM was similar between groups. Biceps CSA was positively correlated to fiber CSA (R = 0.75) and fiber number (R = 0.55). This suggests that adaptations to resistance training may be complex and involve fiber hypertrophy and fiber number (e.g., proliferation). Alternatively, since the muscle characteristics before training are not known, these apparent adaptations might be genetically determined attributes.     

Effects of resistance training on elbow flexors of highly competitive bodybuilders.

The influence of gender on muscular adaptation of the elbow flexors to 24 wk of heavy resistance training was studied in five male bodybuilders (MB) and five female bodybuilders (FB) who were highly competitive. Muscle cross-sectional area (CSA), fiber area, and fiber number were determined from the biceps brachii, and voluntary elbow flexor torque was obtained at velocities of contraction between 0 and 300 degrees/s. Biceps and flexor CSA was 75.8 and 81% greater, respectively, in MB than in FB, but muscle CSA was not significantly altered by the training program in either group. Because estimated fiber number and the volume density of nonmuscle tissue were similar in MB and FB, most of the gender difference in muscle CSA appeared to be due to greater absolute mean fiber areas in MB (10.51 and 10.68 x 10(3) microns 2 pre- and posttraining, respectively) than in FB (5.33 and 5.96 x 10(3) microns 2 pre- and posttraining, respectively). In neither MB nor FB did fiber type achieve further hypertrophy during the 24-wk training program. These data suggest that the extent of any change in muscle mass or muscle fiber characteristics is minimal after a bodybuilder of either gender has attained a high degree of muscle mass and a highly competitive status.     

Architectural and histochemical analysis of the biceps brachii muscle of the horse.

The biceps brachii of horses is a complex muscle subdivided into two heads which may subserve distinct functions. The lateral head contains a large percentage of type I myofibers. This region is largely composed of short fibers (5-7 mm long) arranged in a pinnate fashion and heavily invested with connective tissue. The medial head contains fewer type I fibers and is composed of relatively longer myofibers (15-20 mm long), also arranged in a pinnate fashion but less heavily invested with connective tissue. It is hypothesized that the lateral muscle head of biceps brachii contributes to the postural role of the muscle in the forelimb passive stay apparatus. The medial head, with its longer fibers and generally fast fiber population may be most important during dynamic activity such as walking, trotting and running.     

Voluntary strength and muscle characteristics in untrained men and women and male bodybuilders

Eight untrained women (F), 13 untrained men (M), and 11 male bodybuilders (BB) did maximal elbow flexions on an isokinetic dynamometer at velocities of 30, 120, 180, 240, and 300 degrees/s, from which impact torque (IT), peak torque (PT), and work (W) were measured. Biceps and total flexor cross-sectional area (CSA) were measured by computerized tomographic scanning. Muscle fiber area, fiber composition, and collagen volume density were determined from single needle biopsies of biceps brachii. Biceps fiber number was estimated as the ratio of biceps CSA (corrected for connective tissue) to mean fiber area. PT and W decreased at higher velocities in M and BB but not in F; consequently, the correlation between CSA and PT and W was lower at 300 degrees/s (r = 0.58, 0.60) than 30 degrees/s (r = 0.80, 0.79). The ratio of PT to flexor CSA was similar in all groups at 30 degrees/s, whereas F had greater ratios than M and BB at the remaining velocities. F had greater W/CSA ratios than M and BB at all velocities. IT increased at higher velocities in all groups; the increase was greater in F and M than in BB. In contrast to PT and W, the correlation between IT and CSA was greater at 300 degrees/s (r = 0.67) than 30 degrees/s (r = 0.58), and there were no differences among groups in the IT/CSA ratios. Flexor CSA correlated negatively with the ratio of IT, PT, and W to CSA. Muscle fiber composition failed to correlate with any measure of strength. M and BB had greater biceps area, fiber number, and fiber area than F.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interfiber tension transmission in series-fibered muscles of the cat hindlimb

Several muscles of the cat hindlimb, including biceps femoris and tenuissimus, are composed of short, in-series muscle fibers with tapered intrafascicular terminations. Tension generation and transmission within such muscles requires that active fibers should be mechanically coupled in series via myomyous junctions, specialized connective tissue attachments, or the endomysium. This report establishes that the tapered fibers of the cat biceps femoris and tenuissimus muscles have insignificant numbers of either myomyous or specialized connective tissue junctions. Tension appears to be transmitted in a distributed manner across the plasmalemma of the tapered (and probably the non-tapered) portions of the fibers to the connective tissue of the endomysium, which is therefore an essential series elastic element in these muscles. Subplasmalemmal dense plaques were identified and may play a role in transmembrane force transmission. In addition to the endomysium, passive muscle fibers may also serve to transmit tension between active fibers, and therefore should also be considered to be series elastic elements.     

Effects of prolonged undernutrition on structure and function of the diaphragm

The present study examined the effect of prolonged undernutrition on diaphragmatic structure and force-generating ability. Studies were performed on 58 Syrian hamsters in which the feed was reduced by 33% for a 4-wk period. Sixty animals fed a similar diet ad libitum served as controls. Diaphragm muscle structure was assessed from its mass (wet and dry weight), thickness, fiber composition, and fiber size. Isometric force produced in vitro by isolated muscle strips in response to electrical stimulation of the phrenic nerve was examined over a range of muscle lengths (length-tension relationship). In undernourished animals, body weight decreased 25 +/- 5%. Diaphragm wet and dry weight, muscle thickness, and the cross-sectional area of fast-glycolytic (FG) and fast-oxidative (FO) fibers were significantly less in undernourished than control animals and correlated with reductions in body weight. The cross-sectional area of slow-oxidative (SO) fibers was the same in the two groups. The percentage of FG fibers in undernourished animals was decreased slightly and the percentage of SO fibers increased. Maximum isometric tension was reduced in undernourished animals as compared with controls, but the position and shape of the length-tension relationship was the same in the two groups. Reductions in muscle force appeared to be explained by decreases in muscle mass, since tension corrected for cross-sectional area or tissue weight was the same in the two groups. Therefore muscle mechanical efficiency appeared to be unaffected by undernutrition. These data indicate that prolonged undernutrition causes deleterious changes in diaphragm muscle structure that impair its ability to generate force.     

Regionalized adaptations and muscle fiber proliferation in stretch-induced enlargement

The relative contribution of increases in fiber area to stretch-induced muscle enlargement was evaluated in the slow tonic fibers of the anterior latissimus dorsi of adult Japanese quails. A weight corresponding to 10% of the bird's body mass was attached to one wing. Thirty days of stretch in 34 birds averaged 171.8 +/- 13.5% increase in muscle mass and 23.5 +/- 0.8% increase in muscle fiber length. The volume density of noncontractile tissue increased in middle and distal regions of stretch-enlarged muscles. Mean fiber cross-sectional area increased 56.7 +/- 12.3% in the midregion of stretched muscles. Further analysis indicated slow beta-fiber hypertrophy occurred in proximal, middle, and distal regions; however, fast alpha-type fiber hypertrophy was limited to middle regions of stretched muscles. Stretched muscles had a significant increase in the frequency of slow beta-fibers that were less than 500 microns 2 in all regions and fast alpha-type fibers in middle and distal regions. Total fiber number was determined after nitric acid digestion of connective tissue in 10 birds. Fiber number increased 51.8 +/- 19.4% in stretched muscle. These results are the first to clearly show that muscle fiber proliferation contributes substantially to adult skeletal muscle stretch-induced enlargement, although we do not know whether the responses of the slow tonic anterior latissimus dorsi might be similar or different from mammalian twitch muscle.     

Effect of long-term hypergravitation on the skeletal-muscular tissue in rats

The space flight or simulated gravitational unloading lead to the muscle atrophy, slow-to-fast transformation of muscle fibers and myofibrillar damages both in humans and animals (1, 7, 13, 17). This process could be prevented by the exercise training during space flight (1), (partly) by periodic weight support during unloading (13). It has been demonstrated in these studies that there is some level of force production necessary for the maintenance of skeletal muscle properties. It is known that adaptation to the physical training frequently induces augmentation in cross-sectional area (CSA) of muscle fibers (MF), transformation of fibers, augmentation of mitochondrial volume density, and increase in absolute volume of myofibrillas. Numerous observations suggest importance of gravitational loading in regulating muscle mass. The centrifuging is believed to be useful for preventing muscle functional and structural losses under microgravity. But there are few studies designed to investigate effect of artificial gravity on the skeletal musculature (2, 7). Our objective was to investigate structural adaptation in slow-twitch soleus muscle (percentage of connective tissue and central nuclei, fiber size, myosin heavy chain isotope, myofibrillar proteins and mitochondria volume density) after 19 and 33 days of hypergravity.     

Morphometric analysis of capillary geometry in pigeon pectoralis muscle

The objective of the study was to examine the relationship(s) between the size and the geometry of the capillary network in the flight muscle of pigeon (Columbia livia). To this end, we used morphometry to analyze the degree of anisotropy (i.e., orientation) of capillaries with respect to the axis of the muscle fibers in perfusion-fixed samples of pigeon pectoralis muscles with large difference in capillary density. Capillary number per fiber cross-sectional area (range, 1,491-5,680 mm-2) depended on fiber size (aerobic fibers, 304-782 microns 2; glycolytic, 1,785-2,444 microns 2), as well as sarcomere length (1.69-2.20 microns), and the relative sectional area of aerobic and glycolytic fibers (aerobic, 42-84% of total fiber area). The degree of tortuosity of capillaries, i.e., their bending or sinuosity relative to the muscle fiber axis, was primarily a function of sarcomere length. In spite of large differences in capillary density, capillary orientation at a given sarcomere length was remarkably similar among samples. In addition to capillaries running parallel to the muscle fiber axis, a unique arrangement of branches running perpendicular to the muscle fiber axis was found in all samples. This arrangement yielded a large circumferential distribution of capillary surface around the muscle fibers. Compared to mammalian limb muscles examined over a 10-fold range of capillary density (range, 450-4,670 mm-2), the degree of anisotropy of capillaries was greater in all samples of pigeon M. pectoralis. In the pigeon, there was no increase in the amount of capillary surface area available for exchange per microvessel as a result of a greater degree of capillary tortuosity in samples with larger capillary density (capillary number per fiber cross-sectional area greater than 4,000 mm-2), as compared to samples with a capillary density less than 4,000 mm-2.     

Essential role of satellite cells in the growth of rat soleus muscle fibers

Effects of gravitational loading or unloading on the growth-associated increase in the cross-sectional area and length of fibers, as well as the total fiber number, in soleus muscle were studied in rats. Furthermore, the roles of satellite cells and myonuclei in growth of these properties were also investigated. The hindlimb unloading by tail suspension was performed in newborn rats from postnatal day 4 to month 3 with or without 3-mo reloading. The morphological properties were measured in whole muscle and/or single fibers sampled from tendon to tendon. Growth-associated increases of soleus weight and fiber cross-sectional area in the unloaded group were approximately 68% and 69% less than the age-matched controls. However, the increases of number and length of fibers were not influenced by unloading. Growth-related increases of the number of quiescent satellite cells and myonuclei were inhibited by unloading. And the growth-related decrease of mitotically active satellite cells, seen even in controls (20%, P > 0.05), was also stimulated (80%). The increase of myonuclei during 3-mo unloading was only 40 times vs. 92 times in controls. Inhibited increase of myonuclear number was not related to apoptosis. The size of myonuclear domain in the unloaded group was less and that of single nuclei, which was decreased by growth, was larger than controls. However, all of these parameters, inhibited by unloading, were increased toward the control levels generally by reloading. It is suggested that the satellite cell-related stimulation in response to gravitational loading plays an essential role in the cross-sectional growth of soleus muscle fibers.     

The resilience of the size principle in the organization of motor unit properties in normal and reinnervated adult skeletal muscles

Henneman's size principle relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement. After nerve injury and surgical repair, the relationship between motoneuron size and the number and size of the muscle fibers that the motoneuron reinnervates is initially lost but returns with time, irrespective of whether the muscles are self- or cross-reinnervated by the regenerated axons. Although the return of the size relationships was initially attributed to the recovery of the cross-sectional area of the reinnervated muscle fibers and their force per fiber, direct enumeration of the innervation ratio and the number of muscle fibers per motoneuron demonstrated that a size-dependent branching of axons accounts for the size relationships in normal muscle, as suggested by Henneman and his colleagues. This same size-dependent branching accounts for the rematching of motoneuron size and muscle unit size in reinnervated muscles. Experiments were carried out to determine whether the daily amount of neuromuscular activation of motor units accounts for the size-dependent organization and reorganization of motor unit properties. The normal size-dependent matching of motoneurons and their muscle units with respect to the numbers of muscle fibers per motoneuron was unaltered by synchronous activation of all of the motor units with the same daily activity. Hence, the restored size relationships and rematching of motoneuron and muscle unit properties after nerve injuries and muscle reinnervation sustain the normal gradation of muscle force during movement by size-ordered recruitment of motor units and the process of rate coding of action potentials. Dynamic modulation of size of muscle fibers and their contractile speed and endurance by neuromuscular activity allows for neuromuscular adaptation in the context of the sustained organization of the neuromuscular system according to the size principle.     

Muscle fiber architecture of the dog diaphragm

Boriek, Aladin M., Charles C. Miller III, and Joseph R. Rodarte. Muscle fiber architecture of the dog diaphragm.J. Appl. Physiol. 84(1): 318-326, 1998.Previous measurements of muscle thickness and length ratio ofcostal diaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte.J. Appl. Physiol. 77: 2065-2070,1994) suggested, but did not prove, discontinuous muscle fiberarchitecture. We examined diaphragmatic muscle fiber architecture usingmorphological and histochemical methods. In 15 mongrel dogs, transversesections along the length of the muscle fibers were analyzedmorphometrically at ×20, by using the BioQuant System IVsoftware. We measured fiber diameters, cross-sectional fiber shapes,and cross-sectional area distributions of fibers. We also determinednumbers of muscle fibers per cross-sectional area and ratio ofconnective tissue to muscle fibers along a course of the muscle fromnear the chest wall (CW) to near the central tendon (CT) for midcostalleft and right hemidiaphragms, as well as ventral, middle, and dorsalregions of the left costal hemidiaphragm. In six other mongrel dogs,the macroscopic distribution of neuromuscular junctions (NMJ) onthoracic and abdominal diaphragm surfaces was determined by stainingthe intact diaphragmatic muscle for acetylcholinesterase activity. Theaverage major diameter of muscle fibers was significantly smaller, andthe number of fibers was significantly larger midspan between CT and CWthan near the insertions. The ratio of connective tissues to musclefibers was largest at CW compared with other regions along the lengthof the muscle. The diaphragm is transversely crossed by multiplescattered NMJ bands with fairly regular intervals offset in adjacentstrips. Muscle fascicles traverse two to five NMJ, consistent withfibers that do not span the entire fascicle from CT to CW. Theseresults suggest that the diaphragm has a discontinuous fiberarchitecture in which contractile forces may be transmitted among themuscle fibers through the connective tissue adjacent to the fibers.

     

Muscle hypertrophy response to resistance training in older women.

We conducted a 12-wk resistance training program in elderly women [mean age 69 +/- 1.0 (SE) yr] to determine whether increases in muscle strength are associated with changes in cross-sectional fiber area of the vastus lateralis muscle. Twenty-seven healthy women were randomly assigned to either a control or exercise group. The program was satisfactorily completed and adequate biopsy material obtained from 6 controls and 13 exercisers. After initial testing of baseline maximal strength, exercisers began a training regimen consisting of seven exercises that stressed primary muscle groups of the lower extremities. No active intervention was prescribed for the controls. Increases in muscle strength of the exercising subjects were significant compared with baseline values (28-115%) in all muscle groups. No significant strength changes were observed in the controls. Cross-sectional area of type II muscle fibers significantly increased in the exercisers (20.1 +/- 6.8%, P = 0.02) compared with baseline. In contrast, no significant change in type II fiber area was observed in the controls. No significant changes in type I fiber area were found in either group. We conclude that a program of resistance exercise can be safely carried out by elderly women, such a program significantly increases muscle strength, and such gains are due, at least in part, to muscle hypertrophy.     

Motor unit fibre density in extremely hypertrophied skeletal muscles in man. Electrophysiological signs of muscle fibre hyperplasia

Single fibre electromyography (SFEMG) was performed on the left quadriceps, extensor digitorum communis, and biceps brachii muscles of four highly trained bodybuilders (BB). Muscle fibre areas, as assessed from biopsy samples, were similar in the BB as in age-matched habitually active male controls despite large differences in limb circumferences. This indicates a greater muscle fibre number in BB. An abnormally high muscle fibre density (FD) was recorded in two subjects exposed to extremely severe exercise for 14 years or longer, while those with normal FDs had been training extensively for a considerably shorter time (4-6 years). The abnormal FD was due either to an ephaptic transmission of action potentials between adjacent muscle fibres, or a muscle fibre hyperplasia, or most probably a combination of both, in response to a long-term functional overload. From the present results one cannot say whether the suggested muscle fibre hyperplasia had any major influence on the muscle hypertrophy per se, since all BB possessed large muscle volumes.     

Changes in total fiber numbers of disused soleus muscle on rats

In this study, by use of technique that was modified from Morey method, we discussed the histological influence on the soleus muscle of the rats caused by disuse. This study is characterized by the calculating of total numbers of muscle fibers. ST (slow-twitch) and FT (fast-twitch) fibers in total muscular cross-sectional area were classified by their difference in intensity of staining of actomyosin adenosinetriphosphatase (myosin ATPase). During the experiment, average fiber diameter of ST and FT fibers declined when compared to control group (p less than 0.01). A 54% decrease in the total number of ST fibers was observed in the experimental group (p less than 0.01). Conversely, the total number of FT fibers increased to 362% of the control value (p less than 0.01). These results of the changes evoked in ST and FT fibers indicate 34% decrease in total muscular cross-sectional area, and showed that muscular function shifted toward a faster muscle in disused soleus muscle.     

Fine structure of the nephridial muscle layer cells of Phascolosoma granulatum (Sipuncula)

The nephridial muscle layer of Phascolosoma granulatum consists of a network of longitudinal and circular cells separated by connective tissue matrix. The muscle fibers are densely packed with thick and thin myofilaments, among which are scattered cytoplasmic dense bodies. The nucleus and noncontractile cytoplasmic organelles occupy a lateral projection from the contractile portion of the fiber. Cytoplasmic dense bodies are the result of a clustering of an indeterminate number of the thin actin filaments that fill the cytoplasm between thick filaments. Attached to the cytoplasmic face of the cell membrane are membrane-associated electron-dense plaques. These sites are linked to the contractile myofilaments by narrow filamentous bridges. Extracellular narrow filaments extend from these plaques to collagen fibers of the connective tissue matrix. Differences in length of the dense plaques may be related to differences in thick myofilament diameter in three types of muscle fiber, types A, B and C, statistically distinguished by mean fiber size differences. The plaques may serve as connecting links for the transmission of tension from contractile units to the connective tissue of the muscle layer. © 1993 Wiley-Liss, Inc.     

Myosin heavy chain composition of muscle spindles in human biceps brachii.

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.     

The arrangement and function of octopus arm musculature and connective tissue

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.     

Skeletal muscle fiber regeneration following heterotopic autotransplantation in cats.

Whole 3 g extensor digitorum longus (EDL) muscles of cats were autotransplanted. The EDL muscles were either transplanted without denervation prior to transplantation (normal transplants) or denervated 3 to 4 weeks prior to transplantation (pre-denervated transplants). A few peripheral skeletal muscle fibers survived transplantation but most fibers degenerated and then regenerated as the transplant became revascularized. Both normal and pre-denervated muscles regenerated successfully and by 50 days after transplantation fibers which had reinnervated showed high and low myofibrillar ATPase activity. Compared to controls, the smaller mean fiber cross-sectional area of the transplants was due to the large number of small fibers, but some fibers in the transplant were larger than any fibers observed in the controls. Transplants regained 57 percent of the muscle mass of the controls. Contraction and half relaxation times of transplanted muscles were slower than controls, but peak isometric tetanus tension per cm² of muscle was nearly normal. Fifty to 170 days after transplantation, muscles showed low oxidative capacity and fatigued rapidly.