Relation between muscle mass, motor units and type of training in master athletes.

OBJECTIVE: The aim of this study was to measure the number of motor units and muscle mass in power-trained and endurance-trained master athletes compared with community-dwelling older adults. METHODS: Seventy-five master athletes (52 power- and 23 endurance-trained athletes) were recruited at the 2012 European Veteran Athletics Championships in Zittau (Germany). One hundred and forty-nine community-dwelling older adults served as controls. In all participants, the motor unit number index (MUNIX) in the hypothenar muscle and whole body muscle mass was determined by bioelectrical impedance analysis (BIA). RESULTS: In both male and female master athletes, there were significant negative correlations between age and muscle mass (female: r = -0·510, P = 0·002; male: r = -0·714, P<0·001). Master athletes showed a weak correlation (r = -0·295, P = 0·010) between MUNIX and age. Master athletes exhibited significantly higher values than the control group with regard to both muscle mass (P = 0·002) and motor units (P = 0·004). Subanalysis showed that only power trained master athletes had both a larger muscle mass (P<0·001) and a higher MUNIX (P = 0·014) than the control group. Among the master athletes, power-trained athletes had a larger (P<0·001) muscle mass than endurance-trained athletes. CONCLUSIONS: The present data of master athletes are compatible with the hypothesis of an age-related decline in whole body muscle mass and motor units. Nevertheless, the data suggest that the master athletes' high level of physical activity may protect motoneurons. In addition, power training seems to have a positive effect on muscle mass and could therefore be an effective method of training to prevent sarcopenia.

Similar changes in muscle fiber phenotype with differentiated consequences for rate of force development: endurance versus resistance training.

Resistance training has been shown to positively affect the rate of force development (RFD) whereas there is currently no data on the effect of endurance training on RFD. Subjects completed ten weeks of either resistance training (RT, n=7) or endurance cycling (END, n=7). Pre and post measurements included biopsies obtained from m. vastus lateralis to quantify fiber phenotype and fiber area and isokinetic dynamometer tests to quantify maximal torque (Nm) and RFD (Nm/s) at 0-30, 0-50, 0-100 and 0-200ms during maximal isometric contraction for both knee extensors and flexors. Both groups increased the area percentage of type IIa fibers (p<.01) and decreased the area percentage of type IIx fibers (p=.05), whereas only RT increased fiber size (p<.05). RT significantly increased eccentric, concentric and isometric strength for both knee extensors and flexors, whereas END did not. RT increased 200ms RFD (p<.01) in knee flexor RFD and a tendency towards an increase at 100ms (p<.1), whereas tendencies towards decreases were observed for the END group at 30, 50 and 100ms (p<.1), resulting in RT having a higher RFD than END at post (p<.01). In conclusion, resistance training may be very important for maintaining RFD, whereas endurance training may negatively impact RFD.

Fiber typing in aging muscle.

It is accepted widely that fast-twitch muscle fibers are preferentially impacted in aging muscle, yet we hypothesize that this is not valid when aging muscle atrophy becomes severe. In this review, we summarize the evidence of fiber type-specific effect in aging muscle and the potential confounding roles of fibers coexpressing multiple myosin heavy-chain isoforms and their histochemical identification.

Muscle fibre-type dependence of neuronal nitric oxide synthase-mediated vascular control in the rat during high speed treadmill running.

We have recently shown that nitric oxide (NO) derived from neuronal NO synthase (nNOS) does not contribute to the hyperaemic response within rat hindlimb skeletal muscle during low-speed treadmill running. This may be attributed to low exercise intensities recruiting primarily oxidative muscle and that vascular effects of nNOS-derived NO are manifest principally within glycolytic muscle. We tested the hypothesis that selective nNOS inhibition via S-methyl-l-thiocitrulline (SMTC) would reduce rat hindlimb skeletal muscle blood flow and vascular conductance (VC) during high-speed treadmill running above critical speed (asymptote of the hyperbolic speed versus time-to-exhaustion relationship for high-speed running and an important glycolytic fast-twitch fibre recruitment boundary in the rat) principally within glycolytic fast-twitch muscle. Six rats performed three high-speed treadmill runs to exhaustion to determine critical speed. Subsequently, hindlimb skeletal muscle blood flow (radiolabelled microspheres) and VC (blood flow/mean arterial pressure) were determined during supra-critical speed treadmill running (critical speed + 15%, 52.5 ± 1.3 m min(-1)) before (control) and after selective nNOS inhibition with 0.56 mg kg(-1) SMTC. SMTC reduced total hindlimb skeletal muscle blood flow (control: 241 ± 23, SMTC: 204 ± 13 ml min(-1) (100 g)(-1), P < 0.05) and VC (control: 1.88 ± 0.20, SMTC: 1.48 ± 0.13 ml min(-1) (100 g)(-1) mmHg(-1), P < 0.05) during high-speed running. The relative reductions in blood flow and VC were greater in the highly glycolytic muscles and muscle parts consisting of 100% type IIb+d/x fibres compared to the highly oxidative muscles and muscle parts consisting of 35% type IIb+d/x muscle fibres (P < 0.05). These results extend our understanding of vascular control during exercise by identifying fibre-type-selective peripheral vascular effects of nNOS-derived NO during high-speed treadmill running.

The influence of fiber size distribution of type IIB on carcass traits and meat quality in pigs.

The effects of detailed characteristics such as the size and proportion of type IIB fibers in longissimus thoracis muscle on carcass traits and pork quality were investigated. A total of 96 pigs were classified into four groups by the proportion of different IIB fiber sizes. Group NS (high proportion of both small- and normal-sized IIB fibers) had a higher total number (136.4) and density (231.31) of type IIB fibers, backfat thickness (37.20 mm) and intramuscular fat content (4.77%) than the other groups (P<0.05), whereas Group NS had the lowest values of cross-sectional area (3413.85 µm2) and diameter (60.15 µm) of type IIB fiber among the groups (P<0.05). Pig muscles with higher percentage of large IIB fibers exhibit tougher, lighter and more exudative meat than pig muscles with a higher proportion of small- or normal-sized IIB fibers. Therefore, an increase in the proportion of large IIB fibers causes poor quality of pork.

Misclassification of hybrid fast fibers in resistance-trained human skeletal muscle using histochemical and immunohistochemical methods.

Sixteen healthy untrained women participated in a 6-week progressive resistance training program to compare 2 common methods of classifying fiber types. The women were a subset from a previous study and were randomly divided into 2 groups: traditional strength training (TS, n = 9) and non-exercising control (C, n = 7). The TS group performed 3 lower limb exercises (leg press, squat, and knee extension) using 6-10 repetitions maximum 2 days per week for the first week and 3 days per week for the remaining 5 weeks (17 total workouts). Pre- and posttraining vastus lateralis muscle biopsies were analyzed for fiber type composition using 2 popular methods: myosin adenosine triphosphatase (mATPase) histochemistry and myosin heavy chain (MHC) immunohistochemistry. Six fiber types (I, IC, IIC, IIA, IIAX, and IIX) were delineated using each method separately and in combination. Because of the subjective nature of each method (visual assessment of staining intensities), IIAX fibers expressing a small amount of MHCIIa were misclassified as type IIX using mATPase histochemistry, whereas those expressing a small amount of MHCIIx were misclassified as type IIA using MHC immunohistochemistry. As such, either method used separately resulted in an underestimation of the type IIAX fiber population. In addition, the use of mATPase histochemistry alone resulted in an overestimation of type IIX, whereas there was an overestimation of type IIA using MHC immunohistochemistry. These fiber typing errors were most evident after 6 weeks of resistance training when fibers were in transition from type IIX to IIA. These data suggest that the best approach to more accurately determine muscle fiber type composition (especially after training) is the combination of mATPase histochemical and MHC immunohistochemical methods.

Division of motor units into fast and slow of the basis of profile of 20 Hz unfused tetanus.

In the medial gastrocnemius muscle of intact rats, division of motor units (MUs) into slow (S) or fast (F) types is typically based on presence of a sag phenomenon in 40 Hz unfused tetanic contraction. MUs with sag are classified as F, while those without sag as S. However, in rats one month after spinal cord injury this phenomenon almost completely disappears and cannot be used as a basis for MUs differentiation, whereas the twitch contraction time increased significantly. Analysis of myosin heavy chain (MHC) isoform composition confirmed transformational changes of muscle fibres after spinal cord transection and indicated unchanged proportion of type I MHC isoforms, disappearance of type IIa MHC isoforms, and increase of type IIb MHC isoforms. We proposed an additional method for division of MUs into types when standard criteria are not applicable. It was observed that relative effectiveness of force summation during 20 Hz tetanus, described as a ratio of the force of the last contraction of this tetanus to the force of the first contraction, did not change after spinal cord injury. This ratio for S MUs both in intact and spinal rats exceeded 2.0, whereas for F units was lower than 2.0. Calculations of this ratio made for better fused tetani, evoked by 30 Hz or 40 Hz stimulation, showed overlapping values. We conclude that this 20 Hz tetanus index appears to be an alternative method useful for division of motor units into S and F types.

Fiber type composition and maximum shortening velocity of muscles crossing the human shoulder.

A study of the fiber type composition of fourteen muscles spanning the human glenohumeral joint was carried out with the purpose of determining the contribution of fiber types to overall muscle cross-sectional area (CSA) and to estimate the maximum shortening velocity (V(max)) of those muscles. Muscle biopsies were procured from 4 male cadavers (mean age 50) within 24 hr of death, snap frozen, mounted, and transversely sectioned (10 microm). Slides were stained for myofibrillar ATPase after alkaline preincubation. Photoimages were taken of defined areas (100 fibers) using the Bioquant system, and fiber type and CSA were measured from these images. Staining for mATPase produced three different fiber types: slow-oxidative (SO), fast-oxidative-glycolytic (FOG), and fast-glycolytic (FG). On average, the muscle fiber type composition ranged from 22 to 40% of FG, from 17 to 51% of FOG, and from 23 to 56% of SO. Twelve out of the 14 muscles had average SO proportions ranging from 35 to 50%. V(max) was calculated from the fiber type contribution relative to CSA and shortening velocity values taken from the literature. The maximum velocities of shortening presented here provide a physiological basis for the development of human shoulder musculoskeletal models suitable for predicting muscle forces for functionally relevant tasks encompassing conditions of muscle shortening and lengthening.

Horse soleus muscle: postural sensor or vestigial structure?

The soleus muscle of horses is rather diminutive with respect to the overall size of adjacent synergist muscles in the hind limb of the horse. Whether or not such a muscle might be vestigial or may be providing some essential function has not been determined. We have studied the horse's soleus muscle using histochemical (ATPase), immunocytochemical (myosin isoform identification), and SDS-PAGE analysis to demonstrate that it is largely composed of 100% type I, presumed slow-twitch fibers. Only one soleus muscle studied (out of 13 adult horses) contained any type II muscle fibers. Given this consistent high percentage of slow-oxidative fibers, we hypothesized that the soleus muscle could have a significant role in proprioceptive function, essentially functioning as a proprioceptive organ instead of a significant force-generating muscle during locomotion. We tested this by examining three whole soleus muscles and assessing their muscle spindle content, which proved to have a spindle index of about 12. This value provided equivocal support for the hypothesis since it did not approach values reported for other mammalian proprioceptive muscles that were approximately 40-50 spindles per gram of muscle mass. Other parameters, such as motoneuron number and muscle unit size, may be useful in understanding these data.

Skeletal muscle fiber composition of the English sparrow (Passer domesticus).

Substrate utilization by English sparrow skeletal muscle has been extensively studied in our lab. However, there are few published studies on the muscle fiber composition of English sparrow wing and gastrocnemius muscles. The objective of the present study was to examine the fiber type composition of a variety of muscles in the English sparrow. The classification of a muscle fiber as fast glycolytic, slow oxidative, or fast oxidative glycolytic provides insight into the physiological function of muscles. Therefore, we completed mATPase and NADH stains on four muscles of the sparrow wing, as well as the gastrocnemius muscle, to characterize these muscle fiber types. Results show that the fibers of extensor digitorum communis, extensor metacarpi ulnaris, and extensor metacarpi radialis are homogeneous fast oxidative. The fibers of the supinator are homogeneous fast oxidative in 62.5% of samples, and heterogeneous (45.2% fast oxidative, 54.8% fast nonoxidative) in 37.5% of samples. Whereas the gastrocnemius muscle fibers are heterogeneous (10% fast oxidative, 64% fast nonoxidative, 26% slow oxidative) in all muscles examined.

The effect of a unilateral muscle transplantation on the muscle fiber type and the MyHC isoform content in unoperated hind limb slow and fast muscles of the inbred Lewis rats.

To reveal the effect of foreign innervation and altered thyroid status on fiber type composition and the myosin heavy chain (MyHC) isoform expression in the rat slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles, a method of heterochronous isotransplantation was developed. In this experimental procedure, the SOL or EDL muscles of young inbred Lewis rats are grafted either into the host EDL or SOL muscles of adult rats of the same strain with normal or experimentally altered thyroid status. To estimate the extent of fiber type transitions in the transplanted muscles, the SOL and EDL muscle from the unoperated leg and unoperated muscles from the operated leg could be legitimately used as controls, but only when the experimental procedure itself does not affect these muscles. To verify this assumption, we have compared the fiber type composition and the MyHC isoform content of unoperated contralateral SOL and EDL muscles and ipsilateral unoperated SOL muscle of experimental rats after unilateral isotransplantation into the host EDL muscle with corresponding muscles of the naive rats of the same age and strain. We provide compelling evidence that the unilateral heterochronous isotransplantation has no significant effect on the fiber type composition and the MyHC isoform content of unoperated muscles of experimental animals. Hence, these muscles can be used as controls in our grafting experiments.

Spatial fiber type distribution in normal human muscle Histochemical and tensiomyographical evaluation.

The variability of fiber type distribution in nine limb muscles was examined with histochemical and tensiomyographical (TMG) methods in two groups of 15 men aged between 17 and 40 years. The aim of this study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles in function to depth and to predict fiber type proportions with a non-invasive TMG method. The distribution of different fiber types varied within the muscles, as a function of depth, with a predominance of type 2b fibers at the surface and type 1 fibers in deeper regions of the muscle. For all the analyzed muscles the contraction times measured at stimulus intensity 10% of supramaximal stimulus (10% MS) were significantly (p<0.05) shorter than the contraction times measured at 50% of supramaximal stimulus intensity (50% MS). The Pearson's correlation coefficient between percentage of type 1 muscle fibers measured at the surface of the muscle and contraction time at 10% MS, obtained by TMG was statistically significant (r=0.76,P<0.01). Also the Pearson's correlation coefficient between percentage of type 1 muscle fibers measured in the deep region of the muscle and contraction time at 50% MS obtained by TMG was also statistically significant (r=0.90,P<0.001). These findings suggest that the contraction time obtained by TMG may be useful for non-invasive examining of muscle fiber types spatial distribution in humans.

Kinetic effects of fiber type on the two subcomponents of the Huxley-Simmons phase 2 in muscle.

The Huxley-Simmons phase 2 controls the kinetics of the first stages of tension recovery after a step-change in fiber length and is considered intimately associated with tension generation. It had been shown that phase 2 is comprised of two distinct unrelated phases. This is confirmed here by showing that the properties of phase 2(fast) are independent of fiber type, whereas those of phase 2(slow) are fiber type dependent. Phase 2(fast) has a rate of 1000-2000 s(-1) and is temperature insensitive (Q(10) approximately 1.16) in fast, medium, and slow speed fibers. Regardless of fiber type and temperature, the amplitude of phase 2(fast) is half (approximately 0.46) that of phase 1 (fiber instantaneous stiffness). Consequently, fiber compliance (cross-bridge and thick/thin filament) appears to be the common source of both phase 1 elasticity and phase 2(fast) viscoelasticity. In fast fibers, stiffness increases in direct proportion to tension from an extrapolated positive origin at zero tension. The simplest explanation is that tension generation can be approximated by two-state transition from attached preforce generating (moderate stiffness) to attached force generating (high stiffness) states. Phase 2(slow) is quite different, progressively slowing in concert with fiber type. An interesting interpretation of the amplitude and rate data is that reverse coupling of phase 2(slow) back to P(i) release and ATP hydrolysis appears absent in fast fibers, detectable in medium speed fibers, and marked in slow fibers contracting isometrically. Contracting slow and heart muscles stretched under load could employ this enhanced reversibility of the cross-bridge cycle as a mechanism to conserve energy.

Muscle-fiber characteristics in adult mouse-tongue muscles.

To clarify functions of the mouse-tongue muscles, proteins such as myocin heavy chain (MHC) 2a and MHC-2b, which are isoforms of the fast-twitch fiber type myosin heavy chain, in the lateral margin of the tongue were observed by reverse transcription polymerase chain reaction and immunohistochemical analyses. The main MHC isoform in the superior longitudinal muscle of the tongue was MHC-2b, with the fastest function and the main MHC isoform in the transverse muscle of the tongue was MHC-2a. These findings suggested that the fastest function is necessary for the superior longitudinal muscle of the tongue, which is useful for moving the tongue in and out of the mouth in the sagittal direction, showing different cellular biological properties of the myofibers from those of the transverse muscle of the tongue.

Cytophotometrical and immunohistochemical analysis of soft palate muscles of children with isolated cleft palate and combined cleft lip and palate.

Palatal muscle biopsies from the cleft margin of children were subjected to cytophotometrical and immunohistochemical analysis. Muscle fiber types were classified according to the enzyme activity of myofibrillic adenosine triphosphatase, glycerol-3-phosphate-dehydrogenase and succinate dehydrogenase assessed cytophotometrically. Fiber type-related immunoreactivity of nitric oxide synthase (NOS) isoforms I, II, III, as a physiological modulator of skeletal muscle function, was related to the oxidative and glycolytic activity of the muscle fibers. Fast oxidative glycolytic fibers with high oxidative activity showed strong NOS I immunoreactivity, whereas fast glycolytic fibers with high glycolytic activity were stronger immunolabelled for NOS III. NOS II expression was similar in all fiber types. No differences in NOS immunoreactivity were found between the two investigated forms of deformity. Additionally to the usual skeletal muscle fiber types, a slow tonic fiber type was for the first time identified in cleft palate muscles. Comparison of two forms of cleft palate, isolated cleft palate and combined cleft lip and palate has shown decreased enzyme activities in muscle fibers of palatal muscles from combined cleft lip and palate. Fast oxidative glycolytic fibers were mainly effected. Cytophotometrical and immunohistochemical analysis indicated a depressed performance of the cleft palatal muscles from combined cleft lip and palate as a stronger deformity compared with isolated cleft palate.

A curve-fitting procedure to explain changes in muscle force-velocity relationship induced by hyperactivity.

Experiments have shown that a period of hyperactivity induces changes in the muscle force-velocity relationship. The goal of this study was to explain such changes by taking into account that the myosin heavy chain (MHC) composition of a muscle is a primary determinant of its shortening velocity. For this purpose a mathematical model was developed where the force-velocity relationship of the whole muscle was built by summing the force contributions of individual components at each of a series of shortening velocities. An individual force-velocity relationship was assigned to each component, i.e. each type of MHC. Experimental data were obtained on control and hyperactivated epitrochlearis muscles from rats. In the controls rats, fitting of the model with experimental data was satisfactory. In hyperactivated muscles, parameters of the fastest MHC component had to be modified. This improved the fit between model and experimental data and accounted for possible changes in myosin light chain composition.

Giant muscle fibres in pigs with different Ryr1 genotype.

This study examined the frequency, morphological and immunohistochemical characteristics of the giant fibres in the longissimus muscle of local Krsko polje pigs with different Ryr1 genotypes. Giant fibres were round-shaped and had significantly increased cross-sectional area compared with normal muscle fibres. Only fast-twitch glycolytic fibres were affected, usually showing enhanced succinate dehydrogenase activity. On the ultrastructural level, the dilation of the sarcoplasmic reticulum, swelling of mitochondria and destruction of myofilaments was observed. The incidence of giant fibres was the highest in Ryr1 dimutant pigs (Ryr1 nn), which also exhibited lower muscle pH1 than heterozygous (Ryr1 Nn) or pigs with the wild Ryr1 gene (Ryr1 NN). However, the giant fibres were also present in pigs free of Ryr1 gene mutation. Our results suggest that the giant fibre syndrome depends mostly upon the rate and intensity of early post-mortem glycolysis, which results in acidity of muscle tissue. We suppose that the giant fibre formation is a result of excessive intracellular lactate accumulation in some fast-twitch glycolytic fibres. This process could also explain the ultrastructural alterations and the consequent changes in the oxidative enzymes and myofibrillar ATPase staining pattern observed in our and some previous studies.

Fibre type composition of soleus and extensor digitorum longus muscles in normal female inbred Lewis rats.

We have analysed the fibre type composition of soleus and extensor digitorum longus (EDL) muscles of normal female 4-6-month-old inbred Lewis rats. This rat strain is used in our ongoing study of the effects of thyroid hormone on myosin heavy chain (MyHC) isoform expression. On the basis of the mATPase reaction, soleus muscles contained 96.1 +/- 2.9% of type 1 fibres supplemented by 2A fibres. EDL muscles contained type 1 (5.5 +/- 1.0%), type 2A (18.8 +/- 1.7%) and type 2B (75.7 +/- 2.2%) fibres. Immunohistochemical analysis and SDS gel electrophoresis confirmed that most fibres in the soleus muscle expressed the type 1 (slow) MyHC isoform and that only a small proportion of fibres expressed the fast 2a MyHC isoform. Immunohistochemical analysis and SDS gel electrophoresis demonstrated that almost half of the 2B fibres of EDL muscles expressed the 2x/d MyHC isoform. In both muscle types, many fibres expressed more than one MyHC isoform. The content of slow fibres in the soleus muscle of female inbred Lewis rats was slightly higher than that reported for Wistar rats, but was considerably higher than that of Sprague-Dawley rats, whereas substantial differences were not found in the proportion of slow and fast fibre types in EDL muscles in these strains.

Analyses of muscular activity, energy metabolism, and muscle fiber type composition in a patient with bilateral masseteric hypertrophy.

Hyperwork of the masseter muscles due to habitual parafunction is thought to induce masseteric hypertrophy (so called work hypertrophy). However, the causes underlying this disease are not yet fully understood. Recently, we had a patient with bilateral masseteric hypertrophy, and we performed a partial excision of the masseter muscles. In this patient's case, we examined muscular activity, energy metabolism, and fiber type composition of the masseter muscles using electromyograms (EMG), 31P-magnetic resonance spectroscopy (MRS), and enzyme-histochemistry. The EMG showed no hyperactivity, and the 31P-MRS showed normal energy spectral patterns and PCr contents of the masseter muscles. The fiber type composition, however, in the muscles in this case was very different from that in muscles with "work hypertrophy" and also that in normal masseter muscles: 1. Loss of type IIB fibers; 2. Increases in type IIA and in type IM & IIC fibers; and 3. Decrease in type I fibers. The findings suggest that this is not a case of work hypertrophy but a case of compensatory hypertrophy possibly due to a lack of high-tetanus-tension type IIB fibers.