Fibre types in human abdominal muscles.

Histochemical muscle fibre composition was studied in biopsied from the four different muscles of the abdominal wall (rectus abdominis, RA, obliquus externus, OE, obliquus internus, OI, and transversus abdominis, Tr) in 13 normal human subjects (9 females and 4 males, age 24-55 years) undergoing gall-bladder surgery. Muscle fibres were classified as Type I, IIA, IIB or IIC on the basis of their myofibrillar ATPases' pH lability. There were large inter-individual variations in fibre composition, whereas, in general, the differences between the different muscles were minor or non-existent. Mean fibre distribution ranges were 55-58% I, 15-23% 22A, 21-28% IIB, and 0-1% II C fibres. The least fibre diameters were similar for all types and muscles (range of means 50-54 micrometer) except for Tr in which the Type II fibres were smaller (mean 45 micrometer). There was a high correlation in the size of Type I vs. II fibres and Type IIA vs. IIB fibres in all layers. The oxidative potential (NADH-diaphorase staining intensity) appeared high in Type I fibres and low in Type II fibres, irrespective of subgroups. Thus, based on histochemical fibre composition, the different abdominal muscles appear to have a similar functional capacity. However, functional differences between individuals were indicated by the large inter-individual variation in muscle fibre distribution.     

Fibre composition of human intrinsic tongue muscles

The muscle fibre composition of three human intrinsic tongue muscles, the longitudinalis, verticalis and transversus, was investigated in four anterior to posterior regions of the tongue using morphological and enzyme- and immunohistochemical techniques. All three muscles typically contained type I, IIA and IM/IIC fibres. Type I fibres expressed slow myosin heavy chain (MyHC), type II fibres fast MyHC, mainly fast A MyHC, whereas type IM/IIC coexpressed slow and fast MyHCs. Type II fibres were in the majority (60%), but regional differences in proportion and diameter of fibre types were obvious. The anterior region of the tongue contained a predominance of relatively small type II fibres (71%), in contrast to the posterior region which instead showed a majority of larger type I and type IM/IIC fibres (66%). In general, the fibre diameter was larger in the posterior region. This muscle fibre composition of the tongue differs from those of limb, orofacial and masticatory muscles, probably reflecting genotypic as well as phenotypic functional specialization in oral function. The predominance of type II fibres and the regional differences in fibre composition, together with intricate muscle structure, suggest generally fast and flexible actions in positioning and shaping the tongue, during vital tasks such as mastication, swallowing, respiration and speech.     

Coexistence of slow and fast isoforms of contractile and regulatory proteins in human skeletal muscle fibres induced by endurance raining

The distribution of fast and slow isoforms of troponin C, I, and T components and myosin heavy chains was investigated in histochemically typed myofibrillar ATPase intermediate (IM) fibres, that is, fibres that stain after both acid and alkaline pre-incubation in stainings for myofibrillar ATPase. In addition to the previously described IM fibres of types II C and I B, fibres that displayed staining characteristics between types II C and I B were observed and termed type II C–I B. The IM fibres constitute less than 1% of the fibres in normal human limb and abdominal muscles. The IM fibres studied here resulted from extensive endurance training of human triceps brachii muscle (n= 6) and were induced by conversion of a proportion (130) of type II fibres. The immunohistochemical stains of serial sections with antibodies to slow isoforms of troponin I, T, C and myosin heavy chain showed no staining of type II fibres but intense staining of types I and I B fibres, whereas type II C fibres stained with intermediate intensity. The antibodies to fast isoforms of the troponin components and myosin heavy chain did not give rise to staining of type I fibres but dark staining of type II fibres. Type I B fibres stained with intermediate intensity and type II C was either as dark as type II or slightly lighter. Type II C-I B fibres showed staining intensities intermediate between those observed for types I B and IIC in the immunohistochemical stains. It is therefore concluded that training-induced myofibrillar ATPase intermediate human skeletal muscle fibres are characterized by the coexistence of slow and fast isoforms of contractile and regulatory proteins. Changes in the distribution of fast and slow isoforms of several of the myofibrillar proteins appeared to be induced in a co-ordinated manner.     

Fibre types in human lumbar back muscles

The distribution of histochemically identified muscle fibre types was studied in biopsy samples from the two main muscles in the lumbar region of the human erector spinae, the multifidus and the longissimus, in 16 healthy subjects (nine males and seven females, age 20–30 years). Muscle fibres were classified as types I, IIA, IIB or IIC on the basis of the pH lability of their myofibrillar ATPases. There were no differences between the multifidus and the longissimus muscles in the relative occurrence of type I (62 vs. 57%), type IIA (20 vs. 22%) or type IIB fibres (18 vs. 22%), or in the absolute size of fibres (range of mean least diameters 58–66 μm). The oxidative potential (NADH-diaphorase staining intensity) was high in type I and low in type II fibres, irrespective of subgroups, in both muscles. In the females, the type I fibres occupied a relatively larger area (70–75 vs. 54–58% for the males) although the relative number of type I fibres was the same in both sexes. This was due to smaller type II fibres in the females resulting in higher type I/type II area ratios (1.70–1.90 vs. 0.88–0.92 for males). This suggests a difference in functional capacity of lumbar back muscles between males and females. On the other hand, the similarity in histochemical fibre-type distribution between the multifidus and the longissimus muscles does not give support for a functional differentiation between these two anatomically different parts of the lumbar erector spinae in man.     

Muscle fibre size and type distribution in thoracic and lumbar regions of erector spinae in healthy subjects without low back pain: normal values and sex differences

This study sought to investigate the normal muscle fibre size and type distribution of the human erector spinae, both in thoracic and lumbar regions, in a group of 31 young healthy male (n=17) and female (n=14) volunteers. Two percutaneous muscle biopsy samples were obtained under local anaesthesia, from the belly of the left erector spinae, at the levels of the 10th thoracic and 3rd lumbar vertebrae. Samples were prepared for routine histochemistry for the identification of fibre types. Fibre size (cross-sectional area (CSA) and narrow diameter (ND)) was quantified using computerised image analysis. The mean CSA/ND for each fibre type was greater in the thoracic than the lumbar region, but there was no difference between the 2 regions either for percentage type I (i.e. percentage distribution by number), percentage type I area (i.e. relative area of the muscle occupied by type I fibres) or the ratio describing the size of the type I fibre relative to that of the type II. Men had larger fibres than women, for each fibre type and at both sampling sites. In the men, each fibre type was of a similar mean size, whereas in the women the type I fibres were considerably larger than both the type II A and type II B fibres, with no difference between the latter two. In both regions of the erector spinae there was no difference between men and women for the proportion (%) of a given fibre type, but the percentage type I fibre area was significantly higher in the women. The erector spinae display muscle fibre characteristics which are clearly very different from those of other skeletal muscles, and which, with their predominance of relatively large type I (slow twitch) fibres, befit their function as postural muscles. Differences between thoracic and lumbar fascicles of the muscle, and between the muscles of men and women, may reflect adaptive responses to differences in function. In assessing the degree of any pathological change in the muscle of patients with low back pain, it seems clear that (1) sex cannot be disregarded and (2) ‘atrophied’ (using the criteria from other muscles) type II fibres are not necessarily abnormal for the erector spinae, particularly in women.     

Muscle Adaptation to Extreme Endurance Training in Man

To evaluate the effect of extreme endurance training on muscle fibre composition and activities of oxidative enzymes in different fibre types biopsies were taken from vastus lateralis, gastrocnemius and deltoideus of elite orienteers. Comparisons were made between the (trained) leg muscles and the (relatively untrained) arm muscles, and with leg muscles of 16–18 years old boys. The orienteers had the same percentage type I fibres in vastus lateralis and gastrocnemius as in deltoideus, but higher percentage type I fibres in vastus lateralis compared with the controls. The similarity between trained and untrained muscle in the orienteers suggests that training had not caused the high percentage type I fibres which rather might be the result of selection of individuals with the best prerequisites for high oxidative capacity. However, the distribution of type II subgroups in the leg muscles of the orienteers differed from both their own deltoideus and leg muscles of the controls, the relationship IIA/IIB being altered in favour of the more oxidative IIA. The leg muscles of the orienteers also showed an increased occurrence of the normally rare IIC fibre. These latter findings point at the possibility of a training induced alteration in the subgroup pattern. Unlike in the controls there was no significant difference in succinate dehydrogenase activity, measured in single fibres, between type I and II fibres in gastrocnemius of the orienteers. Thus, type II fibres have the ability metabolically to adapt to high oxidative demands. This might to some extent be mediated by a conversion from IIB to IIA form.     

Muscle fibre number is a possible determinant of muscle fibre composition in rats

The purpose of the present study was to investigate whether the muscle fibre composition is related to the number of muscle fibres. To resolve this issue, we developed fast-twitch fibre dominant rats (FFDR) by selective breeding and compared the findings to those of control rats (CR) obtained by random breeding. Percentage of type I fibres of the deep portion of gastrocnemius (DG), soleus (SOL), vastus intermedius (VI), adductor longus (AL), and biceps brachii (BB) muscles in FFDR were lower than CR. Percentage of type IIB fibres in DG, VI and AL and percentage of type IIA fibres of SOL in FFDR were higher than CR. However, fibre composition of plantaris (PLAN), extensor digitorum longus (EDL), rectus abdominis (RA), diaphragm (DIA), and palmaris longus (PL) muscles in FFDR were identical with CR. Total fibre numbers on the cross-sectional area in SOL, PLAN, EDL, AL and PL were counted. Numbers of type I fibres of all those muscles in FFDR were not different from CR. Numbers of type IIA fibres of SOL and AL and of type IIB fibres of AL in FFDR were greater than CR, but there were no significant differences in the number of type IIA or type IIB fibres of PLAN, EDL or PL between the two groups. Based on these observations, it is suggested that there are pleiotropic and muscle-specific effects on muscle fibre composition. In addition, the number of type II fibres is a possible determinant of muscle fibre composition.     

Distribution patterns of muscle fibre types in major muscles of the bull (Bos taurus).

Summary The study describes the variations in distribution and cross-sectional area (fibre size) of three muscle fibre types (I, IIA, IIB) in 34 of the largest muscles of the bull (Bos taurus). The animals had been kept strictly unexercised for one year before slaughter. Representative sampling was done at 15 positions within each muscle, and from 2700 to 4500 fibres were analysed in each muscle. Different intermuscular patterns are described. The overall volume fraction (%) of type I fibres was about 10% higher in the forepart muscles than in the hindpart muscles (41% and 31%, respectively), while the mean content of type IIB fibres was similar. Type I fibres were particularly abundant in antigravity muscles. Of these, the hindlimb muscles contained 50% more type I fibres (by weight) than those of the forelimb. Typical antigravity antagonists contained very few type I fibres. In the thigh cross-section the proportion of type I fibres was highest in the anterior and medial parts, while the IIB fibres tended to be concentrated in the superficial and posterior parts. Intramuscular patterns were revealed, with type I fibres becoming gradually more abundant from superficial to deep regions, while IIB fibres had an opposite distribution. This was particularly evident in the thigh proper and in the scapular region. Within each fasciculus of all the muscles, the I fibres in the muscles of the forepart were on average about 15% larger than those of the muscles in the hindpart. The IIB fibres were on average about 10% larger in the hindpart than in the forepart muscles. A covariation between the proportion of type I and IIB fibres and their cross-sectional area was indicated.     

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.     

Metabolic Characteristics of Fibre Types in Human Skeletal Muscle

Muscle biopsy samples were obtained from healthy subjects in order to evaluate quantitative differences in single fibres of substrate (glycogen and triglyceride) and ion concentrations (Na⁺ and K⁺) as well as enzyme activity levels (succinate-dehydrogenase, SDH; phosphofructokinase, PFK; 3-hydroxyacyl-CoA-dehydrogenase, HAD; myosin ATPase) between human skeletal muscle fibre types. After freeze drying of the muscle specimen fragments of single fibres were dissected out and stained for myofibrillar-ATPase with preincubations at pH's of 10.3, 4.6, and 4.35. Type I (“red”) and II A, B, and C (“white”) fibres could then be identified. Glycogen content was the same in different fibres, whereas triglyceride content was highest in Type I fibres (2–3 × Type II). No significant differences were observed for Na⁺ and K⁺ between fibre types. The activity for the enzymes studied were quite different in the fibre types (SDH and HAD, Type I ? 1.5 × Type II; PFK Type I ? 0.5 × Type II; Myosin ATPase Type I ? 0.4 × Type II). The subgroups of Type II fibres were distinguished by differences in both SDH and PFK activities (SDH, Type II C > A > B; PFK, Type II B > A ? C). It is concluded that contractile and metabolic characteristics of human skeletal fibres are very similar to many other species. One difference, however, appears to be that no Type II fibres have an oxidative potential higher than Type I fibres.     

Histochemical and ultrastructural characteristics of leg muscle fibres in patients with repairative abdominal aortic aneurysm (AAA)

Tibialis anterior (ta) muscle biopsies before and after elective abdominal aortic aneurysm (AAA) repair operation were obtained, in order to observe possible changes after the aortic declamping reperfusion. Open muscle biopsies were taken from each of eight patients (60-75 years old) which were processed for enzyme histochemistry, and for transmission electron microscopy (EM). Morphometric analysis was applied to estimate the number and the area of muscle fibres of each fibre type. Rectus abdominis muscle biopsies were served as controls. Before the operation the predominant elements found were the presence of atrophic muscle fibres, fibre size diversity, localised cellular reactions, increased extent of connective tissue, disappearance, in many cases, of the mosaic pattern, predominance of type I and oxidative fibres, and existence of fibres with core-like structures in the sarcoplasm. Type I fibres consisted of 66.95 +/- 9% of all muscle fibres, the mean cross sectional area of which was 3,372.8 +/- 1,016 microm(2) and of type II fibres was 3,786.5 +/- 6,046 microm(2). After the aortic clamping was performed mitochondrial swelling was found, as well as disorganisation of sarcomeres. After declamping of the aorta, there were also severe edema, local fibre necrosis, and adhesion of leucocytes, whereas muscle fibre areas became 3,935.18 micro 531 microm(2) for type I and 5,804 +/- 1,075 microm(2) for type II. The short ischemic period during aortic clamping and the subsequent reperfusion resulted mainly in ultrastructural changes.     

Fibre type composition of the human psoas major muscle with regard to the level of its origin

The aim of our study was to explore the fibre type composition of the human psoas major muscle at different levels of its origin, from the first lumbar to the fourth lumbar vertebra, and to compare the muscle fibre size and distribution of different fibre types between levels with respect to its complex postural and dynamic function. Muscle samples were collected from 15 young males (younger than 35 years). Serial transverse sections (5 μm) of the samples were cut by cryomicrotome. Type I, IIA and IIX muscle fibres were typed using myosin heavy chain identification. The serial sections were analysed using a light microscope with a magnitude of 100×. The differences between measurements were evaluated using a repeated‐measures anova and Scheffé test for post‐hoc analysis. Our study showed that the human psoas major muscle was composed of type I, IIA and IIX muscle fibres. It had a predominance of type IIA muscle fibres, whereas type I muscle fibres had the largest cross‐sectional area. Type IIX muscle fibres were present as a far smaller percentage and had the smallest cross‐sectional area. Moreover, the fibre type composition of the psoas major muscle was different between levels of its origin starting from the first lumbar to the fourth lumbar vertebra. We conclude that the fibre type composition of the psoas major muscle indicated its dynamic and postural functions, which supports the fact that it is the main flexor of the hip joint (dynamic function) and stabilizer of the lumbar spine, sacroiliac and hip joints (postural function). The cranial part of the psoas major muscle has a primarily postural role, whereas the caudal part of the muscle has a dynamic role.     

Differences of morphometrical parameters in hind limb muscle fibres between ovarectomized and sexually intact female dogs.

Slow and fast twitch fibres of the Mm. tibialis cranialis, semitendinosus and sartorius of seven sexually intact and seven ovarectomized female beagles were histochemically and morphometrically analysed. Along with type I and type IIA fibres, another main type II fibre (IIS), which seems to be peculiar to the dog, was found in the Mm. semitendinosus and tibialis cranialis. Type I fibers comprised 26% and type II fibres 74% of all recorded muscle fibres in the M. tibialis cranialis, 29% (type I) and 71% (type II) in the M. semitendinosus and 51% (type I) and 49% (type II) in the M. sartorius, respectively. The average single profile area and the corresponding mean diameter of fibre types I and II in the investigated hind limb muscles were generally larger in ovarectomized than in sexually intact animals. This was more evident in type II than in type I fibres. However only the type II fibres of the M. tibialis cranialis and sartorius exhibited a statistically significant increase in diameter (p < 0.01 and p < 0.05, respectively). Accordingly, the mean density (number of fibres/mm2) of both fibre types in the hind limb muscles of spayed dogs was generally reduced. Again, this reduction attained statistical relevance in the type I and II fibres of the tibialis cranialis. In addition, the fibre densities of type I in the semitendinosus and type II in the sartorius muscles were also significantly reduced in ovarectomized dogs. In conclusion, ovarectomized beagles showed a generally increased mean diameter of the investigated type I and II hind limb muscle fibres and a concomitant decreased average fibre density of the respective types when compared to sexually intact animals.     

Metabolic characteristics of fibre types in human skeletal muscle.

Muscle biopsy samples were obtained from healthy subjects in order to evaluate quantitative differences in single fibres of substrate (glycogen and triglyceride) and ion concentrations (Na+ and K+) as well as enzyme activity levels (succinate-dehydrogenase, SDH; phosphofructokinase, PFK; 3-hydroxyacyl-CoA-dehydrogenase, HAD; myosin ATPase) between human skeletal muscle fibre types. After freeze drying of the muscle specimen fragments of single fibres were dissected out and stained for myofibrillar-ATPase with preincubations at pH's of 10.3, 4.6, 4.35. Type I ("red") and II A,B, and C ("white") fibres could then be identified. Glycogen content was the same in different fibres, whereas triglyceride content was highest in Type I fibres (2-3 X Type II). No significant differences were observed for Na+ and K+ between fibre types. The activity for the enzymes studied were quite different in the fibre types (SDH and HAD, Type I is approximately 1.5 X Type II; PFK Type I is approximately 0.5 X Type II, Myosin ATPase Type I is approxiamtely 0.4 X Type II). The subgroups of Type II fibres were distinguished by differences in both SDH and PFK activities (SDH, Type II C is greater than A is greater than B; PFK, Type II B is greater than A is approximately C). It is concluded that contractile and metabolic characteristics of human skeletal fibres are very similar to many other species. One difference, however, appears to be than no Type II fibres have an oxidative potential higher than Type I fibres.     

Quantification of fibre type regionalisation: an analysis of lower hindlimb muscles in the rat

Newly developed concepts and methods for the quantification of fibre type regionalisation were used for comparison between all muscles traversing the ankle of the rat lower hindlimb (n = 12). For each muscle, cross‐sections from the proximodistal midlevel were stained for myofibrillar ATPase and classified as type I (‘slow’) or II (‘fast’). For the 11 ‘fast’ muscles (i.e. all except soleus), the muscle outline and the position of each type I fibre were digitised for further computer processing. Two potentially independent aspects of type I fibre regionalisation were evaluated quantitatively: (1) the degree to which type I fibres were restricted to a limited portion of the total cross‐sectional area (‘area‐regionalisation’) ; (2) the extent and direction of the difference (if any) between the centre of the muscle cross‐section and the calculated centre for the type I fibre cluster (‘vector regionalisation’). Statistical analysis showed that type I fibres were vector regionalised in practically all investigated muscles and area regionalised within most of them, the only consistent exceptions being peroneus brevis and peroneus digitorum 4, 5. In muscles with a high degree of area regionalisation the population of type I fibres also had a markedly eccentric intramuscular position (i.e. high vector regionalisation). A significant relationship was observed between the relative position of a muscle within the hindlimb (transverse plane) and the direction and degree of its type I fibre eccentricity. On average, the degree of type I fibre eccentricity was greater for muscles remote from the limb centre than for those situated more centrally. In addition, the intramuscular concentration of type I fibres was typically greatest towards the centre of the limb, the most striking exception being tibialis posterior. For the slow soleus muscle, which is centrally placed within the limb, our analysis concerned the type II fibres, which were found to be weakly vector regionalised but not significantly area regionalised. It is concluded that, within muscles of the rat's lower hindlimb, fibre type regionalisation is a general and graded phenomenon which may reflect differentiating (embryological?) mechanisms of a transmuscular significance. Furthermore, the analysis demonstrated the usefulness of our new methods and concepts for the quantification of fibre type regionalisation.     

Segmental fibre type composition of the rat iliopsoas muscle

The iliopsoas of the rat is composed of two muscles – the psoas major muscle and the iliacus muscle. The psoas major muscle arises from all the lumbar vertebrae and the iliacus muscle from the fifth and sixth lumbar vertebrae and ilium. Their common insertion point is the lesser trochanter of the femur, and their common action is the lateral rotation of the femur and flexion of the hip joint. Unlike humans, the rat is a quadruped and only occasionally rises up on its hind legs. Therefore, it is expected that the fibre type composition of the rat iliopsoas muscle will be different than that of humans. The iliopsoas muscle of the rat is generally considered to be a fast muscle. However, previous studies of the fibre type composition of the rat psoas muscle showed different results. Moreover, very little is known about the composition of the rat iliacus muscle. The aim of our study was to examine the fibre type composition of the rat iliopsoas muscle in order to better understand the complex function of the listed muscle. The psoas major muscle was examined segmentally at four different levels of its origin. Type I, IIA, IIB and IIX muscle fibres were typed using monoclonal antibodies for myosin heavy chain identification. The percentage of muscle fibre types and muscle fibre cross‐sectional areas were calculated. In our study we showed that in the rat iliopsoas muscle both the iliacus and the psoas major muscles had a predominance of fast muscle fibre types, with the highest percentage of the fastest IIB muscle fibres. Also, the IIB muscle fibres showed the largest cross‐sectional area (CSA) in both muscles. As well, the psoas major muscle showed segmental differences of fibre type composition. Our results showed changes in percentages, as well as the CSAs of muscle fibre types in cranio‐caudal direction. The most significant changes were visible in type IIB muscle fibres, where there was a decrease of percentages and the CSAs from the cranial towards the caudal part of the muscle. From our results it is evident that the rat iliopsoas muscle has a heterogeneous composition and is composed of all four muscle fibre types. Primarily, it is a fast, dynamic muscle with a predominance of fast type IIB muscle fibres with the largest CSAs. The composition of the rat psoas major muscles changes in a cranio‐caudal direction, thus pointing to a more postural role of the caudal part of the muscle.     

Calcium sensitivity and myofibrillar protein isoforms of rat skinned skeletal muscle fibres

We investigated the calcium sensitivity for tension generation of different fibre types and the possible correlation between calcium sensitivity and the presence of distinct regulatory protein and myosin light chain (MLC) isoforms in rat skinned skeletal muscle fibres. Fibre types 1, 2A and 2B were identified by electrophoretic analysis of myosin heavy chain (MHC) isoforms. Fibres showing more than one MHC isoform were discarded. Type 1 fibres from the soleus showed a higher pCa (–log₁₀ [Ca], where [ ] denotes concentration) threshold and a lower slope of pCa/tension curve than type 2 extensor digitorum longus (EDL) fibres; between type 2 fibres, type 2B showed the higher slope of pCa/tension curve. Type 1 fibres from different muscles showed similar calcium sensitivities when containing only the slow set of regulatory proteins and MLC; when both slow and fast isoforms were present, calcium sensitivity shifted toward fast type fibre values. Type 2A fibres from different muscles showed a similar calcium sensitivity, independently of the set (purely fast or mixed) of regulatory proteins and MLC. It is suggested that when both fast and slow isoforms of regulatory proteins and of MLC are present in a muscle fibre, calcium sensitivity is dictated mainly by the fast isoforms.     

Fibre type regionalisation in lower hindlimb muscles of rabbit, rat and mouse: a comparative study

The topographical distribution of different fibre types in muscles of the lower hindlimb in rabbits and mice was quantitatively determined. The results were compared to those previously obtained, using the same new quantification methods, in homologous muscles of the rat. Type I fibres ('slow') were identified using myofibrillar ATPase histochemistry and mapped out at the mid proximo-distal level for 11 'fast' muscles in the rabbit and 7 'fast' muscles in the mouse. For the slow soleus muscle the procedure was undertaken for the type II fibres. Furthermore, for 5 of the 'fast' muscles from each animal species (extensor digitorum longus; flexor digitorum and hallucis longus; gastrocnemius medialis; peroneus longus; tibialis anterior), several more proximal and distal cross-sectional levels were also analysed. All the investigated 'fast' muscles showed a significant degree of topographical eccentricity in the midlevel distribution of type I fibres. For most muscles, the direction of this 'vector regionalisation' of type I fibres was similar between the three animal species. For homologous muscles, the degree of vector regionalisation was significantly different: mouse > rat > rabbit. The relative area of the region containing the type I fibres, inversely related to the degree of 'area regionalisation', was also significantly different: mouse < rat < rabbit. Also within each animal species, muscles with a marked degree of vector regionalisation tended to show a marked area regionalisation. Proximo-distal differences in type I fibre density were observed in all the three species of animals; also these patterns showed marked inter-species differences. The findings demonstrate the general occurrence of, and systematic relationships between, different aspects of type I fibre regionalisation. The observed interspecies differences suggest that the expression of this phenomenon is adapted to differing functional needs.     

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.     

Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.