Capillary supply of the muscle fibre population in hindlimb muscles of the cat

Comparative analyses of the fibre content (FG, FOG, and SO fibres) and the capillary density (the number of capillaries surrounding individual fibres and the capillary/fibre ratio) were performed in hind limb muscles of the cat. Cross-sections from the tenuissimus, the biceps femoris, the lateral head (LG) and the medial head (MG) of the gastrocnemius and the soleus were cut in a cryostat. The sections were stained histochemically for the NADH₂-diaphorase and alkaline (pH 9.4) actomyosin ATPase activity, which enables differentiation of different types of fibres. The endothelium of the capillaries was identified via staining for unspecific alkaline ATPase activity. The number of capillaries surrounding each individual muscle fibre had a positive correlation, first to the oxidative capacity and secondly to the average diameter of the fibres. The thin tenuissimus muscle did not differ in this respect from the thicker muscles. The highest proportion of SO fibres was found in the soleus and the MG muscles. FG fibres of two different types were dominating the fibre mass in the biceps femoris and the LG muscles, while the tenuissimus contained more FOG fibres than these muscles. In general the FG fibres had a larger diameter than the FOG and the SO fibres. The soleus and the MG muscles contained larger fibres than the other examined muscles. FG fibres were surrounded by fewer capillaries than FOG and SO fibres. The soleus and the MG muscles, with a higher percentage of SO fibres and also larger fibres, had the largest number of capillaries around the fibres and the highest capillary/fibre ratio.     

Ultrastructural and metabolic characteristics of single muscle fibres belonging to the same type in various muscles in rats

Summary Single muscle fibres from soleus (SOL) as well as extensor digitorum longus (EDL) muscles from Wistar male rats in relaxing solution were divided into three types by their histochemical features — slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), or fast-twitch glycolytic (FG) fibres. The relationship between ultrastructural profiles (mitochondrial volume, number, and Z-line width) and metabolic profiles (glycolytic and oxidative enzymes' activity) were analysed using the same types of fibres dissected from different SOL and EDL muscles using stereological and biochemical techniques. The Z-line width is specialized in different fibre types. Fast-twitch (FG and FOG) fibres have narrow Z-line width compared to slow-twitch (SO) fibre in SOL and EDL muscles. A significant difference was observed between SOL muscle SO and FOG fibres and EDL muscle SO and FOG fibres. All glycolytic (lactate dehydrogenase (LDH), phosphofructokinase (PFK), pyruvate kinase (PK) and creatine kinase (CK) activities in FOG fibres from EDL muscles were significantly higher (p<0.01) than those dissected from SOL muscles. The oxidative enzyme (succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) activity in SO and FOG fibres dissected from SOL muscle were significantly higher (p<0.01) than those dissected from EDL muscles. Mitochondrial volume and number in SO fibres dissected from SOL muscle were significantly higher (volume,p<0.01, number,p<0.01) than those dissected from EDL muscles. A significant difference was not observed in mitochondrial volume of FOG fibres between SOL and EDL muscles. Significant positive correlation was observed in FOG (p<0.05) and FG (p<0.01) fibres between mitochondrial volume and number dissected from EDL muscle.The results suggest that the same type of single muscle fibres in different muscles have different ultrastructural and metabolic profiles, and these profiles resembled those of the fibre types primarily constituting those muscles.     

Different metabolic responses to exercise training programmes in single rat muscle fibres

Summary The aim of this report is to elucidate the effects of exercise training on metabolic properties of different muscle fibre types of the rat hindlimb. Single muscle fibres were dissected from soleus (SOL) or extensor digitorum longus (EDL) muscles of Wistar strain male rats trained on a treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic). Then glycolytic and oxidative enzymes (CK, LDH, PFK, PK, SDH, and MDH) activities were measured biochemically. Slow,-type fibres (SO) were hypertrophied following endurance training and fast-twitch fibres (FOG and FG) were hypertrophied following sprint training. In EDL muscles the distribution of the slow-type fibres was reduced following the sprint training. The activity of glycolytic enzymes increased significantly in the fast-type fibres (FOG and FG) following sprint training, while oxidative enzymes activities increased in both fast (FOG and FG) and slow (SO) muscle fibres following the endurance training. Neither glycolytic nor oxidative enzymes' activities always increased equally in all types of fibre following exercise training. Consequently, the metabolic profiles in each type of single muscle fibre were affected differently by different intensities of exercise training. These results suggest that the functional (enzymes activity) and structural (muscle fibre hypertrophy) changes of skeletal muscle fibre following exercise training appeared gradually, and would be controlled by different factors.     

Differences in distribution of myofiber types between the supraspinatus and infraspinatus muscles of sheep

Background: The m. supraspinatus stabilizes the shoulder joint to bear the body weight, and the m. infraspinatus assists in extension and flexion of the joint in sheep. Postural muscles have many SO myofibers, whereas locomotory muscles have numerous fast-twitch myofibers. In sheep the distribution of myofiber types within the two muscles, necessary for a better understanding of postural function, remains to be clarified. Methods: Muscle samples were removed from the whole transverse sections of the dorsal, middle, and ventral compartments of the m. supraspinatus and m. infraspinatus of sheep. Myofibers were classified into FG, FOG, SO-1, and SO-2 myofibers by histochemical methods. Results: The distribution of SO myofibers changed more greatly in the m. supraspinatus (15.0–99.1%) than in the m. infraspinatus (24.5–62.3%). SO myofibers were concentrated markedly in the caudal and deep regions near the spine and fossa of the scapula in the m. supraspinatus and distributed more in the medial part than in the lateral part in the m. infraspinatus. Such changes were caused by increases in percentage of SO-2 myofibers and not SO-1 myofibers. The craniolateral regions of the m. supraspinatus and the caudolateral regions of the m. infraspinatus had many fast-twitch (FOG plus FG) myofibers suited for rapid extension and flexion of the shoulder joint. Conclusions: The m. supraspinatus has the compartmentalized, deep, and caudal regions occupied by SO myofibers, which seem to be specialized for maintenance of the joint extension. The medial region of the m. infraspinatus may assist in the joint stabilization. © 1995 Wiley-Liss, Inc.     

Distribution of microspheres in plantaris muscles of resting and exercising rats as a function of fiber type

The purpose of this study was to determine histologically the distribution of microspheres (MSs) (14 μm), and hence the relative distribution of blood flow, in rat plantaris muscle relative to the fiber types (fast-twitch-oxidative-glycolytic [FOG], fast-twitch-glycolytic [FG], and slow-twitch-oxidative [SO]). Three conditions were investigated: (1) preexercise standing; (2) treadmill locomotion at 15 m/min (fast walking); and (3) treadmill locomotion at 60 m/min (moderate galloping). The MS suspension (containing 1 × 10⁶ MSs) was infused into the ascending aorta via a catheter in the carotid artery under each of the 3 conditions so that MSs were distributed to the tissues in proportion to their respective blood flows. Sections (20 m?m) of the plantaris muscle were cut and assayed for reduced nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) and myofibrillar adenosine triphosphatase (ATPase) activities so the fibers could be typed as SO, FOG, or FG. MSs were located in the NADH-TR sections, and the fibers next to the MSs were classified and counted. The observed numbers of fibers of each type in each condition that were adjacent to MSs were compared to the predicted number of adjacent fibers based on the assumption the MSs were randomly distributed in the tissue. This analysis demonstrated that MSs (and blood flows) were preferentially distributed to SO fibers during preexercise, to SO and FOG fibers during slow locomotion, and to FOG fibers during fast locomotion. The data support the contention that blood flow is distributed in muscles of conscious animals as functions of fiber type and exercise intensity.     

Activities of creatine kinase isoenzymes in single skeletal muscle fibres of trained and untrained rats

Biochemical changes in the creatine kinase isoenzyme compositions in single muscle fibres of different types in rats were induced by endurance running training. Single muscle fibres were dissected from the soleus and extensor digitorum longus muscles of Wistarstrain male rats trained on a motor-driven treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic), and the activities of total creatine kinase and its four isoenzymes (CK-MM, -MB,-BB, and mitochondrial creatine kinase) were measured. The endurance training did not affect the total creatine kinase activity, but resulted in significantly increased activities of CK-MB and CK-BB in SO and FOG fibres, and the mitochondrial enzyme activity in FOG and FG fibres. Endurance training induced biochemical changes in the isoenzyme compositions, specifically in FOG fibres. These results suggest that changes in creatine kinase isoenzymes with endurance training reflect changes in the energy metabolism in the different muscle fibres, supporting the hypothesis that the different isoenzymes play different roles in energy transduction.     

Histochemical fiber type composition in 12 skeletal muscles of miniature pigs

In 12 skeletal muscles of adult miniature pigs histochemical fiber type analysis was performed. Longissimus dorsi, psoas major, diaphragm and fore- and hind-limb muscles were composed from SO, FOG and FG fiber types showing characteristic clump distribution. The muscles differed in the first place by the frequencies of SO and FG types (7-70% and 19-79% respectively); the proportion of FOG type varied considerably less (10-32%). On the other hand in the masseter muscle only SO and FOG types with a mosaic distribution were demonstrated (28% and 72% respectively). In the majority of muscles the prevailing fiber type has also the largest diameter, but the differences between individual types were mostly insignificant. The largest diameter was found in the masseter muscle fibers--around 70 microns, which is 20-30 microns more than in the other muscles.     

The proximal border of the human respiratory unit,as shown by scanning and transmission electron microscopy and light microscopical cytochemistry

The muscles of the pectoral girdle in domestic animals attach the forelimbs to the trunk and function as the suspensory apparatus. In the present study the composition of the pectoral girdle musculature of sheep by myofiber types was examined. Myofibers showing a strong reaction for alkali-stable myosin ATPase were classified into fast-twitch/glycolytic (FG) myofibers with a weak activity for NADH tetrazolium reductase (NADH-TR) and fast-twitch/oxidative/glycolytic (FOG) myofibers with a moderate and strong NADH-TR activity. Myofibers showing a weak reaction for alkali-stable myosin ATPase and a strong activity for NADH-TR were classified as slow-twitch/oxidative (SO) myofibers. The SO myofibers that showed a granular and striped pattern of diformazan deposits in NADH-TR activity were classified as SO-1 myofibers, whereas the SO myofibers characterized by a reticular pattern of diformazan deposits were classified as SO-2 myofibers. The trapezius, rhomboideus cervicis, and pectoralis descendens muscles situated superficially in the cranial regions of the back and chest had about 50% SO (SO-1 plus SO-2) myofibers. The deeply situated serratus ventralis cervicis and thoracis muscles had 37.5% SO myofibers. These five muscles included more SO-2 myofibers with large diameters than did all other muscles, and had about 50% and more cross-sectional area of SO myofibers. The other muscles had less than 32% SO myofibers and fewer SO-2 myofibers. The FOG and FG myofibers accounted for 50% or less in the muscles examined. Many muscles of the pectoral girdle had many fast-twitch (FOG plus FG) myofibers; they seem to meet locomotory requirements. In the pectoral girdle musculature, the SO myofibers were not necessarily distributed more in the deep regions than in the superficial regions. The distribution of SO myofibers appears to meet postural requirements for stabilizing the shoulder and brachium and for supporting the trunk.     

Differentiation of rat skeletal muscle fibres during development and ageing

The purpose of the present study was to determine at which point in the period from embryonic day 21 up to postnatal day (PD) 75, the different fibre types and subtypes are detectable in rat extensor digitorum longus, soleus and gastrocnemius muscles using immunohistochemical, enzyme histochemical and cytophotometrical methods. Moreover, fibre type-specific changes in metabolic profile and changes in fibre type population during postnatal development were analysed. Before birth, no clear differentiation of fibre types was found. At PD 1, slow and fast fibres were typed by antibodies against neonatal, slow and fast myosin heavy chains (MHCs). At PD 8, the different ATPase activities of slow and fast MHCs after alkaline preincubation were detected histochemically. At PD 21, differences in acid stability of ATPase activity of fast MHC isoforms revealed the fast subtypes IIA and IIB (including IIX). At this age, also differences in metabolic properties (oxidative and glycolytic enzyme activities) of fibres were detected for the first time by cytophotometry classifying the fibres into SO, FOG I, FOG II and FG. Before the age of 21 days, the fast fibres were metabolically undifferentiated. During further development and ageing, the population of FG fibres with high glycolytic activity increased at the expense of FOG fibres suggesting FOG to FG transformation. Cytophotometrical measurements revealed that the muscle fibres developed their highest contractile, oxidative and glycolytic activity at PD 21, the time of weaning. In this way, muscle fibres may be prepared for the higher demands for posture and mobility after leaving the nest.     

Effect of growth on muscle capillarity and fiber type composition in rat diaphragm

Summary The effect of growth on the capillarity and fiber type composition of the diaphragm, soleus and extensor digitorum longus (EDL) muscles of rats weighing between 55 and 330 g have been studied. Muscle samples obtained from the anesthetized rat were rapidly frozen and sliced transversely in a cryostat. The sections were stained histochemically by the SDH method and the myosin ATPase method after preincubation at pH 4.3 to typify fibers (FG, FOG and SO fibers). To visualize capillaries, the myosin ATPase method after preincubation at pH 4.0 was used. The percentage of FOG fibers decreased in all muscles with growth. While the FG and SO fibers increased in the diaphragm, SO fibers increased in the soleus, and FG fibers increased in the EDL. The capillary density showed a hyperbolic decrease with growth in all muscles, while the number of capillaries around each fiber increased in all muscles with growth. It is concluded that growth causes the changing properties of the motoneurons and the new capillary formation in the diaphragm muscle, as well as the soleus and EDL muscles.     

Muscle fibre composition of ratvastus intermedius following immobilisation at different muscle lengths

The knee joints of one hind limb of male Wistar rats (250–300 g) were immobilised in a plaster cast for up to 3 weeks with the limb in either the fully extended or flexed position Serial frozen sections ofvastus intermedius muscle were stained histochemically for myosin ATPase, succinic dehydrogenase, phosphorylase and capillaries. Muscle fibres were classified as either slow twitch oxidative (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG).Fibre area of SO fibres (3019 ²) decreased by 50% following 3 weeks' immobilisation in the stretched (1532 ²) and shortened (1517 ²) positions. Atrophy of FOG fibres (2456 ²) was greater in muscles immobilised in the stretched (750 ²) than shortened (1185 ²) positions. Evidence has been obtained that muscle fibre number remained unchanged following immobilisation. Control muscles contained 70.4% SO fibres, 29.6% FOG fibres and >0.5% FG fibres. Immobilisation produced an increased proportion of high myosin ATPase staining fibres. The percentage occurrence of FOG fibres increased to 65% and 83% respectively, in muscles immobilised for 3 weeks in the stretched and shortened positions. Mechanisms for the transformation of muscle fibre types and the influence of muscle length on the properties of immobilised muscle are discussed.     

Histochemical fiber types in the thigh muscles of 4 chicken inbred lines

Distribution, frequency and diameter of fiber types were ascertained in the thigh muscles of I, C, W and M chicken inbred lines. The iliotibialis posterior muscle is composed of FOG and FG types uniformly distributed over the whole muscle cross-section. Three fiber types are demonstrated in the sartorius and semimembranosus muscles. In the superficial regions, the FOG and FG types are present only. Avian SO type appears down into the depth of the muscles. The frequency of this type and FOG type gradually increases in this direction whereas the proportion of FG type decreases. Semimembranosus muscle shows generally higher percentage of FOG fibers and lower frequency of remaining two types. Differences between inbred lines in fiber type frequencies are mostly insignificant. However, the M line with the highest live and muscle weights shows larger fiber diameter of FG and FOG types than I, C and W lines.     

Comparative histochemical study of prosimian primate hindlimb muscles. II. Populations of fiber types

The populations of fiber types in hindlimb muscles of the tree shrew (Tupaia glis), lesser bushbaby (Galago senegalensis), and the slow loris (Nycticebus coucang) were described and an attempt was made to correlate populations of fiber types and locomotor patterns. Muscle fibers were assigned to one of the following groups: fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), and slow-twitch oxidative (SO). Histochemical techniques for the demonstration of alkaline- and acid-stable ATPases, succinic dehydrogenase, and mitochondrial á-glycerophosphate dehydrogenase were used in the classification of muscle fibers. Results indicated that the FG fiber type is the predominant fiber type in muscles used for jumping, the FOG fiber type is predominant in muscles used for running, and the SO fiber type occurs in high percentages in postural muscles. The SO fiber was also the most common fiber in muscles of the slow loris--a species that exhibits a slow, deliberate, sustained locomotor pattern. Intramuscular regional variations in populations were seen in some larger muscles of the tree shrew, but not in the lesser bushbaby and slow loris. Our results did not support the contentions of others that analogous muscles in different species have similar populations of fiber types.     

Ultrastructural and metabolic profiles on single muscle fibers of different types after hindlimb suspension in rats

The relationships between ultrastructural and metabolic profiles in different types of single muscle fiber after hindlimb suspension in rats were examined. Glycolytic (lactate dehydrogenase, LDH; phosphofructokinase, PFK) and oxidative (succinate dehydrogenase, SDH; malate dehydrogenase, MDH) enzyme activities in extensor digitorum longus (EDL) and soleus (SOL) muscles were measured. Relative mitochondrial and lipid droplet volumes were also measured in single muscle fiber of different types. Glycolytic enzyme activity in EDL muscle and oxidative enzyme activity in soleus muscle decreased following suspension for 2 weeks. LDH and PFK activities in fast-twitch (FG, fast-twitch glycolytic; FOG, fast-twitch oxidative glycolytic) fibers and oxidative enzymes in FOG and FG fibers decreased following suspension. Relative mitochondrial volume decreased significantly in all types (SO, slow-twitch oxidative; FOG, and FG) of fibers following suspension. The mitochondrial volume in SO fiber of the control group was significantly (p less than 0.01) higher than that of suspended group; however, SDH and MDH activities were not different between the control and suspended groups. The structural and metabolic changes following hindlimb suspension were influenced by different factors, respectively. Changes in ultrastructural and metabolic profiles in response to the hindlimb suspension differed according to the type of fibers.     

Substrate oxidation specificity in different types of mammalian muscle.

The maximal capacity of homogenates prepared from cardiac, fast-oxidative-glycogenolytic (FOG) vastus lateralis, fast-glycogenolytic (FG) vastus lateralis, and slow-oxidative (SO) soleus muscle to oxidize pyruvate, palmitate, alpha-glycerophosphate, and acetoacetate was assessed by measuring oxygen consumption under conditions of nonlimiting substrate and cofactors. Pyruvate oxidation varied eight-fold among the muscle types and was highest in cardiac, followed by FOG, SO, and FG muscle. Palmitate was oxidized at 97%, 85%, 77%, and 57% of the relative rate for pyruvate in cardiac, FOG, SO, and FG muscle, respectively. In contrast, alpha-glycerophosphate oxidation rates were highest in FG muscle, followed by cardiac, FOG, and SO muscle. Although cardiac muscle possessed the highest absolute rate for acetoacetate oxidation, it had the lowest capacity relative to pyruvate (19%), whereas SO muscle possessed the highest (61%). FOG and FG muscle had similar relative capacities for this substrate (30%). These results provide further evidence to suggest that mammalian muscle types are differentiated in terms of both mitochondrial mass and substrate oxidation specificity.     

Histoenzymology and morphometry of the masticatory muscles of tufted capuchin monkey (Cebus apella Linnaeus, 1758)

Samples of the anterior and posterior regions of the masseter and temporal muscles and of the anterior belly of the digastric muscle of 4 adult male tufted capuchin monkeys (Cebus apella) were removed and stained with HE and submitted to the m-ATPase reaction (with alkaline and acid preincubation) and to the NADH-TR and SDH reactions. The results of the histoenzymologic reactions were similar, except for acid reversal which did not occur in fibers of the fast glycolytic (FG) type in the mandibular locomotor muscles. FG fibers had a larger area and were more frequent in all regions studied. No significant differences in frequency or area of each fiber type were detected, considering the anterior and posterior regions of the masseter and temporal muscles. The frequency of fibers of the fast oxidative glycolytic (FOG) and slow oxidative (SO) types and of FOG area differed significantly between the anterior belly of the digastric muscle and the mandibular locomotor muscle. The predominance of fast twitch (FG and FOG) fibers and the multipenniform and bipenniform internal architecture of the masseter and temporal muscles, respectively, are characteristics that permit the powerful bite typical of tufted capuchin monkeys.     

Profiles of creatine kinase isoenzyme compositions in single muscle fibres of different types

Summary Creatine kinase (CK) isoenzyme compositions of different types of single muscle fibres dissected from soleus (SOL) and extensor digitorum longus (EDL) muscles from rats were examined. CK isoenzymes were separated into cytoplasmic (CK-MM, CK-MB, CK-BB) (muscle, brain and hybrid types, respectively) and mitochondrial (m-CK) isoenzymes. Total CK and CK-MM activities showed the highest activities in fast-twitch glycolytic fibres (FG), lower in fast-twitch oxidative glycolytic (FOG) and the lowest in slow-twitch oxidative (SO) fibres. Conversely, the activity of m-CK was highest in SO, lowest in FG and intermediate in FOG fibres. The activity of CK-MB was highest in SO and lower in FOG and FG fibres. However, the activities of total CK and CK isoenzymes in a single muscle fibre type were not distinguishable from those of another type, and the profiles of CK isoenzyme compositions from the same type of single muscle fibres overlapped over a considerable range. The relationships between the four CK isoenzymes activities in single muscle fibres of different types were not similar. These results suggest that CK isoenzymes of single muscle fibres of different types play different roles in intracellular energy metabolism. Therefore, it is supposed that the CK isoenzyme compositions of single muscle fibres are suitable for their contractive and metabolic properties.     

Effect of hyperthyroidism on the contractile and histochemical properties of fast and slow twitch skeletal muscle in the rat

Male Wistar rats were made hyperthyroid by intraperitoneal injections on alternate days for 1–6 weeks, of 200 g/kg triiodothyronine (T₃). The effects of this treatment on the contractile properties of the soleus, a slow twitch and the extensor digitorum longus (EDL), a fast twitch skeletal muscle, were studied in vivo in the anaesthetized animal. Post mortem, serial frozen sections of both muscles were stained histochemically for myosin ATPase, succinic dehydrogenase and phosphorylase. Muscle fibres were classified as either slow twitch (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG).Elevation of plasma T₃ levels is associated with progressive alterations in the muscle fibre populations of both muscles. In the soleus there is conversion of SO to FOG fibres while in the EDL the main effect is FG to FOG conversion. There are also marked changes, mainly confined to the soleus muscle, in contractile properties; progressive increases in isometric twitch and tetanic tension and in the rates of contraction and relaxation during both twitch and tetanus.The effect of T₃ on slow muscle contractility may be related to its effect on muscle phenotype. However, changes in the former precede detectable fibre population changes. T₃ may influence properties such as the Ca²⁺ binding activity of sarcoplasmic reticulum of existing slow twitch fibres before the later changes associated with the interconversion of fibre types occur.     

Differences in ultrastructural and metabolic profiles within the same type of fibres in various muscles of young and adult rats

Single fibres from tibialis anterior, extensor digitorum longus, gastrocnemius and soleus muscles in young (4–week–old) and adult (35–week–old) Wistar male rats were classified into three types on the basis of their enzyme–histochemical features: slow–twitch oxidative (SO), fast–twitch oxidative and glycolytic (FOG) and fast–twitch glycolytic (FG) fibres. Ultrastructural (volume density of mitochondria: V_mt and Z line width) and metabolic (phosphofructokinase: PFK and succinate dehydrogenase: SDH activities) profiles were measured. PFK activity in all types of fibres¹ was higher in adult rats, and the difference between the two age–groups (adult/young) was largest between FG, FOG and SO fibres respectively. SDH activity and V_mt were lower in adult rats in a similar way in all fibres. A significant positive correlation was observed between the V_mt and SDH activity in both age–groups. This positive correlation was very specific in fast–twitch and slow–twitch fibres. Changes in the V_mt did not relate directly to the changes in fibre cross–sectional area. The overall pattern indicates that glycolytic capacity of fast–twitch fibres in flexor muscles (TA and EDL) is higher than in extensor muscles (GC and SOL), and that oxidative capacity of all types of fibre in extensor muscles is higher than in flexor muscles. These profiles were changed by growth, and may be related to the specific differences in pattern of activity of each skeletal muscle, and may reflect differences in the recruitment order of different muscles.     

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.