Two-dimensional changes of muscle fiber types in growing rat hind limb

In the present study, we examined the changes in two-dimensional distribution of fiber types in the whole area of the rat skeletal muscle and the effect of growth on this distribution. Muscles of rats aged 3 (body weight 58 g), 4 (89 g), 8 (276 g), 12 (312 g), 18 weeks (368 g), and 6 months (450 g) were stained for myofibrillar adenosine triphosphatase (mATPase) with preincubation at pH 4.35. Muscle fibers were classified into type I (slow oxidative), IIA (fast oxidative), IIB (fast glycolytic), and IIX (fast oxidative glycolytic). The x-y coordinates of each fiber were used to analyze the growth-related changes using an image analyzing system. In the tibialis anterior (TA) muscle, type I fibers were predominant in the deep and middle regions at 3 to 4 weeks of age, but became restricted to the deeper region with growth. In the extensor digitorum longus (EDL) muscle, type I fibers were predominant in the deep region at 3 to 8 weeks of age, but decreased gradually with growth and completely disappeared at 6 months of age. Compared with the TA and EDL, type I fibers of the soleus (SOL) muscle were spread throughout the muscle and the number of these fibers tended to increase with growth. Type IIA and IIX fibers of the SOL decreased in number and became restricted to the superficial region with growth. No type IIB fibers were detected in the SOL throughout life. Our results indicated that the growing process influences the distribution, proportion and characteristics of individual muscle fiber types in the rat hind limb muscles.     

Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition

Skeletal muscles have a heterogeneous fiber type composition, which reflects their functional demand. The daily muscle use and the percentage of slow‐type fibers have been shown to be positively correlated in skeletal muscles of larger animals but for smaller animals there is no information. The examination of this relationship in adult rats was the purpose of this study. We hypothesized a positive relationship between the percentage of fatigue‐resistant fibers in each muscle and its total duration of use per day. Fourteen Wistar strain male rats (410–450 g) were used. A radio‐telemetric device was implanted to record muscle activity continuously from the superficial masseter, deep masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time) exceeding specified levels of the peak activity (2, 5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of the fibers by means of immunohistochemical staining. At lower activity levels (exceeding 2 and 5% of the peak activity), the duty time of the anterior belly of digastric muscle was significantly (P < 0.01) longer than those of the other muscles. The anterior belly of digastric muscle also contained the highest percentage of slow‐type fibers (type I fiber and hybrid fiber co‐expressing myosin heavy chain I + IIA) (ca. 11%; P < 0.05). By regression analysis for all four muscles, an inter‐muscular comparison showed a positive relationship between the duty time (exceeding 50% of the peak activity) and the percentage of type IIX fibers (P < 0.05), which demonstrate intermediate physiological properties relative to type IIA and IIB fibers. For the jaw muscles of adult male rats, the variations of fiber type composition and muscle use suggest that the muscle containing the largest amounts of slow‐type fibers (the anterior belly of digastric muscle) is mainly involved in low‐amplitude activities and that the amount of type IIX fibers is positively related to the generation of large muscle forces, validating our hypothesis.     

Regional differences in fiber characteristics in the rat temporalis muscle

The behavioral differences in muscle use are related to the fiber type composition of the muscles among other variables. The aim of this study was to examine the degree of heterogeneity in the fiber type composition in the rat temporalis muscle. The temporalis muscle was taken from 10‐week‐old Wistar strain male rats (n = 5). Fiber types were classified by immunohistochemical staining according to their myosin heavy chain content. The anterior temporalis revealed an obvious regional difference of the fiber type distribution, whereas the posterior temporalis was homogeneous. The deep anterior temporalis showed a predominant proportion of type IIA fibers and was the only muscle portion displaying slow type fibers (< 10%). The other two muscle portions, the superficial anterior and posterior temporalis, did not differ significantly from each other and contained mainly type IIB fibers. Moreover, the deep anterior temporalis was the only muscle portion showing slow type fibers (< 10%). In the deep portion, type IIX fibers revealed the largest cross‐sectional area (1943.1 ± 613.7 µm²), which was significantly (P < 0.01) larger than those of type IIA and I + IIA fibers. The cross‐sectional area of type IIB fibers was the largest in the remaining two muscle portions and was significantly (P < 0.01) larger than that of type IIX fibers. In conclusion, temporalis muscle in rats showed an obvious heterogeneity of fiber type composition and fiber cross‐sectional area, which suggests multiple functions of this muscle.     

Cortisone-induced changes in myosin heavy chain distribution in respiratory and hindlimb muscles

In this study the effects of administration of cortisone acetate (100 mg kg^-1 body weight subcutaneously for 11 days) on distribution and cross-sectional area of different fibre types of rat skeletal muscles were investigated. Diaphragm, parasternal intercostal (PI), extensor digitorum longus (EDL) and soleus muscles were examined in cortisone treated animals (CA) in comparison with ad libitum controls (CTRL) and pair-fed (PF) controls. Four fibre types (I or slow and IIA, IIX, IIB or fast) were identified on the basis of their myosin heavy chain composition using a set of monoclonal antibodies. In CA rats the reduction of cross-sectional area was above 30% in IIX fibres of diaphragm, IIB fibres of PI and in all fast fibres of EDL. In all muscles slow fibres were spared from atrophy. Significant variations in fibre type distribution were found in the muscles of CA rats when compared to CTRL. The percentage of IIB fibres decreased in EDL, PI and diaphragm. This decrease was accompanied by an increase in the percentage of IIA fibres in the same muscles. No changes in the percentage of slow fibres and of fast IIX fibres were observed in EDL, PI and diaphragm of CA rats in comparison with CTRL. In soleus of CA rats the proportion of IIA fibres was lower than in CTRL. In EDL of PF rats atrophy of IIA fibres and changes in fibre type distribution were similar to those observed in CA rats. In diaphragm, PI and soleus of PF rats no significant decrease in fibre cross-sectional area nor significant changes in fibre distribution were found in comparison with CTRL rats.     

Segmental Distribution of Myosin Heavy Chain Isoforms Within Single Muscle Fibers

Despite many studies looking at the distribution of myosin heavy chain (MHC) isoforms across a transverse section of muscle, knowledge of MHC distribution along the longitudinal axis of a single skeletal muscle fiber has been relatively overlooked. Immunocytochemistry was performed on serial sections of rat extensor digitorum longus (EDL) muscle to identify MHC types I, IIA, IIX, IIY, and IIB. Sixteen fascicles which contained a total of 362 fibers were randomly and systematically sampled from the three EDL muscles. All MHC type I and type II isoforms were expressed. Segmental expression occurred within a very limited segment. MHC isoform expression followed the accepted traditional order from I?IIA?IIX?IIB, however, in some samples expression of an isoform was circumvented from IIB to I or from I to IIB directly. Segmental distribution of MHC isoforms along a single muscle fiber may be because of the myonuclear domain. Anat Rec, 300:1636–1642, 2017. © 2017 Wiley Periodicals, Inc.     

Adaptation of rat jaw muscle fibers in postnatal development with a different food consistency: an immunohistochemical and electromyographic study

The development of the craniofacial system occurs, among other reasons, as a response to functional needs. In particular, the deficiency of the proper masticatory stimulus affects the growth. The purpose of this study was to relate alterations of muscle activity during postnatal development to adaptational changes in the muscle fibers. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a solid (hard-diet group) or a powder (soft-diet group) diet for 63 days. A radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time), the total burst number and their average length exceeding specified levels of the peak activity (5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of fibers by means of immunohistochemical staining and their cross-sectional area was measured. All muscle fibers were identified as slow type I and fast type IIA, IIX or IIB (respectively, with increasing twitch contraction speed and fatigability). At lower activity levels (exceeding 5% of the peak activity), the duty time of the anterior belly of the digastric muscle was significantly higher in the soft-diet group than in the hard-diet group (P < 0.05). At higher activity levels (exceeding 20 and 50% of the peak activity), the duty time of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.05). There was no difference in the duty time of the anterior temporalis muscle at any muscle activity level. The percentage of type IIA fibers of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P< 0.01) and the opposite was true with regard to type IIB fibers (P< 0.05). The cross-sectional area of type IIX and type IIB fibers of the superficial masseter muscle was significantly smaller in the soft-diet group than in the hard-diet group (P < 0.05). There was no difference in the muscle fiber composition and the cross-sectional area of the anterior belly of the digastric and anterior temporalis muscles. In conclusion, for the jaw muscles of male rats reared on a soft diet, the slow-to-fast transition of muscle fiber was shown in only the superficial masseter muscle. Therefore, the reduction in the amount of powerful muscle contractions could be important for the slow-to-fast transition of the myosin heavy chain isoform in muscle fibers.     

Distribution Pattern of Muscle Fiber Types in the Perivertebral Musculature of Two Different Sized Species of Mice

Many physiological parameters scale with body size. Regarding limb muscles, it has been shown that the demands for relatively faster muscles, less postural work, and greater heat production in small mammals are met by lower proportions of Type I and conversely higher proportions of Type II fibers. To investigate possible adaptations of the perivertebral musculature, we investigated the proportion, spatial distribution, and cross‐sectional area (csa) of the different muscle fiber types in the laboratory and harvest mouse. Serial cross sections from the posterior thoracic to the lumbo‐sacral region were prepared and Type I, IIA, and IIB fibers identified using enzymehistochemistry. The general distribution of Type I and IIB fibers, as well as the more or less equal distribution of IIA fibers, resembles the pattern found in other mammals. However, the overall proportion of Type I fibers was very low in the laboratory mouse and particularly low in the harvest mouse. Muscular adaptations to a small body size were met primarily by increased Type IIA fiber proportions. Thereby, not all muscles or muscle regions similarly reflected the expected scaling effects. However, our results clearly show that body size is a critical factor when fiber‐type proportions are compared among different sized mammals. Anat Rec, 293:446–463, 2010. © 2010 Wiley‐Liss, Inc.     

Scaling of the ankle extensor muscle‐tendon units and the biomechanical implications for bipedal hopping locomotion in the post‐pouch kangaroo Macropus fuliginosus

Bipedal hopping is used by macropods, including rat‐kangaroos, wallabies and kangaroos (superfamily Macropodoidea). Interspecific scaling of the ankle extensor muscle‐tendon units in the lower hindlimbs of these hopping bipeds shows that peak tendon stress increases disproportionately with body size. Consequently, large kangaroos store and recover more strain energy in their tendons, making hopping more efficient, but their tendons are at greater risk of rupture. This is the first intraspecific scaling analysis on the functional morphology of the ankle extensor muscle‐tendon units (gastrocnemius, plantaris and flexor digitorum longus) in one of the largest extant species of hopping mammal, the western grey kangaroo Macropus fuliginosus (5.8–70.5 kg post‐pouch body mass). The effective mechanical advantage of the ankle extensors does not vary with post‐pouch body mass, scaling with an exponent not significantly different from 0.0. Therefore, larger kangaroos balance rotational moments around the ankle by generating muscle forces proportional to weight‐related gravitational forces. Maximum force is dependent upon the physiological cross‐sectional area of the muscle, which we found scales geometrically with a mean exponent of only 0.67, rather than 1.0. Therefore, larger kangaroos are limited in their capacity to oppose large external forces around the ankle, potentially compromising fast or accelerative hopping. The strain energy return capacity of the ankle extensor tendons increases with a mean exponent of ~1.0, which is much shallower than the exponent derived from interspecific analyses of hopping mammals (~1.4–1.9). Tendon safety factor (ratio of rupture stress to estimated peak hopping stress) is lowest in the gastrocnemius (< 2), and it decreases with body mass with an exponent of −0.15, extrapolating to a predicted rupture at 160 kg. Extinct giant kangaroos weighing 250 kg could therefore not have engaged in fast hopping using ‘scaled‐up’ lower hindlimb morphology of extant western grey kangaroos.     

Dependence of cross-bridge kinetics on myosin light chain isoforms in rabbit and rat skeletal muscle fibres

Cross-bridge kinetics underlying stretch-induced force transients was studied in fibres with different myosin light chain (MLC) isoforms from skeletal muscles of rabbit and rat. The force transients were induced by stepwise stretches (< 0.3% of fibre length) applied on maximally Ca²⁺-activated skinned fibres. Fast fibre types IIB, IID (or IIX) and IIA and the slow fibre type I containing the myosin heavy chain isoforms MHC-IIb, MHC-IId (or MHC-IIx), MHC-IIa and MHC-I, respectively, were investigated. The MLC isoform content varied within fibre types. Fast fibre types contained the fast regulatory MLC isoform MLC2f and different proportions of the fast alkali MLC isoforms MLC1f and MLC3f. Type I fibres contained the slow regulatory MLC isoform MLC2s and the slow alkali MLC isoform MLC1s. Slow MLC isoforms were also present in several type IIA fibres. The kinetics of force transients differed by a factor of about 30 between fibre types (order from fastest to slowest kinetics: IIB > IID > IIA ≫ I). The kinetics of the force transients was not dependent on the relative content of MLC1f and MLC3f. Type IIA fibres containing fast and slow MLC isoforms were about 1.2 times slower than type IIA fibres containing only fast MLC isoforms. We conclude that while the cross-bridge kinetics is mainly determined by the MHC isoforms present, it is affected by fast and slow MLC isoforms but not by the relative content of MLC1f and MLC3f. Thus, the physiological role of fast and slow MLC isoforms in type IIA fibres is a fine-tuning of the cross-bridge kinetics.     

Innervation distribution pattern,nerve ending structure,and fiber types in pigeon skeletal muscle

Four fiber types have been characterized in different pigeon skeletal muscles according to their innervation pattern (nerve ending structure and innervation distribution) and histochemical properties (SDH and m-ATPase activities). All fast fibers, types IIA and IIB, present aggregated distribution of their nerve endings with “en plaque” structures and very low innervation frequencies. The two kinds of slow fibers recognized are multiple innervated and present higher innervation frequencies. However, type I fibers have nerve terminals in small knobs with uniform localization, whereas type III fibers present “en grappe” nerve endings, which tend to be randomly distributed. Fiber type composition of skeletal muscles has been found closely related to their biomechanical function. Fast fibers are predominant in muscles with an active role in locomotive movements, whereas slow fibers are mainly or exclusively located in postural muscles. © 1993 Wiley-Liss Inc.     

Fiber differentiation of the human laryngeal muscles using the inhibition reactivation myofibrillar ATPase technique

Summary The aim of the present study was to further subdivide the type II fibers of the human thyroarytenoid and posterior cricoarytenoid muscles by means of a modified myosin ATPase reaction. In order to understand the functioning of these highly strained muscles better, it is important to know the respective percentage of fatigue-resistant type IIA fibers and fatigable type IIB fibers. The material comprised the larynges of seven laryngectomized males aged between 45 and 70 years and four laryngectomized females aged between 39 and 72 years. After having been frozen in nitrogen, 10-m-thick sections were cut from the laryngeal muscles in a cryostat. The pH-lability of the enzyme that can be utilized in a classical myosin ATPase reaction permits a differentiation between fiber types I, IIA and IIB. Evidently, this is not possible with every human muscle. The fiber types IIA and IIB of the thyroarytenoid and the posterior cricoarytenoid muscles could be clearly distinguished by means of the inhibition reactivation myofibrillar ATPase technique. Using this method, the myosin ATPase enzyme was initially inhibited by hydroxymer curibenzoate and subsequently reactivated by cysteine. Regarding the incidence of type I and IIA fibers, there was a statistically significant difference between the thyroarytenoid and the posterior cricoarytenoid muscles. The type IIA fiber content was statistically significantly higher in the arytenoid muscle than in the posterior cricoarytenoid muscle. The percentage of type IIB fibers was low, not only in the thyroarytenoid muscle and the posterior cricoarytenoid muscle but also in the other laryngeal muscles. The share of fiber types I, IIA and IIB in the thyroarytenoid muscles varied greatly from one patient to another. This was also true for the other laryngeal muscles. This aspect may be especially significant with regard to an individual's vocal character and vocal fatigability under stress.     

Cell size and oxidative enzyme activity of rat biceps brachii and triceps brachii muscles

Fiber-type distributions, cross-sectional areas, and oxidative enzyme activities of type-identified fibers in the biceps brachii and triceps brachii muscles of 10-week-old male Wistar rats were determined and compared with those in the soleus and plantaris muscles. The soleus and plantaris muscles consisted of two (I and IIA) and three (I, IIA, and IIB) types of fibers, respectively. The deep regions of the biceps brachii and triceps brachii muscles consisted of three types of fibers, while the surface regions of those muscles consisted only of type IIB fibers. The cross-sectional areas of fibers in the deep and surface regions of the plantaris muscle and in the deep regions of the biceps brachii and triceps brachii muscles were in the rank order of type I = type IIA < type IIB, while the oxidative enzyme activities of fibers in the deep and surface regions of the plantaris muscle and in the deep region of the triceps brachii muscle were in the rank order of type IIB < type I = type IIA. These results indicate that fiber-type distributions, cross-sectional areas, and oxidative enzyme activities are muscle type- and region-specific. Therefore, the metabolic and functional significance of the biceps brachii and triceps brachii muscles, especially in the surface regions, where only type IIB fibers are located, in those muscles, appears to be determined by their fibers having larger cells and lower oxidative enzyme activity.     

Thyroid hormone regulation of myosin heavy chain isoform composition in young and old rats,with special reference to IIX myosin

The effects of 4 weeks of thyroid hormone (3,5,3′-triiodothyronine, T₃) treatment on the myosin heavy chain (MHC) composition were compared in the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles from young (3–6 months) and old (20–24 months) male albino rats. Four MHC isoforms were separated on silverstained 6% sodium dodecyl sulphate polyacrylamide gel electrophoresis. According to immunoblotting experiments with specific MHC monoclonal antibodies, the four MHCs corresponded to types I, IIB, IIX and IIA. In the soleus, the type I MHC content was higher in the old than in the young animals, and the type IIA content lower. Type IIX myosin was observed in some young control soleus, but not in old ones. After T₃ treatment, the content of type I MHC decreased substantially in both young and old animals and that of type IIA increased. After T₃ treatment, type IIX myosin was observed in both young and old animals, with a slighty higher IIX myosin content in old age, but the age-related different in the contents of types I and IIA was diminished. In EDL, the type IIX MHC content was significantly higher in the old animals, at the significantly by T₃ treatment in EDL, either in young or old animals. In conclusion, an age-related motor unit transformation is observed in both the slow-twich soleus and the fast-twitch EDL and the capacity for MHC isoform switching in response to T₃ treatment is not impaired in old age.     

Succinate dehydrogenase activities of fibers in the rat extensor digitorum longus, soleus, and cardiac muscles

Succinate dehydrogenase (SDH) activities and cross-sectional areas (CSAs) of different types of fibers in the superficial (EDLs) and deep (EDLd) regions of the extensor digitorum longus and soleus (SOL) muscles and the left ventricular muscle of the heart (HEART) of 10-week-old male rats were determined using quantitative histochemistry and a computer-assisted image processing system. The fibers were classified as type I, type IIA, type IIB, or type IIC according to their histochemically assessed adenosine triphosphatase activities. The mean SDH activity was higher and the mean CSA was smaller in type IIA fibers than in type IIB fibers in both the EDLs and EDLd. The mean SDH activity of type IIA fibers in the SOL was higher than that of type I fibers. Fibers in the HEART showed the highest mean SDH activity and the smallest mean CSA among all fiber types in the muscles examined. There was an inverse correlation between CSA and SDH activity for the different fiber types in different muscles. These data suggest that the SDH activity of fibers in muscle is fiber type- and size-specific, and that the highest SDH activity of fibers in the left ventricular muscle of the heart contributes to their functional properties, i.e., high fatigue resistance.     

PGC-1α mRNA level and oxidative capacity of the plantaris muscle in rats with metabolic syndrome, hypertension, and type 2 diabetes

We examined the fiber profiles and the mRNA levels of peroxisome proliferator-activated receptors (PPARα and PPARδ/β) and of the PPARγ coactivator-1α (PGC-1α) in the plantaris muscles of 15-week-old control (WR), metabolic syndrome (CP), hypertensive (SHR), and type 2 diabetic (GK) rats. The deep regions in the muscles of SHR and GK rats exhibited lower percentages of high-oxidative type I and IIA fibers and higher percentages of low-oxidative type IIB fibers compared with WR and CP rats. The surface regions in the muscles of CP, SHR, and GK rats exhibited lower percentages of high-oxidative type IIA fibers and higher percentages of low-oxidative type IIB fibers compared with WR rats. The muscles of SHR and GK rats had lower oxidative enzyme activity compared with WR rats. The muscles of SHR rats had the lowest PPARδ/β mRNA level. In addition, the muscles of SHR and GK rats had lower PGC-1α mRNA level compared with WR and CP rats. We concluded that the plantaris muscles of rats with hypertension and type 2 diabetes have lower oxidative capacity, which is associated with the decreased level of PGC-1α mRNA.     

Adaptation of rat extensor digitorum longus muscle to gamma irradiation and overload

The right extensor digitorum longus (EDL) muscle of growing male rats was overloaded by ablation of its synergist tibialis anterior (TA) muscle. Four weeks later, the overloaded muscle was heavier and contained larger type IIA, IIX and IIB fibres than either untreated contralateral muscle or control muscle from an untreated animal. The myonuclear-to-myoplasmic volume ratio was maintained in the overloaded muscle. Overloaded EDL muscle, previously subjected to a dose of irradiation sufficient to sterilise satellite cells, and EDL muscle which had been only irradiated, were significantly lighter and contained significantly smaller fibres than controls, though a significant amount of normal EDL muscle growth did occur following either treatment. The myonuclear-to-myoplasmic volume ratio of the irradiated muscles was smaller than in controls. Overloaded muscle, with or without prior irradiation, possessed a smaller proportion of fibres containing IIB myosin heavy chain (MHC) and a larger proportion of fibres containing IIA and IIX MHC; a significant percentage of these fibres coexpressed either type IIA and IIX MHC or type IIX and IIB MHC. Thus in the absence of satellite cell mitosis, muscles of young rats possess a limited capacity for normal growth but not for compensatory hypertrophy. Adaptations in MHC gene expression to chronic overload are completely independent of satellite cell activity.     

Expression of a fast myosin heavy chain mRNA in individual rabbit skeletal muscle fibers with intermediate oxidative capacity.

In situ hybridization (ISH) of myosin heavy chain (MHC) mRNA, immunofluorescent detection of MHC protein, and oxidative enzyme histochemistry were performed on the same fibers in serially sectioned rabbit skeletal muscle. By combining these three techniques quantitatively, on a fiber-by-fiber basis, fibers that expressed mRNA complementary to a fast MHC cDNA pMHC24-79 of unknown subtype (Maeda et al., 1987) were classified into fiber types with respect to slow myosin expression and oxidative capacity. As expected, slow fibers had low hybridization to pMHC24-79. Fast fibers were divided into three subtypes. mRNA from the low oxidative fibers (fast-glycolytic, IIB) did not hybridize with pMHC24-79. Fast fibers whose mRNA hybridized best to pMHC24-79 were mainly in the intermediate range of oxidative capacity (probably IIX). The fast fibers with the highest oxidative capacity had low hybridization to this MHC mRNA (probably IIA). Thus, pMHC24-79 was identified as a clone of a fast isomyosin, tentatively designated as the fast IIX with intermediate oxidative capacity. The expression of more than a single species of fast and slow isomyosin mRNAs in classically defined fiber type was considered in interpreting these results.     

Muscle fiber type distribution in multifidus muscle in cases of lumbar disc herniation

A study was conducted to analyze the distribution and diameter of muscle fiber types in samples of the medial paravertebral lumbar muscle, i.e., multifidus muscle, obtained from 76 patients who underwent surgery for disc herniation. The samples were compared with 41 control samples of corresponding muscle tissue taken from 41 young healthy subjects who had died a sudden death. Histochemical analysis of fibers associated with myofibrillar adenosine triphosphatase (ATPase) revealed the presence of Type I fibers (slow-twitch fibers) and of Type IIA and IIB fibers (fast-twitch fibers) in both the experimental and control samples. The respective percentage of muscle fibers was calculated and their diameters were measured. Type I fibers predominated in both groups and were significantly larger in diameter than Type IIA and IIB fibers. Both fast-twitch fiber types were distributed in almost equal proportions in the healthy women. In the healthy men, Type IIA fibers prevailed. In the healthy females, the percentage of Type I fibers was found to be slightly higher than in the males, but the diameter of all fiber types was respectively smaller. In the females who had undergone surgery, Type I fibers were significantly larger in diameter than those of the healthy subjects. On the other hand, the diameters of all muscle fiber types were significantly larger, and the percentage of both fast-twitch fibers were Idwer in the samples from men who underwent surgery, as compared to the healthy tissue samples. The morphometric changes in the multifidus muscle at the level of the protruded disc observed by the histochemical method for demonstration of myofibrillar ATPase could not be related to the compressed nerve root in the majority of cases in our study.