Activity of some enzymes of energy metabolism during denervation and reflex atrophy in rat slow and fast muscles

The loss of muscle weight in the soleus (SOL) and extensor digitorum longus (EDL) muscles was compared after denervation and in the course of reflex muscle atrophy induced by unilateral fracture of metatarsal bones of the paw and local injection of 0.02 ml turpentine oil subcutaneously. This so-called reflex atrophy is significantly greater after 3 days than that after denervation. Seven days after the nociceptive stimulus, reflex and denervation atrophy are grossly similar in both muscles. This also applies in case that the nociceptive stimulus had been repeated on the third day. The EDL:SOL enzyme activities of energy supply metabolism reflect the differences between a glycolytic-aerobic (EDL) and predominantly aerobic type (SOL) of muscle. No consistent changes were found in either type of atrophy after 3 days. In 7 days' denervation, the activity of hydroxyacetyl-CoA-dehydrogenase (HOADH) and citrate synthase (CS) was decreased in the SOL, while glycerolphosphate:NAD dehydrogenase (GPDH) was enhanced. In the EDL, the activity of triosephosphate dehydrogenase (TPDH), GPDH, malate dehydrogenase (MDH), CS and HOADH was decreased. Acid phosphatase (AcP) was greatly increased in both muscles. Seven days after application of the nociceptive stimulus, all enzyme activities were altered in a grossly analogous manner as after denervation.     

Myosin heavy chain isoform composition in striated muscle after denervation and self-reinnervation

The total content of myosin heavy chains (MHC) and their isoform pattern were studied by biochemical methods in the slow-twitch (soleus) and fast-twitch (extensor digitorum longus) muscles of adult rat during atrophy after denervation and recovery after self-reinnervation. The pattern of fibre types, in terms of ultrastructure, was studied in parallel. After denervation, total MHC content decreased sooner in the slow-twitch muscle than in the fast-twitch. The ratio of MHC-1 and the MHC-2B isoforms to the MHC-2A isoform decreased in the slow and the fast denervated muscles, respectively. After reinnervation of the slow muscle, the normal pattern of MHC recovered within 10 days and the type 1 isoform increased above the normal. In the reinnervated fast muscle, the 2B/2A isoform ratio continued to decrease. Traces of the embryonic MHC isoform, identified by immunochemistry, were found in both denervated and reinnervated slow and fast muscles. A shift in fibre types was similar to that found in the MHC isoforms. Within 2 months of recovery a tendency to normalization was observed. The results show that (a) MHC-2B isoform and the morphological characteristics of the 2B-type muscle fibres are susceptible to lack of innervation, similar to those of type 1, (b) during muscle recovery induced by reinnervation the MHC isoforms and muscle fibres shift transiently to type 1 in the soleus and to type 2A in the extensor digitorum longus muscles, and (c) the embryonic isoform of MHC may appear in the adult skeletal muscles if innervation is disturbed.     

Effects of undernutrition on diaphragm fiber size, SDH activity, and fatigue resistance

The influence of prolonged nutritional deprivation on the succinate dehydrogenase (SDH) activity and cross-sectional areas of individual fibers in the rat diaphragm and deep portion of the medial gastrocnemius (MGr) muscles was determined. Fatigue resistance of the diaphragm was measured by means of an in vitro nerve-muscle strip preparation. Fiber SDH activity and cross-sectional area were quantified by means of an image processing system. Diaphragm fatigue resistance was significantly improved in the nutritionally deprived (ND) group. In both muscles, nutritional deprivation resulted in a significant decrease in fiber cross-sectional area (both type I and II), type II fibers showing greater atrophy. The SDH activities of type I and II fibers in the diaphragm were not affected by nutritional deprivation. This contrasted with a significant decrease in the SDH activity of both type I and II fibers in the MGr of ND animals. An assessment of the interrelationships between fiber atrophy and fiber SDH activity revealed a greater effect of malnutrition on those diaphragm type II fibers that had the lowest relative SDH activities and the largest cross-sectional areas. By comparison, the effect of malnutrition on type I and II fibers in the MGr was nonselective with regard to fiber SDH activity. We conclude that the enhanced diaphragm fatigue resistance in the ND animals does not result from an increase in the oxidative capacity of muscle fibers and is best explained by the pattern of diaphragm muscle fiber atrophy.     

Periodic weight support effects on rat soleus fibers after hindlimb suspension

The morphological and histochemical properties of the rat soleus were studied after 1 wk of hindlimb suspension, one model that removes the weight-bearing function of the hindlimbs. To examine the effectiveness of weight support activity in maintaining soleus mass, fiber size, and succinate dehydrogenase (SDH) activity, the hindlimbs of adult male Sprague-Dawley rats were suspended (HS) and half of these rats were walked on a treadmill for 40 min/day (10 min every 6 h) at 5 m/min and a 19 degree grade (HS-WS). Significant reductions in soleus mass and fiber size were found after 1 wk of HS. Weight support activity decreased the atrophic response by approximately 50%. In the alkaline myofibrillar adenosine triphosphatase (ATPase) dark-staining fibers, SDH activity was higher in the HS than control rats, whereas it was similar to control in the HS-WS rats. Total SDH activity (SDH activity X cross-sectional area) in fibers staining lightly for ATPase in HS and HS-WS rats was lower than in control rats, whereas in the darkly stained ATPase fibers it was similar among the three groups. No changes were observed in fiber type percentages after 1 wk of HS or HS-WS. The results suggest that short-duration, daily weight support activity can ameliorate, but not prevent, soleus atrophy induced by HS. Furthermore, fiber cross-sectional area is more responsive to periodic weight support in dark than light ATPase fibers. These results also demonstrate that muscle fiber atrophy need not be associated with a loss in SDH activity.     

Studies of human antagonistic muscles after rupture of the tendon of biceps brachii

The fibre type ratio and the cross sectional areas of fibres were studied in triceps brachial muscle after rupture of tendon of the biceps brachial muscle in man. On the healthy and injured sides of the m. triceps brachii, the mean value of type 1 fibres was 51.9% and 52.4%, respectively. The mean cross sectional area of fibre on the two sides of the triceps muscle varied considerably among individuals without any significant correlation to injury of the biceps tendon.     

Structural and functional responses to prolonged hindlimb suspension in rat muscle

The purpose of this study was to investigate alterations in structural and functional properties in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats after 1, 2, and 5 wk of tail suspension. Maximal O2 uptake was 19% lower after 5 wk suspension. Loss of muscle mass was greater in SOL (63%) than in EDL (22%) muscle. A reduction of type I distribution was accompanied by an increase of intermediate fiber subgroups (int I in SOL, int II in EDL). The cross-sectional area of all three fiber types was reduced by hypokinesia. The decrease in capillaries per fiber in SOL was greater than the decrease in citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities after 5 wk. No alteration in lactate dehydrogenase activity was noted. In EDL, no changes in fiber area, capillarization, and enzymatic activities occurred. Energy charge remained unchanged (0.91) whatever the muscle. These results suggest that type I fibers showed an earlier and greater susceptibility than type II fibers to suspension which is also accompanied by a decreased aerobic capacity.     

Reduced glycogen phosphorylase activity in denervated hindlimb muscles of rat is related to muscle atrophy and fibre type

Changes in the activity of muscle glycogen synthase or phosphorylase (GP) may be responsible for the deregulation of glycogen synthesis and storage which occurs in diabetes mellitus. To clarify the relationship between muscle atrophy, fibre type, insulin-stimulated glucose uptake and GP activity during insulin resistance, we used sciatic nerve severance to induce insulin resistance in rat hindlimb muscles and compared the above parameters in muscles with a range of fibre types. Changes were analysed by comparison with the contralateral hindlimb, which bears more weight due to denervation of the opposing limb, as well as the sham-operated and contralateral limb of a separate rat. Denervation caused a decrease in insulin-stimulated glucose uptake by 1 day after denervation and a decline of GP activity after 7 days in all muscles investigated. GP activity decreased by 73% in soleus, 36% in red gastrocnemius, 35% in tibialis and 13% in white gastrocnemius, which was related to the degree of muscle atrophy and inversely related to the overall GP activity in non-denervated muscles. GP activity in muscles of the contralateral limb from the denervated rat did not differ from either hindlimb of the sham-operated rat. We conclude that the fibre-type related reduction in insulin-stimulated glucose uptake of denervated muscle determines the change in its metabolism and it is this metabolic change which determines the mechanism, rate and degree of muscle atrophy, which is directly related to the decline in GP activity.     

Adaptations in metabolic capacity of rat soleus after paralysis.

To determine whether long-term reductions in neuromuscular activity result in alterations in metabolic capacity, the activities of oxidative, i.e., succinate dehydrogenase (SDH) and citrate synthase (CS), and glycolytic, i.e., alpha-glycerophosphate dehydrogenase (GPD), enzyme markers were quantified in rat soleus muscles 1, 3, and 6 mo after a complete spinal cord transection (ST). In addition, the proportional content of lactate dehydrogenase (LDH) isozymes was used as a marker for oxidative and glycolytic capacities. The myosin heavy chain (MHC) isoform content of a fiber served as a marker of phenotype. In general, MHC isoforms shifted from MHC1 toward MHC2, particularly MHC2x, after ST. Mean SDH and CS activities were higher in ST than control at all time points. The elevated SDH and CS activities were indicative of an enhanced oxidative capacity. GPD activities were higher in ST than control rats at all time points. The increase in activity of SDH was larger than GPD. Thus the GPD-to-SDH (glycolytic-to-oxidative) ratio was decreased after ST. Compared with controls, total LDH activity increased transiently, and the LDH isozyme profile shifted from LDH-1 toward LDH-5, indicative of an enhanced glycolytic capacity. Combined, these results indicate that 1) the metabolic capacities of soleus fibers were not compromised, but the interrelationships among oxidative and glycolytic capacity and MHC content were apparently dissociated after ST; 2) enhancements in oxidative and glycolytic enzyme activities are not mutually exclusive; and 3) chronic reductions in skeletal muscle activity do not necessarily result in a reduced oxidative capacity.     

Structural and metabolic properties of rat muscle exposed to weightlessness aboard Cosmos 1887.

Male Wistar rats were subjected to 12.5 days of weightlessness aboard Cosmos 1887. Histomorphometric and biochemical analyses were investigated in soleus (SOL), plantaris (PL) and extensor digitorum longus (EDL) muscles of flight rats (group F) and compared with data from two groups of terrestrial controls: one group living free in a vivarium (group V) and another subjected to a flight simulation except for the state of weightlessness (group S). Relative to groups V and S, no alteration in the percentage distribution of fibres had occurred in SOL, PL or EDL, after the flight. In SOL muscles from group F animals, cross-sectional areas of all fibre types were reduced to a greater extent (-40%) than capillary to fibre ratio (-24%) leading to a higher capillary density (+33%) than in V and S groups. In PL, type I, IIA and IIB fibre cross-sectional areas were less decreased (-25%). In EDL, only fast-twitch fibre cross-sectional areas showed an average decrease of 30%. Capillary per fibre ratio was reduced by 15% and 28% respectively in PT and EDL muscles from group F rats compared to control groups V and S. Citrate synthase and 3-hydroxyacyl-coenzyme A dehydrogenase activities remained unchanged in SOL, PL and EDL following spaceflight. These findings indicate greater atrophy and functional alterations (capillarity) compared to those observed after 7 days of microgravity on Cosmos 1667.     

Arrest of developmental conversion of type II to type I fibres after suspension hypokinesia

Summary The effects of hypokinesia and of the lack of gravity on muscle fibres, fibre type composition and myosin light chain pattern, as well as on muscle mechanoreceptors were investigated in the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of young growing and adult rats after suspension hypokinesia (SH) of their hind limbs. The animals were suspended by their tail so that their hind limbs were relieved of their normal weight-bearing function for 3–6 weeks.In normal 3-to 4-week-old rats the SOL contained about 50% type I fibres and their percentage increased up to about 80% until the 10th week, with simultaneous reduction of type IIA fibres. After 3 to 6 weeks of suspension treatment maintained from 3-to, 4-week-old rats up to 6 to 10 weeks of age, the SOL still only contained about 50% of type I fibres. The content of fast LC₁ and LC₂ in the SOL of 6-week-old rats after 3 weeks of suspension was higher than that of control litter-mates reflecting the higher occurrence of IIA fibres in the suspended solei. No changes in fibre type composition were observed after SH performed in adult rats.SH thus leads, in young animals, to the arrest of conversion of type IIA to type I fibres resulting in the persistence of the fibre type composition and of the myosin light chain pattern corresponding to those present in the SOL at the time of the onset of suspension. In both young and adult rats, SH markedly decreased the mass and the mean cross-sectional area of the SOL, mainly due to the severe atrophy of type I fibres. We observed no signs indicating conversion of type I back to type IIA muscle fibres due to the SH either in young or adult animals.In contrast to profound changes in the SOL, no significant differences were found in the EDL in any of the parameters studied.No changes in the investigated parameters of muscle spindles and tendon organs were observed after SH, performed either in young or in adult rats.We thus conclude that SH leads to muscle atrophy and that it influences mainly or exclusively type I fibres in muscles with a postural function such as the SOL. It is suggested that in young rats SH arrests changes in the SOL motoneurones, which remain unable to ensure the normal developmental transformation of type IIA into type I fibres, thus preventing conversion of the SOL into a typical slow-twitch muscle.     

PGC-1α and FOXO1 mRNA levels and fiber characteristics of the soleus and plantaris muscles in rats after hindlimb unloading

Fifteen-week-old rats were subjected to unloading induced by hindlimb suspension for 3 weeks. The peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and forkhead box-containing protein O1 (FOXO1) mRNA levels and fiber profiles of the soleus and plantaris muscles in rats subjected to unloading (unloaded group) were determined and compared with those of age-matched control rats (control group). The body weight and both the soleus and plantaris muscle weights were lower in the unloaded group than in the control group. The PGC-1α mRNA was downregulated in the soleus, but not in the plantaris muscle of the unloaded group. The FOXO1 mRNA was upregulated in both the soleus and plantaris muscles of the unloaded group. The oxidative enzyme activity was reduced in the soleus, but not in the plantaris muscle of the unloaded group. The percentage of type I fibers was decreased and the percentages of type IIA and IIC fibers were increased in the soleus muscle of the unloaded group, whereas there was no change in fiber type distribution in the plantaris muscle of the unloaded group. Atrophy of all types of fibers was observed in both the soleus and plantaris muscles of the unloaded group. We conclude that decreased oxidative capacity and fiber atrophy in unloaded skeletal muscles are associated with decreased PGC-1α and increased FOXO1 mRNA levels.     

On the stability of key enzymes of energy metabolism in muscle biopsies

Enzymes of energy metabolism were tested for stability depending on different storage conditions (-20, -80 degrees C). To avoid problems due to the different fiber type composition of human muscle, we selected two muscles from rabbit. The m. psoas consists almost exclusively of type 2B fibers, and the m. soleus consists almost exclusively of type 1 fibers. Enzyme activities were measured from small aliquots of these muscles at various time points up to 1 year after sacrificing the animal. Enzymes from anaerobic metabolism were stable for more than 1 year, independent of whether the muscle was stored at -20 or -80 degrees C. Oxidative enzymes, such as succinate dehydrogenase, citrate synthetase, or cytochrome c oxidase (COX) decrease in activity at -20 degrees C and, to a lesser degree, at -80 degrees C. In addition, mitochondria were isolated from freshly taken muscle and stored at -80 degrees C. Oxidative enzymes were surprisingly stable for more than 1 year, with the exception of COX which decreased by 60% of its original activity in mitochondria from m. soleus.     

Enzyme profiles of single muscle fibers never exposed to normal neuromuscular activity.

Do muscle fiber properties commonly associated with fiber types in adult animals and the population distribution of these properties require normal activation patterns to develop? To address this issue, the activity of an oxidative [succinic dehydrogenase (SDH)] and a glycolytic [alpha-glycerophosphate dehydrogenase (GPD)] marker enzyme, the characteristics of myosin adenosinetriphosphatase (myosin ATPase, alkaline preincubation), and the cross-sectional area of single fibers were studied. The soleus and medial gastrocnemius of normal adult cats were compared with cats that 6 mo earlier had been spinally transected at T12-T13 at 2 wk of age. In control cats, SDH activity was higher in dark than light ATPase fibers in the soleus and higher in light than dark ATPase fibers in the medial gastrocnemius. After transection, SDH activity was similar to control in both muscles. GPD activity appeared to be elevated in some fibers in each fiber type in both muscles after transection. The cross-sectional areas most affected by spinal transection were light ATPase fibers of the soleus and dark ATPase fibers of the medial gastrocnemius, the predominant fiber type in each muscle. These data demonstrate that although the muscle fibers of cats spinalized at 2 wk of age presumably were never exposed to normal levels of activation, the activity of an oxidative marker enzyme was maintained or elevated 6 mo after spinal transection. Furthermore, although the absolute enzyme activities in some fibers were elevated by transection, three functional protein systems commonly associated with fiber types, i.e., hydrolysis of ATP by myosin ATPase and glycolytic (GPD) and oxidative (SHD) metabolism, developed in a coordinated manner typical of normal adult muscles.     

Histopathological changes and oxidative damage in type I and type II muscle fibers in rats undergoing paradoxical sleep deprivation

Backgroundprevious studies have shown that muscle atrophy is observed after sleep deprivation (SD) protocols; however, the mechanisms responsible are not fully understood. Muscle trophism can be modulated by several factors, including energy balance (positive or negative), nutritional status, oxidative stress, the level of physical activity, and disuse. The metabolic differences that exist in different types of muscle fiber may also be the result of different adaptive responses. To better understand these mechanisms, we evaluated markers of oxidative damage and histopathological changes in different types of muscle fibers in sleep-deprived rats.MethodsTwenty male Wistar EPM-1 rats were randomly allocated in two groups: a control group (CTL group; n = 10) and a sleep deprived group (SD group; n = 10). The SD group was submitted to continuous paradoxical SD for 96  h; the soleus (type I fibers) and plantar (type II fiber) muscles were analyzed for histopathological changes, trophism, lysosomal activity, and oxidative damage. Oxidative damage was assessed by lipid peroxidation and nuclear labeling of 8-OHdG.ResultsThe data demonstrated that SD increased the nuclear labeling of 8-OHdG and induced histopathological changes in both muscles, being more evident in the soleus muscle. In the type I fibers there was signs of tissue degeneration, inflammatory infiltrate and tissue edema. Muscle atrophy was observed in both muscles. The concentration of malondialdehyde, and cathepsin L activity only increased in type I fibers after SD.ConclusionThese data indicate that the histopathological changes observed after 96 h of SD in the skeletal muscle occur by different processes, according to the type of muscle fiber, with muscles predominantly composed of type I fibers undergoing greater oxidative damage and catabolic activity, as evidenced by a larger increase in 8-OHdG labeling, lipid peroxidation, and lysosomal activity.     

Skeletal muscle changes after endurance training at high altitude.

The effects of endurance training on the skeletal muscle of rats have been studied at sea level and simulated high altitude (4,000 m). Male Wistar rats were randomly assigned to one of four groups: exercise at sea level, exercise at simulated high altitude, sedentary at sea level, and sedentary at high altitude (n = 8 in each group). Training consisted of swimming for 1 h/day in water at 36 degrees C for 14 wk. Training and exposure to a high-altitude environment produced a decrease in body weight (P less than 0.001). There was a significant linear correlation between muscle mass and body weight in the animals of all groups (r = 0.89, P less than 0.001). High-altitude training enhanced the percentage of type IIa fibers in the extensor digitorum longus muscle (EDL, P less than 0.05) and deep portions of the plantaris muscle (dPLA, P less than 0.01). High-altitude training also increased the percentage of type IIab fibers in fast-twitch muscles. These muscles showed marked metabolic adaptations: training increased the activity levels of enzymes involved in the citric acid cycle (citrate synthase, CS) and the beta-oxidation of fatty acids (3 hydroxyacyl CoA dehydrogenase, HAD). This increase occurred mainly at high altitude (36 and 31% for HAD in EDL and PLA muscles; 24 and 31% for CS in EDL and PLA muscles). Training increased the activity of enzymes involved in glucose phosphorylation (hexokinase). High-altitude training decreased lactate dehydrogenase activity. Endurance training performed at high altitude and sea level increased the isozyme 1-to-total lactate dehydrogenase activity ratio to the same extent.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphofunctional characteristics of enzyme activity of the muscles in ontogenesis and after physical load

By means of histochemical methods using cytospectrophotometer in femoral muscles of white rats 1-, 3- and 12-month-old enzymatic activity of anaerobic and aerobic cycles has been estimated. The greatest changes occur after 20 days of physical load in 1- and 12-month-old animals. The semitendinous muscle (ventral origin), consisting mainly of red muscle fibers, works chiefly in aerobic regime and possesses a more manifested succinate dehydrogenase activity, and the quadriceps muscle (dorsal origin) consists principally of white fibers and its lactate dehydrogenase activity changes more noticeably.     

Metabolic variability within individual fibres of the cat tibialis posterior and diaphragm muscles

Summary Variance in succinate dehydrogenase activity along the transverse and longitudinal axes of fibres from the cat tibialis posterior and diaphragm muscles was determined in order to estimate the three-dimensional distribution of mitochondria within single fibres. The variance (coefficient of variation) in succinate dehydrogenase activity along the transverse fibre axis was greatest in type IIB fibres from both muscles. Intracellular compartmentalization (i.e. subsarcolemmal vs central core differences in succinate dehydrogenase activity) was observed only in type II fibres from the tibialis posterior; the succinate dehydrogenase activity of the subsarcolemmal region was significantly greater than that of the central core. The extent of succinate dehydrogenase variance along the longitudinal fibre axis was dependent on the total length of the fibre segment analyzed, the muscle, and fibre type. The coefficient variation for short fibre segments, i.e. 40 m, was significantly lower than that for much longer fibre segments (840 m). Significant differences in the coefficient variation for 840 m fibre segments were observed between the diaphragm and tibialis posterior muscles. The longitudinal variance in succinate dehydrogenase activity was higher in diaphragm muscle fibres. The succinate dehydrogenase variance along the longitudinal axis of type II fibres was significantly greater than that of the type I fibre population. These results indicate that mitochondria are heterogeneously distributed within muscle fibres. Possible functional implications of such intrafibre metabolic variance are discussed.     

Trypanosoma cruzi: histochemical characterization of parasitized skeletal muscle fibers

C57B1/6 mice were infected with Brasil strain Trypanosoma cruzi trypomastigotes. The leg muscles of the mice were serial-sectioned with a cryostat, and individual fibers were classified histochemically as type I or type II on the basis of succinic dehydrogenase or adenosine triphosphatase activity. Although markedly more type II fibers were present in the leg muscles, the percentage of infected type I fibers was nearly five-fold higher than type II. This is the first demonstration of a preferential in vivo distribution of T. cruzi in muscle fibers based upon muscle type.     

Skeletal muscle fiber, nerve, and blood vessel breakdown in space-flown rats

Histochemical and ultrastructural analyses were performed postflight on hind limb skeletal muscles of rats orbited for 12.5 days aboard the unmanned Cosmos 1887 biosatellite and returned to Earth 2 days before sacrifice. The antigravity adductor longus (AL), soleus, and plantaris muscles atrophied more than the non-weight-bearing extensor digitorum longus, and slow muscle fibers were more atrophic than fast fibers. Muscle fiber segmental necrosis occurred selectively in the AL and soleus muscles; primarily, macrophages and neutrophils infiltrated and phagocytosed cellular debris. Granule-rich mast cells were diminished in flight AL muscles compared with controls, indicating the mast cell secretion contributed to interstitial tissue edema. Increased ubiquitination of disrupted myofibrils implicated ubiquitin in myofilament degradation. Mitochondrial content and succinic dehydrogenase activity were normal, except for subsarcolemmal decreases. Myofibrillar ATPase activity of flight AL muscle fibers shifted toward the fast type. Absence of capillaries and extravasation of red blood cells indicated failed microcirculation. Muscle fiber regeneration from activated satellite cells was detected. About 17% of the flight AL end plates exhibited total or partial denervation. Thus, skeletal muscle weakness associated with spaceflight can result from muscle fiber atrophy and segmental necrosis, partial motor denervation, and disruption of the microcirculation.     

Morphological changes in continuously stretched skeletal muscles in sheep

The effects of continuous elongation of skeletal muscles were studied on six sheep who underwent a lengthening osteotomy of the right tibia. Open muscle biopsies were taken from the biceps femoris muscle preoperatively (Group A), after 5 weeks of bone distraction (Group B) and after another 5 weeks without further distraction (Group C). The size and distribution of type 1 (slow-twitch) and type 2 (fast-twitch) muscle fibres were determined from sections stained for myofibrillar ATPase activity. All sections were also evaluated by light microscopy, especially with regard to myopathic changes. The type 2 fibres showed a significant decrease in size from group A to B and from group B to C. The reduction in fibre size from group A to C was 44.2%. The type 1 fibres, on the other hand, showed no significant differences in mean fibre size between the groups. However, there were considerable individual variations in type 1 fibre size between the groups. The distribution of both fibre types was similar in groups A and B (appr. 17% type 1 fibres) whereas the relative number of type 1 fibres was reduced to 12.4% in group C (P less than 0.01). Myopathic changes, i.e. muscle fibre necroses, were not seen in any of the groups. It is concluded that the type 2 fibre atrophy is mainly caused by muscular inactivity during the postoperative period, but an additional effect of continuous stretching of the muscle cannot be excluded.(ABSTRACT TRUNCATED AT 250 WORDS)