Three days of intermittent stretching after muscle disuse alters the proteins involved in force transmission in muscle fibers in weanling rats

The aim of this study was to determine the effects of intermittent passive manual stretching on various proteins involved in force transmission in skeletal muscle. Female Wistar weanling rats were randomly assigned to 5 groups: 2 control groups containing 21- and 30-day-old rats that received neither immobilization nor stretching, and 3 test groups that received 1) passive stretching over 3 days, 2) immobilization for 7 days and then passive stretching over 3 days, or 3) immobilization for 7 days. Maximal plantar flexion in the right hind limb was imposed, and the stretching protocol of 10 repetitions of 30 s stretches was applied. The soleus muscles were harvested and processed for HE and picrosirius staining; immunohistochemical analysis of collagen types I, III, IV, desmin, and vimentin; and immunofluorescence labeling of dystrophin and CD68. The numbers of desmin- and vimentin-positive cells were significantly decreased compared with those in the control following immobilization, regardless of whether stretching was applied (P<0.05). In addition, the semi-quantitative analysis showed that collagen type I was increased and type IV was decreased in the immobilized animals, regardless of whether the stretching protocol was applied. In conclusion, the largest changes in response to stretching were observed in muscles that had been previously immobilized, and the stretching protocol applied here did not mitigate the immobilization-induced muscle changes. Muscle disuse adversely affected several proteins involved in the transmission of forces between the intracellular and extracellular compartments. Thus, the 3-day rehabilitation period tested here did not provide sufficient time for the muscles to recover from the disuse maladaptations in animals undergoing postnatal development.     

Bouts of passive stretching after immobilization of the rat soleus muscle increase collagen macromolecular organization and muscle fiber area

This study evaluated the effect of short bouts of stretching on the soleus muscle after immobilization, by measuring the birefringence of the intramuscular connective tissue (ICT) and the muscle fiber area. Thirty rats were divided into five groups: the left soleus was immobilized in the shortened position; after immobilization the animals remained free; after immobilization, the soleus was stretched daily (10 stretches of 60 sec followed by 30 sec rest); after immobilization, the soleus was stretched 3 times a week; control. Immobilization caused a loss of birefringence of the ICT and of muscle fiber area and only daily stretching increased both compared with the control (p< 0.01). In conclusion, short daily bouts of stretching after immobilization induced molecular reorganization of the collagen bundles and muscle fiber hypertrophy in the rat soleus.     

Post-immobilization eccentric training promotes greater hypertrophic and angiogenic responses than passive stretching in muscles of weanling rats

This study investigated how different types of remobilization after hind limb immobilization, eccentric exercise and passive static stretching, influenced the adaptive responses of muscles with similar function and fascicle size, but differing in their contractile characteristics. Female Wistar weanling rats (21 days old) were divided into 8 groups: immobilized for 10 days, maintaining the ankle in maximum plantar flexion; immobilized and submitted to eccentric training for 10 or 21 days on a declining treadmill for 40 min; immobilized and submitted to passive stretching for 10 or 21 days for 40 min by maintaining the ankle in maximum dorsiflexion; control of immobilized; and control of 10 or 21 days. The soleus and plantaris muscles were analyzed using fiber distribution, lesser diameter, capillary/fiber ratio, and morphology. Results showed that the immobilization reduced the diameter of all fiber types, caused changes in fiber distribution and decreased the number of transverse capillaries in both muscles. The recovery period of the soleus muscle is longer than that of the plantaris after detraining. Moreover, eccentric training induced greater hypertrophic and angiogenic responses than passive stretching, especially after 21 days of rehabilitation. Both techniques demonstrated positive effects for muscle rehabilitation with the eccentric exercise being more effective.     

Immobilization atrophy and membrane properties in rat skeletal muscle fibres

Wet mass, resting membrane potential, frequency of miniature end-plate potentials and the concentration of [³H]ouabain-binding sites were studied after 7 days' immobilization of the rat soleus and extensor digitorum longus (EDL) muscles in the shortened or stretched position and after 3 and 7 days of remobilization. We observed that the loss of muscle mass by 37% in the rat soleus immobilized for 7 days in the shortened position is accompanied by a membrane depolarization of about 5 mV, a decrease in frequency of miniature end-plate potentials by 60 % and a decrease of [³H]ouabain binding by 25%. Only minor changes were found in stretched soleus and in shortened and stretched EDL. After 3 days of remobilization of stretched soleus the muscle mass, [³H]ouabain binding and miniature end-plate potential frequency recovered to control values but the resting membrane potential continued to decrease. All changes induced by immobilization disappeared on day 7 of remobilization.     

Quantitative alterations in intramuscular connective tissue following immobilization: an experimental study in the rat calf muscles

The purpose of the present investigation was to study the effect of immobilization in two different positions, shortened or lengthened, on the connective tissue and capillarization of the rat calf muscles. In 18 rats the left hind limb was immobilized so that the soleus and gastrocnemius muscles were in a lengthened position and tibialis anterior was in a shortened position, and in 18 rats vice versa. The right hind limbs were kept free to serve as controls. After 1, 2, and 3 weeks of immobilization, an equal number of rats were killed and the muscles were analyzed. The connective tissue muscle ratio was measured from histological sections by automatic image analyzer and the number of capillaries per 1000 muscle fibers was also recorded. In the intact muscles the proportions of intramuscular connective tissue in the soleus, gastrocnemius, and tibialis anterior were 2, 3, and 5%, respectively. During the immobilization period of 3 weeks the proportion of connective tissue increased significantly in all muscles, but the only significant difference between the positions was in the soleus muscle in which more connective tissue was found in the lengthened position, 54% vs 30% in the shortened position. The amount of capillaries decreased significantly during immobilization in all muscles to about 65% of normal capillar density, but the position of immobilization seemed to have no effect on this phenomenon. The increase in intramuscular connective tissue during immobilization seems to occur simultaneously with muscle atrophy and loss of muscle capillarity. The stretched position of immobilization seems to have an additional fibrotic effect if the immobilized muscle, such as soleus, consists mainly of type I fibers generally known to be more vulnerable to disuse atrophy than type II fibers.     

Bouts of Passive Stretching after Immobilization of the Rat Soleus Muscle Increase Collagen Macromolecular Organization and Muscle Fiber Area

This study evaluated the effect of short bouts of stretching on the soleus muscle after immobilization, by measuring the birefringence of the intramuscular connective tissue (ICT) and the muscle fiber area. Thirty rats were divided into five groups: the left soleus was immobilized in the shortened position; after immobilization the animals remained free; after immobilization, the soleus was stretched daily (10 stretches of 60 sec followed by 30 sec rest); after immobilization, the soleus was stretched 3 times a week; control. Immobilization caused a loss of birefringence of the ICT and of muscle fiber area and only daily stretching increased both compared with the control (p < 0.01). In conclusion, short daily bouts of stretching after immobilization induced molecular reorganization of the collagen bundles and muscle fiber hypertrophy in the rat soleus.     

Effects of immobilization on the rat soleus muscle in relation to age

A hind limb of young adult, adult and old male Wistar rats (4–5, 6–7 and 20–21 months, respectively) was immobilized for 4 weeks by a plaster cast with the knee and ankle joints in a resting position. Enzyme-histochemical, morphometrical and contractile characteristics of the soleus muscle were compared with those in age-matched controls. A pronounced decrease in muscle mass and cross-sectional muscle fibre area was found at all ages. The degree of atrophy after immobilization did not differ between different fibre types in each age group, but the decrease in fibre area was less pronounced in old animals (i.e. the fibre area was decreased by 49–64, 53–66 and 27–38% in young adult, adult and old animals, respectively). The maximum tetanus force was decreased in all age groups (by 73, 78 and 69% in young adult, adult and old rats, respectively) as was the tetanus tension (i.e. tetanus force divided by muscle fibre cross-sectional area). The contraction time of the isometric twitch was significantly altered, i.e. decreased, only in the youngest age group, although it also tended to decrease in old age. A significant increase in the number and proportion of fibre types intermediate to types I and 11 A, was found in the immobilized muscle of 4–5- and 6–7-month-old animals, but not in that of old ones (i.e. the proportion of intermediate fibres increased by 14, 13 and 2% in young adult, adult and old animals, respectively). Thus, in contrast to the atrophic changes, the contractile alterations after immobilization were not markedly different between young and old age. It is further concluded that the age-related fast-to-slow muscle fibre transition that occurs in normal soleus during maturation and growth can be partly reversed by restrictions of the normal muscle activity and that the ability of the soleus to modulate its fibre-type composition in response to a change in activity may be diminished in old age.     

The effect of long-term immobilization on the motor unit population of the cat medial gastrocnemius muscle

The properties of medial gastrocnemius and soleus muscles and of individual medial gastrocnemius motor units were evaluated after chronic (3–29 weeks) unilateral immobilization of ankle and knee joints in 10 adult female cats. Joint fixation markedly impaired the mobility of the pinned limb and, when prolonged (> 17 weeks; 8 animals), produced marked muscle atrophy and some shortening of whole muscle isometric twitch contraction times (mean contraction time of pinned medial gastrocnemius was 83.3 percent of the contralateral muscle). However, the immobilized muscles were not truly disused, since the patterns of medial gastrocnemius and soleus electromyographic activity during posture and locomotion were qualitatively normal in the 4 animals examined. Despite severe whole muscle atrophy, all but 3 of the 239 individual medial gastrocnemius motor units studied were readily classified into one of the 4 motor unit types (types FF, F(int), FR and S) found in the normal muscle. The histochemical mosaic of immobilized muscles was also the same as in contralateral unoperated muscles. When compared to data from normal cats, there was a highly significant loss of maximum twitch and tetanic force output in all unit types after long-term immobilization (17–29 weeks; N = 187), most marked among the fatigue-resistant unit groups (types FR and S), and mean isometric twitch contraction times were shorter than expected in all unit type groups. The most surprising finding in the present work was that the mean homonymous (medial gastrocnemius) and heteronymous (lateral gastrocnemius-soleus) group Ia excitatory postsynaptic potentials were significantly smaller than expected in all motor unit groups in both short-term and long-term immobilization. The mean axonal conduction velocity of medial gastrocnemius motor axons was unchanged from the normal sample.These results indicate that the essential features that define the motor unit types in normal medial gastrocnemius muscles are resistant to change despite long-term immobilization. We conclude that the motor units types found in normal muscles are robust and essentially unaltered despite severe muscle atrophy. The effects on group Ia excitatory postsynaptic potentials suggest that limb immobilization produces altered patterns of impulse traffic in group Ia afferents that induce significant decreases in synaptic efficacy, although it is unclear whether or not Ia traffic is increased or decreased by immobilization.     

Differential effects of a six-day immobilization on newborn rat soleus muscles at two developmental stages

Our objective was to determine the effects of a six-day immobilization on the musculoskeletal system of the rat during postnatal development at two key periods when the states of innervation are known to be different. This work was undertaken on the soleus muscle since it is well known that postural slow muscles show marked changes after a period of disuse. Thus, the soleus muscle was immobilized in a shortened position either when the innervation was polyneuronal or monosynaptic, respectively from 6 to 12 and from 17 to 23 days. The muscle modifications were followed by ATPase staining and myosin heavy chain (MyHC) isoform identification using monoclonal antibodies and SDS–PAGE. The functional properties of skinned fibre bundles were established by calcium/strontium (Ca/Sr) activation characteristics. In control muscles the maturation was characterized by a progressive increase of adult MyHCs (I and IIA) concomitant with a decrease in both the MyHC neo and the Ca affinit y. Between 6 to 12 days, immobilization of the limb induced an increase in histochemical type IIC fibres. Using antibodies we identified new fibre types, classified as a function of their MyHC isoform co-expression. We observed an increase in expression of both MyHC neo and Ca affinity. From 17 to 23 days, the immobilization induced an increase in Ca affinity and marked changes in the MyHC isoform composition: disappearance of MyHC neo and expression of the fast MyHC IIB isoform, which in normal conditions is never expressed in the soleus muscle. We conclude that an immobilization imposed during polyneuronal innervation delays the postnatal maturation of the soleus muscle, whereas when the immobilization is performed under monosynaptic innervation the muscle evolves towards a fast phenotype using a default pathway for MyHC expression.     

Changes in postnatal skeletal muscle development induced by alternative immobilization model in female rat

The objective of this study was to adapt a model of hind limb immobilization to newly weaned female rats and to determine the morphology of shortened soleus and plantaris muscles. Female Wistar rats were divided into three groups: control zero (n = 3) and control and free (n = 8), animals aged 21 and 31 days, respectively, submitted to no intervention, and immobilized (n = 25), animals aged 21 days submitted to immobilization for 10 days and sacrificed at 31 days of age. The device used for immobilization had advantages such as easy connection, good fit, and low cost. The immobilized rats showed a reduction in muscle fiber area and in connective tissue. The adaptation of this immobilization model originally used for adult rats was an excellent alternative for newly weaned rats and was also efficient in inducing significant hind limb disuse.     

Effects of eccentric exercise in rehabilitation of phasic and tonic muscles after leg immobilization in rats

Eccentric exercise is an essential resource for skeletal muscle rehabilitation following muscle disuse however, abnormalities linked to the tissue recuperation require further research. Our aim was analyze the adaptation ability of rehabilitated muscular tissue in rats during different periods of eccentric training after 10 days of limb immobilization. Twenty-seven Wistar rats were divided into six groups: immobilized 10 days, immobilized and eccentric trained for 10 days, immobilized and eccentric trained for 21 days, and three age-matched control groups. After sacrifice, soleus and plantaris muscles were frozen, cut and stained for general histology using hematoxylin and eosin and Gomori trichrome methods and immunohistochemical methods for fiber typing (mATPase, NADH2-TR), for capillaries (CD31) and intermediate filaments (desmin, vimentin) and high resolution microscopy of resin embedded material. Immobilization resulted in more intense morphological alterations in soleus muscles such as formation of target fibers, nuclear centralization, a reduction in the number of type I fibers, diameter of type I, IIA, IIAD fibers, and capillaries. After 10 days of eccentric training, increases in the nuclear centralization and the number of lobulated fibers were observed. This period was insufficient to reestablish the capillary/fiber (C/F) ratio and distribution of fiber types as that observed in the control group. However, 21 days of rehabilitation allowed the reversal of all morphological and quantitative abnormalities. For the plantaris muscles, 10-days of training restored their basic characteristics. Despite the fact that immobilization affected soleus and plantaris muscles, 10 days of eccentric training was insufficient to restore the morphological characteristics of soleus muscles, which was not the case observed in plantaris muscle.     

Immobilization and therapeutic passive stretching generate thickening and increase the expression of laminin and dystrophin in skeletal muscle

Extracellular matrix and costamere proteins transmit the concentric, isometric, and eccentric forces produced by active muscle contraction. The expression of these proteins after application of passive tension stimuli to muscle remains unknown. This study investigated the expression of laminin and dystrophin in the soleus muscle of rats immobilized with the right ankle in plantar flexion for 10 days and subsequent remobilization, either by isolated free movement in a cage or associated with passive stretching for up to 10 days. The intensity of the macrophage response was also evaluated. One hundred and twenty-eight female Wistar rats were divided into 8 groups: free for 10 days; immobilized for 10 days; immobilized/free for 1, 3, or 10 days; or immobilized/stretched/free for 1, 3, or 10 days. After the experimental procedures, muscle tissue was processed for immunofluorescence (dystrophin/laminin/CD68) and Western blot analysis (dystrophin/laminin). Immobilization increased the expression of dystrophin and laminin but did not alter the number of macrophages in the muscle. In the stretched muscle groups, there was an increase in dystrophin and the number of macrophages after 3 days compared with the other groups; dystrophin showed a discontinuous labeling pattern, and laminin was found in the intracellular space. The amount of laminin was increased in the muscles treated by immobilization followed by free movement for 10 days. In the initial stages of postimmobilization (1 and 3 days), an exacerbated macrophage response and an increase of dystrophin suggested that the therapeutic stretching technique induced additional stress in the muscle fibers and costameres.     

Changes in signalling molecule levels in 10-day hindlimb immobilized rat muscles

AIM: Hindlimb immobilization produces similar percentage decreases in muscle mass in the predominantly type I soleus and type II vastus lateralis muscles. Consequently we hypothesized that the percentage changes in potential regulatory molecules for atrophy would be similar in the two muscle fibre types. METHODS: Therefore, the purpose of the current study was to measure phosphorylated p38 MAPK and JNK, as well as the protein levels of p53, growth arrest and DNA damage-inducible 45 (GADD45), and full-length poly(ADP-ribose) polymerase (PARP) to determine whether their changes in expression in the soleus and vastus lateralis muscles were similar at 10th day of hindlimb immobilization in young rats. RESULTS: Unexpectedly, preferential increases in phosphorylation of p38 MAPK and JNK and in the protein levels of p53, GADD45, as well as decreases in full-length PARP occurred in the soleus muscle, while only p38 phosphorylation increased in the white portion of the vastus lateralis muscle at 10th day of hindlimb immobilization. CONCLUSION: Taken together, these results are interpreted to suggest that some of regulatory processes or kinetics in the atrophy of type I and II muscle fibres during limb immobilization may differ at the 10th day of limb immobilization.     

Role of iron in oxidative stress in skeletal muscle atrophied by immobilization

To clarify the role of iron in oxidative stress in skeletal muscle atrophied by immobilization, we investigated the effect of deferoxamine — an iron-chelating agent. Deferoxamine, iron-saturated deferoxamine and double-distilled water (control) were administered subcutaneously from the 4th day after immobilization via osmotic pumps to male Wistar rats (14 weeks old), one ankle joint of which was immobilized in the extended position. After 12 days' immobilization, soleus — typical slow red muscles were collected from both hind limbs and their levels of thiobarbituric acidreactive substance (TBARS) and glutathione were measured. Deferoxamine suppressed the increase of TBARS and glutathione disulfide in atrophied muscle while iron-saturated deferoxamine did not, which strongly suggests that the iron-chelating action of deferoxamine suppressed the increased oxidative stress. This means that iron plays a very important role in increasing oxidative stress in atrophied muscle. In addition, deferoxamine decreased the degree of atrophy, an effect thought to be mediated by the suppression of oxidative stress.     

Author index

This study examined the time course change of the capillary luminal diameter and the number of capillaries in the rat soleus muscle during hind‐limb suspension. Male Wistar rats were divided into 1 and 3 weeks of hind‐limb suspension (HS) groups (HS‐1 and HS‐3). The HS groups were compared with age‐matched control groups. All morphometric parameters with respect to capillary and muscle fibre cross‐sectional area were determined in perfusion‐fixed soleus muscles. After 1 and 3 weeks of hind‐limb suspension, the mean muscle fibre cross‐sectional area was significantly decreased in HS‐1 (–32.0%) and HS‐3 (–59.3%) compared with age‐matched control groups. Despite a lower capillary‐to‐fibre ratio (HS‐1, –19.3%; HS‐3, –21.2%), the capillary density was unchanged in HS‐1 and significantly increased in HS‐3 compared with age‐matched control groups. The mean capillary luminal diameter was significantly smaller in HS‐1 (–19.9%) and HS‐3 (–21.9%) than in the age‐matched control groups. The capillary‐to‐fibre perimeter ratio which indicates the capillary surface area available for gas exchange between blood and tissue did not significantly differ between control groups and HS groups. In conclusion, the morphometrical adaptations in rat soleus with the suspension involved changes in both the capillary luminal diameter and number of capillaries, and the change in capillary surface area was proportional to the degree of muscle atrophy in HS groups.     

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.     

Changes in signalling molecule levels in 10‐day hindlimb immobilized rat muscles

Aim: Hindlimb immobilization produces similar percentage decreases in muscle mass in the predominantly type I soleus and type II vastus lateralis muscles. Consequently we hypothesized that the percentage changes in potential regulatory molecules for atrophy would be similar in the two muscle fibre types. Methods: Therefore, the purpose of the current study was to measure phosphorylated p38 MAPK and JNK, as well as the protein levels of p53, growth arrest and DNA damage‐inducible 45 (GADD45), and full‐length poly(ADP‐ribose) polymerase (PARP) to determine whether their changes in expression in the soleus and vastus lateralis muscles were similar at 10th day of hindlimb immobilization in young rats. Results: Unexpectedly, preferential increases in phosphorylation of p38 MAPK and JNK and in the protein levels of p53, GADD45, as well as decreases in full‐length PARP occurred in the soleus muscle, while only p38 phosphorylation increased in the white portion of the vastus lateralis muscle at 10th day of hindlimb immobilization. Conclusion: Taken together, these results are interpreted to suggest that some of regulatory processes or kinetics in the atrophy of type I and II muscle fibres during limb immobilization may differ at the 10th day of limb immobilization.     

Morphological changes in the triads and sarcoplasmic reticulum of rat slow and fast muscle fibres following denervation and immobilization

Summary We observed the morphological features of the membrane systems (sarcoplasmic reticulum, transverse tubules and triads) involved with the excitation-contraction coupling in rat soleus and extensor digitorum longus muscle following two disuse protocols: denervation and immobilization. The immobilized positions were: maximum dorsal flexor (soleus were stretched and extensor digitorum longus were shortened), maximum plantar flexor (soleus were shortened and extensor digitorum longus were stretched), and midway between the dorsal flexor and plantar flexor. The arrangement of the membrane systems was disordered following both disuse conditions. Increases in transverse tubule network were apparent; there were clearly more triads than in normal fibres, and pentadic and heptadic structures (i.e., a close approximation of two or three transverse tubule elements with three or four elements of terminal cisternae of sarcoplasmic reticulum) were frequently appeared following both denervation and immobilization. The most notable difference between the influence of denervation and immobilization on the membrane systems is the time at which the pentads and heptads appeared. They appeared much earlier (1 week after denervation) in denervated than in immobilized (3 or 4 weeks after immobilization) muscle fibres. On the other hand, the frequency of pentads and heptads is clearly related to the fibre type (significantly higher in extensor digitorum longus) and to extent of atrophy. The different influences of immobilization in each leg position suggest that disuse, but with neurotrophic factor(s), influences on the membrane systems were affected by sarcomere length, and the neurotrophic factor(s) and muscle activity were not always necessary to form mew membrane systems in disuse skeletal muscle fibres.     

The effect of hypokinesia and hypodynamia on protein turnover and the growth of four skeletal muscles of the rat

An animal suspension model has been used to simulate the weightlessness experienced during space travel. This procedure results in a reduction in the normal shortening (i.e. hypokinesia) and force generation functions of hind limb muscles (i.e. hypodynamia). The ensuing muscle atrophy was studied over 12 days in different muscle types. Slow muscles (e.g. the soleus) underwent a more pronounced atrophy than intermediate (i.e. gastrocnemius) and fast phasic muscles (e.g. extensor digitorum longus). In all muscle types inactivity resulted in a smaller accumulation of DNA and losses of RNA and protein after 5 days. The latter arose from a decrease in the rate of protein synthesis (measured in vivo) and an increase in protein breakdown. Increased specific activities of cathepsins B and D also supported the view that there is an increased proteolysis after hypokinesia and hypodynamia.When the inactive soleus was simultaneously held in a lengthened (stretched) state the atrophy was prevented through a large increase in the fractional rate of protein synthesis. Protein degradation remained elevated with stretch, thereby slowing the growth of these muscles relative to those in pair-fed, ambulatory controls. The much smaller atrophy of the tibialis anterior and extensor digitorum longus muscles in suspended only limbs represented an underestimate of the true atrophic effects of hypokinesia and hypodynamia. In this model gravity pulls the suspended foot into a plantar flexed position, thereby permanently stretching and protecting such flexor muscles. When this influence of stretch was removed a greater atrophy ensued, mainly due to the loss of the stretch-induced stimulation of protein synthesis. Despite this, the inactive fast-twitch muscles still exhibited less atrophy than the gastrocnemius and soleus muscles.     

Muscle differentiation after sciatic nerve transection and reinnervation in adult rats

Reinnervation after peripheral nerve transections generally leads to poor functional recovery. In order to study whether changes in muscles might be a contributing factor in this phenomenon we studied muscle morphology and fibre type distributions after sciatic nerve transection in the rat hind limb. Proximally, before the bifurcation in the tibial and common peroneal nerve, a 12 mm segment of the sciatic nerve was resected, reversed and re-implanted as an autologous nerve graft. After survival periods of 7, 15 and 21 weeks the lateral gastrocnemius, tibialis anterior and soleus muscles were dissected, stained with mATP-ase, and fibre type distributions were studied. In addition, numbers of muscle fibres were counted, and cross sectional areas were calculated. After 7 weeks, cross sectional areas were decreased in all muscles. In the gastrocnemius and tibialis anterior muscles the fibre number remained unaltered but the hypotrophy had been reversed at later ages. The number of muscle fibres in the soleus muscle remained decreased over the entire period of observation. The percentages of type II fibres in the gastrocnemius and tibialis anterior muscles were decreased at 7 and 15 weeks but these again approached normal values at 21 weeks. The type I fibres, however, remained arranged in groups. In the soleus muscle a large increase in the percentage of type II muscle fibres was observed and this remained until 21 weeks. We conclude that a non-selective reinnervation and later readjustments by regression of polyneural innervation may in part explain the changes in distributions of various fibre types.