Exercise-induced respiratory muscle fatigue: implications for performance.

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O(2) and CO(2) transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.     

Hypoxia-induced fibre type transformation in rat hindlimb muscles Histochemical and electro-mechanical changes

Twelve male Sprague-Dawley rats (21 days old) were randomly assigned into two experimental groups: sea level control (CONT) and hypobaric hypoxia (HYPO). The HYPO rats were kept in an hypobaric chamber maintaining a simulated altitude of 4000 m (61.1 kPa). After 10 weeks of treatment, the rat hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] were subjected to histochemical and electro-mechanical analyses. Results indicated that compared to CONT the HYPO SOL muscle had a significantly greater relative distribution of fast-twitch-oxidative-glycolytic (FOG) fibres (28.9% SEM 2.0 vs 18.3% SEM 1.8, P less than 0.01) with a significant decrease in slow twitch oxidative fibre distribution (69.5% SEM 2.4 vs 82.9% SEM 3.1, P less than 0.01). Compared to CONT the HYPO EDL muscle also manifested a significant increase in FOG fibre distribution (51.6% SEM 0.8 vs 46.6% SEM 1.1, P less than 0.01), but this was accompanied by a significant decrease in fast twitch glucolytic fibres (44.3% SEM 0.9 vs 49.2% SEM 1.7, P less than 0.05). These histochemical fibre type transformations accompanied significant and expected changes in the electro-mechanical parameters tested in situ, e.g. maximal twitch force, maximal rate of force development, contraction time, half relaxation time, force: frequency curve, and fatigability. It was concluded that chronic hypobaric hypoxia could have a potent influence upon the phenotype expression of muscle fibres.     

Positive inotropism in mammalian skeletal muscle in vitro during and after fatigue

We tested the hypothesis that positive inotropic factors decrease fatigue and improve recovery from fatigue in mammalian skeletal muscle in vitro. To induce fatigue, we stimulated mouse soleus and extensor digitorum longus (EDL) to perform isometric tetanic contractions (50 impulses x s(-1) for 0.5 s) at 6 contractions x min(-1) for 60 min in soleus and 3 contractions x min(-1) for 20 min in EDL. Muscles were submerged in Krebs-Henseleit bicarbonate solution (Krebs) at 27 degrees C gassed with 95% nitrogen - 5% carbon dioxide (anoxia). Before and for 67 min after the fatigue period, muscles contracted at 0.6 contractions x min(-1) in 95% oxygen - 5% carbon dioxide (hyperoxia). We added a permeable cAMP analog (N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophosphate at 10(-3) mol x L(-1) (dcAMP)), caffeine (2 x 10(-3) mol x L(-1), or Krebs as vehicle control at 25 min before, during, or at the end of the fatigue period. In soleus and EDL, both challenges added before fatigue significantly increased developed force but only caffeine increased developed force when added during the fatigue period. At the end of fatigue, the decrease in force in challenged muscles was equal to or greater than in controls so that the force remaining was the same or less than in controls. EDL challenged with dcAMP or caffeine at any time recovered more force than controls. In soleus, caffeine improved recovery except when added before fatigue. With dcAMP added to soleus, recovery was better after challenges at 10 min and the end of the fatigue period. Thus, increased intracellular concentrations of cAMP and (or) Ca2+ did not decrease fatigue in either muscle but improved recovery from fatigue in EDL and, in some conditions, in soleus.     

Effects of caffeine on mouse skeletal muscle power output during recovery from fatigue.

The effects of 10 mM (high) and 70 microM (physiologically relevant) caffeine on force, work output, and power output of isolated mouse extensor digitorum longus (EDL) and soleus muscles were investigated in vitro during recovery from fatigue at 35 degrees C. To monitor muscle performance during recovery from fatigue, we regularly subjected the muscle to a series of cyclical work loops. Force, work, and power output during shortening were significantly higher after treatment with 10 mM caffeine, probably as a result of increased Ca2+ release from the sarcoplasmic reticulum. However, the work required to relengthen the muscle also increased in the presence of 10 mM caffeine. This was due to a slowing of relaxation and an increase in muscle stiffness. The combination of increased work output during shortening and increased work input during lengthening had different effects on the two muscles. Net power output of mouse soleus muscle decreased as a result of 10 mM caffeine exposure, whereas net power output of the EDL muscle showed a transient, significant increase. Treatment with 70 microM caffeine had no significant effect on force, work, or power output of EDL or soleus muscles, suggesting that the plasma concentrations found when caffeine is used to enhance performance in human athletes might not directly affect the contractile performance of fatigued skeletal muscle.     

Properties of slow- and fast-twitch skeletal muscle from mice with an inherited capacity for hypoxic exercise

Muscle fiber type, myosin heavy chain (MHC) isoform composition, capillary density (CD) and citrate synthase (CS) activity were investigated in predominantly slow-twitch (soleus or SOL) and fast-twitch (extensor digitorum longus or EDL) skeletal muscle from mice with inherited differences in hypoxic exercise tolerance. Striking differences in hypoxic exercise tolerance previously have been found in two inbred strains of mice, Balb/cByJ (C) and C57BL/6J (B6), and their F1 hybrid following exposure to hypobaric hypoxia. Mice from the three strains were exposed for 8 weeks to either normobaric normoxia or hypobaric hypoxia (1/2 atm). Hypoxia exposure led to a slightly higher 2b fiber composition and a lower fiber area of types 1 and 2a in SOL of all mice. In the EDL, muscle fiber and MHC isoform composition remained unaffected by chronic hypoxia. Chronic hypoxia did not significantly affect CD in either muscle from any of the three strains. There were relatively larger differences in CS activity among strains and treatment, and in SOL the highest CS activity was found in the F1 mice that had been acclimated to hypoxia. In general, however, neither differences among strains nor treatment in these properties of muscle vary in a way that clearly relates to inherited hypoxic exercise tolerance.     

Operation Everest II: neuromuscular performance under conditions of extreme simulated altitude

The force output of the ankle dorsiflexors was studied during a 40-day simulated ascent of Mt. Everest in a hypobaric chamber; both electrically activated and maximal voluntary contractions (MVCs) were employed. The purpose of this study was to establish whether, under conditions of progressive chronic hypoxia, there was a decrease in muscle force output and/or increased fatigability. We also attempted to identify the main site of any failure, i.e., central nervous system, neuromuscular junction, or muscle fiber. Muscle twitch torque (Pt), tetanic torque (Po), MVC torque, and evoked muscle compound action potential (M wave) were monitored during 205-s exercise periods in five subjects at three simulated altitudes (760, 335, and 282 Torr). All three types of torque measurement were well preserved at the three altitudes. In some subjects, the responses to stimuli interpolated during repeated MVCs provided evidence of "central" fatigue at altitude. In addition, the rate of fatigue during 20-Hz electrical stimulation was greater (P less than 0.01) at altitude and there was increased fatigability of the twitch (P less than 0.025); however, the M wave amplitude was maintained. We conclude that central motor drive becomes more precarious at altitude and is associated with increased muscle fatigue at low excitation frequencies; the latter is the result, in part, of chronic hypoxia and occurs in the muscle fiber interior because no impairment in neuromuscular transmission could be demonstrated.     

Denervation enhances the physiological effects of the K(ATP) channel during fatigue in EDL and soleus muscle

The objective was to determine whether denervation reduces or enhances the physiological effects of the K(ATP) channel during fatigue in mouse extensor digitorum longus (EDL) and soleus muscle. For this, we measured the effects of 100 microM of pinacidil, a channel opener, and of 10 microM of glibenclamide, a channel blocker, in denervated muscles and compared the data to those observed in innervated muscles from the study of Matar et al. (Matar W, Nosek TM, Wong D, and Renaud JM. Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during fatigue in skeletal muscle. Am J Physiol Cell Physiol 278: C404-C416, 2000). Pinacidil increased the (86)Rb(+) fractional loss during fatigue, and this effect was 2.6- to 3.4-fold greater in denervated than innervated muscle. Pinacidil also increased the rate of fatigue; for EDL the effect was 2.5-fold greater in denervated than innervated muscle, whereas for soleus the difference was 8.6-fold. A major effect of glibenclamide was an increase in resting tension during fatigue, which was for the EDL and soleus muscle 2.7- and 1.9-fold greater, respectively, in denervated than innervated muscle. A second major effect of glibenclamide was a reduced capacity to recover force after fatigue, an effect observed only in denervated muscle. We therefore suggest that the physiological effects of the K(ATP) channel are enhanced after denervation.     

Length-tension relationships are altered in regenerating muscles of the rat after bupivacaine injection.

Intramuscular injection of bupivacaine causes complete degeneration of fibers in extensor digitorum longus (EDL) muscles of rats, followed by complete regeneration within 60 days. Previous studies have shown that regenerated EDL muscles are protected from contraction-induced injury 60 days after bupivacaine injection. It is possible that these regenerated muscles have altered length-tension relations because of fiber remodeling. We tested the hypothesis that length-tension relations are different in bupivacaine-injected and noninjected control muscles. EDL and soleus muscles of the right hindlimb of deeply anesthetized rats were injected with bupivacaine and then allowed to recover for 7, 14, 21, or 60 days (7D, 14D, 21D, 60D), and isometric contractile properties were assessed. Muscles of the contralateral limb were not injected and served as control. EDL muscles recovered from bupivacaine injection more rapidly than soleus muscles, with mass restored to control levels at 21D, and isometric tetanic force (P(o)) restored to control at 60D. In contrast, mass and P(o) of injected soleus muscles was not restored to control even at 60D. In 7D EDL muscles, length-tension curves were shifted leftward compared with control, but in 21D and 60D EDL muscles length-tension curves were right shifted significantly (treatment x muscle length: P < 0.001). Although no clear shift in the position of the length-tension curve was observed in regenerating soleus muscles, force production was enhanced on the descending limb of the curve in 60D soleus muscles (treatment x relative muscle length: P < 0.01). The rightward shift in the length-tension curve of EDL muscles 60 days after bupivacaine injection is likely to contribute to the mechanism for their previously observed protection from contraction-induced injury.     

Skeletal muscle fatigue in vitro is temperature dependent

Our purpose was to determine the effect of temperature on the fatigability of isolated soleus and extensor digitorum longus (EDL) muscles from rats during repeated isometric contractions. Muscles (70-90 mg) were studied at 20-40 degrees C in vitro. Fatigability was defined with respect to both the time and number of stimuli required to reach 50% of the force (P) developed at the onset of the fatigue test. Fatigue was studied during stimulation protocols of variable [force approximately 70% of maximum force (Po)] and constant frequency (28 Hz). Results for soleus and EDL muscles were qualitatively similar, but fatigue times were longer for soleus than for EDL muscles. During the variable-frequency protocol, development of approximately 70% of Po required an increase in stimulation frequency as temperature increased. During stimulation at these frequencies, fatigue time shortened as temperature increased. For both fatigue protocols, the relationship between temperature and the number of stimuli required to reach fatigue followed a bell-shaped curve, with maximum values at 25-30 degrees C. The temperature optimum for maximizing the number of isometric contractions to reach fatigue reflects direct effects of temperature on muscle function.     

Chronic electrical stimulation of nongrafted and grafted skeletal muscles in rats

Our purpose was to determine the effects of chronic electrical stimulation on the structure and function of neve-intact grafts in rats. Fourteen days after grafting, extensor digitorum longus (EDL) grafts (n = 6) and nongrafted EDL muscles (n = 4) were stimulated 8 h/day at 10 Hz for 26 days. Measurements were made subsequently of cytochrome c concentration, capillary density, contraction and relaxation times, developed tension, and the resistance to fatigue. Compared with contralateral nonstimulated grafts, chronically stimulated grafts demonstrated a 65% greater cytochrome c concentration, 45% greater number of capillaries per millimeter squared, 30% greater resistance to fatigue, 35% longer contraction time, 30% longer relaxation time, and 30% lower maximum tetanic tension. The differences that resulted from the stimulation of nongrafted EDL muscles were significant but of less magnitude. Chronic stimulation of 8 h/day provided a mixed stimulus for adaptation that enhanced the metabolic and endurance characteristics of fibers in muscles and grafts, but decreased the total fiber cross-sectional area and development of force.     

Effects of modulators of sarcoplasmic Ca2+ release on the development of skeletal muscle fatigue.

The reduced release of Ca2+ from sarcoplasmic reticulum (SR) is considered a major determinant of muscle fatigue. In the present study, we investigated whether the presence of dantrolene, an established inhibitor of SR Ca2+ release, or caffeine, a drug facilitating SR Ca2+ release, modifies muscle fatigue development. Accordingly, the effects of Ca2+ release modulators were analyzed in vitro in mouse fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles, fatigued by repeated short tetani (40 Hz for 300 ms, 0.5 s(-1) in soleus and 60 Hz for 300 ms, 0.3 s(-1) in EDL, for 6 min). Caffeine produced a substantial increase of tetanic tension of both EDL and soleus muscles, whereas dantrolene decreased tetanic tension only in EDL muscle. In both EDL and soleus muscles, 5 microM dantrolene did not affect fatigue development, whereas 20 microM dantrolene produced a positive staircase during the first 3 min of stimulation in EDL muscle and a slowing of fatigue development in soleus muscle. The development of the positive staircase was abolished by the addition of 15 microM ML-7, a selective inhibitor of myosin light chain kinase. On the other hand, caffeine caused a larger and faster loss of tension in both EDL and soleus muscles. The results seem to indicate that the changes in fatigue profile induced by caffeine or dantrolene are mainly due to the changes in the initial tetanic tension caused by the drugs, with the resulting changes in the level of contraction-dependent factors of fatigue, rather than to changes in the SR Ca2+ release during fatigue development.     

Differential adaptations during growth spurt and in young adult rat muscles

During the post-weaning growth and maturation period (25/90 days after birth), rat limb muscles are submitted to specific adaptations. Our aim was to characterize the mechanical properties of two muscles that are opposite in terms of fibre-type distribution, the soleus and the extensor digitorum longus (EDL) muscles of male Wistar rats. Results showed a fast-to-slow fibre-type transition in soleus while no modification in fibre-type distribution was observed in EDL. A growth-induced increase in muscle force was observed. Soleus underwent an increase in twitch kinetics, but EDL showed no modification. Resistance to fatigue was higher in 90-day-old soleus but not modified in the EDL. Surprisingly, analysis of maximal shortening velocity showed a decrease in both soleus and EDL. Finally, tension/extension curves indicated a growth-induced increase in series elastic stiffness in the two muscles. These results suggest that during this growth period, skeletal muscles are submitted to differential adaptations. Moreover, whereas adaptation of biomechanical properties observed can be explained partly by an adaptation of fibre profile in soleus, this is not the case for EDL. It is suggested that changes in muscle architecture, which are often disregarded, could explain some variations in mechanical properties, especially when muscles undergo an increase in both mass and length.     

Continuous testosterone administration prevents skeletal muscle atrophy and enhances resistance to fatigue in orchidectomized male mice

Androgens promote anabolism in skeletal muscle; however, effects on subsequent muscle function are less well defined because of a lack of reliable experimental models. We established a rigorous model of androgen withdrawal and administration in male mice and assessed androgen regulation of muscle mass, structure, and function. Adult C57Bl/6J male mice were orchidectomized (Orx) or sham-operated (Sham) and received 10 wk of continuous testosterone (T) or control treatment (C) via intraperitoneal implants. Mass, fiber cross-sectional area (CSA), and in vitro contractile function were assessed for fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. After 10 wk, Orx+C mice had reduced body weight gain (P < 0.05), seminal vesicle mass (P < 0.01), and levator ani muscle mass (P < 0.001) compared with Sham+C mice, and these effects were prevented with testosterone treatment. Orx+T mice had greater EDL (P < 0.01) and SOL (P < 0.01) muscle mass compared with Orx+C mice; however, median fiber CSA was not significantly altered in these muscles. EDL and SOL muscle force was greater in Sham+T compared with Orx+C mice (P < 0.05) in proportion to muscle mass. Unexpectedly, Orx+T mice had increased fatigue resistance of SOL muscle compared with Orx+C mice (P < 0.001). We used a rigorous model of androgen withdrawal and administration in male mice to demonstrate an essential role of androgens in the maintenance of muscle mass and force. In addition, we showed that testosterone treatment increases resistance to fatigue of slow- but not fast-twitch muscle.     

Intermuscular interaction via myofascial force transmission: effects of tibialis anterior and extensor hallucis longus length on force transmission from rat extensor digitorum longus muscle.

Force transmission in rat anterior crural compartment, containing tibialis anterior (TA), extensor hallucis longus (EHL) and extensor digitorum longus (EDL) muscles, was investigated. These muscles together with the muscles of the peroneal compartment were excited maximally. Force was measured at both proximal and distal tendons of EDL muscle as well as at the tied distal tendons of TA and EHL muscles (the TA + EHL complex). Effects of TA + EHL complex length and force on proximally and distally measured forces of EDL muscle kept at constant muscle-tendon complex length were assessed. Length changes of EDL muscle were imposed by movement of the proximal force transducer to different positions.Proximal EDL force was unequal to distal EDL force (active as well as passive) over a wide range of EDL muscle-tendon complex lengths. This is an indication that force is also transmitted out of EDL muscle via pathways other than the tendons (i.e. inter- and/or extramuscular myofascial force transmission). At constant low EDL length, distal lengthening of the TA + EHL complex increased proximal EDL force and decreased distal EDL force. At optimum EDL length, TA+EHL active force was linearly related to the difference between proximal and distal EDL active force. These results indicate intermuscular myofascial force transmission between EDL muscle and the TA + EHL complex. The most likely pathway for this transmission is via connections of the intact intermuscular connective tissue network. The length effects of the TA + EHL complex can be understood on the basis of changes in the configuration, and consequently the stiffness, of these connections. Damage to connective tissue of the compartment decreased the proximo-distal EDL force difference, which indicates the importance of an intact connective tissue network for force transmission from muscle fibers to bone.     

Inotropic effects on mammalian skeletal muscle change with contraction frequency

Over the last decade, we have attempted to determine if mammalian skeletal muscle's steady-level force development as established by mechanical and stimulation parameters can be increased or decreased by physiological signals. In these experiments, nitric oxide (NO), endothelin-1 (ET-1), adenosine (Ado), and beta-adrenergic agonists (beta) modified force production in the soleus and (or) the extensor digitorum longus (EDL) of the mouse. NO and beta increased the force produced by 0.5-s tetanic contractions at 0.6 contractions/min in both muscles. While EDL did not respond to either Ado or ET-1, the developed force of the soleus was amplified by Ado but attenuated by ET-1. Increased cAMP analogue concentrations amplified developed force in both muscles, but a cGMP analogue had no effect on either muscle. Following an increase in the contraction frequency of the soleus, the increased force in response to NO disappeared, as did the decreased force to ET-1. The increase in force due to a cAMP analogue disappeared during fatigue but reappeared quickly during recovery. Thus, steady-level developed force can be modified by a number of substances that can be released from locations in the body or muscle. The response to a given compound is determined by a complex interaction of metabolic and intracellular signals on the force-generating cascade.     

Physiological, histological and biochemical properties of rat skeletal muscles in response to hindlimb suspension.

In previous study, we found that the reduced exercise-induced production of reactive oxygen species (ROS) reported in slow-oxidative muscle of hypoxemic rats and also in chronic hypoxemic patients did not simply result from deconditioning. In control rats and after a 3-week period of hindlimb suspension (HS), the slow-oxidative (Soleus, SOL) and fast-glycolytic skeletal muscles (Extensor digitorum longus, EDL) were sampled. We determined the response to direct muscle stimulation (twitch stimulation (TS), Maximal force (Fmax)), twitch amplitude and maximal relaxation rate, tetanic frequency, endurance to fatigue after muscle stimulation (MS), the different fibre types based on their myofibrillar adenosinetriphosphatase (ATPase) activity, and the intra-muscular redox status (Thiobarbituric Acid Reactive Sustances: TBARS, reduced glutathione: GSH, reduced ascorbic acid: RAA). After the 3-w HS period: (1) the contractile properties were modified in SOL only (reduced Fmax and twitch amplitude, increased tetanic frequency); (2) the fibre typology was modified in both muscles (in SOL: increased proportion of IIa and IIc fibres, in EDL: increased proportion of IId/x fibres but decreased proportion of IIb fibres); and (3) only in SOL, the TBARS level increased and the GSH and RAA concentrations decreased at rest and after fatiguing MS. Thus, HS accentuates the exercise-induced ROS production in slow-oxidative muscle in a direction opposite to that measured in chronic hypoxemic rats. This strongly suggests that hypoxemia reduces the ROS production independently from any muscle disuse.     

Measurement of rate constants for the contractile cycle of intact mammalian muscle fibers.

Small, random length changes were applied to bundles of intact fibers from rat and mouse extensor digitorum longus (EDL) and soleus muscles, while they were being tetanically stimulated. With increasing frequency of length changes, EDL muscle stiffness (tension change per unit change in length) increased, then decreased and increased again. The decrease was not seen in the soleus muscles. The EDL frequency-response could be well fitted by three exponential components with apparent rate constants of approximately 25, 150, and 500 s-1 at 20 degrees C. All rate constants increased steadily with temperature and for each 10 degrees C increase in temperature, the rates in the mouse EDL increased by a factor (Q10) between 1.8 and 2.4. With tetanic stimulation, force increased nearly exponentially to a steady level with a rate constant of 24 s-1 at 20 degrees C in mouse EDL muscles, and a Q10 of 2.4. These values correspond closely to the lowest frequency rate constant measured with length perturbations, which suggests that this process may limit the rate of rise of force in intact muscle fibers. During fatigue the high frequency and intermediate frequency rate constants declined, but the low frequency rate constant remained unchanged. These results are discussed in relation to current biochemical models for cross-bridge cycling.     

High energy phosphate concentrations and AMPK phosphorylation in skeletal muscle from mice with inherited differences in hypoxic exercise tolerance

The effect of chronic hypobaric hypoxia (1/2 atmospheric pressure) on high energy phosphate (HEP) compounds was investigated in slow (soleus; SOL) and fast twitch (extensor digitorum longus; EDL) muscle from 3 strains of mice with large differences in hypoxic exercise tolerance (HET). Phosphocreatine concentration ([PCr]) decreased 16–29% following hypoxia in EDL and SOL in all strains, while [ADP] and [AMP] increased. In the EDL, HET was negatively correlated with the PCr/ATP ratio and positively correlated with the ATP/P_i ratio. The free energy of ATP hydrolysis (ΔG_obs) remained constant despite the substantial changes that occurred in HEP profiles. The alteration of HEP set points and preservation of ΔG_obs are consistent with the notion that (1) maximal rates of steady-state ATP turnover are reduced under hypoxia, and (2) HEP perturbations during rest to work transitions are reduced in skeletal muscle from hypoxia acclimated animals. We therefore expected a lower phosphorylation ratio of AMP-activated protein kinase (AMPK-P/AMPK) during stimulation in hypoxic acclimated animals. However, neither the resting nor stimulated AMPK-P/AMPK was influenced by hypoxia, although there were significant differences among strains.     

Protein synthesis in adult skeletal muscle after tenotomy: responses to fasting and insulin infusion.

Muscle growth was established in specific muscles in the hindlimb of adult female rats by tenotomy of the gastrocnemius muscle. Seven days after surgery there was an increase in the wet weight of the soleus (Sol) and plantaris (P) muscles and a decrease in that of the gastrocnemius (G) muscle from the tenotomized limb compared with the respective control muscles from the contralateral limb from the same animal. In all three muscles there was a significant increase in the fractional rate of protein synthesis (ks) in the muscles from the tenotomized limb above the rate of the respective control muscles. In contrast, the extensor digitorum longus (EDL) muscle showed no change in wet weight or ks 7 days after tenotomy of G. Fasting for 12 or 36 h had no significant effect on ks in G, P, or Sol muscles from either the control or tenotomized limbs. In EDL from the control limb, both fasting periods resulted in a significant decrease in ks, although this effect was not seen in the EDL from the tenotomized limbs of the same animals. A subsequent 30-min insulin infusion was similarly ineffectual in G, P, and Sol, with its only effect evident in the EDL from the control limb, where it was sufficient to reverse the decreased ks resulting from the fasting, even though after 36 h fasting the reversal was only partial.