Muscle glycogen repletion and pre-exercise glycogen content: effect of carbohydrate loading in rats previously fed a high fat diet

We have recently reported that rates of muscle glycogen repletion during the early period of recovery were increased by carbohydrate (CHO) loading in rats previously fed a high fat diet. However, the reason for this remained unanswered. The purpose of this study was to examine whether an increase of glycogen utilization due to an elevated pre-exercise glycogen store would enhance rates of glycogen repletion in muscle. Despite an equal degree of glycogen depletion, the rates of glycogen repletion of soleus, red and white gastrocnemius muscles by postexercise administration of glucose (3.0 g · kg^–1 body mass) and citrate (0.5 g · kg^–1 body mass) were faster in the CHO loaded (3 days) rats than in the nonloaded rats, as a result of elevated pre-exercise glycogen content and consequently the greater glycogen utilization. The higher rate of muscle glycogen repletion may in part be explained by increased postexercise glycogen synthase activity.     

Muscle glycogen supercompensation: absence of a gender-related difference

Recently it has been reported that women do not have the capacity to accumulate supranormal levels of muscle glycogen when subjected to a carbohydrate (CHO) loading regimen [Tarnopolsky et al. (1995) J Appl Physiol 78:1360–1368]. Since, in this study, CHO intake relative to body mass in the female subjects was much lower than that in males, our primary aim was to re-examine this issue using subjects fed comparable amounts of CHO. Endurance-trained female and male subjects ingested 12 g CHO·kg^–1 lean body mass·day^–1 in conjunction with the cessation of their daily physical training. A 3-day exposure to this diet resulted in a marked rise in muscle glycogen levels from [mean (SD)] 108 (15) mmol·kg^–1 wet weight to 193 (14) mmol·kg^–1 wet weight and 111 (16) mmol·kg^–1 wet weight to 202 (20) mmol·kg^–1 wet weight in the female participants during the post-menstrual and pre-menstrual phases of their menstrual cycle, respectively, and from 109 (27) mmol·kg^–1 wet weight to 183 (25) mmol·kg^–1 wet weight in males. We conclude that (1) female athletes have the capacity to accumulate supranormal levels of muscle glycogen, and (2) when exercise-trained males and females are fed comparable amounts of CHO relative to lean body mass, there is no gender-related difference in their ability to accumulate supranormal levels of muscle glycogen. Electronic Publication     

Carbohydrate feeding and glycogen synthesis during exercise in man

In 7 male cyclists glycogen synthesis during exercise and rest was studied. Each subject did two exercise trials (A and B), in random order. In both trials, after determining the maximal workload (W _max), intermittent exercise was given to exhaustion. After the exhaustive exercise and taking a muscle biopsy the subjects either exercised at 40%W _max for 3 h (trial A) or rested for 3 h (trial B), during which they consumed approximately 21 of a 25% malto-dextrine drink in both trials. After 3 h rest (trial A) or 3 h of mild exercise (trial B) a second muscle biopsy was taken for total glycogen and histochemistry (ATPase and PAS). Blood glucose and insulin levels were elevated during the first 2 h of exercise (p<0.05). Glycogen depletion was most pronounced in type I and to a less extent in type IIA fibers. In trial A muscle glycogen increased from 136±66 to 199±71 mmol/kg DW, and in trial B from 145±56 to 257±79 mmol/kg DW. During exercise glycogen repletion was restricted to type IIA and IIB fibers, whereas during rest glycogen synthesis occurred both in type I and type II fibers. The present study demonstrates that oral carbohydrate administered during exercise may not only provide substrate for energy metabolism, but can also be utilized for glycogen synthesis in the non-active muscle fibers.     

Skeletal muscle phosphagen and lactate concentrations in ischaemic dynamic exercise

Summary Five young males performed dynamic, submaximal contractions to exhaustion with the quadriceps muscle under arterial occlusion. The work load was 14.7 Watt (W). After 10 min rest with intact arterial circulation, the subjects commenced another bout to exhaustion; this process was repeated until a total of 10–16 bouts had been performed. Muscle biopsies were obtained immediately after the second, fifth, eighth, and last bout as well as 30 min after the last bout. The concentrations of adenosine triphosphate (ATP), creatine phosphate (CP), lactate, and glycogen were measured in each sample and some material underwent histochemical analysis. Muscle lactate was highest following the second work bout [22.9 mmol/kg wet weight (ww)] and gradually declined to 7.0 mmol/kg ww by the end of the last bout. CP level was low in all postexercise samples with the exception of a remarkably high CP (11.7 mmol/kg ww) after the last bout. Glycogen utilization tended to parallel muscle lactate levels, the rate of depletion being most rapid initially. Histochemical staining for glycogen depletion revealed that both type I and II fibres were low in glycogen, although type I was depleted most uniformly. In the first work bouts the high lactate and low CP levels in the total muscle could be responsible for the fatigue; none of these factors seem adequate to explain the development of the fatigue experienced in the later work bouts. It is concluded that muscle fatigue in this type of exercise is not related to substrate depletion or accumulation of metabolites, further that the fibre recruitment pattern is determined by the type and relative severity of performed work rather than local metabolic factors.Supported by grants from the Danish National Association Against Rheumatic Diseases and the Danish Council for Sports Research     

Carbohydrate and fat metabolism in contracting canine skeletal muscle

Summary Isolated canine gracilis muscles were perfused in situ with a free flow (systemic blood flow; FF) or a constant flow (blood from a reservoir; CF). The nervous supply was stimulated electrically for 60 min. A-V-concentration differences for glucose, pyruvate, lactate, glycerol, FFA and O₂ were obtained as well as the concentrations of ATP, CP, glycogen and lactate in the muscle.Resting O₂ uptake ranged from 4 to 11 moles×100 g^–1×min^–1 (FF; CF). A 30- and 5-fold increase in O₂ uptake occurred during stimulation in the FF and CF-experiments, respectively. The release of lactate was, however, the same (20–40 moles×100 g×min^–1) although the muscle lactate concentration was much higher in the CF experiments. In the CF experiment stimulation did not significantly increase glucose uptake which ranged from 0.3 to 3.3 moles×100 g^–1 ×min^–1 at rest. Conversely, stimulation resulted in a 6-fold increase in glucose uptake in the FF experiments. No definite tendency for a FFA uptake or a glycerol release was found in either experiment (FF, CF). Glycogen depletion during the stimulation period amounted to 20–30 moles×g^–1. Thus the glucose uptake could account for only 12% of the carbohydrate utilized during the stimulation period.     

Muscle strength and fatigue after selective glycogen depletion in human skeletal muscle fibers

Summary Two groups of male subjects were studied to examine the effects of different exercise protocols on performance of an isokinetic, short-time strength test, the performance of which is related to fast twitch (FT) muscle fiber recruitment. The laboratory group (LG) (n=10) cycled (30 min, 70% VO₂ max), ran (75 min), and performed repeated bouts of sprint cycling and rapid, maximal contractions of the quadriceps. The marathon group (MG) (n=7) participated in and completed Stockholm's Marathon 1979. A strength test was performed before and within 1–2 h after completion of the group exercise protocol. The m. vastus lateralis was biopsied and muscle fibers classified as slow twitch (ST) or FT. After periodic acid-Schiff staining fibers were qualitatively classified as to glycogen content. In LG significant glycogen depletion occurred in both fiber types and in MG predominantly ST fibers were exhausted of glycogen after the exercise protocol. The glycogen exhaustion from both fiber types in LG was associated with impaired maximal muscular strength produced during a single dynamic contraction, as well as with reduced muscle fatigue patterns. When glycogen exhaustion was induced in ST muscle fibers only in the MG, no impairment was observed for maximal muscular strength but fatigue during 50 consecutive contractions was significantly increased.This study was supported in part by Coca-Cola Export Corporation, Sweden     

Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes

Summary Three female and three male highly trained endurance runners with mean maximal oxygen uptake (VO_2max) values of 60.5 and 71.5 ml·kg^–1·min^–1, respectively, ran to exhaustion at 75%–80% of VO_2max on two occasions after an overnight fast. One experiment was performed after a normal diet and training regimen (Norm), the other after a diet and training programme intended to increase muscle glycogen levels (Carb). Muscle glycogen concentration in the gastrocnemius muscle increased by 25% (P<0.05) from 581 mmol·kg^–1 dry weight, SEM 50 to 722 mmol·kg^–1 dry weight, SEM 34 after Carb. Running time to exhaustion, however, was not significantly different in Carb and Norm, 77 min, SEM 13 vs 70 min, SEM 8, respectively. The average glycogen concentration following exhaustive running was 553 mmol· kg^–1 dry weight, SEM 70 in Carb and 434 mmol·kg^–1 dry weight, SEM 57 in Norm, indicating that in both tests muscle glycogen stores were decreased by about 25%. Periodic acid-Schiff staining for semi-quantitative glycogen determination in individual fibres confirmed that none of the fibres appeared to be glycogen-empty after exhaustive running. The steady-state respiratory exchange ratio was higher in Carb than in Norm (0.92, SEM 0.01 vs 0.89, SEM 0.01; P<0.05). Since muscle glycogen utilization was identical in the two tests, the indication of higher utilization of total carbohydrate appears to be related to a higher utilization of liver glycogen. We have concluded that glycogen depletion of the gastrocnemius muscle is unlikely to be the cause of fatigue during exhaustive running at 75%–80% of VO_2max in highly trained endurance runners. Furthermore, diet- and training-induced carbohydrate supercompensation does not appear to improve endurance capacity in such individuals.     

Glycolytic intermediates in human muscle after isometric contraction

Isometric contraction of the quadriceps muscle sustained to fatigue with a force of 66% of the maximum voluntary contraction force resulted in a mean glycogen utilization of 80.4 (S.D. 58.4) mmol glucosyl units/kg dry muscle (d.m.) and an accumulation of glycolytic intermediates and glucose corresponding to 82.9 (S.D. 17.5) mmol glucosyl units/kg d.m. Accumulation of hexose phosphates (principally glucose 6-phosphate) accounted for 35.4% (S.D. 4.1) of the total increase and lactate for 59.3% (S.D. 2.8). During a 4 min recovery period glucose 6-[hosphate content showed a linear decrease with a half time of 2.0 min and lactate decreased exponentially with a half time of 2.5 min. The rate of lactate disappearance from the muscle was approximately 4 times as fast as that observed previously after maximal bicycle exercise. This was probably due to a lower lactate concentration in blood after isometric contraction resulting in a larger muscle-blood gradient for lactate. Muscle content of free glucose was increased after contraction and increased further during recovery. It is concluded that the glucose increase is confined to the intracellular pool and is an effect of hexokinase inhibition by accumulated glucose 6-phosphate. Occlusion, of the local circulation after the contraction inhibited the recovery processes for lactate and glucose 6-phosphate.     

Do muscle fibre size and fibre angulation correlate in pennated human muscles?

Summary Several studies have reported estimations of the total number of fibres in a muscle, e.g. before and after training or before and after inactivity. In those investigations a combination of computed tomographic estimations of muscle size and morphological studies of fibre size has most often been used. There have been doubts about the reliability of those studies on pennate muscles, since changes in muscle fibre size have been said to alter fibre angulation and thus the number of fibres that will cross a section. If such an alteration in fibre angulation takes place with an increase in fibre size, there ought to be some correlation between fibre size and fibre angulation. The present study was designed to test whether repetitive estimations of muscle fibre angulation could be performed in vivo and if any such correlation could be found between fibre size and fibre angulation. A group of 15 women volunteered to take part in the study. Repeated ultrasonographic recordings were made on five subjects on 3 consecutive days to test the repeatability of ultrasonographic measurement of fibre angulation. Both muscle morphological analyses and ultrasonographic measurements of fibre angulation were performed on the other 10 subjects. Ultrasonographic measurement of fibre angulation was found to be reproducible since no variation between measurements made on different days was found. When trying to correlate muscle fibre size to the muscle fibre angulation, measured ultrasonographically, no significant correlation was found. In conclusion, the study showed that ultrasonographic recordings of fibre angulation are reproducible when performed on different days; furthermore, our results did not reveal any correlation between fibre angulation and fibre size which would have indicated that fibre angulation was dependent on fibre size.Preliminary results of this investigation have been presented at 1988 meeting of The Swedish Society of Medicine     

Glycogen content and glucose uptake in stimulated frog sartorius muscle: Effects of prolonged incubation,insulin, and testosterone

Anin vitro preparation of electrically stimulated sartorius muscle of the frog was used as a model to investigate the effects of insulin and testosterone on the increased glycogen content of frog skeletal muscle that was observedin vivo 48 h after an exhaustive bout of treadmill exercise. Thein vitro preparation could effectively extract glucose from the bathing medium and continue glycogen synthesis for incubation periods up to 60 h, and the electrical stimulation depleted glycogen by 68%. Electrical stimulation enhanced rates of glucose uptake and glycogen synthesis when these variables were measured after prolonged periods of poststimulation incubation but did not cause an overshoot of glycogen storage similar to that observedin vivo. Stimulation did not produce increments in glycogen synthesis that were additive to those of insulin or testosterone. The absence of glycogen overshoot in thein vitro system may have resulted from the absence of neural factors.     

Carbohydrate metabolism and food intake in food restricted rats: effects of an unexpected meal

Rats subjected to a daily single-meal feeding schedule were presented with an unexpected meal at three different times of the day and the effects on the carbohydrate metabolic patterns were determined. The results indicate that it is the actual metabolic moment of the organism, rather than the duration of the inter-meal period and the degree of gastric emptying, that determines the amount of food ingested.     

Non-invasive method to detect motor unit contractile properties and conduction velocity in human vastus lateralis muscle

The contractile properties and conduction velocity of motor units are estimated by using surface array electrodes during voluntary isometric contractions of the human vastus lateralis muscle. The subjects develop and maintain sufficient force to steadily discharge a given motor unit, assisted by visual feedback from an oscilloscope. The torque curve developed around the knee joint is triggered by an individual motor unit and averaged. 31 motor units in five subects are studied. The twitch tension detected ranges from 3 to 27 m Nm with a mean of 12·3 m Nm. The threshold force ranges from 1·88 to 10·12 Nm with a mean of 5·48 Nm, which is 3% of the maximal voluntary contraction. The rise time ranges from 56 to 106 ms with a mean of 83 ms. The mean value of conduction velocity is 4·64 m s⁻¹. The twitch tension is positively correlated to the threshold force (r=0·839, p<0.01), but has no relation to the other parameters. It is concluded that the use of non-invasive surface array electrodes provides the contractile properties of motor units and muscle fibre conduction velocity during weak contractions.     

Light diffraction intensity from muscle fibres in different osmotic solutions: measurement of equilibration time

A new method for evaluating the osmotic equilibration time of striated muscle fibres is described. Single fibres from the semitendinosus muscles of frog were illuminated normally with a He-Ne laser. The first-order diffraction intensity was measured continuously after the tonicity of the fibre's bathing solution was changed abruptly. Hypertonicity decreased while hypotonicity increased the intensity. The time course of the intensity decrease or increase followed closely a simple exponential with a single time constant. It is proposed that the time constant associated with the diffraction intensity transient is a true representation of the equilibration process. A simple diffraction model provides a relationship between diffraction intensity and the diameter of the muscle fibre. This relationship is used to explain the diffraction intensity of fibres in different osmotic solutions.     

In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion. Accepted: 28 June 2000     

Deficit in human muscle strength with cast immobilization: contribution of inorganic phosphate

Metabolic factors have been proposed to explain strength deficits observed in skeletal muscle with immobilization that are not completely accounted for by changes in muscle cross-sectional area (CSA) and neural adaptations. The aim of this study was to quantify changes in the resting inorganic phosphate (Pi) concentration from the medial gastrocnemius muscle during immobilization, reloading and rehabilitation. Additionally, we assessed the contributions of CSA, muscle activation and Pi concentration to plantar flexor torque during rehabilitation following immobilization. Eight persons with a surgically stabilized ankle fracture participated. Subjects were immobilized for 6-8 weeks and subsequently participated in 10 weeks of rehabilitation. Localized (31)P-Magnetic resonance spectroscopy, magnetic resonance imaging, isometric torque and activation testing were performed on the immobilized and uninvolved limbs. At 6 weeks of immobilization, significant differences were noted between the immobilized and uninvolved limbs for the Pi concentration and the Pi/PCr ratio (P < 0.05). From 6 weeks of immobilization to 3-5 days of reloading, the increase in Pi concentration (15%, P = 0.26) and Pi/PCr (20%, P = 0.29) was not significant. During rehabilitation, the relative contributions of CSA, muscle activation and Pi concentration to plantarflexor torque were 32, 44 and 40%, respectively. Together, CSA, muscle activation and Pi concentration accounted for 76% of the variance in torque (P < 0.01). In summary, our findings suggest that immobilization, independent of reloading, leads to a significant increase in the resting Pi concentration of human skeletal muscle. Additionally, alterations in resting Pi concentration may contribute to strength deficits with immobilization not accounted for by changes in muscle CSA or neural adaptations.     

Contralateral muscle fatigue in human quadriceps muscle: evidence for a centrally mediated fatigue response and cross-over effect

The purpose of this investigation was to examine the effects of voluntary muscular fatigue in one lower limb and determine whether a ‘cross-over’ of fatigue is evident in the contralateral limb. Twenty-eight subjects (13 males and 15 females) performed a series of voluntary and evoked isometric contractions of both the dominant (exercised) and non-dominant (non-exercised) leg extensor muscles, prior to and after a fatigue protocol consisting of a 100-s sustained maximal isometric contraction (MVC) performed by the dominant limb only. Force values and surface electromyography (EMG) from the vastus lateralis muscle were obtained allowing for the determination of twitch and compound action potential (M-wave) values. Maximal twitch tension and peak-to-peak amplitude were significantly decreased after the fatigue test in the dominant limb, as was maximal voluntary force (∼65 N reduction), EMG activity (∼0.1 mV decrease) and voluntary activation (∼17% decline). However, no significant changes were observed in the non-dominant limb with respect to twitch and M-wave properties nor in MVC force. The voluntary activation of the non-dominant limb decreased significantly by 8.7% after the fatigue test, which was performed only on the dominant limb. The results of the present study suggest that the decrease in force production in the exercised limb was primarily related to peripheral fatigue mechanisms, with central fatigue making a lesser contribution. Centrally mediated mechanisms appear to be the sole contributor to fatigue in the non-exercised limb suggesting an anticipatory fatigue response and a ‘cross-over’ of central fatigue between the exercised and non-exercised contralateral limb.     

Myogenic stem cells: regeneration and cell therapy in human skeletal muscle

Human skeletal muscle has been considered as an ideal target for cell-mediated therapy. However, the positive results obtained in dystrophic animal models using the resident precursor satellite cell population have been followed by discouraging evidences obtained in the clinical trials involving Duchenne muscular dystrophy patients. This text reviews the recent advances that many groups have achieved to identify from the stem cell compartment putative candidates for cell therapy. We focused our attention on stem cells with myogenic potential which might be able to improve transplantation efficiency and therefore could be used as a therapeutic tool for neuromuscular diseases.     

Reflex actions of muscle afferents on fusimotor innervation in decerebrated cats: an assessment of beta contributions

Summary The existence of beta innervation in many cat muscles raises the possibility that spindle afferent discharge will excite beta motoneurons, augmenting spindle afferent discharge and thereby closing a positive feedback loop. In order to evaluate the strength of such a loop through beta motoneurons and muscle spindles, the stretch responses of muscle spindle afferents from medial gastrocnemius (MG) and soleus (SOL) muscles were studied in decerebrated cats before and after dorsal root section. If a positive feedback loop were operational, the spindle afferent stretch response should be diminished following dorsal root section by an amount related to the magnitude of positive feedback. After dorsal root section, the static positional sensitivities of our MG spindle afferent sample were significantly reduced for 72% (13/18, p < 0.05) of the afferents, and dynamic rate/length slopes were decreased for 88% (8/9) of a subset of the afferents studied. Similar reductions for 6 afferents from SOL were not found. To apportion these afferent changes to reflex excitation of either gamma or beta motoneurons, we recorded the stretch responses of gamma and alpha-type fibers in the same preparation. (We assume that the population of alpha-type fibers includes beta fibers). In keeping with other reports, alpha fibers were much more responsive to stretch than gamma fibers. The mean positional sensitivity for alpha fibers (1.29 ± 0.92 pps/mm, n = 15) was greater (p < 0.05) than that of gamma fibers (0.49 ± 0.93 pps/mm, n = 18). Because of these differences in sensitivity, beta motoneurons are more likely (than gamma motoneurons) to be involved in a positive feedback loop, although some gamma contribution is also likely. Using equations based on a beta position regulating scheme, differences in spindle positional sensitivity were used to estimate beta loop gain. The average loop gain was estimated to be 0.41 (n = 18). The contribution of such a beta configuration to reducing the sensitivity of muscle to changes in load and muscle properties is evaluated.Supported by NIH no 1 P01NS17489, and NS21180 (WZR)     

Phasic and tonic stretch reflexes in muscles with few muscle spindles: human jaw-opener muscles

 We investigated phasic and tonic stretch reflexes in human jaw-opener muscles, which have few, if any, muscle spindles. Jaw-unloading reflexes were recorded for both opener and closer muscles. Surface electromyographic (EMG) activity was obtained from left and right digastric and superficial masseter muscles, and jaw orientation and torques were recorded. Unloading of jaw-opener muscles elicited a short-latency decrease in EMG activity (averaging 20 ms) followed by a short-duration silent period in these muscles and sometimes a short burst of activity in their antagonists. Similar behavior in response to unloading was observed for spindle-rich jaw-closer muscles, although the latency of the silent period was statistically shorter than that observed for jaw-opener muscles (averaging 13 ms). Control studies suggest that the jaw-opener reflex was not due to inputs from either cutaneous or periodontal mechanoreceptors. In the unloading response of the jaw openers, the tonic level of EMG activity observed after transition to the new jaw orientation was monotonically related to the residual torque and orientation. This is consistent with the idea that the tonic stretch reflex might mediate the change in muscle activation. In addition, the values of the static net joint torque and jaw orientation after the dynamic phase of unloading were related by a monotonic function resembling the invariant characteristic recorded in human limb joints. The torque-angle characteristics associated with different initial jaw orientations were similar in shape but spatially shifted, consistent with the idea that voluntary changes in jaw orientation might be associated with a change in a single parameter, which might be identified as the threshold of the tonic stretch reflex. It is suggested that functionally significant phasic and tonic stretch reflexes might not be mediated exclusively by muscle spindle afferents. Thus, the hypothesis that central modifications in the threshold of the tonic stretch reflex underlie the control of movement may be applied to the jaw system. Received: 11 October 1996 / Accepted: 17 March 1997