Influence of an active pre-stretch on fatigue of skeletal muscle

Summary In activities such as running, many muscles of the lower extremities appear to be actively stretched before they are allowed to shorten. In this study we investigated the effect of an active pre-stretch on the fatigability of muscles. Thus muscle contractions were compared in which shortening was preceded by an active isometric phase or by an active stretch. Rat medial gastrocnemius muscle-tendon complexes (with arrested blood flow) performed a series of ten repeated contractions (1·s^–1) with either an active stretch or an isometric phase preceding the shortening. Contraction duration (0.45 s), and shortening duration (0.3 s), distance (6 mm) and velocity (20 mm·s^–1) were the same in both types of contraction. Work output during the ten shortening phases was approximately 40% higher in the contractions with an active pre-stretch; in contrast, high-energy phosphate utilization was similar. Over the ten repeated contractions reduction of work output during the shortening phases of both types of contraction was similar in absolute terms (approx. 9.5 mJ). It is suggested that all the extra work performed during the shortening phases after a pre-stretch originated from sources other than cross-bridge cycling, which are hardly affected by fatigue. However, reduction of work output in relative terms, which is how the reduction is often expressed in voluntary exercise, was less after a pre-stretch (26% vs 32%), giving the impression of protection against fatigue by an active pre-stretch.     

The effect of unloading on protein synthesis in human skeletal muscle

Atrophy of skeletal muscle is observed in response to immobilization and lack of weight-bearing. The aim of this study was to investigate the effect of immobilization on muscle protein synthesis and associated biochemical parameters in skeletal muscle of healthy volunteers employing a standardized model of lower limb unloading. One leg was unloaded for 10 days, and percutaneous muscle biopsies were taken before and at the end of the unloading period. The capacity for protein synthesis, as reflected by the concentration of RNA, decreased by 16% (P < 0.05) although the fractional synthesis rate (FSR) of protein was not significantly changed after 10 days of unloading. Furthermore there was an increase in the concentration of the free branched chain amino acids in muscle by 48% (P < 0.05).     

Improving human skeletal muscle myosin heavy chain fiber typing efficiency

Single muscle fiber sodium dodecyl sulfate polyacrylamide gel-electrophoresis (SDS-PAGE) is a sensitive technique for determining skeletal muscle myosin heavy chain (MHC) composition of human biopsy samples. However, the number of fibers suitable to represent fiber type distribution via this method is undefined. Muscle biopsies were obtained from the vastus lateralis (VL) of nine resistance-trained males (25 ± 1 year, height = 179 ± 5 cm, mass = 82 ± 8 kg). Single fiber MHC composition was determined via SDS-PAGE. VL fiber type distribution [percent MHC I, I/IIa, IIa, IIa/IIx, and total “hybrids” (i.e. I/IIa + IIa/IIx)] was evaluated according to number of fibers analyzed per person (25 vs. 125). VL fiber type distribution did not differ according to number of fibers analyzed (P > 0.05). VL biopsy fiber type distribution of nine subjects is represented by analyzing 25 fibers per person. These data may help minimize cost, personnel-time, and materials associated with this technique, thereby improving fiber typing efficiency in humans.     

Absence of an effect of fatigue on muscle efficiency during high-intensity exercise in rat skeletal muscle

The effect of fatigue was studied on rat skeletal muscle efficiency during maximal dynamic exercise of 10s duration. After the initial 4s of exercise, power output decreased rapidly to 46.2 ± 6.7% (mean ± SD; n=6) after 6s of stimulation and further to 17.5 ± 5.8% in the last contraction. Both the rates of total work output and high-energy phosphate consumption decreased with increasing exercise duration. As a result muscle efficiency was not affected by exercise time in the present experiments. This result indicates that fatigue in severe maximal exercise is induced by a feed-back mechanism, which in the case of high ATP utilisation rates will reduce ATP splitting probably by reducing Ca²⁺-release from the sarcoplasmic reticulum.     

Alterations of mechanical characteristics of human skeletal muscle during strength training

Summary To investigate the influence of strength training on the mechanical characteristics of human skeletal muscle, 14 male subjects went through training of combined heavy concentric and eccentric contractions three times a week for 16 weeks. The strength training program consisted mainly of dynamic exercises for leg extensors with loads of 80 to 120% of one maximum repetition. The force-time curves produced during various vertical jumps were the basis for calculation of various mechanical parameters. In addition to a great increase (p<0.001) in maximal isometric force, heavy resistance strength training also caused significant (p<0.05–0.01) increases in heights and in various mechanical parameters in positive work phases of vertical and drop jumps. The increase in positive force during a fast dynamic contraction was correlated (p<0.01) with the reduced time to produce a certain submaximal force level in isometric condition. No changes in the elastic properties of the muscle were observed as judged from the difference between the counter-movement and squat jumps. When the training was followed by the 8-week detraining period a great decrease (p<0.001) in maximal force took place, but only minor changes (ns) were observed in fast force production.Supported in part by the grants from The Finnish Olympic Committee and Central Sport Federation     

Potentiation of the mechanical behavior of the human skeletal muscle through prestretching

Force–velocity and power–velocity curves in a vertical jump involving movements around several joints were derived from vertical ground reaction forces and knee angular velocities. The jumps were performed with weights from 10 to 160 kg added on the shoulders. The obtained curves from a semi–squatting static starting position resembled those reported for isolated muscles or single muscle groups. Vertical jumps were also performed in the conditions where the shortening of the leg extensors was preceded by prestretching of the active muscles either through a preparatory counter–movement or dropping down on the force–platform from the various heights ranging from 20 to 100 cm. Prestretching modified through a range of velocities the force–velocity and power–velocity curves by increasing both the ground reaction forces and the calculated mechanical power. Thus the results are similar to those reported in isolated muscles. In studies with isolated muscle preparation the nervous connections have not been intact and therefore it is suggested that increase in the performance of the skeletal muscles through prestretching, in the conditions of the present study, was attributed to the combined effects of the utilization of stored elastic energy and the reflex potentiation of muscle activation.     

The influence of extra load on the mechanical behavior of skeletal muscle

Summary Eleven international jumpers and throwers engaged in year round training were divided into experimental (n=6) and control (n=5) groups. The experimental group was tested before and after a 3 weeks simulated hypergravity period, and again 4 weeks after the hypergravity period. The high gravity condition was created by wearing a vest weighing about 13% of the subjects body weight. The vest was worn from morning to evening including the training sessions, and only removed during sleep. The daily training of all subjects consisted of classical weight training and jumping drills. No changes in the ordinary training program were allowed in the experimental group, except for the use of the vest. Vertical jumps, drop jumps and a 15 s continuous jumping test were used to measure the explosive power characteristics of the subjects. After the hypergravity period the experimental subjects demonstrated significant (5–10%, P<0.05–0.01) improvements in most of the variables studied: however, 4 weeks after cessation of the high gravity period they tended to return towards the starting values. No changes were observed in the results of the control group. The improvement observed in the experimental subjects was explained as fast adaptation to the simulated high gravity field. It is suggested that adaptation had occurred both in neuromuscular functions and in metabolic processes.Supported by grants from Italian Track and Field Association, the Central Sport Federation of Finland, and Ministry of Education (Finland)     

The effect of ageing and exercise on skeletal muscle function

The contractile and selected biochemical properties of fast- and slow-twitch skeletal muscle were studied at 9, 18, and 28 months of age in sedentary and regularly exercised rats. The isometric twitch duration was prolonged with aging in both the fast- and slow-twitch muscle. This effect was primarily due to a prolonged one-half relaxation time (1/2RT), which developed late in life. Regular exercise tended to further prolong the twitch duration, particularly in the slow-twitch soleus. Surprisingly, twitch and tetanic tension (Po), peak rate of tension development and decline, and the maximal shortening velocity were all unaltered between 9 and 28 months of age. Furthermore, regular exercise (running or swimming) had little or no effect on these properties. The prolonged 1/2RT with aging could not be explained by a decreased rate of Ca2+ sequestration by the sarcoplasmic reticulum, as the rate of Ca2+ uptake measured in muscle homogenates was unaltered in any of the muscles studied between 9 and 28 months. The degree of muscle fatigue (decline in Po) with 30 min of contractile activity in the slow-twitch soleus was not affected by aging. However, lactate reached two-fold higher levels and glycogen fell to considerably lower levels in the muscles of the old rats. This suggests an increased glycolysis and glycogen utilization during contractile activity in aged rats.     

Potentiation of the mechanical behavior of the human skeletal muscle through prestretching.

Force-velocity and power-velocity curves in a vertical jump involving movements around several joints were derived from vertical ground reaction forces and knee angular velocities. The jumps were performed with weights from 10 to 160 kg added on the shoulders. The obtained curves from a semi-squatting static starting position resembled those reported for isolated muscles or single muscle groups. Vertical jumps were also performed in the conditions where the shortening of the leg extensors was preceded by prestretching of the active muscles either through a preparatory counter-movement or dropping down on the force-platform from the various heights ranging from 20 to 100 cm. Prestretching modified through a range of velocities the force-velocity and power-velocity curves by increasing both the ground reaction forces and the calculated mechanical power. Thus the results are similar to those reported in isolated muscles. In studies with isolated muscle preparation the nervous connections have not been intact and therefore it is suggested that increase in the performance of the skeletal muscles through prestretching, in the conditions of the present study, was attributed to the combined effects of the utilization of stored elastic energy and the reflex potentiation of muscle activation.     

The effect of skeletal muscle ventricle pouch pressure on muscle blood flow.

Skeletal muscle powered assist ventricles (SMV) are being investigated in animal studies as a treatment for heart failure. Muscle fatigue is almost always dependent upon muscle capillary blood flow. This study examined the relationship between SMV intrapouch pressure and blood flow to the circumferential muscle in a working SMV with a mock circulation. The unconditioned rectus abdominis muscle was used to create an in situ SMV in five dogs. Muscle blood flow was measured by both the radioactive microsphere and the electromagnetic flow probe method as the pouch pressure was varied between 10 and 70 mmHg and as the SMV was stimulated to contract at a rate of 20 min-1. The correlation coefficient for the two methods was 0.908. At pouch pressures of 10, 40, and 70 mmHg, the respective blood flow values were 22.60 +/- 2.50 (1 SEM), 12.20 +/- 2.10, and 4.40 +/- 0.74 ml min-1 (p less than 0.05). When they were corrected for muscle weight, the mean blood flow values at these same pouch pressures were 0.28 +/- 0.03, 0.15 +/- 0.03, and 0.05 +/- 0.01 ml min-1 g-1, respectively (p less than 0.05). SMV output was measured for each pouch pressure that was tested. Pouch output, expressed as ml min-1, was 458 +/- 20 (1 SEM) at an SMV diastolic pouch pressure of 10 mmHg, 309 +/- 22 at a pouch pressure of 40 mmHg, and 103 +/- 6 at a pouch pressure of 70 mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of methylprednisolone on intracellular calcium of normal and dystrophic human skeletal muscle cells.

Clinical trials have shown that a glucocorticoid, the methyiprednisolone (PDN), has a beneficial effect on muscle strength and function in Duchenne muscular dystrophy (DMD) patients. The aim of this study was to test if the effect of PDN could be mediated via a possible action on intracellular calcium. The intracellular calcium activity, at rest and during calcium mobilizing drug superfusion protocols was recorded in normal and dystrophic human cocultured muscle cells. PDN (10 microM) pretreatment induced an elevation of the resting calcium concentration of 51, 34 and 38% in proliferating normal myoblasts, DMD myoblasts and DMD myotubes, respectively, while normal myotubes resting [Ca2+]i was not altered.     

Lack of effect of moderate hypoxaemia on human postural reflexes to skeletal muscle

The effect of moderate hypoxemia upon postural sympathetic vasoconstrictor reflexes in skeletal muscle was studied in five healthy young students, aged 20-30 years. The vasoconstrictor response to head-up tilt was studied in brachio radial muscle kept at heart level and in the anterior tibial muscle. The local sympathetic veno-arteriolar axon reflex was studied in the anterior tibial muscle placed at heart level and lowered 30-50 cm below heart level. Muscle blood flow was measured by the local 133Xe wash-out technique. The measurements were carried out with the subject breathing atmospheric air and with the subject breathing 10-11% oxygen. No sign of orthostatic intolerance was seen during hypoxaemia as arterial blood pressure remained constant in the tilted position. Hypoxaemia did not alter the head-up tilt induced vasoconstriction in brachio-radial and anterior tibial muscles. The vasoconstriction elicited by the local veno-arteriolar reflex was slightly reduced during hypoxaemia. The results suggest that central orthostatic vasoconstrictor reflexes to muscle are essentially normal during moderate hypoxaemia, but local veno-arteriolar reflexes are slightly attenuated. This does not, however, significantly alter blood pressure control during head-up tilt.     

The effect of high-intensity exhaustive exercise studied in isolated mitochondria from human skeletal muscle

Six young men performed five 1-min bicycle exercise bouts to exhaustion. Muscle lactate increased to congruent with 114 mmol x kg(-1) dwt and pH decreased to congruent with 6.6. Mitochondria were prepared from a needle biopsy sample taken from m. vastus lateralis immediately after the last exercise bout. No significant effect of exhaustion on the proton permeability and amount of cytochromes c and aa3 in isolated mitochondria was detected. The activities of the following enzymes and systems were not altered either: citrate synthase, succinate dehydrogenase, cytochrome oxidase, succinate + glutamate respiration, malate + glutamate respiration, the respiratory chain, and the reactions involved in ATP synthesis. Thus, the mitochondria did not appear globally altered upon exhaustion. However, the following NAD-linked activities were significantly lowered: pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, glutamate dehydrogenase and fatty acid beta-oxidation. The activities of alpha-glycerophosphate dehydrogenase and exo-NADH oxidase, enzymes that might catalyze the oxidation of sarcoplasmic NADH, were increased. These changes may be due to the action of reactive oxygen species, protons and Ca2+. Transient opening of the permeability transition pore may also be involved. Some effects may have been reversed during isolation of the mitochondria and the changes in mitochondrial function in situ upon exhaustion may have been more extensive than observed.     

The effect of muscle extracts on the contracture response of skeletal muscle to acetylcholine

Summary Preincubation of normal rat soleus muscles in vitro with homogenates prepared from mixed leg muscles which had been denervated 4 days previously resulted in an increase in the contracture response to acetylcholine. After 30 min incubation a 1.5-fold increase was observed. Homogenates of normally innervated muscles did not increase the response. The active principles of the denervated muscles were found to reside in the cytosol fraction. An approximately 2-fold increase was observed upon incubation with the cytosol for 30 min; incubation for longer periods resulted in a subsequent decrease in the response. The effect of the denervated muscle cytosol was concentration-dependent and heat-labile. Normal muscle cytosol also increased the soleus muscle response to acetylcholine but this fraction was less effective than denervated muscle cytosol. The response of control muscles incubated in Krebs-Henseleit solution was found to decrease with time.Commercially obtained phospholipases C and D increased the response of normal soleus muscles approximatcly 2-fold. Phospholipase A, lipase, trypsin, collagenase and a bacterial protease had no effect, lysozyme produced a small but consistent increase in the response to acetylcholine.