The interaction of cations with the calcium-binding site of troponin

Is there any correlation between muscle fatigue and serum lactate dehydrogenase levels in prediabetic individuals?

Aim: Lactate dehydrogenase (LDH) is an enzyme that acts as a catalyst in the conversion of lactate to pyruvate which is abundantly found in liver, kidney, heart and muscles. Previous studies have all shown a strong positive correlation between muscle fatigue and increased serum LDH levels with type 2 diabetes mellitus but no study has actually assessed the same for prediabetes. The basic objective of this study, thus, is to find out the correlation between muscle fatigue and serum LDH levels in prediabetic individuals. Methods: A cross-sectional study was conducted in adults between 24–60 years old who were classified as prediabetic individuals as per norms established by American Diabetes Association. A total of 100 prediabetic individuals were selected for the study. Fatigability was calculated as a function of work done by the pleximeter finger of the dominant hand using Mosso’s ergograph. The study was conducted at Rajarshi Dashrath Autonomous State Medical College, Ayodhya. Results: Out of 100 prediabetic participants, 50% were males with a mean age of 44.14 ± 10.91 years and remaining 50% were females with a mean age of 41.12 ± 11.5 years. Overall, the average work done by the participants was 2.9 ± 1.2 weight lifted•total distance moved (kg•m) with an average serum LDH level of 323.84 ± 26.82 unit/litre (U/L). Conclusions: This study aimed at assessing the correlation between muscle fatigue and serum LDH levels in prediabetic individuals so that further work can be initiated to improve the quality of life in prediabetics that maybe drastically hampered due to easy fatigability in prediabetic individuals.

EFFECTS OF BLOOD FLOW RESTRICTION IN LARGE AND SMALL MUSCLE GROUPS

ABSTRACT Introduction It is known that strength training brings improvements in health and sports performance by causing muscle hypertrophy and increased strength, as well as modifying some hemodynamic and physiological factors. Several strength training methodologies have been developed, one of which is vascular occlusion. There are few studies with large muscle groups due to poor adherence to the training style and the fact that vascular occlusion of large muscle groups is more difficult. Objective To verify and compare the hemodynamic effects of exercise with and without vascular occlusion in different muscle groups. Methods Quantitative crossover study, with cross-sectional and field procedures. The sample consisted of 10 physically active healthy male and female subjects between 18 and 30 years of age. With the cross-over design, all the volunteers participated in 3 groups: intervention with vascular occlusion, intervention without vascular occlusion and the control group. Results Overall, lactate and cholesterol remained elevated after 15 minutes of recovery and blood glucose and blood pressure did not vary among the groups. Conclusion Vascular occlusion training is an effective method for manipulating hemodynamic variables. Evidence level II; Clinical study.

Hyperventilation-Aided Recovery for Extra Repetitions on Bench Press and Leg Press

Sakamoto, A, Naito, H, and Chow, CM. Hyperventilation-aided recovery for extra repetitions on bench press and leg press. J Strength Cond Res 34(5): 1274-1284, 2020-Hyperventilation (HV)-induced alkalosis, an ergogenic strategy, improved repeated pedaling sprint performance through enhanced H removal. However, it did not confer beneficial effects on other forms of exercises. This study investigated the benefits of HV-aided recovery on lifting repetitions and joint velocity during resistance training involving multiple joints and both concentric and eccentric contractions. Eleven power-trained men (mean ± SD age: 22.5 ± 4.3 years, training experience: 8.3 ± 3.6 years) performed 6 sets each of bench press and leg press at 80% 1 repetition maximum. Each set was continued until failure, with a 5-minute recovery between sets. In protocol A, HV was implemented for 30 seconds before the first, third, and fifth sets of each exercise (HV-aided recovery), whereas spontaneous breathing continued throughout the recovery before the second, fourth, and sixth sets (control recovery). In protocol B, the order of the HV and control recoveries was reversed. For both protocols, reductions in repetitions (range: -4.7% to -22.5%) and velocity (range: -23.1% to -37.7%) were consistently observed after control recovery (p < 0.05), whereas HV-aided recovery resulted in increased repetitions (range: +21.3% to +55.7%) and velocity (range: +6.3% to +15.3%) (p < 0.05) or no reductions in these measures from the previous set. The total repetitions performed across 6 sets (protocols A and B combined) were greater after the HV-aided than control recovery (p ≤ 0.001) in bench press (44 ± 10 vs. 36 ± 10 reps, increased by 27.1 ± 24.1%) and leg press (64 ± 9 vs. 50 ± 15 reps, increased by 35.2 ± 29.5%). Hyperventilation-aided recovery may boost the effectiveness of resistance training through increased training volume and lifting velocity.

Effect of Resistance Exercise Performed to Volitional Failure on Ratings of Perceived Exertion

The purpose of this investigation was to assess the effect of resistance exercise performed to volitional failure on ratings of perceived exertion (RPE) using power as an indication of fatigue. 12 male participants ( M age= 21.9 yr., SD = 1.3) performed one set of back squats at three different intensities (50%, 70%, and 90% of one repetition maximum) for both a pre-determined number of repetitions (3) and to volitional failure. RPE was significantly different between sets at 50%, 70%, and 90% when performed to a pre-determined number of repetitions, but not during volitional failure. A decrease in power between the first and the last repetitions in the volitional failure sets suggests that fatigue may confound the relationship between RPE and intensity.

Influence of hyperoxia on muscle metabolic responses and the power-duration relationship during severe-intensity exercise in humans: a 31P magnetic resonance spectroscopy study

Severe-intensity constant-work-rate exercise results in the attainment of maximal oxygen uptake, but the muscle metabolic milieu at the limit of tolerance (T(lim)) for such exercise remains to be elucidated. We hypothesized that T(lim) during severe-intensity exercise would be associated with the attainment of consistently low values of intramuscular phosphocreatine ([PCr]) and pH, as determined using (31)P magnetic resonance spectroscopy, irrespective of the work rate and the inspired O(2) fraction. We also hypothesized that hyperoxia would increase the asymptote of the hyperbolic power-duration relationship (the critical power, CP) without altering the curvature constant (W). Seven subjects (mean +/- s.d., age 30 +/- 9 years) completed four constant-work-rate knee-extension exercise bouts to the limit of tolerance (range, 3-10 min) both in normoxia (N) and in hyperoxia (H; 70% O(2)) inside the bore of 1.5 T superconducting magnet. The [PCr] (approximately 5-10% of resting baseline) and pH (approximately 6.65) at the limit of tolerance during each of the four trials was not significantly different either in normoxia or in hyperoxia. At the same fixed work rate, the overall rate at which [PCr] fell with time was attenuated in hyperoxia (mean response time: N, 59 +/- 20 versus H, 116 +/- 46 s; P < 0.05). The CP was higher (N, 16.1 +/- 2.6 versus H, 18.0 +/- 2.3 W; P < 0.05) and the W was lower (N, 1.92 +/- 0.70 versus H, 1.48 +/- 0.31 kJ; P < 0.05) in hyperoxia compared with normoxia. These data indicate that T(lim) during severe-intensity exercise is associated with the attainment of consistently low values of muscle [PCr] and pH. The CP and W parameters of the power-duration relationship were both sensitive to the inspiration of hyperoxic gas.

Neural drive and electromechanical alterations in the fatiguing diaphragm

It is suggested that respiratory failure in the compromised circulation might occur as a result of respiratory muscle fatigue in the presence of adequate neural drive and muscle excitation. As the cardiac output decreases acidosis develops and ventilation increases, resulting in an increase in the work of breathing, which requires the delivery of large supplies of energy. As these demands cannot be met by the energy supply, because of low cardiac output, the diaphragm fails as a force generator and respiratory failure ensues. Diaphragmatic fatigue may occur in normal subjects if the pressure developed with each breath is greater than 40% of the maximum transdiaphragmatic pressure and hypoxia predisposes the diaphragm to fatigue. Diaphragmatic fatigue, as in other skeletal muscles, might be located either at the neuromuscular junction or distal to it and can be detected either by phrenic stimulation or by frequency analysis of the myoelectric signal. Phrenic stimulation shows that after fatigue the diaphragm develops less force at any frequency of stimulation, but the loss of force at low frequencies persists for a longer period than at high frequencies. Frequency analysis of the electromyogram reveals that the power spectrum shifts to lower frequencies. This shift occurs long before the diaphragm fails as a force generator.

Muscle metabolic responses to exercise above and below the "critical power" assessed using 31P-MRS

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using (31)P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3-18 min) for estimation of the CP (mean +/- SD, 20 +/- 2 W). Subsequently, the subjects exercised at work rates 10% below CP (<CP) for 20 min and 10% above CP (>CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P(i) concentration ([P(i)]), and pH, were estimated using (31)P-MRS. All subjects completed 20 min of <CP exercise without duress, whereas the limit of tolerance during >CP exercise was 14.7 +/- 7.1 min. During <CP exercise, stable values for [PCr], [P(i)], and pH were attained within 3 min after the onset of exercise, and there were no further significant changes in these variables (end-exercise values = 68 +/- 11% of baseline [PCr], 314 +/- 216% of baseline [P(i)], and pH 7.01 +/- 0.03). During >CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P(i)] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 +/- 16% of baseline [PCr], 564 +/- 167% of baseline [P(i)], and pH 6.87 +/- 0.10, all P < 0.05 vs. <CP exercise). These data support the hypothesis that the CP represents the highest constant work rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e., H(+) concentration and [P(i)]), which have been associated with the fatigue process.

Effects of sodium citrate ingestion before exercise on endurance performance in well trained college runners

OBJECTIVE

To test the hypothesis that sodium citrate administered two hours before exercise improves performance in a 5 km running time trial.

METHODS

A total of 17 male well trained college runners (mean (SD) O(2)MAX 61.3 (4.9) ml/kg/min) performed a 5 km treadmill run with and without sodium citrate ingestion in a random, double blind, crossover design. In the citrate trial, subjects consumed 1 litre of solution containing 0.5 g of sodium citrate/kg body mass two hours before the run. In the placebo trial, the same amount of flavoured mineral water was consumed.

RESULTS

The time required to complete the run was faster in the citrate trial than the placebo trial (1153.2 (74.1) and 1183.8 (91.4) seconds respectively; p = 0.01). Lower packed cell volume and haemoglobin levels were found in venous blood samples taken before and after the run in the citrate compared with the placebo trial. Lactate concentration in the blood sample taken after the run was higher in the citrate than the placebo trial (11.9 (3.0) v 9.8 (2.8) mmol/l; p<0.001), and glucose concentration was lower (8.3 (1.9) v 8.8 (1.7) mmol/l; p = 0.02).

CONCLUSION

The ingestion of 0.5 g of sodium citrate/kg body mass shortly before a 5 km running time trial improves performance in well trained college runners.

Magnesium-calcium exchange in cardiac troponin C bound to cardiac troponin I

Understanding the process of Ca(2+)/Mg(2+)exchange during muscle excitation and relaxation is fundamental to elucidating the mechanism of Ca(2+)-regulated muscle contraction. During the resting phase, the C-domain of cardiac troponin C may be occupied by either Ca(2+)or Mg(2+). Here, complexes of recombinant cardiac troponin C(81-161) and the N terminus of cardiac troponin I, representing residues 33-80, were generated in the presence of saturating Mg(2+). Heteronuclear multi-dimensional nuclear magnetic resonance experiments were used to obtain backbone assignments of the Mg(2+)-loaded complex. In the presence of cardiac troponin I, the affinity of site IV for Mg(2+)is increased. Comparison of Mg(2+)and Ca(2+)-loaded complexes reveals that chemical shift differences are primarily localized to metal-binding sites III and IV, defining positions within these sites that have distinct Ca(2+)/Mg(2+)conformations. The observed transition from the Mg(2+)-loaded to Ca(2+)-loaded form demonstrates that sites III and IV fill simultaneously with Ca(2+)displacing Mg(2+). However, even in the absence of excess Ca(2+), Mg(2+)does not readily displace Ca(2+)in the isolated binary complex. Thus, the Mg(2+)-loaded conformer may only represent a small fraction of the total cardiac troponin C found in the sarcomere.

Effects of drafting during short-track speed skating

Short-track speed skating involves pack-style racing where five to seven skaters may be on the ice at once. Since average speed for a 3000-m event may exceed 35 km.h-1, drafting may be beneficial. However, the short (111 m) oval track could limit effective drafting space, and high forces required in cornering may compromise potential benefits. We evaluated heart rate (HR)-lactate (LA) responses and post-drafting 3-lap sprint performance using 18 National Team and developmental skaters. Two 4-min trials, one drafting and one leading at 8.8 m.s-1, were performed. In addition, six skaters performed three 3-lap sprints, rested, immediately after a 4-min drafting trial at 9.2 m.s-1, and immediately after an unaided 4-min trial at 9.2 m.s-1. Results demonstrated lower HR and LA responses during drafting (174.0 +/- 9.0 and 5.56 +/- 2.18 vs 180.4 +/- 8.7 and 7.75, P < 0.05) at 8.8 m.s-1. After 4-min trials at 9.2 m.s-1, HR deltas were 6 bpm, lactate values were 9.00 +/- 1.84 and 5.22 +/- 1.18 for unaided and drafting, respectively. Sprint performance was better following drafting (33.46 +/- 1.19 vs 34.03 s, P < 0.05). HR and LA deltas during the 8.8 m.s-1 trials ranged from 0.8 to 12.4 and -0.18 to 5.37, respectively, indicating that some skaters were more effective drafters than others. These results suggest that drafting could be an important strategy in short-track speed skating, and drafting technique should be emphasized in training.

The acute reversal of a diet-induced metabolic acidosis does not restore endurance capacity during high-intensity exercise in man

The present experiment was designed to investigate whether a diet-induced metabolic acidosis was a major factor in the earlier onset of fatigue during high-intensity exercise. Six healthy males cycled to exhaustion at a workload equivalent to 95 percent of maximum oxygen uptake on four separate occasions. Exercise tests were performed after an overnight fast and each test was preceded by one of four experimental conditions. Two experimental diets were designed, either to replicate each subject's own normal diet [N diet, mean (SD) daily energy intake (E) = 13 (0.7) MJ, 14.5 (0.8) percent protein (Pro), 37.5 (2.2) percent fat (Fat) and 47.5 (2.1) percent carbohydrate (CHO)], or a low-carbohydrate diet [E = 12.6 (0.8) MJ, 33.6 (1.3) percent Pro, 64.4 (1.5) percent Fat and 2.2 (0.4) percent CHO]. These diets were prepared and consumed within the department over a 3-day period. Over a 3-period prior to the exercise trial subjects ingested either NaHCO(3) or CaCO(3) (3.6 and 3.0 mmol*kg body mass), thus giving four experimental conditions: N diet and treatment, N diet and placebo, low-CHO diet and treatment and low-CHO diet and placebo. Treatments were assigned using a randomised protocol. Arterialised venous blood samples were taken for the determination of acid-base status and metabolite concentrations at rest prior to exercise and at intervals for 30 min following exhaustion. Consumption of the low-CHO diet induced a mild metabolic acidosis which was reversed by the ingestion of NaHCO(3). Blood pH, bicarbonate (HCO-(3)) and base excess (BE) were higher following NaHCO(3) ingestion after the normal diet than all of the other experimental conditions (P <0.01). Exercise time following the low-CHO diet was less than on the normal diet conditions (P <0.05): bicarbonate ingestion had no effect on exercise time on either of the diet conditions. Post-exercise blood pH, HCO-(3); and BE were higher following the ingestion of NaHCO(3) irrespective of the pre-exercise diet (P <0.05). Blood lactate concentration was higher 2 min after exercise following the N diet with NaHCO(3) when compared to the low-CHO diets with either NaHCO(3) or placebo (P <0.05). Plasma ammonia accumulation was not significantly different between experimental conditions. These data confirm previous data showing that the ingestion of a low-CHO diet reduces the capacity to perform high-intensity exercise, but it appears that the metabolic acidosis induced by the low-CHO diet is not the cause of the reduced exercise capacity observed during high-intensity exercise under these conditions.

Fatigability during repetitive maximal knee extensions in 14-year-old boys

The isokinetic forces, during 50 repeated maximal knee extensions with a constant velocity of 3.14 rad.s-1, and muscle cross-sectional area (CSA) of the quadriceps femoris muscles were measured for boys aged 14 years (n = 26) and young adult men (n = 26). As representative scores in the maximal session, the mean values of force (F) of every five consecutive and all trials were calculated. The CSA was measured by using a B-mode ultrasound technique at the midpoint of the thigh length (lt). The average values of F at the 1st-5th contractions were 193 (SEM 12) N for the boys and 303 (SEM 13) N for the young adults. The average decline of F with 50 contractions, expressed as a percentage of the value in the 1st-5th trial, was higher in the young adults than in the boys: F for the young adults was reduced by 48 (SEM 2.9)%, for the boys by 36 (SEM 3.1)%. The F of every five consecutive and all trials were significantly correlated to the product of CSA and lt (CSA.lt) in separate groups: for the boys r = 0.762-0.894 (P < 0.01), for the young adults r = 0.598-0.837 (P < 0.01). In a trial range between the 1st-5th and 11th-15th contractions, the young adults showed significantly higher values in the ratio of F to CSA.lt (F.CSA-1.lt-1) than the boys. However, the difference between groups of the ratio on and after the 16th-20th trial and for all trials became insignificant. Thus, at least for 50 maximal repeated knee extensions, the 14-year-old boys were inferior to the young adults in their ability to produce force during the earlier sessions even when the difference in muscle size was allowed for. The inferiority in the boys might be attributed to a lower reliance on glycolysis as pointed out in previous biochemical studies.

Sodium bicarbonate versus Carbicarb in canine myocardial hypercarbic acidosis

The objective of this study was to compare the in vivo effects of sodium bicarbonate (NaHCO3) and Carbicarb infusion on regional contractile performance and acid-base status in the setting of hypercarbic acidosis. Animals (N = 9) were anesthetized and paralyzed using sodium pentothal, halothane, and pancuronium bromide, and mechanically ventilated with an air-O2 mixture so that arterial PO2 was > or = 300 mm Hg. Following beta-adrenergic blockade, alveolar ventilation was gradually reduced over a 50-minute period to increase arterial PCO2 to 60 to 80 mm Hg. Each of the following solutions was then infused in consecutive order directly into the left anterior descending artery coronary artery for 15 minutes: (1) 8.4% NaHCO3 at 2 mL/min; (2) 5% sodium chloride at 2 mL/min, equivalent to NaHCO3 in osmolality; (3) 6.3% Carbicarb at 0.5 mL/min, equivalent to NaHCO3 in buffer capacity; and (4) 6.3% Carbicarb at 2 mL/min, equivalent to NaHCO3 in volume. Regional stroke work analog (ultrasonic dimension transducers), interstitial myocardial pH (Khuri electrode), coronary blood flow (doppler flow probe), and hemodynamic/metabolic variables (heart rate, blood pressure, arterial and coronary venous blood gases) were measured at 1, 5, 10, and 15 minutes during each infusion and 10 minutes after the infusion was discontinued, ie, at 25 minutes. Animals were allowed to recover for 45 minutes between interventions. Values at each time point were compared with baseline for statistical significance. Small reductions in interstitial myocardial pH (P < .05) and stroke work (P > .05) were observed within 1 minute of NaHCO3 administration. Both parameters increased significantly from baseline levels thereafter, ie, interstitial myocardial pH at 5 minutes and stroke work at 15 minutes. Infusion of Carbicarb invariably was associated with an increase (P < .05) in interstitial myocardial pH. Stroke work increased (P < .05) during low-dose Carbicarb administration, but infusion of the higher dose was accompanied by a biphasic response, ie, an increase (P < .05) from 0 to 5 minutes, followed by a gradual decrease that achieved statistical significance 10 minutes after termination of the infusion. End-diastolic length was inversely proportional to changes in stroke work, and coronary blood flow varied directly with changes in coronary venous Pco2. Myocardial O2 consumption decreased (P < .05) during Carbicarb infusion, but changes during NaHCO3 did not reach statistical significance. Our findings lend support to the hypothesis that intramyocardial pH determines myocardial function independent of CO2 production by buffer therapy.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin reverses the effects of acidosis on the intracellular Ca2+ transient and contractility in ferret myocardium

Endothelin may play an important role in modulating myocardial contractility under certain pathophysiological conditions. To determine whether endothelin beneficially modulates myocardial contractility in the common clinical condition of acidosis, we compared the effects of endothelin-1 on intracellular Ca2+ transients and isometric contractions under normal (extracellular pH [pH(o)] 7.4) and acidotic (pH(o) 6.4) conditions in ferret papillary muscles (n = 33) loaded with the Ca(2+)-regulated bioluminescent indicator aequorin. A pH(o) of 6.4 was induced by replacing 92% of HCO3- with Cl- in the bathing medium. The effects of endothelin at pH(o) 6.4 differed from the effects at pH(o) 7.4 in that 1) the minimally effective concentration of endothelin was 30-fold lower (1 x 10(-10) M at pH(o) 6.4; 3 x 10(-9) M at pH(o) 7.4) and the concentration-response curve of endothelin was significantly shifted to the left with a decrease in log EC50 from -7.83 +/- 0.13 to -8.92 +/- 0.10 (p less than 0.001), indicating an increased sensitivity of myocardium to endothelin; 2) endothelin produced an increase of approximately 375% in tension development at pH(o) 6.4 (approximately 62% at pH(o) 7.4) (p less than 0.001) without increasing peak [Ca2+]i (approximately 13% increase at pH(o) 7.4, p less than 0.001), indicating an increase in myofilament Ca2+ responsiveness; and 3) endothelin significantly abbreviated (approximately -19%, p less than 0.001) the prolonged intracellular Ca2+ transient induced by acidosis (pH(o) 6.4). In addition, pretreatment with 10 microM of the Na(+)-H+ exchange inhibitor 5-(N-methyl-N-isobutyl)-amiloride significantly attenuated endothelin-induced effects on the intracellular Ca2+ transient and contraction during acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen delivery-independent effect of blood flow on diaphragm fatigue

To determine the effect of blood flow on diaphragm fatigue independent of oxygen delivery, the left hemidiaphragm was vascularly isolated in 14 pentobarbital-anesthetized, mechanically ventilated dogs. Fatigue (decline in tension generation) of the left diaphragm was induced by phrenic nerve stimulation at 10 Hz, 12/min, duty cycle of 0.5 for 8 min. Two stimulation periods separated by 30 min of rest were performed in each animal. Diaphragmatic O2 delivery during the two periods was the same. In Group 1 (n = 8), the diaphragm was autoperfused from the femoral artery (high O2-low flow) during the first stimulation period. The tension generated by the diaphragm during this period declined progressively to 47.7% of initial values. In the second period in this group, the diaphragm was pump perfused with arterial blood, diluted with an equal volume of 6% dextran at a flow rate twice that of the first period (low O2-high flow). Tension in this period declined to 76% of initial tension (p less than 0.05 compared with high O2-low flow). In Group 2 (n = 6), stimulation performed while perfusing the diaphragm in the first period with diluted arterial blood at a flow rate twice that recorded during autoperfusion (low O2-high flow) produced a decline in tension to 70% of the initial values. In the second period, the diaphragm was perfused with undiluted arterial blood at a flow rate equal to 50% of that of the first period (high O2-low flow). Tension during this period declined to 56% of initial values (p less than 0.05 compared with low O2-high flow).(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle fatigue: conduction or mechanical failure?

It is well documented that repeated voluntary activity or electrical stimulation of skeletal muscle results in a decline in force production or power output. However, the precise physiological causes of "muscle fatigue" are not yet well understood. It is conceivable that the mechanism(s) may lie either in the conduction of action potentials in the central and peripheral nervous systems or in the transformation of the electrical event into mechanical force production by the muscle itself. In fact, none of the components of the electrical pathway from generation of impulses in the brain to their conduction over the neuron and the excitable membranes of the muscle can as yet be ruled out as potential contributors to the fatigue process. Relative to that on conduction failure, more information exists concerning the possibility that a defect in the excitation contraction coupling process in skeletal muscle, e.g., intracellular acidosis, inadequate supply of energy for contraction, or a disruption in Ca2+ homeostasis may also be significant in compromising force production following sustained activity. Despite this, the amount of conflicting data derived from these experiments has hindered the resolution of this question. In the future more attention must be given to such issues as the type of activity used to elicit fatigue and the fiber composition of the muscles studied. This is imperative as these factors clearly impact the nature of correlations between the biochemical and physiological events in muscle that are required to support prospective fatigue mechanisms.

Differential activation of myofibrils during fatigue in phasic skeletal muscle cells

In fatigued muscles the T-system is swollen; thus the action potential may fail to travel along the T-system or the T-tubule terminal cisternae signal may fail to bring about TC Ca2+ release. This would lead to a decrease in the number of myofibrils activated and in force development, but if fatigue is the result of a generalized process, all the myofibrils would be affected equally leading to a lower activation of all of them. We have investigated this possibility in isolated twitch muscle fibres by giving them repetitive tetanic stimulations until fatigue developed. The behaviour of myofibrils was followed with cinemicrophotography. Before fatigue, no lack of shortening of myofibrils could be found. During fatigue groups of myofibrils became wavy. When exposed to caffeine, the wavy myofibrils disappeared and tension similar to the control developed. The tension-caffeine concentration relationship was shifted to the left after development of fatigue. In low Na+ solution fatigue developed faster and after reintroducing normal Ringer, tension recovered substantially. K-contractures were smaller during fatigue. These results indicate that in this type of fatigue, a step in the EC coupling chain of events is involved in its development.

The relationship between anaerobic threshold and electromyographic fatigue threshold in college women

The purpose of this study was to investigate the relationship between anaerobic threshold (Th(an)) and muscle fatigue threshold (EMGFT) as estimated from electromyographic (EMG) data taken from the quadriceps muscles (vastus lateralis) during exercise on a cycle ergometer. The subjects in this study were 20 female college students, including highly trained endurance athletes and untrained sedentary individuals, whose fitness levels derived from their maximal oxygen consumption ranged from 24.9 to 62.2 ml.kg-1.min-1. The rate of increase in integrated EMG (iEMG) activity as a function of time (iEMG slope) was calculated at each of four constant power outputs (350, 300, 250, 200 W), sufficiently high to bring about muscle fatigue. The iEMG slopes so obtained were plotted against the exercise intensities imposed, resulting in linear plots which were extrapolated to zero slope to give an intercept on the power axis which was in turn interpreted as the highest exercise intensity sustainable without electromyographic evidence of neuromuscular fatigue (EMGFT). The Th(an) was estimated from gas exchange parameters during an incremental exercise test on the same cycle ergometer. The mean results indicated that oxygen uptake (VO2) at Than was 1.39 l.min-1, SD 0.44 and VO2 at EMGFT was 1.33 l.min-1, SD 0.57. There was no significant difference between these mean values (P greater than 0.05) and there was a highly significant correlation between VO2 at Than and VO2 at EMGFT (r = 0.823, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of glycogen depletion and supercompensation on the physical working capacity at the fatigue threshold

The purpose of this investigation was to determine the effect of glycogen depletion and supercompensation on the physical working capacity at the fatigue threshold (PWCFT). Ten adult males (mean age 23 years, SD 3) volunteered as subjects for this study. During the first laboratory visit the subjects performed a maximal bicycle ergometer test for the determination of maximum oxygen consumption (VO2max). Between 48 and 72 h later, the subjects pedaled to exhaustion at a power output which corresponded to a mean of 76% of VO2max (range, 72-80%) for the purpose of glycogen depletion. For the next 3 days, the subjects were fed a 10.5 MJ.day-1 low carbohydrate diet which consisted of 7.5% carbohydrates, 22.0% protein and 70.5% fat. The subjects then performed an incremental cycle ergometer test to the onset of fatigue or PWCFT, which was estimated from integrated electromyographic voltages of the vastus lateralis muscle. For the next 3 days the subjects were fed a 10.5 MJ high carbohydrate diet which consisted of 72.2% carbohydrates, 12.4% protein and 15.4% fats for the purpose of glycogen supercompensation. The subjects then performed a second PWCFT test. A paired t-test indicated that there was no significant (p greater than 0.05) difference between the means of the PWCFT values (depletion 246 W, SD 30; supercompensation 265 W, SD 28) and they were highly correlated at r = 0.884. The results of this investigation suggested that the methods commonly used to affect glycogen depletion or supercompensation had no effect on PWCFT.

Effect of hypoxia on diaphragm blood flow, oxygen uptake, and contractility

Recent studies examining the effects of hypoxia on diaphragm function have reached conflicting conclusions, with some reports suggesting an adverse effect of even mild hypoxemia while others indicate that the diaphragm may be extremely resistant to hypoxic stress. Diaphragm tension was not, however, directly measured nor was diaphragm length controlled in these previous reports, and it seems possible that methodologic limitations may have been responsible for these discrepant results. The purpose of the present study was to examine the effects of graded, steady-state hypoxia on diaphragm blood flow, oxygen extraction, oxygen consumption, and contractility using an in situ canine diaphragm strip preparation that permitted direct and continuous measurement of diaphragm length, tension, and blood flow. Measurements were made with the diaphragm at rest, during normoxia (PaO2, 90 to 160 mm Hg), mild hypoxia (PaO2, 45 to 60 mm Hg), and severe hypoxia (PaO2, 25 to 35 mm Hg); measurements were made with the diaphragm at rest, during rhythmic contractions at a tension time index (TTI) of 0.05, and with contractions at a TTI of 0.15. Decreases in arterial oxygenation resulted in progressive increases in blood flow and in the fractional extraction of oxygen in both resting and contracting diaphragm strips. At all levels of activity tested, blood flow and fractional extraction increased sufficiently to keep diaphragm oxygen consumption constant despite reductions in arterial oxygen content. Diaphragm contractility, as assessed from the tension generated in response to a range of electrical stimuli (1 to 80 Hz), was unaffected by hypoxia for trials performed with the diaphragm at rest and contracting at a TTI of 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships between muscle lactate accumulation and surface EMG activities during isokinetic contractions in man

In order to investigate the relationship between metabolic state and myoelectrical activity in working muscle during short term intense exercise, eleven healthy males performed isokinetic knee extensions at an angular velocity of 180 deg X sec-1 for 30 and 60 s. The median frequency (MF) of the surface electromyogram (EMG) recorded from vastus lateralis was decreased while the time lag of torque production after the onset of electrical activity (EMD) was increased during exercise. These changes (MF and EMD) corresponded well to muscle lactate accumulation in the same muscle. Over the exercise period, the integrated EMG/knee extension peak torque ratio (E/T ratio) was increased, which indicated a decrease in the efficiency of electrical activity. It was concluded that the changes in the frequency components of the EMG and in the contractile property of the muscle during short term intense exercise correlated with lactate accumulation in the identical muscle, and that the decrease in efficiency of the electrical activity in the muscle suggested peripheral fatigue.

The effects of dietary manipulation on blood acid-base status and the performance of high intensity exercise

The effect of a pattern of exercise and dietary modification, which is normally used to alter muscle glycogen content, upon the acid-base status of the blood and the ability to perform high intensity exercise was studied. Eleven healthy male subjects cycled to exhaustion on an electrically braked cycle ergometer at a workload equivalent to 100% of their maximal oxygen uptake (VO2max) on three separate occasions. The first exercise test took place after a normal diet (46.2 +/- 6.7% carbohydrate (CHO)), and was followed by prolonged exercise to exhaustion to deplete muscle glycogen stores. The second test was performed after three days of a low carbohydrate diet (10.1 +/- 6.8% CHO) and subsequently after three days of a high CHO diet (65.5 +/- 9.8% CHO) the final test took place. Acid-base status and selected metabolites were measured on arterialised venous blood at rest prior to exercise and during the post-exercise period. Exercise time to exhaustion was longer after the normal (p less than 0.05) and high (p less than 0.05). CHO dietary phases compared with the low CHO phase. Resting pre-exercise pH was higher after the high CHO diet (p less than 0.05) compared with the low CHO diet. Pre-exercise bicarbonate, PCO2 and base excess measurements were higher after the high CHO treatment compared with both the normal (p less than 0.01, p less than 0.05, p less than 0.01 respectively) and low CHO phases (p less than 0.001, p less than 0.01, p less than 0.001 respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatigue from high- and low-frequency muscle stimulation: role of sarcolemma action potentials

This study compared the effect of high- (75 Hz, 1 min) and low- (5 Hz, 1.5 min) frequency stimulation on sarcolemmal action potentials of rat phrenic nerve-diaphragm preparations, measured in vitro at 25 degrees C. High-frequency stimulation reduced peak tetanic tension to 21 +/- 1% (means +/- SE) of initial, whereas 5 Hz stimulation produced less of a decline (71 +/- 2% of initial). Despite an initial faster rate of force recovery after 75-Hz stimulation, tetanic tension was still significantly depressed at 0.25 and 1 min relative to the values after 5-Hz stimulation (P less than 0.05). Resting membrane potential, and action potential overshoot and area were not significantly altered by fatigue. Action potential amplitude (AMP) was initially depressed by repetitive stimulation but increased significantly during recovery (P less than 0.05). No significant difference occurred in AMP recovery between the high- vs. low-frequency stimulation groups. The rate of rise and fall of the action potential was reduced after fatiguing stimulation but increased significantly with time (P less than 0.05). Moreover, the time to peak height of the action potential was prolonged by fatigue but significantly declined to resting values with time (P less than 0.05). During recovery, fatigue from high-frequency stimulation was associated with a greater prolongation in duration and time to baseline of the action potential relative to low-frequency stimulation (P less than 0.05). Action potential variables altered by stimulation generally recovered within 1 to 3 min, whereas peak tetanic tension did not completely return to resting values until 10 to 15 min of recovery. We conclude that high- and low-frequency stimulation elicits virtually identical perturbations in sarcolemmal action potentials, and thus changes in surface membrane properties cannot explain the decreased tetanic tension that follows 75-Hz stimulation. It appears that events distal to the sarcolemma are responsible for fatigue from both high- and low-frequency stimulation.

Effect of altered arterial O2 tensions on muscle metabolism in dog skeletal muscle during fatiguing work

These experiments were conducted to determine whether changes in arterial O2 tension are related to changes in muscle metabolism during fatiguing contractions. Arterial and venous circulation to the gastrocnemius muscle (n = 8) was isolated, and the Achilles tendon was attached to a force transducer. Each muscle was electrically stimulated through the sciatic nerve for three 2-min periods of fatiguing contractions separated by 8 min of rest. The arterial O2 tensions were altered for each work period (mean PO2 = 44, 72, 391 Torr). Arterial and venous samples were drawn to measure lactate, O2 and CO2 concentrations (Van Slyke analysis), and [H+]. Muscle biopsies were taken to measure muscle [H+] (homogenate method) and lactate. Fatigue was evaluated as the decline in tension from peak initial tension. At the end of the contraction periods, values were significantly different (P less than 0.05) between the low arterial O2 tension and the high for flow [84 +/- 6 (mean +/- SE) vs. 70 +/- 8 ml X 100 g-1 X min-1], muscle lactate contraction (44 +/- 10 vs. 26 +/- 4 mmol/kg dry wt), and lactate release (122 +/- 12 vs. 57 +/- 14 mumol X 100 g-1 X min-1). O2 uptake and the rate of fatigue were not different among treatments during contractions. Muscle [H+] increased (work [H+] minus rest [H+]) to a significantly greater extent during low arterial O2 tensions compared with high (P less than 0.05). We conclude that alterations in arterial O2 tension during fatiguing contractions induce changes in blood and muscle acid-base status and in muscle metabolism that are independent of O2 uptake.

Fatigue in isometric contraction in a single muscle fibre: a compartmental calcium ion flow model

Fatigue in muscle is a complex biological phenomenon which has so far eluded a definite explanation. Many biochemical and physiological models have been suggested in the literature to account for the decrement in the ability of muscle to sustain a given level of force for a long time. Some of these models have been critically analysed in this paper and are shown to be not able to explain all the experimental observations. A new compartmental model based on the intracellular calcium ion movement in muscle is proposed to study the mechanical responses of a muscle fibre. Computer simulation is performed to obtain model responses in isometric contraction to an impulse and a train of stimuli of long duration. The simulated curves have been compared with experimentally observed mechanical responses of the semitendinosus muscle fibre of Rana pipiens. The comparison of computed and observed responses indicates that the proposed calcium ion model indeed accounts very well for the muscle fatigue.

The influence of muscle metabolic characteristics on physical performance

This study describes the influence of muscle fiber type composition, enzyme activities and capillary supply on muscle strength, local muscle endurance or aerobic power and capacity. Muscle biopsies were obtained from m. vastus lateralis in thirteen physically active men. Histochemical staining procedures were applied to assess the percentage of fast twitch (FT) fibers, muscle fiber area, and capillary density. Also, the activity of citrate synthase (CS), creatine kinase (CK), hexokinase (HK), lactate dehydrogenase (LDH), and phosphofructokinase (PFK) were analysed using fluorometrical assays. Peak torque at 'low' and 'high' angular velocities was measured during leg extension. Similarly, muscle fatigue (e.g. peak torque decline) and recovery from a short-term exercise task were measured during maximal, voluntary consecutive leg extensions. Aerobic power (VO2max) and aerobic capacity (e.g. onset of blood lactate concentration; OBLA), as defined by a blood lactate concentration of 4 mol X 1(-1) were measured during cycling. Peak torque at a high angular velocity was positively correlated with % FT area (p less than 0.001). Fatigue and recovery were correlated with LDH X CS-1 (p less than 0.001). WOBLA was best correlated with PFK and PFK X CS-1 (p less than 0.001). Hence, muscle strength was partly determined by fiber type composition whereas local muscle endurance, recovery and aerobic capacity reflect mainly capillary supply and the activity of key enzymes involved in aerobic and anaerobic metabolism.

Sex differences in endurance capacity and metabolic response to prolonged, heavy exercise

In order to test for possible sex differences in endurance capacity, groups of young, physically active women (n = 6) and men (n = 7) performed bicycle ergometer exercise at 80% and 90% of their maximal oxygen uptakes (VO2 max). The groups were matched for age and physical activity habits. At 80% VO2 max the women performed significantly longer (P less than 0.05), 53.8 +/- 12.7 min vs 36.8 +/- 12.2 min, respectively (means +/- SD). Mid-exercise and terminal respiratory exchange ratio (R) values were significantly lower in women, suggesting a later occurrence of muscle glycogen depletion as a factor in their enhanced endurance. At 90% VO2 max the endurance times were similar for men and women, 21.2 +/- 10.3 min and 22.0 +/- 5.0 min, respectively. The blood lactate levels reached in these experiments were only marginally lower (mean differences 1.5 to 2 mmol X l-1) than those obtained at VO2 max, suggesting high lactate levels as a factor in exhaustion. The changes in body weight during the 80% experiments and the degree of hemoconcentration were not significantly different between men and women.

The effects of Ni2+ on ionic currents and tension generation in frog ventricular muscle

The effect of Ni2+ on E-C coupling events of the frog ventricular muscle were studied using a single sucrose gap voltage clamp technique. The results showed that Ni2+ increased the overshoot potential and depressed and prolonged the plateau of the action potential. Ni2+ also increased the dependence of the overshoot potential on [Na]0 from 18 to 58 mV per decade. In the presence of Ni2+, TTX blocked both the upstroke and the plateau of the action potential. The combination of TTX and Ni2+ suppressed the tension-voltage relation, the time-dependent outward currents and K+ efflux. While suppression of the tension-voltage relation by Ni2+ alone was reversed by increasing [Ca]0, the effects of Ni2+ plus TTX are not reversed by addition of Ca2+. The results suggest that Ni2+ may alter the action potential by slowing the inactivation of the Na+ current and blocking the inward Ca2+ current. Although the tension-suppressant effects of Ni2+ could be attributed to the inhibition of a slowly inactivating Ca2+ current, the effects of Ni2+ in the presence of TTX were less readily explained. Several possible mechanisms are considered which are all consistent with the hypothesis that development of tension in ventricular strips is mediated by both a Ca2+ current and a Ca2+ counter-transport system.

The effects of changes of pH on intracellular calcium transients in mammalian cardiac muscle

The calcium-sensitive photoprotein aequorin was micro-injected into cells of rat, ferret, rabbit and cat papillary muscles. Aequorin light emission is a function of free intracellular calcium concentration. The changes in intracellular calcium concentration [( Ca2+]i) and tension accompanying changes of pH have been studied. When the solution perfusing the papillary muscle was changed from Tyrode solution equilibrated with 5% CO2 to Tyrode solution equilibrated with 15% CO2, developed tension showed a rapid fall followed by a slower rise to a steady state which was less than the control. However the calcium transient associated with each contraction increased monophasically to a new steady state. When the external pH was held constant during exposure to 15% CO2 (by increasing the [HCO3-]), the initial fall of tension was reduced and the slow recovery of tension was greater than when CO2 alone was changed. The amplitude of the calcium transient increased monophasically to a new steady state which was greater than control, but less than when [CO2] alone was increased. If [HCO3-] was decreased while maintaining [CO2] at 5%, there was a slow monophasic decline in developed tension, and a small increase in peak light. Alkaloses produced by changing the [HCO3-]/[CO2] ratio produced similar results but the changes observed were in the opposite direction to those described above. The effects of changes of pHo can be explained if pHi affects tension by two mechanisms. The first mechanism, which is responsible for the rapid change in tension, is not associated with a change in [Ca2+]i. The second mechanism leads to a slower and smaller change in tension, in the opposite direction to the first, and is due to a change in the intracellular calcium transient.

Ca2+-binding studies of the phosphoprotein from rat-incisor dentine

Rat incisor dentine was demineralized and extracted with 0.25 M EDTA containing protease inhibitors. The extract was purified by chromatography on DEAE-cellulose and sulfonated polystyrene. The Ca2+-finding properties of the phosphoprotein were studied by dynamic dialysis and by using a Ca2+-selective electrode. Two different binding sites were detected with Kd = 0.9 X 10(-7) M and 1.1 X 10(-5) M and displaying a Ca2+-binding capacity of 127 and 176 mol bound Ca2+/mol protein, respectively, assuming a molecular weight of 30 000. Upon enzymatic dephosphorylation of the phosphoprotein, the highest affinity sites disappeared and those with the lowest affinity were reduced. The optimum for Ca2+ binding by the phosphoprotein occurred at pH 8.2. The specificity of the Ca2+ ion interaction with the phosphoprotein was investigated by studying the competitive nature of other divalent and monovalent cations. It was found that Ca2+ ions were to a large extent displaced from the phosphoprotein by other cations in physiological concentrations.

EMG frequency spectrum, muscle structure, and fatigue during dynamic contractions in man

Fatigue of the vastus lateralis muscle was studied in healthy well-conditioned students, who differed considerable regarding their muscle fibre type distribution. Muscle force decline during repeated maximum voluntary knee extensions at a constant angular velocity (180 degree X s-1 or rad X s-1), using isokinetic equipment, was taken as the criterion for the degree of fatigue. In an attempt to study quantitative as well as qualitative changes in the EMG pattern, integrated EMG (IEMG) and the frequency of the mean power (MPF), computed from the power spectral density function (PSDF), were analysed. It was found that individuals with muscles made up of a high proportion of fast twitch (FT) muscle fibres demonstrated higher peak knee extension torque, and a greater susceptibility to fatigue than did individuals with muscles mainly composed of slow twitch (ST) muscle fibres. An IEMG decline (p less than 0.01) was demonstrated during 100 contractions in individuals rich in FT fibres. Only a slight, but not significant, reduction in IEMG occurred in individuals with high percentage of ST fibres. Concomitantly, MPF decreased (p less than 0.001) in individuals with a high percentage of FT fibres, while their opposites demonstrated only a slight decrease (non-significant). It is suggested that muscle conctraction failure might also be related to qualitative changes in the motor unit recruitment pattern, and that these changes occur more rapidly in muscles composed of a high proportion of FT muscle fibres than in muscles composed to a high proportion of ST fibres.

The relationship between pH and the amount of calcium bound to glycerinated muscle fibers

Variation of pH over the range 6.2--7.4 had no effect on the Ca2+ titration curve of glycerinated rabbit psoas muscle fibers. Thus the effect of H+ on muscle contraction is not due to a simple H+-Ca2+ competition for binding sites.

Muscle fatigue and its relation to lactate accumulation and LDH activity in man

The lactate concentration in different muscle fibre types was determined in biopsy specimens from human vastus lateralis muscle after 30 and 60 s of maximal dynamic leg exercise. In addition, muscle fibre type distribution, total lactate dehydrogenase (LDH) activity, and isozymes of LDH were determined. In accordance with previous studies (Thorstensson and Karlsson 1976, Nilsson et al. 1977) it was found that an increasing proportion of slow twitch (ST) fibres corresponded to better sustained muscle force. Lactate was found preferentially in fast twitch (FT) fibres after 30 s, but after 60 s this difference was abolished. Differences between the two main muscle fibre types in muscle lactate, total LDH activity, and M-LDH activity were correlated to muscle fatigue. It was concluded that lactate or associated pH changes primarily in FT fibres could be one factor responsible for the impaired muscle function.

Effect of ouabain on myocardial metabolic and contractile responses to coronary ligation

The effect of pretreatment with ouabain (40 microgram/kg, i.v.) on myocardial metabolic and contractile responses to regional ischemia induced by coronary artery ligation was studied in the canine left ventricle. In control dogs, ischemia increased activity of phosphorylase a and the levels of glucose-6-phosphate and lactate, and decreased the levels of glycogen and phosphocreatine, without affecting the levels of adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate (AMP). Ouabain increased the activity of phosphorylase a. In ouabain-treated dogs, ischemia did not further increase the phosphorylase a activity but it increased the epicardial AMP level. Other metabolic responses to ischemia in ouabain-treated dogs were similar to those in control dogs. In control dogs, myocardial contractile force decreased by about 10% after ischemia, but blood pressure and heart rate remained unchanged. Ouabain increased contractile force by about 32%. In ouabain-treated dogs, ischemia decreased contractile force by about 54% without affecting blood pressure and heart rate. It is concluded that ouabain increases the activity of the myocardial phosphorylase a and that the inotropic action of ouabain can be nullified by coronary artery ligation.

Physical exercise after induced alkalosis (bicarbonate or tris-buffer)

The influence of bicarbonate and Tris-buffer infusions on the performance capacity for maximal, brief exercise (400 m run) was studied using 10 normal males in their twenties. Run time, maximal lactate concentration and heart rate remained unchanged after the buffer infusions. As a result of the induced elevated buffering capacity, the average pH after exercise was about 0.1 unit higher. Corresponding values for base excess and standard bicarbonate were found. The arterial pCO2 was higher after infusion as a result of the active respiratory compensation. Since the reduction in the work-related metabolic acidosis by the buffering substances caused no improvement in performance, the importance of pH as the performance-limiting factor must be questioned because the investigation gave no evidence for alterations of intracellular pH.