Physiological responses during prolonged exercise at the power output corresponding to the blood lactate threshold

Heart rate (HR) and oxygen uptake (VO2) at the mechanical power (W) corresponding to the capillary blood lactate ([la]cap) of 4 mmol.l-1 (Wlt) were measured in 34 healthy male subjects during incremental exercise (Winc). On the basis of these measurements, the subjects were asked to cycle at Wlt for 60 min (steady-state exercise, Wss). Twenty subjects could not reach the target time (mean exhaustion time, te, 38.2 min, SD 5.3), while 6 of the 14 remaining subjects declared themselves exhausted at the end of exercise. The final [la]cap if the two groups of exhausted subjects were 5.3 mmol.l-1, SD 2.3 and 4.3 mmol.l-1, SD 1.1, respectively. At the end of Wss, [la]cap and HR were significantly lower in the 8 unexhausted subjects than in the other subjects. This group also had a lower HR at Wlt during Winc. The HR and VO2 appeared to be higher during Wss than during Winc. When all subjects were ranked according to their te during Wss, Wlt (expressed per kilogram of body mass) was found to be negatively related to te. In conclusion, during Winc, measurements of physiological variables at fixed [la]cap give a poor prediction of their trends during Wss and of the relative te; at the same work load [la]cap can be quite different in the two experimental conditions. Furthermore, resistance to exercise fatigue at Wlt seems lower in the fitter subjects.     

Human blood neutrophil responses to prolonged exercise with and without a thermal clamp.

The purpose of this study was to investigate the effects of prolonged exercise with and without a thermal clamp on neutrophil trafficking, bacterial-stimulated neutrophil degranulation, stress hormones, and cytokine responses. Thirteen healthy male volunteers (means +/- SE: age 21 +/- 1 yr; mass 74.9 +/- 2.1 kg; maximal oxygen uptake 58 +/- 1 ml x kg(-1) x min(-1)) completed four randomly assigned, 2-h water-immersion trials separated by 7 days. Trials were exercise-induced heating (EX-H: water temperature 36 degrees C), exercise with a thermal clamp (EX-C: 24 degrees C), passive heating (PA-H: 38.5 degrees C), and control (CON: 35 degrees C). EX-H and EX-C was comprised of 2 h of deep water running at 58% maximal oxygen uptake. Blood samples were collected at pre-, post-, and 1 h postimmersion. Core body temperature was unaltered on CON, clamped on EX-C (-0.02 degrees C), and rose by 2.23 degrees C and 2.31 degrees C on EX-H and PA-H, respectively. Exercising with a thermal clamp did not blunt the neutrophilia postexercise (EX-C postexercise: 9.6 +/- 1.1 and EX-H postexercise: 9.8 +/- 1.0 x 10(9)/liter). Neutrophil degranulation decreased (P < 0.01) similarly immediately after PA-H (-21%), EX-C, and EX-H (-28%). EX-C blunted the circulating norepinephrine, cortisol, granulocyte-colony stimulating factor, and IL-6 response (P < 0.01) but not the plasma epinephrine and serum growth hormone response. These results show a similar neutrophilia and decrease in neutrophil degranulation after prolonged exercise with and without a thermal clamp. As such, the rise in core body temperature does not appear to mediate neutrophil trafficking and degranulation responses to prolonged exercise. In addition, these results suggest a limited role for cortisol, granulocyte-colony stimulating factor, and IL-6 in the observed neutrophil responses to prolonged exercise.     

Plasma and muscle amino acid and ammonia responses during prolonged exercise in humans

Plasma and muscle amino acid (AA) and ammonia (NH3) responses were measured during prolonged submaximal exercise in humans. Increased NH3 production during submaximal exercise has been attributed to the activity of the purine nucleotide cycle, without consideration of any possible contribution from AA. Six men cycled at 75% of maximal O2 uptake until exhaustion on two occasions after 2.5 days of ingestion of a high-carbohydrate or mixed diet. Plasma samples (antecubital vein) and muscle biopsies (vastus lateralis) were obtained at rest and during exercise and analyzed for plasma and muscle NH3 and AA as well as muscle metabolites. There were no significant diet effects in these parameters, so the majority of results focus on the effects of exercise. Plasma and muscle NH3 increased significantly from the onset and continued to increase throughout exercise. The total and total essential [AA] of muscle were significantly increased at exhaustion, whereas both the plasma and muscle branched-chain AA contents were unchanged. This suggests that protein catabolism was occurring during exercise and the branched-chain AA were used for energy and NH3 production.     

Plasma amino acid and ammonia responses to altered dietary intakes prior to prolonged exercise in humans.

This study examined the effects of altered dietary intakes on amino acid and ammonia (NH3) responses prior to and during prolonged exercise in humans. Six male recreational cyclists rode to exhaustion at 75% of VO2max following 3 days on a low carbohydrate (LC), mixed (M), or high carbohydrate (HC) diet in a latin square design. There were differences (p less than 0.05) in exercise times among all treatments (58.8 +/- 3.7, 112.1 +/- 7.3, and 152.9 +/- 10.3 min for the LC, M, and HC treatments, respectively). The rate of increase in plasma NH3 during exercise was greater (p less than 0.05) during the LC trial. The LC trial was also characterized by higher (p less than 0.05) resting plasma concentrations of branched chain amino acids (BCAA) and a greater decrease in these amino acids during exercise (p less than 0.05), as compared with the other two treatments. Both plasma BCAA and NH3 were susceptible to dietary manipulations. These findings suggest that limited carbohydrate availability in association with increased BCAA availability results in enhanced BCAA metabolism during exercise. This is reflected in a greater rate of increase in plasma NH3 and is consistent with the hypothesis that a significant fraction of the NH3 released during a prolonged, submaximal exercise bout is from amino acid catabolism.     

Effects of the menstrual cycle phase on the blood lactate responses to exercise

The effects of menstrual cycle phase on the blood lactate response to exercise were examined in eumenorrheic women (n=9). Exercise tests were performed at the mid-follicular and mid-luteal points in the menstrual cycle (confirmed by basal body temperature records and hormone levels). Blood lactates were measured at rest and during the recovery from exercise. Resting lactates were not different between the exercise tests; however, recovery lactates were significantly (p < 0.05) lower in the luteal compared to the follicular phase. The mechanism for these differences is unclear, but may be related to an estrogen mediated increased lipid metabolism inducing a concurrent reduction in carbohydrate metabolism. The present findings question the use of blood lactate monitoring as a suitable technique to measure exercise intensity in eumenorrheic women.     

Physiological responses to prolonged physical exercise in dogs

1. Duration of exercise until exhaustion is significantly correlated with body weight of dogs. 2. Rectal temperature (Tre) achieved at the end of the effort depends on the resting value of Tre, but the exercise-induced increases in Tre are unrelated to the initial Tre. 3. The magnitude of exercise-induced decrease in blood glucose (BG) level is positively correlated with the resting BG level and negatively correlated with the elevations of the plasma free fatty-acid (FFA) concentration. 4. A significant positive relationship is found between the exercise-induced increases in the plasma FFA levels and noradrenalinaemia.     

Physiological responses to prolonged exercise in ultramarathon athletes

The physiological responses of 10 ultramarathon athletes to prolonged exercise at the highest intensity level they could sustain for 4 h have been examined. Energy expenditure for the 4 h of exercise was 14,146 +/- 1,789 kJ, of which 63% was provided by the oxidation of fat. Plasma free fatty acids rose, but the changes in blood lactate concentration (delta 0.2 mmol/l) and exchange ratio (delta 0.05) were small, and the postexercise glycogen content (130 +/- 42 mumol/g) of the vastus lateralis muscles was estimated to be 37-53% of normal resting values. During exercise O2 intake (VO2) increased with time from the 50th to 240th min, the rise becoming significant (P less than 0.01) after 110 min of work. The change in VO2 was equivalent to a rise in relative intensity (%VO2max) of +9.1% and a change of speed of 1.49 km/h. A rise in cardiac frequency compensated for a fall in stroke volume (SV), so that cardiac output was maintained, and the increases in rectal temperature (Tre) (delta 0.63 degree C) and sweat loss (3.49 +/- 0.50 kg, equivalent to 5.5% of body wt) and the decreased mean skin temperature (Tsk) (-1.22 degree C) were within tolerable limits during exercise. Following exercise there was a loss (-25%) of ability to generate voluntary force of the quadriceps femoris, though electrically evoked mechanical properties of the muscle remained unchanged. The results suggest that neither thermal nor cardiovascular factors are limiting to prolonged (4 h) exercise, although the ability to utilize fat as a fuel may be important in ultradistance athletes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma hypoxanthine and ammonia in humans during prolonged exercise

In this study we examined the time course of changes in the plasma concentration of oxypurines [hypoxanthine (Hx), xanthine and urate] during prolonged cycling to fatigue. Ten subjects with an estimated maximum oxygen uptake (VO2(max)) of 54 (range 47-67) ml x kg(-1) x min(-1) cycled at [mean (SEM)] 74 (2)% of VO2(max) until fatigue [79 (8) min]. Plasma levels of oxypurines increased during exercise, but the magnitude and the time course varied considerably between subjects. The plasma concentration of Hx ([Hx]) was 1.3 (0.3) micromol/l at rest and increased eight fold at fatigue. After 60 min of exercise plasma [Hx] was >10 micromol/l in four subjects, whereas in the remaining five subjects it was <5 micromol/l. The muscle contents of total adenine nucleotides (TAN = ATP+ADP+AMP) and inosine monophosphate (IMP) were measured before and after exercise in five subjects. Subjects with a high plasma [Hx] at fatigue also demonstrated a pronounced decrease in muscle TAN and increase in IMP. Plasma [Hx] after 60 min of exercise correlated significantly with plasma concentration of ammonia ([NH(3)], r = 0.90) and blood lactate (r = 0.66). Endurance, measured as time to fatigue, was inversely correlated to plasma [Hx] at 60 min (r = -0.68, P < 0.05) but not to either plasma [NH(3)] or blood lactate. It is concluded that during moderate-intensity exercise, plasma [Hx] increases, but to a variable extent between subjects. The present data suggest that plasma [Hx] is a marker of adenine nucleotide degradation and energetic stress during exercise. The potential use of plasma [Hx] to assess training status and to identify overtraining deserves further attention.     

Peak blood ammonia and lactate after submaximal, maximal and supramaximal exercise in sprinters and long-distance runners

The purpose of this study was to elucidate the difference in peak blood ammonia concentration between sprinters and long-distance runners in submaximal, maximal and supramaximal exercise. Five sprinters and six long-distance runners performed cycle ergometer exercise at 50% maximal, 75% maximal, maximal and supramaximal heart rates. Blood ammonia and lactate were measured at 2.5, 5, 7.5, 10 and 12.5 min after each exercise. Peak blood ammonia concentration at an exercise intensity producing 50% maximal heart rate was found to be significantly higher compared to the basal level in sprinters (P less than 0.01) and in long-distance runners (P less than 0.01). The peak blood ammonia concentration of sprinters was greater in supra-maximal exercise than in maximal exercise (P less than 0.05), while there was no significant difference in long-distance runners. The peak blood ammonia content after supramaximal exercise was higher in sprinters compared with long-distance runners (P less than 0.01). There was a significant relationship between peak blood ammonia and lactate after exercise in sprinters and in long-distance runners. These results suggest that peak blood ammonia concentration after supramaximal exercise may be increased by the recruitment of fast-twitch muscle fibres and/or by anaerobic training, and that the processes of blood ammonia and lactate production during exercise may be strongly linked in sprinters and long-distance runners.     

Metabolic and ventilatory responses to steady state exercise relative to lactate thresholds

The metabolic and ventilatory responses to steady state submaximal exercise on the cycle ergometer were compared at four intensities in 8 healthy subjects. The trials were performed so that, after a 10 min adaptation period, power output was adjusted to maintain steady state VO2 for 30 min at values equivalent to: (1) the aerobic threshold (AeT); (2) between the aerobic and the anaerobic threshold (AeTAnT); (3) the anaerobic threshold (AnT); and (4) between the anaerobic threshold and VO2max (AnTmax). Blood lactate concentration and ventilatory equivalents for O2 and CO2 demonstrated steady state values during the last 20 min of exercise at the AeT, AeAnT and AnT intensities, but increased progressively until fatigue in the AnTmax trial (mean time = 16 min). Serum glycerol levels were significantly higher at 40 min of exercise on the AeAnT and the AnT when compared to AeT, while the respiratory exchange ratios were not significantly different from each other. Thus, metabolic and ventilatory steady state can be maintained during prolonged exercise at intensities up to and including the AnT, and fat continues to be a major fuel source when exercise intensities are increased from the AeT to the AnT in steady state conditions. The blood lactate response to exercise suggests that, for the organism as a whole, anaerobic glycolysis plays a minor role in the energy release system at exercise intensities upt to and including the AnT during steady state conditions.     

Relationship between plasma ammonia and blood lactate concentrations after maximal treadmill exercise in circumpubertal girls and boys

The purpose of the study was to define a relationship between plasma ammonia [NH3]pl and blood lactate concentrations [la-]b after exercise in children and to find out whether the [NH3]pl, determined during laboratory treadmill tests, may be useful as a predictor of the children's sprint running ability. A group of 20 girls and 14 boys trained in athletics or swimming and 8 untrained boys, aged 13.2 to 13.7 years, participated in the study. Their [NH3]pl and [la-]b were measured before and after incremental maximal treadmill exercise. In addition, the subjects' running performance was tested in 30-, 60- and 600- or 1000-m runs under field conditions. The [NH3]pl during the treadmill runs increased by 20.1 (SD 17.3), 24 (SD 16.7) and 10 (SD 4.3) mumol.l-1 in the girls, the trained boys and the untrained boys, respectively. The postexercise [NH3]pl correlated positively with [la-]b (r = 0.565 in the girls and 0.812 in the boys) and treadmill speed attained during the test (r = 0.489 in the girls and 0.490 in the boys). Significant correlations were also found between [NH3]pl obtained during the treadmill test and the times of 30- and 60-m runs (r = -0.676 and -0.648, respectively) in the boys but not in the girls. A comparison of the present data with those reported previously in adults showed that increases in [NH3]pl during maximal exercise in children would seem to be lower than in adult subjects both in absolute values and in relation to [la-]b.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle metabolism during prolonged physical exercise in dogs

In order to provide reference data, adenine nucleotide, creatine phosphate, glycogen, glycolytic intermediates and lactate muscle contents were measured in 49 dogs under resting conditions and during prolonged physical exercise of moderate intensity performed until exhaustion. Both the resting and exercise values of the measured variables were remarkably similar to those described in human subjects, except muscle lactate content which achieved higher values during submaximal exercise in dogs than in men.