Influence of moderate cold exposure on blood lactate during incremental exercise

Summary This study examined the effect of exposure of the whole body to moderate cold on blood lactate produced during incremental exercise. Nine subjects were tested in a climatic chamber, the room temperature being controlled either at 30°C or at 10°C. The protocol consisted of exercise increasing in intensity in 35 W increments every 3 min until exhaustion. Oxygen consumption (VO₂) was measured during the last minute of each exercise intensity. Blood samples were collected at rest and at exhaustion for the measurement of blood glucose, free fatty acid (FFA), noradrenaline (NA) and adrenaline (A) concentrations and, during the last 15 s of each exercise intensity, for the determination of blood lactate concentration [la^–]_b. TheVO₂ was identical under both environments. At 10°C, as compared to 30°C, the lactate anaerobic threshold (Th_{an, la} ^–) occurred at an exercise intensity 15 W higher and [Th_{an, la} ^–]_b was lower for submaximal intensities above the Th_{an, la} ^– Regardless of ambient temperature, glycaemia, A and NA concentrations were higher at exhaustion while FFA was unchanged. At exhaustion the NA concentration was greater at 10°C [15.60 (SEM 3.15) nmol·l^–1] than at 30°C [8.64 (SEM 2.37) nmol·l^–1]. We concluded that exposure to moderate cold influences the blood lactate produced during incremental exercise. These results suggested that vasoconstriction was partly responsible for the lower [la^–]_b observed for submaximal high intensities during severe cold exposure.     

Effect of muscle mass on lactate formation during exercise in humans

To elucidate the mechanisms of lactate formation during submaximal exercise, eight men were studied during one- (1-LE) and two-leg (2-LE) exercise (approximately 11-min cycling) using the catheterization technique and muscle biopsies (quadriceps femoris muscle). The absolute exercise intensity and thus the energy demand for the exercising limb was the same [mean 114 (SEM 7) W] during both 1-LE and 2-LE. At the end of exercise partial pressure of O₂ and O₂ saturation in femoral venous blood were lower and arterial adrenaline and noradrenaline were higher during 2-LE than during 1-LE. Mean arterial blood lactate concentration increased to 10.8 (SEM 0.8) (2-LE) and 5.2 (SEM 0.4) mmol · 1^–1 (1-LE) after 10 min of exercise. The intramuscular metabolic response to exercise was attenuated during 1-LE [mean, lactate = 49 (SEM 9); glucose 6-P = 3.3 (SEM 0.3); nicotinamide adenine dinucleotide, reduced = 0.17 (SEM 0.02); adenosine 5-diphosphate 2.7 (SEM 0.1) mmol · kg dry mass^–1] compared to 2-LE [76 (SEM 6); 6.1 (SEM 0.7); 0.21 (SEM 0.02); 3.0 (SEM 0.1) mmol · kg dry mass^–1, respectively]. To elucidate whether the lower plasma adrenaline concentration could contribute to the attenuated metabolic response, additional experiments were performed on four of the eight subjects with infusion of adrenaline during 1-LE (1-LEE). Average plasma adrenaline concentration was increased during 1-LEE and reached 2–4 times higher levels than during 2-LE. Post-exercise muscle lactate and glucose 6-P contents were higher during 1-LEE than during 1-LE and were similar to those during 2-LE. Also, leg lactate release was elevated during 1-LEE versus 1-LE. It was concluded that during submaximal dynamic exercise the intramuscular metabolic response not only depended on the muscle power output, but also on the total muscle mass engaged. Plasma adrenaline concentrations and muscle oxygenation were found to be dependent upon the working muscle mass and both may have affected the metabolic response during exercise.     

Reproducibility of plasma catecholamine concentrations at rest and during exercise in man

Summary The purpose of this study was to test the reproducibility of plasma norepinephrine (NE) and epinephrine (E) concentrations, at rest and during exercise, in man. Twelve young men were evaluated on two occasions (one week apart) at rest in supine and sitting positions and during dynamic exercise on bicycle ergometer: 5 min at a low intensity workload (heart rate=131–133 bt min^–1) and 5 and 20 min at a higher intensity (174–175 bt min^–1). Mean plasma NE and E concentrations were not significantly different (p<0.05) on the two occasions in any of the experimental situations. However large within-subject variations were present, and the standard errors of a single measurement corrected for the variability of the catecholamine assay, ranged from 14 to 50% for NE and 14 to 37% for E. These results indicate that the mean plasma NE and E concentrations observed in a group of subjects are reproducible from one week to the other, but that individual plasma NE and E concentrations are not. This lack of reliability of a single determination of plasma catecholamine concentrations might be due to cyclic variations of plasma NE and E concentrations over time.Supported by grants from FCAC and FRSQ, Québec and NSERC, Canada.     

The effects of graded exercise on plasma proenkephalin peptide F and catecholamine responses at sea level

Summary The purpose of this study was to evaluate the effects of graded treadmill exercise on plasma preproenkephalin peptide F immunoreactivity and concomitant catecholamine responses at sea level (elevation, 50 m). Few data exist regarding the sea-level responses of plasma peptide F immunoreactivity to exercise. Thirty-five healthy men performed a graded exercise test on a motor-driven treadmill at the relative exercise intensities of 25, 50, 75, and 100% of maximum oxygen consumption (VO_2max). Significant (P<0.05) increases above rest were observed for plasma peptide F immunoreactivity and norepinephrine at 75 and 100% of the VO_2max and at 5 min into recovery. Significant increases in plasma epinephrine were observed at 75 and 100% of VO_2max. Whole blood lactate significantly increased above resting values at 50, 75, and 100% of the VO_2max and at 5 min into recovery. These data demonstrate that exercise stress increases plasma peptide F immunoreactivity levels at sea level. While the exercise response patterns of peptide F immunoreactivity are similar to catecholamines and blood lactate responses, no bivariate relationships were observed. These data show that sea-level response patterns to graded exercise are similar to those previously observed at moderate altitude (elevation, 2200 m).Human subjects participated in these studies after giving their free and informed voluntary consent. Investigators adhered to AR 70-25 and USAMRDC regulation 70-25 on Use of Volunteers in Research. The views, opinion, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy, or decision, unless to designated by other official documentation.     

Continuous monitoring of extracellular lactate concentration by microdialysis lactography for the study of rat muscle metabolism in vivo

A method is described for the measurement and on-line monitoring of muscular extracellular lactate concentration in both anaesthetized and freely moving rats. This method is based on microdialysis sampling and lactic dehydrogenase-catalysed nicotinamide adenine dinucleotide, reduced (NADH)-fluorescence detection techniques. In vivo calibration revealed a resting extracellular lactate concentration of 1.92±0.13 mmol/l (± SEM) in the gastrocnemius muscle of adult male Wistar rats (n=6), while the average whole-blood lactate level was 0.76±0.12 mmol/l (± SEM). This measured extracellular lactate concentration was 1.73-times higher than that deduced from the arterial lactate concentration. Blocking glycolysis with iodoacetate reduced the extracellular lactate concentration to 52±6% (± SEM, n= 4) of the resting level. The extracellular lactate concentration in rat gastrocnemius muscle had increased to significantly (P0.05) different levels, 2.4±0.03 (± SEM) or 4.0±0.55 (± SEM) times the control value, 1 h after aortic clamping (n=3) or cardiac arrest (n=3), respectively. Stimulation of the sciatic nerve induced elevations of the extracellular lactate concentration in the tibialis anterior muscle which were linearly related to the recorded isometric force-time integral. We also monitored on-line the changes in extracellular lactate concentration in the tibialis anterior muscle of a swimming rat. Our results indicate that microdialysis lactate reflects also intracellular metabolism. Lactography may be a useful alternative to biopsies and nuclear magnetic resonance spectroscopy in clinical medicine and physiology for the monitoring of metabolism in vivo.     

Continuous monitoring of lactate during exercise in humans using subcutaneous and transcutaneous microdialysis

We have evaluated the possibility of monitoring the plasma lactate concentration in human volunteers during cycle ergometer exercise using subcutaneous and transcutaneous microdialysis. In transcutaneous microdialysis, the relative increase in dialysate lactate concentration exceeded that of plasma lactate concentration by a factor of 6 during exercise due to exercise-induced lactate secretion in sweat. During exercise the subcutaneous microdialysis dialysate lactate concentration underestimated the plasma lactate concentration possibly due to diffusion limitation or adipose tissue lactate production. While it was demonstrated that microdialysis can be used for on-line lactate monitoring, neither subcutaneous nor transcutaneous dialysate lactate concentration were linearly related to the plasma lactate concentration during exercise, and it was found therefore that it was not possible to monitor directly plasma lactate concentration during exercise.     

Cardiovascular reflexes during sustained handgrip exercise: role of muscle fibre composition,potassium and lactate

Summary Six healthy men performed sustained static handgrip exercise for 2 min at 40% maximal voluntary contraction followed by a 6-min recovery period. Heart rate (f _c), arterial blood pressures, and forearm blood flow were measured during rest, exercise, and recovery. Potassium ([K⁺]) and lactate concentrations in blood from a deep forearm vein were analysed at rest and during recovery. Mean arterial pressure (MAP) andf _c declined immediately after exercise and had returned to control levels about 2 min into recovery. The time course of the changes in MAP observed during recovery closely paralleled the changes in [K⁺] (r=0.800,P<0.01), whereas the lactate concentration remained elevated throughout the recovery period. The close relationship between MAP and [K⁺] was also confirmed by experiments in which a 3-min arterial occlusion period was applied during recovery to the exercised arm by an upper arm cuff. The arterial occlusion affected MAP whilef _c recovered at almost the same rate as in the control experiment. Muscle biopsies were taken from the brachioradialis muscle and analysed for fibre composition and capillary supply. The MAP at the end of static contraction and the [K⁺] appearing in the effluent blood immediately after contraction were positively correlated to the relative content of fast twitch (% FT) fibres (r=0.886 for MAP vs %FT fibres,P<0.05 andr=0.878 for [K⁺] vs %FT fibres,P<0.05). Capillary to fibre ratio showed an inverse correlation to % FT fibres (r=–0.979,P<0.01). These results indicated that activation of FT rather than slow twitch fibres during static contraction induced a more marked arterial pressure reflex. It was concluded that the arterial pressure reflex would seem to be mediated through stimulation of unmyelinized free nerve endings in the contracted muscle. The [K⁺] would appear to be a more likely candidate than lactate as a mediator for this pressure reflex.     

Lactate concentrations in human skeletal muscle biopsy, microdialysate and venous blood during dynamic exercise under blood flow restriction

The intramuscular microdialysate lactate concentration during dynamic exercise with various degrees of blood flow restriction and its relation to lactate concentration in skeletal muscle biopsy and venous blood were studied. Nine healthy males performed three one-legged knee extension exercises (Ex 1–3). Blood flow was restricted stepwise by applying supra-atmospheric pressure over the working leg. Microdialysate mean (range) lactate concentrations at the end of the exercise periods were 3.2 (0.5–6.6), 4.4 (1.1–9.8) and 7.9 (1.1–11.6) mmol·l^–1 during unrestricted, moderately restricted and severely restricted blood flow respectively. There was a significant correlation between microdialysate and venous lactate concentrations at the end of all three exercise periods. Microdialysate lactate concentration correlated significantly to skeletal muscle biopsy lactate concentration at the end of Ex 1. In conclusion, microdialysate lactate concentration in the working muscle increased step-wise with increasing blood flow restriction. It showed a better correlation to venous than to muscle biopsy lactate, which is possibly partly explained by the characteristics of diffusion between body compartments and differences in time resolution between the methods used.An erratum to this article can be found at     

The effects of exercise training and denervation on the morphology of intrafusal muscle fibers

Summary The morphological effects of daily bouts of exercise and denervation on teres minor intrafusal muscle fibers were investigated in male Sprague-Dawley rats. After denervation, nuclear bag and nuclear chain muscle fiber cross-sectional area atrophied only 25 and 33% of the amount experienced by extrafusal fibers. Of the two fiber types, the nuclear chain fibers appeared to be more responsive to the effects of exercise than the nuclear bag fibers; however, this trend for enlargement had no statistical significance. Length measurements did not reveal any marked changes of any fiber type to the experimental conditions of this study. It was concluded that the possible differences in function and innervation of the nuclear bag and nuclear chain fibers could partly account for these findings.Supported in part by funds provided by the Graduate College.     

Plasma lactate concentration increases as a parabola with delay during ramp exercise

This study presents an elementary model of a system which relates plasma lactate concentration ([La^–]) during ramp exercise to its rate of accumulation (R _c) within its extramuscular distribution space (S). Under the parsimonious assumptions that R _c increases linearly with time (t) with a kinetic delay (), and that the volume of S is constant, it is shown that plasma [La^–] increases as a parabola with the kinetic delay when t increases. This elementary system model describes changes in plasma [La^–] observed in five healthy young subjects during ramp exercise on the cycle ergometer (1 W every 2 s) with great accuracy (r>0.99) with very small residuals (average value less than 0.01 mmol · l^–1), randomly distributed around the fitting curves. The delay between the beginning of exercise and the onset of increase in R _c could be due to the fact that at the corresponding work rates: (1) rate of lactate appearance (R _a), which is equal to the rate of lactate disappearance (R _d), is not modified from rest, since the exercising muscles work in fully aerobic conditions (hypothesis of the anaerobic threshold); or (2) the increase in R _a is associated with a similar increase in R _d. An alternate or complementary hypothesis is that, during ramp exercise, plasma [La^–] could reflect metabolic events within the muscles, with a significant delay.     

The influence of extracellular buffer concentration and propionate on lactate efflux from frog muscle

Lactate efflux from frog sartorius muscles was measured following a lactate load of about 18 mol · g^–1 induced by a 4-min period of stimulation. Lactate efflux rate was buffer concentration dependent. The initial efflux rate increased from about 150 nmol · g^–1 · min^–1 in 1 mM MOPS buffer to 400 nmol · g^–1 · min^–1 in 25 mM MOPS buffer. The addition of 20 mM propionate reduced mean intracellular pH by about 0.2 units and increased lactate efflux rate by 70% at the highest buffer concentration and 400% at the lowest buffer concentration. The observed results are in reasonable agreement with predictions based on a model in which net efflux is limited by diffusion of both buffer and lactate in the extracellular space. Transmembrane lactate efflux appears to consist of two components, one of which is proton linked and carried either by undissociated lactic acid or coupled proton-lactate transport, the other being carried by independent lactate ions.     

The effect of inspiratory muscle training upon maximum lactate steady-state and blood lactate concentration

Several studies have reported that improvements in endurance performance following respiratory muscle training (RMT) are associated with a decrease in blood lactate concentration ([Lac]_B). The present study examined whether pressure threshold inspiratory muscle training (IMT) elicits an increase in the cycling power output corresponding to the maximum lactate steady state (MLSS). Using a double-blind, placebo-controlled design, 12 healthy, non-endurance-trained male participants were assigned in equal numbers to an experimental (IMT) or sham training control (placebo) group. Cycling power output at MLSS was initially identified using a lactate minimum protocol followed by a series of constant power output rides (2.5% increments) of 29.5 min duration; MLSS was reassessed following six weeks of IMT or sham IMT. Maximum inspiratory mouth pressure increased significantly (26%) in the IMT group, but remained unchanged in the placebo group. The cycling power output corresponding to MLSS remained unchanged in both groups after the intervention. After IMT, [Lac]_B decreased significantly at MLSS power in the IMT group [–1.17 (1.01) mmol l^–1 after 29.5 min of cycling; mean (SD)], but remained unchanged in the placebo group [+0.37 (1.66) mmol l^–1]. These data support previous observations that IMT results in a decrease in [Lac]_B at a given intensity of exercise. That such a decrease in [Lac]_B was not associated with a substantial (>2.5%) increase in MLSS power is a new finding suggesting that RMT-induced increases in exercise tolerance and reductions in [Lac]_B are not ascribable to a substantial increase in the lactate threshold.     

Delayed appearance of blood lactate with reduced frequency breathing during exercise

Summary The purpose of the present study was to investigate the blood lactate (LA^–) responses to hypoventilation induced by reduced frequency breathing (RFB) during recovery from exercise. Five male subject performed 16 4 min cycling bouts alternating with 16 min rest periods. Exercise intensities were chosen at power outputs corresponding to 30% at 2mMLA^–, at 4 mMLA^–, and 90% in each subject. Breathing frequency was voluntarily controlled starting 10 s before each 3rd min of exercise and maintained throughout the rest of the exercise period. Four different breathing patterns at each exercise intensity were used: normal breathing (NB), breathing every 4 s, breathing every 8 s, and maximal RFB. Except for the NB trials, subjects held their breath at functional residual capacity during each breathing interval. The concentration difference of LA^– between the 3rd min sample and the 4th min sample was defined as the lactate change during exercise ( LA^–ex), and that between the 4th min sample and the sample at the 3rd min after the end of the exercise as the lactate change during recovery ( LA^–rec). An ANOVA showed significant (p<0.05) differences in breathing procedures only in LA^–rec. LA^–rec seemed to increase as compared to NB only at at 4 mMLA^– and 90% , while LA^–ex remained unchanged as compared to NB in spite of reduced V _A. These results might indicate that RFB inhibited lactate removal from working muscles during exercise.     

Effect of various types of acute exercise and exercise training on the insulin sensitivity of rat soleus muscle measured in vitro

Effects of acute exercise varying in duration and intensity, as well as of two training regimes (endurance and sprint training) on the sensitivity of the soleus muscle of rat to insulin was measured in vitro and compared in rats. As an index of the muscle insulin sensitivity the hormone concentration in the incubation medium which would produce half maximum stimulation of lactate production (LA) and glycogen synthesis was determined. A single bout of moderate endurance exercise (60 min treadmill running at 20 m×min^–1, 0° inclination) increased the rate of LA production at the hormone concentrations used and increased the sensitivity of the process to insulin at 0.25 and 2 h but not 24 h after termination of exercise. Similar though less pronounced effects were found after heavy endurance exercise (30 min at 25 m×min^–1, 10°), but sprint exercise (6×10 s bouts at 43 m×min^–1, 0°) had no influence on the insulin sensitivity of the soleus muscle. The rate of glycogen synthesis in vitro was accelerated after endurance exercise, but the sensitivity of this process to insulin was unaffected by the preceding exercise. Endurance training for 5 weeks caused marked enhancement of sensitivity of both LA production and glycogen synthesis to insulin, which persisted for at least 48 h after the last training session. No changes in the soleus muscle sensitivity to insulin were found after sprint training. It is concluded that the increased insulin sensitivity of glucose utilization by skeletal muscle which occurs after endurance exercise and particularly during endurance training can substantially contribute to improved carbohydrate tolerance. Sprint exercise does not produce any changes in muscle insulin sensitivity, at least in the soleus muscle of the rat.Dedicated to the late Professor Stanislaw Kozlowski     

Effect of ageing on the ventilatory response and lactate kinetics during incremental exercise in man

We investigated the effects of age on breathing pattern, mouth occlusion pressure, the ratio of mouth occlusion pressure to mean inspiratory flow, and venous blood lactate kinetics during incremental exercise. Mouth occlusion pressure was used as an index of inspiratory neuromuscular activity, and its ratio to mean inspiratory flow was used as an index of the “effective impedance” of the respiratory system. Nine elderly male subjects [mean (SD) age: 68.1 (4.8) years] and nine young male subjects [mean (SD) age: 23.4 (1.3) years] performed an incremental exercise test on a bicycle ergometer. After a warm-up at 30 W, the power was increased by 30 W every 1.5 min until exhaustion. Our results showed that at maximal exercise, power output, breathing pattern, and respiratory exchange values, with the exception of tidal volume and the “effective impedance” of the respiratory system, were significantly higher in the young subjects. The power output and oxygen consumption values at the anaerobic threshold were also significantly higher in the young men. At the same power output, the elderly subjects showed significantly higher values for minute ventilation, respiratory equivalents for oxygen uptake and carbon dioxide output (CO₂), mean inspiratory flow, occlusion pressure and lactate concentration than the young subjects. At the same CO₂ below the anaerobic threshold (0.5, 0.75, 1.00 and 1.25 l · min⁻¹), minute ventilation and lactate concentration were also significantly higher in the elderly subjects. We observed a significantly higher minute ventilation at CO₂ values of 0.5, 0.75, 1.00 (P < 0.001) and 1.25 l · min⁻¹ (P < 0.05) in the elderly men, and a significantly higher lactate concentration at CO₂ values of 1.00 (P < 0.05) and 1.25 l · min⁻¹ (P < 0.01). In conclusion, the ventilatory response in elderly subjects is elevated in comparison with that in young subjects, both below and above the anaerobic threshold. This study demonstrates for the first time that this ventilatory increase, both below and above the threshold, is partly due to an increased lactate concentration. Received: 30 March 1999 / Accepted: 24 June 1999     

Peak oxygen consumption and lactate threshold in full mask versus mouth mask conditions during incremental exercise

Respirator masks vary in inhalation and exhalation resistance, and in dead volume. It is believed that these factors may contribute significantly to an early anaerobic threshold in mask wearers during maximal exercise. Very little is known concerning the effect of respirator masks on maximal oxygen consumption and the lactate threshold (LT). The purpose of the present study was to assess peak LT and the ventilatory threshold (VT) of 14 experienced cyclists performing two maximal cycle exercise protocols while wearing a full respirator mask (FM) (M17 type) and a mouth mask (MM). was 10% lower under FM conditions. Peak values for ventilation , respiratory rate (f _bpeak) and tidal volume (V _Tpeak) were all significantly lower under with FM versus MM conditions. Performance time and maximal heart rate (f _cpeak) were not different between mask conditions. The LT and VT when expressed in % and the lactate concentration (mmol · l⁻¹ at LT and VT were not significantly different across mask conditions. Bland-Altman plots demonstrated longer inhalation times, decreasedf _r values and greater oxygen extraction under FM conditions. Thus, perhaps due to the increased inhalation resistance of the FM condition, subjects were unable to attain their “normal” despite similar performance times and maximalf _c. Furthermore, despite a diminished with FM, LT and VT appeared to be the same as with a MM.     

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

Summary The purpose of the study was to define a relationship between plasma ammonia [NH₃]_p1 and blood lactate concentrations [1a^–]_b after exercise in children and to find out whether the [NH₃]_p1, 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]_p1 and [1a^–]_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]_p1 during the treadmill runs increased by 20.1 (SD 17.3), 24 (SD 16.7) and 10 (SD 4.3) mol·1^–1 in the girls, the trained boys and the untrained boys, respectively. The postexercise [NH₃]_p1 correlated positively with [1a^–]_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]_p1 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]_p1 during maximal exercise in children would seem to be lower than in adult subjects both in absolute values and in relation to [1a^–]_b. The present data would also suggest that [NH₃]_p1 reflects involvement of anaerobic processes during maximal treadmill exercise in circumpubertal children but it has a small practical value for predictiton of their sprint running ability.     

Blood lactate and pyruvate concentrations,and their ratio during exercise in healthy children: developmental perspective

Blood concentrations of lactate normally increase during and after intense exercise as does the ratio of concentrations of lactate to pyruvate (L:P). Since there appear to be differences in blood lactate concentrations on exercise, in muscle metabolic enzyme activities, and in anaerobic capacity between children and adults, we speculated that there would be age related differences in lactate and pyruvate concentrations, and their ratio among children. Whole blood concentrations of lactate and pyruvate were measured in 28 healthy children aged 7–17 years, split into three age groups: less than 11, 11–14, and 15–17 years. Blood was drawn at rest, immediately after 6 min of exercise at one-third and two-thirds of maximum work capacity (W _max), and 20 min after completion of work. Lactate and pyruvate concentrations increased significantly from rest to exercise at two-thirds W _max [72% of peak oxygen consumption ( O_2peak)]. Whereas greater increments in lactate concentration were seen with groups of increasing age, exercise-related increments in pyruvate concentrations were no different among age groups. There was a significant rise in L:P ratio on exercise, with greater increments found from the youngest to the oldest group. There were no sex differences. We concluded that in healthy children exercising at 70% Of O_2peak there is a rise in blood lactate concentration in excess of that of pyruvate, such that the L:P ratio rises to a degree determined by age. This suggests age dependent changes, perhaps coincident with puberty, in pathways involved in lactate production and/or elimination.     

Breakdown of high-energy phosphate compounds and lactate accumulation during short supramaximal exercise

Summary Muscle ATP, creatine phosphate and lactate, and blood pH and lactate were measured in 7 male sprinters before and after running 40, 60, 80 and 100 m at maximal speed. The sprinters were divided into two groups, group 1 being sprinters who achieved a higher maximal speed (10.07±0.13 m ·s^–1) than group 2 (9.75±0.10 m ·s^–1), and who also maintained the speed for a longer time. The breakdown of high-energy phosphate stores was significantly greater for group 1 than for group 2 for all distances other than 100 m; the breakdown of creatine phosphate for group 1 was almost the same for 40 m as for 100 m. Muscle and blood lactate began to accumulate during the 40 m exercise. The accumulation of blood lactate was linear (0.55±0.02 mmol · s^–1 ·1^–1) for all distances, and there were no differences between the groups. With 100 m sprints the end-levels of blood and muscle lactate were not high enough and the change in blood pH was not great enough for one to accept that lactate accumulation is responsible for the decrease in running speed over this distance.We concluded that 1) in short-term maximal exercise, performance depends on the capacity for using high-energy phosphates at the beginning of the exercise, and 2) the decrease in running speed begins when the high-energy phosphate stores are depleted and most of the energy must then be produced by glycolysis.     

Effect of exercise to rest ratio on plasma lactate concentration at work rates above and below maximum oxygen uptake

Summary The metabolic and physiological responses to different exercise to rest ratios (E: R) (2:1, 1: l, 1:2) of eight subjects exercising at work rates approximately 10% above and below maximum oxygen uptake ( ) were assessed. Each of the six protocols consisted of 15 1-min-long E : R intervals. Total work (kJ), oxygen uptake ( ), heart rate (f _c and plasma lactate concentrations were monitored. With increases in either E : R or work rate, andf _c increased (P <0.05). The average (15 min) andf _c ranged from 40 to 81 %, and from 62 to 91% of maximum, respectively. Plasma lactate concentrations nearly doubled at each E : R when work rate was increased from 90 to 110% of and ranged from a low of 1.8 mmol -I^–1 (1: 2–90) to a high of 10.7 mmol·1^–1 (2:1–110). The 2:1–110 protocol elicited plasma lactate concentrations which were approximately 15 times greater than that of rest. These data suggest that plasma lactate concentrations during intermittent exercise are very sensitive to both work rate and exercise duration.