Objective This study analysed cardiopulmonary, metabolic and rating of perceived exertion (RPE) responses during exercise bouts performed below, at and above the second lactate threshold (LT2) intensity.
Methods 10 healthy men performed constant workloads to exhaustion at the first lactate threshold (LT1), LT2 and 25% of the difference between LT2 and maximal aerobic power output (TW25%) identified during an incremental test. The time to exhaustion (TE) was 93.8 (18.0), 44.5 (16.0) and 22.8 (10.6) min at LT1, LT2 and TW25%, respectively (p < 0.001). Metabolic and cardiopulmonary parameters and RPE data were time normalised to the exercise bout duration. The correlation between the slope of these variables and TE was calculated.
Results Differences were found for respiratory exchange ratio (RER), RPE and potassium at LT1; RER, RPE, norepinephrine and potassium at LT2; and ventilation, respiratory rate (RR), RPE, lactate and potassium at TW25%. Except for RR, no cardiopulmonary or metabolic parameter increased significantly after 50% of the exercise duration, indicating a physiological steady state. VO2, heart rate and lactate at exhaustion in all exercise bouts were significantly lower than values reached in the maximal incremental test. The slope of most metabolic variables was not correlated to TE in LT1, LT2 and TW25%, whereas the slope of RPE was significantly correlated to TE (r = −0.72 to −0.84; p < 0.05) for the three exercise intensities.
Conclusion Contrary to traditional suggestions, exercise at LT1, LT2 and TW25% intensities is performed and terminated in the presence of an overall physiological steady state.
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Two general physiological responses are usually expected during a constant exercise performed below and above the second lactate threshold (LT2). At or below the LT2 intensity, continuous accumulation of blood lactate concentration (BLC) is not observed as the exercise progresses, allowing a BLC steady state to be reached.1 2 This would indicate that an adequate oxygen supply allows the rate of lactate production to be balanced by the rate of its oxidative removal, suggesting that a physiological steady state was attained.3 Above the LT2, a progressive increase in BLC and other physiological variables is observed throughout the exercise.1 2 This suggests that a complete physiological steady state cannot be attained at exercise intensities above the LT2.3
Nevertheless, some studies did not identify a complete physiological steady state even when exercising at the LT2 or maximal lactate steady state (MLSS) intensity. Some authors have reported a progressive increase in ammonia,4 epinephrine-norepinephrine,5 heart rate (HR)6 and rating of perceived exertion (RPE).5 6 However, as their exercise protocols had a fixed duration, it is possible that the time intervals used to assess the physiological variables may not have allowed evaluating a similar phase of the physiological adjustments.4,–,6 Factors such as training background may affect the time to exhaustion (TE) among individuals.7 8
Recently, Baron et al9 provided support to this suggestion, as no progressive increase in cardiopulmonary variables was found after 20% of the TE during an exercise at the MLSS intensity, when data were time normalised. Furthermore, exhaustion occurred while submaximal oxygen uptake (VO2), HR and BLC values were attained. This suggested that the lack of a complete physiological steady state previously reported during exercise at LT24 or MLSS5 6 might be just a time artefact and not a physiological event. Furthermore, a complete physiological steady state could not be verified at this intensity because only cardiopulmonary variables were time normalised. Additionally, the occurrence of a complete physiological steady state during an exercise below or immediately above this intensity was not investigated. Moreover, whether exhaustion would have occurred while reaching VO2, HR and BLC maximal values still needs verification.
Therefore, the purpose of this study was to analyse cardiopulmonary, metabolic and RPE responses in data normalised to the exercise duration during bouts performed at intensities below, above and at the LT2. We hypothesised that there should be a progressive increase in these variables during constant exercises above the LT2 intensity, but not at or below the LT2 intensity, when data are time normalised. This progressive increase in exercising above the LT2 intensity would lead to the rate of increase on cardiopulmonary and metabolic variables to be linearly correlated to the TE, allowing attainment of maximal BLC, VO2 and HR values.
Ten healthy men were recruited to participate in this study (28.1 years (4.5), 177 cm (4.6), 82.0 kg (9.7) and 14.1% fat mass (5.8)). The benefits and risks of the protocol were explained, and a written consent was obtained after approval of the Local Ethics Committee.
A maximal incremental test was completed at the first visit. Three constant trials corresponding to LT1 and LT2 intensities, and to 25% of the difference between LT2 and peak power output (WPEAK) (referred as TW25%) were randomly performed thereafter (72 h). These constant trials were carried out (48–72 h interval between them) to measure cardiopulmonary, metabolic and RPE responses at regular intervals. All tests were performed (23–25°C) with a pedal cadence between 60 and 70 rpm (cycle ergometer; Godart-Statham, Bilthoven, Holland). Exhaustion was defined as the inability to maintain the pedal cadence above 60 rpm. Subjects were asked to refrain from intense exercise and consumption of caffeine, alcoholic beverages or any stimulating substances for at least 24 h before each experimental session.
Maximal incremental test
After a 3-min warm-up at 50 W, the workload was increased by 20 W every 3 min, until exhaustion. As the lactate steady state takes about 3 min with increases of 20 W during an incremental test,10 lengthened stage and reduced workload increment were chosen in order to provide a more reliable lactate threshold determination. Applying similar protocols, no significant difference has been reported between LT2 and MLSS workloads.1 2 10 Verbal encouragement was provided to ensure that a maximal effort was reached. At the end of each stage, 25 µl of arterialised blood was drawn from the vasodilated ear lobe (Finalgon) and immediately analysed (YSI 1500 Sport; Yellow Springs, Ohio, USA) for the BLC determination whereas RPE was assessed according to the Borg scale. Gas exchange (Quark b2; Cosmed, Rome, Italy) and HR (S810i; Polar, Kempele, Finland) were continuously sampled.
BLC-workload plotting allowed LT1 and LT2 to be mathematically determined by linear regression after visual identification of two intercepts.2 Gas exchange was averaged over 10-s intervals, and the VO2MAX was calculated taking the three highest VO2 values obtained in last 60 s of the test.11 The WPEAK was identified as the maximal power output attained during the test.
Constant workload trials
Three constant bouts were performed until exhaustion. Water intake was allowed ad libitum during the exercises. Trials were performed at the same time of the day and subjects were instructed to maintain their normal diet and the same postprandial state (>2 h) before the experimental sessions. Before each trial, a catheter was inserted in a brachial vein for drawing venous blood samples. The blood samples (10 ml) were drawn before, every 5 min between 0 and 30 min of the exercise and every 15 min afterwards in order to determine the concentration of different metabolites. The samples were allocated in refrigerated tubes and centrifuged (1690g at 3°C) immediately after collection. The supernatant was removed and stored at −80°C for subsequent analysis. Gas exchange–related variables (averaged over 10-s intervals) and HR were assessed continuously whereas the RPE (arbitrary units, a.u.) was measured at the instant of drawing blood.
BLC was determined by electrochemical analysis (YSI 1500 Sport; Yellow Springs, Ohio, USA). Plasma ammonia (Boehringer Mannheim, Germany) and glucose concentrations (Biotechnical, São Paulo, Brazil) were assessed by photometric analysis. Plasma catecholamine concentrations were measured by high-performance liquid chromatography. Plasma potassium, sodium and chloride concentrations were assessed by ion-selective determination (AVL 9180; Roche, São Paulo, Brazil). Differences in the TE among the constant exercise bouts caused a distinct number of points on the metabolic parameters and RPE assessments for each curve. Thus, these variables were plotted as a function of a %TE: 0%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 60%, 75% and 95% (LT1), 0%, 15%, 25%, 40%, 55%, 70% and 95% (LT2) and 0%, 20%, 40%, 60%, 80% and 95% (TW25%). Maximum deviations of 2.5% and 5% between values from 0% to 40% and 40% to 100%, respectively, of the TE were allowed. Because of continuous measures, cardiopulmonary variables were plotted as 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% of TE.
Values obtained at exhaustion in the constant trials were compared with exhaustion values of the maximal incremental test. Individual values for each physiological variable from the fifth minute of exercise were regressed against time to obtain the slope of the linear equation. Physiological parameters linearly correlated to the TE would suggest an absence of steady state during the exercise bout. Because the respiratory exchange ratio (RER) values in TW25% exceed 1.0, oxidation rates of carbohydrate (CHOOXIDATION = 4.58 VCO2 (l/min) − 3.23 VO2 (l/min)) and fat (FATOXIDATION = 4.58 VCO2 (l/min) − 3.23 VO2 (l/min)) were estimated only for LT1 and LT2 exercise bouts and expressed in g/min.
Data are reported as mean (±SE). Gaussian distribution was verified by Shapiro–Wilk's test. A comparison of the physiological parameters during LT1 (n = 8; two subjects excluded due to technical problems), LT2 (n = 10) and TW25% (n = 10) trials was performed using a number of mixed models with time as a fixed factor and subjects as a random factor (with Bonferroni correction). Differences in power output, TE and values of physiological variables obtained at exhaustion in the different trials and in the maximal incremental test were assessed by one-way analysis of variance with post hoc Scheffe's test. The correlations of slopes of physiological parameters with TE were assessed by Pearson's correlation coefficient. All analyses were carried out in SPSS (17.0) software and the statistical significance was accepted at p < 0.05.
Maximum values obtained in the incremental test were as follows: WPEAK = 221.9 ± 8.2 W; VO2MAX = 3430.8 ± 105.5 ml/min (41.8 ± 4.0 ml/kg/min); HRMAX = 178.5 ± 4.4 bpm; RPEMAX = 19.5 ± 0.7 a.u. and BLCPEAK was 8.8 ± 2.7 mmol/l. This likely reflects the lengthened stage duration and reduced workload increment. With regard to constant trials, there were significant differences between the workloads when expressed in terms of W, %WPEAK, VO2 (ml/min), %VO2MAX and TE (p < 0.001) (table 1).
With regard to cardiopulmonary and metabolic parameters, the overall trend was a physiological steady state (figures 1–3) in the last half of the exercise bouts, except for RR in the TW25% (figure 1). In LT1 bout, significant changes in RER and plasma potassium concentrations were found (figure 1–2) for only up to 30% of the TE reaching a steady state condition afterwards. CHOOXIDATION decreased and FATOXIDATION increased from 2.6 (±0.1 g/min) and 0.02 (±0.01 g/min) to 1.8 (±0.1 g/min) and 0.28 (±0.01 g/min) between 10% and 100% of the TE, respectively. In LT2, RER, plasma norepinephrine and potassium concentrations (figures 1–3) showed significant changes only until 20% of the TE, so that a complete physiological steady state occurred from 20% to 100% of the TE (figures 1–3). CHOOXIDATION decreased and FATOXIDATION increased from 3.4 (±0.09 g/min) and 0.04 (±0.05 g/min) to 2.9 (±0.1 g/min) and 0.23 (±0.02 g/min) between 10% and 100% of the TE, respectively. In TW25% bout, RR significantly varied up to 80% of the TE (figure 1), and VE, BLC and plasma potassium concentrations (figures 1 and 2) significantly varied up to 40% of the TE. Thus, a consistent physiological steady state occurred after 40% of the TE in TW25% bout (figures 1–3).
On the other hand, RPE progressively increased until 30%, 55% and 60% of the exercise duration in LT1, LT2 and TW25%, respectively (figure 3). All these analyses presented a moderate or large effect size (>0.5), with a statistical power of ≥0.8.
VO2, HR and BLC at exhaustion in the LT1, LT2 and TW25% exercise bouts were lower than values obtained at exhaustion in the maximal incremental test (p < 0.05). Values obtained at exhaustion in LT1, LT2 and TW25% corresponded to 57%, 78% and 82% (VO2), 81%, 91% and 91% (HR) and 16%, 48% and 56% (BLC) compared with those obtained at exhaustion in the maximal incremental test. For RPE, exhaustion values in LT1, LT2 and TW25% corresponded to 87%, 98% and 98% of the RPE obtained at exhaustion in the maximal incremental test.
Correlations of cardiopulmonary and metabolic parameters with TE did not show a consistent pattern in the LT1, LT2 and TW25% intensities (tables 2 and 3). Specifically, in the TW25% trial, only the slopes of VE, HR, RR, BLC and sodium were significantly correlated with TE whereas the slope of RPE was associated with TE at all exercise intensities (table 2).
In the present study, a consistent steady state in cardiopulmonary and metabolic variables was found in the LT1 and LT2 exercise bouts. Furthermore, consistent steady state was also observed in the TW25%. Overall results showed no significant time effect after 50% of the TE and the rate of increase in most metabolic variables was not associated with the TE even in the TW25%. Consequently, exercise terminated at intensities ranging from LT1 to TW25%, whereas VO2, HR and BLC values were lesser than values obtained at exhaustion in the maximal incremental test.
As traditionally accepted, a physiological steady state is reached during constant exercises performed at LT1 and LT2 intensities,1 which would indicate that the exercise can be prolonged due to the absence of a progressive metabolite accumulation and low CHO depletion.3 Initially, our results may support this hypothesis, because most cardiopulmonary and metabolic parameters showed no change in LT1 and LT2 intensities after 30% of the TE. Additionally, reductions in CHOOXIDATION and increases in FATOXIDATION together with a decrease in RER occurred during these intensities. This substrate oxidation towards fat as exercise progressed might be related to the increase in norepinephrine, as a previous study reported that reductions in RER were roughly accompanied by elevations in the norepinephrine and free fatty acid concentrations when subjects cycled at 68% VO2MAX.12 Yet, despite demonstrating a consistent physiological steady state and indicating low CHO utilisation during the LT1 and LT2 trials, exercises terminated at 93.8 and 44.5 min, respectively. Therefore, the cause of exercise termination during exercise at this intensity range requires further investigation.
It is also traditionally accepted that constant exercise performed above the LT2 should lead to a progressive metabolic accumulation and increased cardiopulmonary responses as exercise progresses.1 3 Metabolic accumulation, such as increases in BLC,2 potassium13 and ammonia,5 has been considered as the most likely cause of exercise termination during exercises above the LT2.7 14 This interpretation is based on the evidence such as the suppression of the activity of glycolytic enzymes as a result of increased intracellular lactate,15 decreased excitability of sarcolemmal and t-tubular membranes caused by increased extracellular potassium,16 and reduced mitochondrial respiration and Krebs cycle intermediates associated with increased intracellular ammonia.17 However, significant increases occurred only in lactate and potassium before 50% of the exercise bout duration.
With regard to cardiopulmonary variables, VE and RR changed significantly during the TW25% trial until 30% and 80% of the TE, respectively. The greater increase in plasma potassium and BLC during the TW25% bout might have provided an additional stimulus for the cardiopulmonary system, increasing the VE and RR responses.18,–,20 Nevertheless, as the overall physiological steady state was observed during TW25%, it is difficult to understand how metabolite accumulation and increased cardiopulmonary responses could be associated with the exercise termination at this exercise intensity. Changes in these variables occurred only up to 50% of the exercise bout duration. Hence, it might be suggested that exhaustion in exercise above the LT2 cannot be related to the lack of a physiological steady state.
The exhaustion in the TW25% trial occurred while reaching 56% (BLC), 82% (VO2) and 91% (HR) of values during the maximal incremental test. It was not possible to determine whether other parameters reached maximal values at exhaustion in TW25% because they were not assessed during the maximal incremental test. Yet, most of these reached lower exhaustion values than peak values as reported by others.21,–,23 Additionally, the fact that only slopes of VE, HR, RR, lactate and sodium were associated with the TE in the TW25% trial suggests an absence of a progressive increase in most physiological variables. These data do not entirely suggest a progressive increase in metabolite and respiratory parameters, or attainment of BLC, VO2 and HR maximal values, during exercises above the LT2.
RPE at exhaustion during the constant trials was close to RPE obtained at exhaustion in the maximal incremental test, corresponding to 87%, 98% and 98% in LT1, LT2 and TW25%, respectively. Additionally, the slope of RPE was strongly correlated to the TE in all these trials. In fact, a linear increase in RPE during exercise has been reported in several conditions,24 25 indicating the exercise time remaining until exhaustion.26 Taken together, the presence of a physiological steady state after 50% of the TE in all exercise bouts and the RPE results could support an integrative, centrally regulated effort model that regulates the body homeostasis based on the remaining exercise time. In this model, RPE would be a key tool used by the brain to ensure that exercise is performed within safety limits.7 8 26
Finally, differences in metabolic variables might have occurred because measurements were obtained from venous instead of arterial blood. However, venous measurements can represent the metabolic state during prolonged exercise because the arteriovenous difference for most variables during prolonged exercise from moderate to high intensities is close to zero.27,–,29 In addition, although previous studies have reported exhaustion values of core temperature and pH that could not be considered as causes of fatigue,9 further information of the cause of exercise termination at intensities ranging from LT1 to TW25% could be provided with these two variables.
In summary, a consistent overall physiological steady state was observed during exercise to exhaustion at LT1 and LT2 intensities, but especially at TW25%, when data are time normalised. Most variations observed in physiological variables occurred before 50% of the exercise bout duration. Thus, the absence of an increase in cardiopulmonary and metabolic parameters during the whole exercise duration did not lead to the rate of increase on these variables to be linearly correlated to the TE. Additionally, the exercise terminated while reaching submaximal values of BLC, VO2 and HR.
▶ According to traditional interpretation, exercise performed at or below the second lactate threshold can be prolonged, ensured by a consistent physiological steady state, such that progressive metabolite accumulation and low substrate depletion does not occur.
▶ Exercises performed above the second lactate threshold are limited by the progressive metabolite accumulation, which should lead to the exercise termination when reaching maximal values in some physiological variables.
What this study adds
▶ The present study provides evidence of consistent physiological steady state during exercises performed below, at or above the second lactate threshold.
▶ Results demonstrated that a consistent physiological steady state occurred in the second half of the exercise performed above the second lactate threshold intensity. Exercise termination occurred while reaching submaximal values in some physiological variables. The rate of increase in rating of perceived exertion was associated with the exercise termination in all exercise bouts.
The authors thank Professor Dulce Casarini for technical assistance. FOP is grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brazil) for his PhD scholarship.
Funding This study received financial support from the FAPESP (Foundation of Aids to Scientific Research of the State of São Paulo, Brazil), process 2006/60641-6. TDN is supported by the Discovery Health, the Medical Research Council and the University of Cape Town.
Competing interests None.
Patient consent Obtained.
Ethics approval This study was conducted with the approval of the University of São Paulo.
Provenance and peer review Not commissioned; externally peer reviewed.
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