W-X50, a new method to determine aerobic endurance
55 W-X50, a new method to determine aerobic endurance
Abstract
This study was designed to analyse the curve of lactate in response to in-cremental exercise as a sigmoid.To this end, 25 male volunteers, aged 23.2 ± 2.1 years old, 71.6 ± 6 1 kg body mass, 180 ± 10 cm tall, underwent an incremental test to determine the X50; they also underwent submaximal constant load tests to determine maximal lactate steady state (MLSS). The exercise intensity in Watts at X50 (W-X50) was determined based on the dependence between the concentra-tion of lactate and exercise intensity, which were analyzed in a sigmoid funcconcentra-tion (Boltzmann function). The results showed that the intensities of W-X50 (162.16 ± 33.59 Watts) and W-MLSS (157.80 ± 31.55 Watts) were not significantly different at p <0.05. In addition, W-X50 has a higher correlation with the W-MLSS (r = 0.8, p<0.05) and the W-peak (r = 0.92, p <0.05) than does the OBLA (r = 0, 77, p <0.05 for W-MLSS; r = 0.79, p <0, 05, W-peak). Based on these results, one can con-clude that (1) lactate kinetics can be analyzed in the form of a sigmoid; (2) X50 intensity can be used to determine the W-MLSS.
Key-words: aerobic endurance, incremental protocol, one maximal repetition, vo-luntary exhaustion
56 INTRODUCTION
The Maximal Lactate Steady State (MLSS) [17] became widely used in the diagnosis of the performance of various endurance events, mainly due to the strong correlation found with activities lasting between 30 and 60 minutes [9], but the requirement for several days of tests restricts its practicality [15, 26]. Several studies have been performed to validate this measure using a single day test, with fixed or incremental load [3, 13, 14, 17, 19, 20, 28, 29, 30, 32]. TOKMAKIDIS [33]
compared several methods for determining lactate threshold using a single day test (visual profile of the lactate curve; concentration 4 mM (OBLA), 1 mM above the baseline, log-log curve, 45o tangent to the exponential regression) and al-though they all correlate (r> 90), the results failed to demonstrate a single intensity related to the threshold. Moreover, the parameters of the incremental test will af-fect the outcome, including magnitude of increment, duration of each step [12, 31, 33]. Therefore, all of these techniques will detect an intensity of exercise that is reasonably close to the lactate threshold, but individual variability results in discre-pancies when each measurement is compared with the lactate threshold (or more practically, MLSS).
Using another approach, Billat et al. [8], suggested that MLSS could be estimated by applying two loads of 20 minutes flat out to 65% and 85% of maximal aerobic velocity (MAV), separated by 40 minutes of complete rest. The validity of this method was also questioned because it is based on the linear kinetics of the in-crease in blood lactate, whereas it is believed that the inin-crease in blood lactate is exponential [18, 25]. However, both exponential kinetics of blood lactate as the existence of a break point on his curve has not been fully elucidated in the litera-ture [6, 7].
In addition, Yoshida [34] showed that the diffusion kinetics of lactate may confuse the results of an incremental test because the duration of each stage does not coincide with the time taken to complete distribution of lactate (as determined for 2 and 4 mM concentrations). The exponentiality of lactate curve during an in-cremental test may be related to the sum of the concentrations observed between subsequent stages, due to insufficient time for complete diffusion between charges
57 [34]. Furthermore, the exponential nature of the curve indicates a trend towards infinite increase in blood lactate, which could lead to an error in its interpretation.
Due to these criticisms, the kinetic model of the exponential increase in blood lac-tate has been harshly criticised [15].
To give a new insight into the kinetics of lactate in the incremental tests, our laboratory aims to analyze the kinetics of blood lactate curve a sigmoid form.
When load testing sets on separate days are performed until exhaustion in the same intensities used in the incremental test, no difference in maximum lactate was found at intensities above 90% VO2 max, as well as supra-maximal intensities (100% - 130% VO2 max) [11, 22, 27]. These data suggest that possibly every indi-vidual has a maximum concentration of lactate. Furthermore, when plotted the maximum concentration of lactate every fixed load test to exhaustion, the kinetics of lactate show sigmoid shape. The point of half-transition of the sigmoid curve of the lactate concentration would represent exercise intensity where there is the be-ginning of the slowdown in accumulation of lactate with the same intensity, reach-ing the plateau in the lactate concentration in intensity near the maximum. In the incremental test, the sigmoid is not seen probably because the sum of the concen-trations of lactate per stage. Thus, we hypothesized that the MLSS can be pre-dicted through an analysis of the sigmoid curve of the lactate produced by incre-mental test.
Based on these findings, this study aims to (1) analyze the kinetics of lactate as a sigmoid in an incremental test and (2) determine the exercise load at the point of half-transition of the sigmoid (W-X50) as a new method for assessment of exercise intensity at the MLSS (W-MLSS).
METHOD
Experimental Approach to the Problem
In the present study, we recruited male students of physical education for performing an incremental test and submaximal several tests of 30 minutes dura-tion. To test our hypotheses, Blood samples were taken for blood lactate analysis to determine the intensity of the X50, OBLA and MLSS. To obtain reliable
meas-58 ures for OBLA and MLSS, an incremental test and load fixed design was selected according to proposed recommendations (9,7). All tests were performed in the morning, within an environmental chamber (Rüssels®) in order to control the tem-perature (22 °C).
Subjects
Twenty-five healthy, physically active men (23.2 ± 2.1 years, 71.6 ± 6.1 kg, 180 ± 10 cm, with VO2max between 40 and 60 ml kg-1min-1) underwent an incre-mental test to determination of W-X50 and OBLA and several sub-maximal load fixed tests to determination of MLSS. This study was approved by the Ethics Committee in Research of the Federal University of Minas Gerais (COEP 355/05) and complied with all rules established by the National Health Council (Res.
196/96) for research involving human subjects.
Procedures
The volunteers took 4-5 visits to the laboratory for this study. The first visit was to perform the incremental test and the other visits to determine MLSS. The volunteers were instructed not to drink beverages with caffeine or alcohol and not to perform vigorous physical activity 24 hours prior to each test, besides having at least 8 hours of sleep the night before the test. In addition, the volunteers were required to ingest 500 ml of water two hours before testing [1], time required to ensure the state hydrated and empty the bladder. All tests were performed in the morning, between 8 and 10 hours, within an environmental chamber (Rüssels®) in order to control the temperature (22 °C).
Incremental Test
The incremental test began with a power of 60 W, with an increase of 15 W every three minutes, held in fixed cadence at 60 rpm. The end of the test was de-termined by voluntary exhaustion or inability to maintain the 60 rpm. Blood sam-ples (25 μl ) were collected from the earlobe at the end of each stage.
The peak power (W-Peak) was calculated according to the equation pro-posed by Kuipers et al. [21]:
59
where W1 is the power of the last complete stage, W2 is the power corresponding to the increments of each stage, and t is the time (given in seconds) taken for the incomplete step.
Maximal Lactate Steady State (MLSS) Test
The MLSS tests were performed every 48 hours, with initial load corre-sponding to concentration of 3.5 mM lactate obtained in the incremental test. An increase of 15 watts was added to the exercise load of each submaximal test to destabilise blood lactate (increase greater than 1 mM in blood lactate concentra-tion between the tenth and thirtieth minutes). The submaximal test was also per-formed at a temperature of 22 °C. Samples of 25 µl blood were collected before the start of the exercise session at every five minutes until the end of each sub-maximal test (30 min).
Determination of W-X50
The relationship between exercise intensity and blood lactate was analyzed by sigmoid regression (Boltzmann function) using the software Origen-Pro 7.5 (Mi-crocal, Northampton, MA) given by the following equation:
)/dX max X (X
ax m in
m L
e 1
L L L
50
(1)
The Lmin and Lmax represent the minimum and maximum concentrations of lactate and were used for determination of blood lactate equivalent to 50% of max-imum (L50). The exercise intensity in watts equivalent to L50 was determined as WX50.
To analyze the quality of the curve adjustments was used the sum of square residual (SRQ) and the mean squared error (MSE) of the waste. While the SRQ was obtained by summing the squared difference between measured values and predicted lactate, the MSE was obtained by SRQ corrected by the difference
be-60 tween the number of points contained in the curve and the degrees of freedom for each mathematical function.
Figure 1 illustrates the typical kinetics of lactate in an incremental test to a voluntary, which were used to determine the value of W-X50.
**Figure 1, HERE**
Determination of Onset of Blood Lactate Accumulation (OBLA)
In the same incremental test, the intensity of exercise at the onset of blood lactate accumulation (OBLA) (W-OBLA) was determined by linear interpolation from the fixed point of 4 mM.
Determination of Maximal Lactate Steady State (MLSS)
The exercise intensity at MLSS (W-MLSS) was determined by the greatest power at which blood lactate remained stable (without increasing by more than 1 mM during the last twenty minutes of the test) [9, 17].
Analysis of Blood Lactate
Measurements of blood lactate were made using a lactimeter YSL 2300 STAT (Yellow Springs, OH, USA). The lactate concentration was always ex-pressed in mM.
Statistic Analysis
All descriptive results are reported as mean ± standard deviation (SD). The assumption of normality was verified using Kolmogorov-Smirnov‟s test. The rela-tionship between blood lactate and exercise intensity was determined by sigmoidal regression, using the OriginPro 7.5 program (Microcal, Northampton, MA). The quality of the fitting curve was analyzed using the sum of the squared residuals (SSR), which was obtained by summing the squared difference between meas-ured values and values predicted by the sigmoidal regression curve. The deter-mined parameters (W-MLSS, W-OBLA, W-PEAK and W-X50) were always
ex-61 pressed as mean ± standard deviation. Student‟s t-test for dependent variables was used to compare the values of MLSS with the values of X50 and of W-OBLA. Differences between values were considered significant when p<0.05. The Wilcoxon test was used to compare the concentrations of lactate in the W-X50 and W-MLSS. The Pearson test was used to verify the correlation between W-MLSS, W-OBLA, W-PEAK and W-X50. The significance level was p <0.05. The level of agreement was analyzed using the technique proposed by Bland and Altman [10], considering the MLSS as the gold standard.
RESULTS
The values of SSR and r for the relation between lactate concentration and exercise intensity, as examined by the Boltzmann function, were 1.97 ± 1.09 and 0.96 ± 0.01, respectively, indicating that the ratio of lactate concentration versus exercise intensity can also be analyzed in the sigmoid.
The values (mean and standard deviation) of W-X50, W-MLSS, W-OBLA and W-Peak are shown in Table 1.
**TABLE 1, HERE**
The lactate concentrations at exercise intensities of W-X50 and W-MLSS were 5.53 ± 1.18 and 5.84 ± 1.84 mM, respectively. There were no statistically significant differences between those values (p<0.05).
Table 2. Pearson correlation coefficients (r) between W-X50, W-MFEL, W-OBLA and W-Peak.
**TABLE 2, HERE**
Significant correlations were observed between W-X50, W-MLSS, W-OBLA and W-Peak. (Table 2), with the agreement between W-MLSS and W-X50 con-firmed by the technique by Bland and Altman [10] (Figure 2).
**FIGURE 2, HERE**
62 DISCUSSION
Although widely accepted the use of kinetics in the exponential curve analy-sis of lactate in the incremental tests [18, 25], as seem above, the results of this study also indicated that the same curve can be analyzed in the form sigmoid.
Nevertheless, many factors must be responsible for the absence of sigmoidal be-haviour in the variation of blood lactate as a function of exercise intensity.
The feeling of unbearable stress can lead the individual to stop the test even before it reaches the metabolic exhaustion associated with the production of lactate [16, 24]. Variables related to the type of protocol used in the incremental test, such as the duration of each stage and the variation of power between stages can alter the kinetics of blood lactate formation and removal. The lactate produced in the previous stage can influence the concentration in the subsequent step. The proper diffusion kinetics of lactate may confuse the results of an incremental test because the duration of each stage does not coincide with the time taken for com-plete diffusion of lactate, especially for concentrations of 2 and 4 mM [34].
In a sigmoid correlation analysis, blood lactate would tend to level off at the exercise load at which the volunteer reaches exhaustion, creating a plateau from intensities close to VO2 max. Indeed, several studies conducted rectangular tests to exhaustion at supra-threshold to supra-maximal intensities (90 to 140%). Re-sponse values were compared among themselves and with the values obtained in respective incremental tests; the maximum individual concentration of lactate did not differ between the intensities, indicating that regardless of the protocol used, each individual has a maximum concentration of lactate [11, 22, 27]. This agrees with the hypothesis that the kinetics of lactate production should be expressed as a sigmoid curve.
Different types of evidence presented in this paper show that the midpoint of the sigmoidal curve of dependence between lactate and exercise intensity (X50) may represent the maximal lactate steady state (MLSS). First, there was no sig-nificant difference between W-X50 and W-MLSS, with both higher than the intensi-ties OBLA (Table 1). Furthermore, there was a strong correlation between W-X50, W-MLSS and W-Peak (Table 2). Although OBLA is a valid method for determining
63 the MLSS, it has been strongly questioned because several studies have demon-strated that lactate concentration at MLSS is not the same in all subjects, ranging from 2 to 10 mM. Based on these data, it was suggested that MLSS determination should be performed by individualized approaches rather than using fixed bLa threshold concepts [15]. The difference between the method OBLA and the X50 to determine the MLSS, is that the determination of the intensity of the X50 takes into consideration all the data of lactate, in addition to respecting the individuality, since each individual has a different concentration of lactate in the X50.
Second, blood lactate levels did not differ significantly between W-X50 (5.53
± 1.18 mM) and W-MLSS (5.84 ± 1.84 mM). The classic study of HECK et al. [17]
showed that the mean lactate concentration in the intensity of MLSS is 4mM rang-ing from 3.0 to 5.5 mM. Although the lactate concentration at the MLSS intensity is not related to performance, it appears to be dependent on the muscle mass in-volved. Beneke [4] and Beneke and Duvillard [5] found that lactate concentration at MLSS is dependent on the motor pattern of exercise, finding that the lactate concentration was lower in rowers (3.1 mM) when compared with cyclists (5.4 mM) and speed skaters (6.6 mM) (no significant difference between cyclists and ska-ters). Based on this data the authors suggest individualised assessments to de-termine the MLSS. Our results were similar to those of Beneke [4] for blood lactate at MLSS and W-X50 for cyclists.
The possibility of using W-X50 to predict the W-MLSS is a discovery of great importance because it allows the determination of MLSS in a single day, as the major problem in the conventional determination of this parameter is the re-quirement for several days of tests [26].
One limitation of this study was that the volunteers did not carry out a test to exhaustion in MLSS and X50 intensity. Baron et al. [2] demonstrated with a sam-ple of 11 volunteers that the time to exhaustion at W-MLSS was 55 ± 8.5 min.
Mognoni et al. [23] found a mean time to voluntary exhaustion at W -OBLA of 38.2 minutes and a final lactate concentration of 5.3 ± 2.3 mM, working with a sample of 20 individuals out of 34 eligible volunteers, suggesting that the W-OBLA overes-timated the W-MLSS.
64 Despite the practical importance of the equivalence between X50 and W-MLSS observed in this study, the origin of this agreement must be evaluated in further studies on the subject.
Based on these results, it can be concluded that the intensity of MLSS can be identified in a single day by using the intensity of X50 obtained through analysis of the sigmoid curve of lactate, confirming our hypothesis.
PRACTICAL APPLICATIONS
Assessment of aerobic endurance through the blood lactate responses is extremely important for the practice of cyclic aerobic training (running, cycling, tri-athlon, rowing). Among the possibilities for use of blood lactate to assess aerobic performance, the intensity of the maximum lactate steady state (MLSS) (gold standard) is one of the most relevant, since its gain reflects increased aerobic ca-pacity. The determination of this intensity is essential for training prescription, since it has a high correlation with the cyclical performance of exercises lasting between 30 and 60 minutes [9]. However, its practicality is questioned because of the involvement of several days of testing at submaximal its realization [26].
Thus, the discovery of this work to predict the intensity of the MLSS through the use of W-X50, is extremely valuable, since it is determined in a single day of incremental test. This increases the practicality of the test, reducing the need for multiple days of assessment, which helps coaches and athletes. That helps another fact of using the X50, is that the value is found through a mathematical formula, so does not need the presence experienced evaluators to determine the strict point of it occurrence.
The W-X50 also has high practicality in conducting re-tests during the sea-son, due only be necessary to carry out an incremental test, thus facilitating the verification of changes in performance. Several other tests have also been vali-dated for predicting the MLSS in a single day, however, all were heavily criticized because of the dependence of the protocol [12, 31, 33]. Another great advantage of predicting the MLSS by W-X50 is that the latter uses the entire data in their de-termination, thereby respecting individual differences in blood lactate
concentra-65 tion, unlike other widely used method in the evaluation and monitoring of training, the OBLA, which called for exercise intensity where lactate concentration reached 4mM as the MLSS intensity. The use of any fixed point, without observing all of the data, violates the principle of biological individuality, thus treating all subjects as equal. This is not true, because each individual responds differently to exercise stress.
66 Table 3.1: Power (Watts) of exercise intensities given by X50, MLSS, W-OBLA and W-Peak.
Parameter Mean SD P*
W-X50 162.16 33.59 a
W-MLSS 157.80 31.55 b
W-OBLA 144.30 37.32 a,b
W-Peak 205.80 32.26 a,b
*Differences represented by the same letter were statistically significant (Student‟s t-test, N=25, p<0.01).
67 Table 3.2: Pearson correlation coefficients (r) between W-X50, W-MFEL, W-OBLA and W-Peak.*
W-X50 W-MLSS W-OBLA W-Peak
W-X50 - 0.80 0.79 0.92
W-MLSS 0.80 - 0.77 0.87
W-OBLA 0.79 0.77 - 0.83
W-Peak 0.92 0.87 0.83 -
*All correlations were statistically significant (p<0.01, N=25).
68
Figure 3.1: Dependence of lactate concentration on exercise intensity in an incremental test. W-X50 represents the inten-sity of exercise when the variation in lactate concentration reached 50% of the maximum concentration of lactate ([Lact] max) n=24.
69
Figure 3.2: Limit of agreement between W -MLSS and W-X50, using the technique proposed by Bland and Altman.
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