Manipulating graded exercise test variables affects the validity of the lactate threshold and V ˙ ⁢ O 2 ⁢ peak

Abstract

Background: To determine the validity of the lactate threshold (LT) and maximal oxygen uptake (${\dot{\mathrm{V}}\mathrm{O}}_{2\max }$) determined during graded exercise test (GXT) of different durations and using different LT calculations. Trained male cyclists (n = 17) completed five GXTs of varying stage length (1, 3, 4, 7 and 10 min) to establish the LT, and a series of 30-min constant power bouts to establish the maximal lactate steady state (MLSS). ${\dot{\mathrm{V}}\mathrm{O}}_2$ was assessed during each GXT and a subsequent verification exhaustive bout (VEB), and 14 different LTs were calculated from four of the GXTs (3, 4, 7 and 10 min)-yielding a total 56 LTs. Agreement was assessed between the highest ${\dot{\mathrm{V}}\mathrm{O}}_2$ measured during each GXT (${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$) as well as between each LT and MLSS. ${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$ and LT data were analysed using mean difference (MD) and intraclass correlation (ICC).

Results: The ${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$ value from GXT1 was 61.0 ± 5.3 mL.kg-1.min-1 and the peak power 420 ± 55 W (mean ± SD). The power at the MLSS was 264 ± 39 W. ${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$ from GXT3, 4, 7, 10 underestimated ${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$ by ~1-5 mL.kg-1.min-1. Many of the traditional LT methods were not valid and a newly developed Modified Dmax method derived from GXT4 provided the most valid estimate of the MLSS (MD = 1.1 W; ICC = 0.96).

Conclusion: The data highlight how GXT protocol design and data analysis influence the determination of both ${\dot{\mathrm{V}}\mathrm{O}}_{2\mathrm{peak}}$ and LT. It is also apparent that ${\dot{\mathrm{V}}\mathrm{O}}_{2\max }$ and LT cannot be determined in a single GXT, even with the inclusion of a VEB.

Fig 1. Representative blood lactate curve with 14 LTs calculated from GXT₄ (participant #9).

The power of the MLSS was 302 W and the blood lactate concentration was 2.85 mmol^.L^-1. Log-log = power at the intersection of two linear lines with the lowest residual sum of squares; log = using the log-log method as the point of the initial data point when calculating the D_max or Modified D_max; poly = Modified D_max method calculated using a third order polynomial regression equation; exp = Modified D_max method calculated using a constant plus exponential regression equation; OBLA = onset of blood lactate accumulation; B + absolute value = the intensity where blood lactate increases above baseline.

Fig 2

(A-D) Forrest Plots of the difference (ES ± 95% CI) between the MLSS and the power calculated from the 13 lactate thresholds derived from (A) GXT₃, (B) GXT₄, (C) GXT₇ and (D) GXT₁₀ (52 in total and excluding log-log). The solid vertical bar represents no difference from the MLSS and the dashed vertical bars represents the threshold between a trivial and small difference (ES = 0.2) established by Cohen (50) and Hopkins (49). log = using the log-log method as the initial data point when calculating the D_max or Modified D_max; poly = Modified D_max method calculated using a third order polynomial regression equation; exp = Modified D_max method calculated using a constant plus exponential regression equation; OBLA = onset of blood lactate accumulation.

Fig 3. Bland-Altman plots displaying agreement between measures of the power associated with the RCP regression equation (RCP_MLSS) calculated from GXT₁ and the MLSS.

The differences between measures (y-axis) are plotted as a function of the mean of the two measures (x-axis) in power (Watts). The horizontal solid line represents the mean difference between the two measures (i.e., bias). The two horizontal dashed lines represent the limits of agreement (1.96 x standard deviation of the mean difference between the estimated lactate threshold via the RCP_MLSS and the maximal lactate steady state). The dotted diagonal lines represent the boundaries of the 95% CI for MLSS reliability (CV = 3.0%; 95%; CI = 3.8%) calculated from Hauser et al., 2014) (RCP = respiratory compensation point).

Fig 4. Bland-Altman plots displaying agreement between measures of the power associated with the baseline plus 1.5 mmol^.L^-1 calculated from GXT₃ and the MLSS.

The differences between measures (y-axis) are plotted as a function of the mean of the two measures (x-axis) in power (Watts). The horizontal solid line represents the mean difference between the two measures (i.e., bias). The two horizontal dashed lines represent the limits of agreement (1.96 x standard deviation of the mean difference between the lactate threshold and the maximal lactate steady state). The dotted diagonal lines represent the boundaries of the 95% CI for MLSS reliability (CV = 3.0%; 95%; CI = 3.8%) calculated from Hauser et al., 2014).

Fig 5

(A-D) Bland-Altman plots displaying agreement between measures of the power associated with the (A) OBLA 2.5 mmol^.L^-1, (B) Modified D_max, (C) Log-Poly-Modified D_max, (D) Log-Exp-Modified D_max calculated from GXT₄ and the MLSS. The differences between measures (y-axis) are plotted as a function of the mean of the two measures (x-axis) in power (Watts). The horizontal solid line represents the mean difference between the two measures (i.e., bias). The two horizontal dashed lines represent the limits of agreement (1.96 x standard deviation of the mean difference between the lactate threshold and the maximal lactate steady state). The dotted diagonal lines represent the boundaries of the 95% CI for MLSS reliability (CV = 3.0%; 95%; CI = 3.8%) calculated from Hauser et al., 2014) (log = Modified D_max method using the log-log method as the point of the initial lactate point; poly = Modified D_max method calculated using a third order polynomial regression equation; exp = Modified D_max method calculated using a constant plus exponential regression equation; OBLA = onset of blood lactate accumulation.).

Fig 6

(A-C) Bland-Altman plots displaying agreement between measures of the power associated with the (A) OBLA 2.5 mmol^.L^-1 (GXT₇), (B) OBLA 3.0 mmol^.L^-1 (GXT₇), (C) Log-Exp-Modified D_max calculated from GXT₇ and the MLSS. The differences between measures (y-axis) are plotted as a function of the mean of the two measures (x-axis) in power (Watts). The horizontal solid line represents the mean difference between the two measures (i.e., bias). The two horizontal dashed lines represent the limits of agreement (1.96 x standard deviation of the mean difference between the lactate threshold and the maximal lactate steady state). The dotted diagonal lines represent the boundaries of the 95% CI for MLSS reliability (CV = 3.0%; 95%; CI = 3.8%) calculated from Hauser et al., 2014) (log = Modified D_max method using the log-log method as the point of the initial lactate point; exp = Modified D_max method calculated using a constant plus exponential regression equation; OBLA = onset of blood lactate accumulation.).

Fig 7

(A-B) Bland-Altman plots displaying agreement between measures of the power associated with the (A) OBLA 3.0 mmol^.L^-1, (B) OBLA 3.5 mmol^.L^-1 calculated from GXT₁₀ and the MLSS. The differences between measures (y-axis) are plotted as a function of the mean of the two measures (x-axis) in power (Watts). The horizontal solid line represents the mean difference between the two measures (i.e., bias). The two horizontal dashed lines represent the limits of agreement (1.96 x standard deviation of the mean difference between the lactate threshold and the maximal lactate steady state). The dotted diagonal lines represent the boundaries of the 95% CI for MLSS reliability (CV = 3.0%; 95%; CI = 3.8%) calculated from Hauser et al., 2014) (OBLA = onset of blood lactate accumulation.).