Modeling lactate threshold in cycling-influence of sex, maximal oxygen uptake, and cost of cycling in young athletes

Abstract

Purpose: Understanding physiological determinants of lactate threshold 2 (LT2) is crucial for tracking adaptations and deriving individualized training recommendations in cycling. Therefore, the study investigated: 1. the accuracy of modeling power output at LT2 in young athletes of both sexes using maximal oxygen uptake ( $\dot{V}{\text{O}}_{2_{\text{peak}}}$ ), fractional utilization of $\dot{V}{\text{O}}_{2_{\text{peak}}}$ (% $\dot{V}{\text{O}}_{2_{\text{peak}}}$ ), and oxygen cost of cycling (Cc); 2. the influence of Cc determination on the model accuracy; 3. the influence of the model predictors and inclusion of maximal lactate accumulation rate ( $\dot{c}L{a}_{\text{max}}$ ) on power at LT2 depending on sex.

Methods: Eighty-three cyclists and triathletes (22 females, 61 males; age [median and IQR]: 14.6 [13.8-17.6] years, $\dot{V}{\text{O}}_{2_{\text{peak}}}$ [mean ± SD]: 59.2 ± 6.5 mL⋅kg^-1⋅min^-1) performed an incremental test to determine power at LT2, $\dot{V}{\text{O}}_{2_{\text{peak}}},$ % $\dot{V}{\text{O}}_{2_{\text{peak}}}$ at LT2, and Cc (assessed at 3 W⋅kg^-1, 75% $\dot{V}{\text{O}}_{2_{\text{peak}}},$ and 90% LT2).

Results: Modeled and experimentally determined power at LT2 demonstrated excellent agreement for all, male and female athletes (ICC $\ge$ 0.961), with Cc at 90% LT2 providing the highest accuracy (ICC $\ge$ 0.986). The three physiological determinants explained $\ge$ 98% of the variance in power at LT2, with the largest unique contribution from $\dot{V}{\text{O}}_{2_{\text{peak}}}$ (62 and 67% of total $R^2$ ), followed by Cc (8 and 34%) and % $\dot{V}{\text{O}}_{2_{\text{peak}}}$ at LT2 (5 and 12%) in males and females, respectively, while $\dot{c}L{a}_{\text{max}}$ did not improve the regression.

Conclusion: $\dot{V}{\text{O}}_{2_{\text{peak}}},$ % $\dot{V}{\text{O}}_{2_{\text{peak}}}$ at LT2 and Cc accurately predict power at LT2 in young cycling athletes independent of sex, with determination of Cc at 90% LT2 providing the highest accuracy. While $\dot{V}{\text{O}}_{2_{\text{peak}}}$ contributes most to LT2 in both sexes, Cc appears more important in young females.

Keywords: Aerobic capacity; Endurance performance; Gross efficiency; Maximal lactate accumulation rate; Maximal metabolic steady state; Performance diagnostics; Youth athletes.

Fig. 1

Bland-Altman plots showing the absolute differences between modeled power output at lactate threshold 2 using oxygen cost of cycling (a) at a fixed power of 3 W⋅kg^–1 (_modLT2_fix), (b) at 75% of maximal oxygen uptake ($_{\text{mod}}^{}{\text{LT2}}_{\%\dot{V}{\text{O}}_{2_{\text{peak}}}}$), and (c) at 90% of lactate threshold 2 (_modLT2_%LT2) vs. power output at lactate threshold 2 (LT2) determined by modified maximal deviation method. The individual data of male (N = 61) and female (N = 22) athletes are presented by blue triangles and red circles, respectively. The solid line indicates mean difference (fixed bias), the dotted lines mark the limits of agreement (mean ± 1.96-fold standard deviation; random bias), and the dashed line represents the fitted linear regression (proportional bias)

Fig. 2

Scatter plot illustrating the correlation between maximal oxygen uptake ($\dot{V}{\text{O}}_{2_{\text{peak}}}$) and oxygen cost of cycling (${\text{Cc}}_{\%\mathrm{LT}2}$). The straight black line represents the overall correlation for the entire sample, whereas the three subgroups, distinguished by power output at lactate threshold 2 (LT2; i.e., the range was divided into three equal intervals), are depicted by different colored arrays. Within these subgroups, the colored straight lines again indicate the correlation between $\dot{V}{\text{O}}_{2_{\text{peak}}}$ and ${\text{Cc}}_{\%\mathrm{LT}2}$. The size of the dots reflects the performance level based on power output at LT2

Fig. 3

Graphical summary of commonality analyses for the experimentally determined power output at lactate threshold 2 (LT2) across all (N = 83), male (N = 61), and female (N = 22) athletes. The percentage contribution of each unique predictor to the total regression $R^2$ is presented by the black-filled arrows; the dashed lines and external solid lines represent the common effects of two and all predictors in $R^2$, respectively. $\dot{V}{\text{O}}_{2_{\text{peak}}}$: maximal oxygen uptake, ${\text{Cc}}_{\%\mathrm{LT}2}$: oxygen cost of cycling determined at 90% of LT2, %$\dot{V}{\text{O}}_{2_{\text{peak}}}$: fractional utilization of $\dot{V}{\text{O}}_{2_{\text{peak}}}$ at LT2

Fig. 4

Scatter plots showing the Spearman’s rank correlation ρ between age and (a) maximal oxygen uptake ($\dot{V}{\text{O}}_{2_{\text{peak}}}$), (b) Oxygen cost of cycling determined at 90% of power output at lactate threshold 2 (${\text{Cc}}_{\%\mathrm{LT}2}$), (c) fractional utilization of $\dot{V}{\text{O}}_{2_{\text{peak}}}$ at lactate threshold 2, (d) power output at lactate threshold 2 (LT2) determined using the modified maximal deviation method, (e) modeled power output at LT2 using oxygen cost of cycling at 90% of power output at LT2, and (f) the difference between modeled and experimentally determined power output at LT2. Individual data of male (N = 61) and female (N = 22) athletes are presented by blue triangles and red circles, respectively. The straight black line represents the linear regression