Bridging the gap between critical velocity and maximal lactate steady-state

Introduction
The ability to sustain a high metabolic rate for an extended period of time is essential for endurance performance and reflects the interplay between the aerobic and anaerobic metabolism [1]. The most frequently used parameters in that regard are the onset of blood lactate accumulation (OBLA), maximal lactate steady-state (MLSS) and critical velocity (CV). These concepts have been discussed extensively in recent reviews [2,3]. Despite the overwhelming general observation that CV overestimates MLSS, the origin of this discrepancy is still unknown [3]. This study examines the CV-MLSS difference and analyses whether physiological parameters may help to bridge the gap between concepts.

Methods
A total of n=19 well-trained runners/triathletes performed several lab tests to determine maximal lactate accumulation rate (ċLamax), maximal oxygen uptake (V̇O2max), fractional utilization (%V̇O2max), running economy (RE), maximal fat oxidation rate (MFO), OBLA and MLSS. Furthermore, time trials (TTs) over 1, 2 and 3 km were performed on an outdoor track to determine CV and D’ by the linear t(d) model [4]. Post-exercise lactate concentration was recorded immediately after the TTs as well as 3 and 6 min afterwards. Repeated measures ANOVA and Bland-Altman plots were used to analyse differences between OBLA, MLSS and CV. The absolute difference between CV and MLSS was correlated with several physiological parameters.

Results
TTs were performed in 174±10, 385±25 and 607±47 s, respectively. The corresponding maximal post-exercise lactate concentrations were 14.8±2.8, 13.8±2.9 and 12.3±2.28 mmol/L, respectively. OBLA (4.25±0.49 m/s), MLSS (4.15±0.49 m/s) and CV (4.66±0.42) differed significantly among each other. Limits of agreement for CV-MLSS ranged between +0.13 and +0.88 m/s, while the mean relative difference was 12.5±5.6%. CV and MLSS shared ~85% of their variance. The only physiological parameters that significantly correlated with the CV-MLSS difference were maximal post-exercise lactate concentrations following the 2 km (r=0.558, p=0.013) and 3 km (r=0.645, p=0.002) TTs. RE demonstrated a positive tendency (r=0.409, p=0.082).

Discussion and Conclusion
Our findings show that individuals with higher post-exercise lactate concentrations after the 2 and 3 km TTs demonstrate larger CV-MLSS differences. High concentrations might reflect the participants’ effort during the TTs which result in a higher estimation of CV. Previous research in soccer players found no significant difference between OBLA and MLSS and a lower mean relative CV-MLSS difference of ~10% [5]. This might be due to the applied TTs (1.5 and 3 km) and/or their lower performance levels in terms of MLSS (3.64±0.36 m/s). Future research should expand the selection of physiological parameters and perform such analyses for the relative CV-MLSS differences.

References
1) Joyner & Coyle (2008) J Physiol
2) Jones et al. (2019) Physiol Rep
3) Dotan (2022) Eur J Appl Physiol
4) Patoz et al. (2021) Eur J Appl Physiol
5) Dendai et al. (2005) J Strength Cond Res