Comparison of modeled lactate threshold with maximal metabolic steady state in running and cycling

Introduction
As endurance performance depends on maximal oxygen uptake (V̇O2peak), fractional utiliza-tion of V̇O2peak at lactate threshold 2 (LT2%), and oxygen cost of movement (C), work rate at LT2 could be accurately modeled using these physiological parameters in running (Støa et al., 2020) and cycling (Støren et al., 2014). Although agreement of LT2 and maximal metabolic steady state (MMSS) has been extensively investigated, evidence of agreement between modeled LT2 and MMSS is still lacking. Therefore, this study compared the accu-racy of modeling LT2 with MMSS in running and cycling using different C determination methods.
Methods
37 endurance trained athletes (26.6 ± 6.8 yrs, 55.6 ± 5.7 mL∙kg-1∙min-1) completed an in-cremental step test to exhaustion on a treadmill (2.4 + 0.4 m·s−1 every 5 min, 30 s rest; n = 16) or cycling ergometer (individual start + 20 W every 3 min; n = 21). V̇O2peak, LT2%, and C were assessed, the latter at lactate threshold 1 (LT1), 80% of V̇O2peak, and LT2 to esti-mate the influence of calculation methods on LT2 modeling. Two to five 30 min constant work rate tests (± 0.1 m·s−1 or ± 10 W) were performed to determine MMSS (i.e., rise in blood lactate level by ≤ 1 mmol∙L-1 between the 10th and 30th min). Modeled LT2 values were checked for agreement with MMSS by Bland-Altman analysis and intraclass correla-tion coefficient (ICC).
Results
Mean differences (± limits of agreement [LoA]) between modeled LT2 and running speed or cycling power at MMSS were -2.8 ± 15.6% and -5.6 ± 14.9% for C at LT1, -1.1 ± 18.0% and -1.5 ± 14.8% for C at 80% V̇O2peak, and -1.5 ± 13.7% and -0.5 ± 12.9% for C at LT2, respectively. Overall C at LT1 and at 80% V̇O2peak showed moderate (ICC ≥ 0.646 and ICC ≥ 0.609) and C at LT2 showed good agreement with MMSS (ICC ≥ 0.760).
Discussion
Modeling LT2 as a function of V̇O2peak, LT2%, and C underestimated MMSS by ≤ 5.6% with at least moderate agreement. Calculating C at LT2 yielded the most accurate MMSS esti-mate. Due to the accuracy of the model and the stability of LT2%, regular measurement of V̇O2peak and C after initial assessment of all three parameters could be a more time-efficient way to evaluate training achievements in terms of MMSS in runners and cyclists.
References
Støa et al. (2020). Factors Influencing Running Velocity at Lactate Threshold in Male and Female Runners at Different Levels of Performance. Front Physiol, 11, 585267.
Støren, Ø., et al. (2014). A Time-Saving Method to Assess Power Output at Lactate Threshold in Well-Trained and Elite Cyclists. J Strength Cond Res, 28(3), 622–629.