Muscle Oxygen Saturation Breakpoints (SmO₂)

SmO₂-NIRS is an optical sensor that measures oxygen saturation in working muscle and records the moments when blood stops covering the needs of muscle mitochondria (BreakPoint 1 and 2).

• SmO₂-breakpoints (1) – the first and second NIRS breakpoints slightly underestimate the corresponding ventilation thresholds (-5 ± 9 W in the cycling test).

Heart Rate (bpm) and Detrended Fluctuation Analysis alpha 1 (DFA a1)

DFA α1 analysis HRV is an algorithm that monitors how the "randomness" of heart rate (RR intervals) changes with increasing workload. A special chest strap with RR interval recording and HRVlogger is used to measure DFA a1:

• α1 = 0.75 (2) – aerobic threshold (VT1/LT1): coincides with LT1 in most studies and is only 1–3 beats·min⁻¹ (or 2–5 W) below VT1.
• α1 = 0.50 (3) – anaerobic threshold (VT2/LT2): lies close to LT2 and is typically 3–6 beats·min⁻¹ / ≈5% VO₂max below VT2.

For training control, DFA a1 0.75/0.50 and SmO₂-breakpoints give almost the same zones as LT1/LT2 and VT1/VT2, with minimal error.

References

1. Feldmann A, Ammann L, Gächter F, Zibung M, Erlacher D. Muscle Oxygen Saturation Breakpoints Reflect Ventilatory Thresholds in Both Cycling and Running. J Hum Kinet. 2022 Sep 8;83:87–97. doi: 10.2478/hukin-2022-0054. PMID: 36157967; PMCID: PMC9465744.
2. Sempere-Ruiz N, Sarabia JM, Baladzhaeva S, Moya-Ramón M. Reliability and validity of a non-linear index of heart rate variability to determine intensity thresholds. Front Physiol. 2024 Feb 5;15:1329360. doi: 10.3389/fphys.2024.1329360. PMID: 38375458; PMCID: PMC10875128.
3. Sheoran S, Stavropoulos-Kalinoglou A, Simpson C, Ashby M, Webber E, Weaving D. Exercise intensity measurement using fractal analysis of heart rate variability: Reliability, agreement and influence of sex and cardiorespiratory fitness. Journal of Sports Sciences. 2024;42(21):2012–2020. https://doi.org/10.1080/02640414.2024.2421691

Ventilatory Thresholds (VT1 & VT2)

Ventilatory thresholds are determined from breath-by-breath gas-exchange during an incremental cardiopulmonary exercise test (CPET).

• VT1 (FeO₂) (1) – first ventilatory threshold: the workload at which expired O₂ fraction (FeO₂) and VE/VO₂ start to rise systematically while VE/VCO₂ and end-tidal CO₂ remain stable, indicating the transition from purely aerobic to mixed aerobic–anaerobic metabolism.
• VT2 (Ve) (1) – second ventilatory threshold (respiratory compensation point): the workload at which minute ventilation (VE) shows a clear second, non-linear increase relative to workload or VCO₂ because of respiratory compensation for metabolic acidosis.
• VT2_DVE (2) – VE-curve method: derived from the VE–time (or VE–workload) curve alone and defined as the workload where VE leaves its previous near-linear trend and enters the main "bend" of the curve—the onset of the sharp upswing in VE, rather than the exact mathematical intersection of the two surrounding slopes.
• VT2_CO₂ (3) – CO₂-based method: the workload where end-tidal CO₂ (PETCO₂) reaches a peak and then falls while VE/VCO₂ begins to rise, indicating the onset of respiratory compensation for metabolic acidosis.

References

1. Wasserman K, Whipp BJ, Koyal SN, Beaver WL. Anaerobic threshold and respiratory gas exchange during exercise. Journal of Applied Physiology. 1973;35(2):236–243.
2. Neder JA, Stein R. A simplified strategy for the estimation of the exercise ventilatory thresholds. Medicine and Science in Sports & Exercise. 2006;38(5):1007–1013.
3. Mezzani A. Cardiopulmonary Exercise Testing: Basics of Methodology and Measurements. Annals of the American Thoracic Society. 2017;14(Supplement_1):S3–S11.