Purpose Training zones are generally assessed by gas-exchange thresholds (GET). Several mathematical analyses of heart rate variability (HRV) are proposed for indirect GET determination. Our study aimed to investigate the accordance of the detrend fluctuation analysis (DFA a1) for determining GET with first (VT1) and second ventilatory (VT2) thresholds in well-trained subjects. Methods Eighteen female and 38 male sub-elite cyclists performed a maximal incremental cycling test of 2-min stage duration with continuous gas exchange and HR measurements. Power output (PO), Oxygen uptake (O2) and HR at VT1 and VT2 were compared with DFA a1 0.75 (HRVT1) and 0.50 (HRVT2). Agreements between PO, O2 and HR values were analyzed using Bland-Altman analysis. Results Large limits of agreement between VT1 and HRVT1 were observed for measures of O2 expressed in mL.min-1.kg-1 [- 21.3; + 14.1], HR [ 39.2; + 26.9] bpm and PO [- 118; + 83] watts. Indeed, agreements were also low between VT2 and HRVT2 for measures of O2 [- 26.7; + 4.3] mL.min-1.kg-1, HR [- 45.5; + 10.6] bpm and PO [- 157; + 35] watts. Our results also showed a sex effect: women obtained worst predictions based on DFA a1 than men for HR (p = 0.014), PO (p = 0.044) at VT1 and (p = 0.045), HR (p = 0.003) and PO (p = 0.004) at VT2. Conclusion There was unsatisfactory agreement between the GET and DFA a1 methods for VT1 and VT2 determination in both sex well-trained cyclists.
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