Training zone demarcation is often based on physiological thresholds of blood lactate concentration or gas exchange under monitored laboratory or field conditions (Skinner and Mclellan
1980; Mann et al.
2013). Apart from the complexity and costs of these procedures, it is recognized that depending on the lactate threshold approach used, testing protocol, type of exercise, or the expertise of the gas exchange test interpreter (Yeh et al.
1983; Shimizu et al.
1991; Meyer et al.
1996), dissimilar threshold calculations can result in differing training recommendations (Jamnick et al.
2018). The need for objective, non-invasive and low-cost markers for threshold demarcation is apparent. Heart rate (HR) variability (HRV) measures have been tested for years as a potential monitoring tool, although mostly during resting conditions and with focus on vagal-related HRV indices (Buchheit
2014; Michael et al.
2017). Recently, studies have endeavored to evaluate the accuracy of HRV correlation properties, in particular, the short-term scaling exponent alpha 1 of Detrended Fluctuation Analysis (DFA-alpha1) for delineation of physiological thresholds during endurance exercise (Gronwald et al.
2020b,
2021; Rogers et al.
2021a). This index represents the fractal, self-similar nature of cardiac beat-to-beat intervals. For DFA-alpha1 calculation, the root mean square fluctuation of the integrated and detrended data is measured in observation windows of different sizes. This is done using a logarithmic plot of the data against the size of the window. The resulting slope of the line relating the (log) fluctuation to the (log) window size represents the scaling exponent (Mendonca et al.
2010). At low exercise intensity, DFA-alpha1 values usually are near 1.0 or slightly above, signifying a well correlated, fractal pattern (Gronwald et al.
2020b). As intensity rises, the index will drop past 0.75 near the aerobic threshold (AT) then approach uncorrelated, random patterns represented by values near 0.50 and below at higher work rates (Rogers et al.
2021a,
b). These observations are attributable to changes in autonomic nervous system balance, namely the withdrawal of the parasympathetic and enhancement of the sympathetic branch as well as the influence of other non-neural factors during endurance exercise (Persson
1996; White and Raven
2014; Qu et al.
2014; Gronwald et al.
2020b). Moreover, DFA-alpha1 appears to be an index that reflects the overall systemic state of internal load (Gronwald and Hoos
2020; Rogers et al.
2021d). In contrast to other typical HRV indexes such as time domain SDNN (total variability as the standard deviation of all normal RR-intervals) which reaches a nadir at the first ventilatory and lactate thresholds (staying suppressed afterward) (Tulppo et al.
1996; Karapetian et al.
2008), DFA-alpha1 possesses a wide dynamic range spanning all exercise intensity domains (Gronwald et al.
2020b). Furthermore, its dimensionless nature allows application independent of an individual’s fitness status and without the need for prior normalization to blood lactate concentration or gas exchange kinetics (Gronwald et al.
2020b; Rogers et al.
2021d,
e). Recent investigations showed agreement in the intensities reached at DFA-alpha1 derived thresholds marked as 0.75 (HRVT1) and 0.50 (HRVT2) with the first (VT1) and second ventilatory threshold (VT2) during an incremental treadmill protocol in recreational male runners (Rogers et al.
2021a,
b). However, since an electrocardiogram (ECG) was used for RR interval data recording with artifact level lower than 3%, it remained unclear whether the same results can also be confirmed in the general application with commercial chest straps and divergent error presence. A follow-up study examining the effects of missed beat artifact on both DFA-alpha1 as well as the HTVT1 showed minimal effects at levels below 5% using the Kubios “automatic” correction method. In addition, there was a small but significant degree of bias of the HRVT1 (4 bpm) between ECG and the Polar H7 (Rogers et al.
2021c). In addition, the HRVT1 was shown to strongly agree with that of the VT1 in a male cardiac disease population (Rogers et al.
2021e). What remains unclear is the question of whether the HRVT1 and HRVT2 thresholds agree with that of the VT1 and VT2 in a group of female participants. If the use of the fixed 0.75 and 0.50 DFA-alpha1 values for threshold identification is confirmed in that demographic, widespread application for intensity distribution monitoring could result. In addition to benefits for routine sports laboratory work, this metric could provide real-time information about absolute exercise intensity based on systemic internal load without prior lactate or gas exchange testing with only HRV monitoring. However, a generalization of the applicability, especially due to the few trials for large parts of the population as well as exercise protocols requires further investigation. There is a particular need for data on women as it has been demonstrated that there are significant gender differences in HRV-related cardiac stress responses and these may also be reflected in non-linear metrics (Adjei et al.
2018). The reasons behind this disparity could be attributed to hormonal, neuroanatomical and cognitive differences in the sexes with age being an essential modulator (Ramaekers
1998; Balhara et al.
2012). Since the gender difference in cardiac autonomic function narrows between the age of 40 and 50 with women becoming menopausal during this age range, the female hormone estrogen is suspected of having a significant influence which can furthermore be observed during the menstrual cycle (Ramaekers
1998; Yildirir et al.
2001). Therefore, this study aims to extend the validity of DFA-alpha1-based intensity thresholds to female participants tested in a cycling ramp protocol.