When measured during incremental exercise tests, heart rate variability (HRV) indices from detrended fluctuation analysis (DFA), particularly the short-term scaling exponent α1, have been associated with cardiorespiratory fitness in cross-sectional settings. This study evaluates the ability of α1-based markers to track within-participant adaptations to endurance training.
Eighteen young healthy participants (9 females) performed incremental treadmill tests (3-min steps starting at 6.0 km/h with 2.0 or 1.5 km/h increases per step for males or females, respectively) before and after a twelve-week training programme (three sessions per week, either high-intensity interval or moderate-intensity continuous programme, depending on the participants). Electrocardiography signals were recorded at 500 Hz during each incremental test. RR intervals were extracted and analysed using DFA in a 2-min centred moving window, updated every 5 s. The temporal evolution of α1 (4-16 beats) was used to determine the time points at which α1 crossed 0.75 (HRV0.75) and 0.5 (HRV0.5), based on linear interpolation. These two times were compared pre- and post-training using paired t-tests, after checking normality. Hedge's g assessed effect size and statistical significance was assumed for p<0.05.
Data from three participants were excluded due to non-monotonic α1 dynamics or absence of threshold crossings. Following training, both HRV0.75 and HRV0.5 increased significantly (+65 [2, 129] s, g=0.54, p=0.044 and+70 [5, 135] s, g=0.56, p=0.038, respectively), indicating a delayed transition toward uncorrelated and anti-correlated heart rate dynamics during exercise.
These results support the use of DFA-derived HRV indices as non-invasive markers of training-induced improvements in cardiorespiratory fitness. It extends findings from the only other longitudinal study measuring HRV0.75 and HRV0.5, which was performed in a context of deconditioning induced by two weeks of bed rest in older healthy participants. This approach requires only an accurate measurement of interbeat intervals and could provide an alternative to gas exchange or lactate measurements.