Evaluating the sensitivity of heart rate variability fractal correlation properties to training load variations: Implications for monitoring training readiness and durability

This study examined the utility of HRV detrended fluctuation analysis alpha-1 (DFA1) to assess readiness-to-train and exercise durability under varying acute training loads. Nineteen trained cyclists completed two 20-minute time-trials (TT) under rested and fatigued conditions. DFA1 was measured during a standardized warm-up (WU), 20-min TT, and standardized cool-down (CD). Power output (PO) and DFA1 responses were compared across conditions, and associations with performance and fitness (W/kg) were examined. DFA1 values declined with increasing WU and CD exercise intensity (p<0.001) and were significantly attenuated following the 20-min TT (p<0.001). While DFA1 profiles did not differ significantly between rested and fatigued conditions, lower pre-TT DFA1 was associated with reduced TT performance (p=0.022; r=0.55), suggesting relevance to training readiness. Additionally, an 18% decline in DFA1 between 10- and 20-min during the TT (p=0.031), and lower post-TT values at matched intensities were observed (p<0.001), indicating physiological perturbation from the 20-min TT. Fitter participants exhibited lower DFA1 values during the 20-min TT (p<0.001; r=-0.77), suggesting a greater capacity to sustain physiological stress. While DFA1 is responsive to exercise intensity and stress, offering potential to assess training readiness and durability, more robust fatigue protocols are needed to validate DFA1 as training load monitoring tool.