Crossover Behavior And Crossover Scale In Detrended Fluctuation Analysis Of Heart Rate Variability During Moderate And Heavy Cycling Exercise

The short-term and long-term scaling exponents alpha1 (a1) and alpha2 (a2) of detrended fluctuation analysis (DFA) of heart rate (HR) variability (HRV) are typically used to denote exercise intensities with fixed crossover scales, although individual scaling of DFA might be present. PURPOSE: The study aims to compare fixed and individual crossover behavior (COB: negative if a1>a2; neutral if a1=a2; positive if a1<a2) and crossover scale (COSn: box size n) of DFA during continuous prolonged cycling bouts of moderate and heavy intensity. METHODS: 17 active male subjects (age: 26.1±4.3years, height: 181.7±6.9cm, weight: 79.3±10.6kg, maximum heart rate (HRmax): 182.3±9.6bpm, peak oxygen consumption (VO2peak): 49.9±7.3ml/kg/min) performed an incremental cycle test for threshold estimation based on blood lactate concentration (BLC; LT1: 2mmol/l; LT2: Baseline+1.5mmol/l) as well as two 20min cycling exercise bouts in randomized order at powers 5% below LT1 (Pmod) and at LT2 (Pheavy). Besides VO2, BLC and HR two experts visually assessed DFA-COB and DFA-COSn of HRV and calculated predetermined (COSn: 11; a1, a2) and individually determined (COSn: ind.; a1-ind, a2-ind) DFA scaling exponents. RESULTS: Pmod and Pheavy yielded substantial mean differences in HR (65.9±4.9 vs. 78.8±6.3%HRmax; d=1.95), VO2 (50.3±6.6 vs. 69.9±10.2%VO2peak; d=1.51), BLC (1.64±0.37 vs. 3.23±0.37mmol/l; d=3.23), a1 (1.02±0.27 vs. 0.85±0.22; d=-1.69) and a2 (0.87±0.18 vs. 0.97±0.17; d=1.32), respectively. In Pmod a negative COB was present for all 17 subjects with COSn of 27±6 (range: 19-39), while in Pheavy COB was negative for 4 subjects, neutral for 5 subjects and positive for 8 subjects with a COSn of 31±20 (range: 12-64). The individually determined COSn yielded a1-ind of 1.21±0.15 vs. 0.93±0.26 (d=-1.24) and a2-ind of 0.53±0.11 vs. 0.86±0.28 (d=1.30) for Pmod and Pheavy, respectively. CONCLUSIONS: Our data support the notion that COB and COSn of DFA might be subject dependent. In addition, scaling behavior seems to shift from a homogenous negative COB during moderate exercise to a more heterogenous and primarily neutral or positive COB during heavy exercise. Future studies should further elucidate the relevance of individual scaling behavior of DFA of HRV for exercise settings.