Fractal scaling properties of heart rate dynamics following resistance exercise training

With aging and disease, there is a breakdown of the natural fractal-like organization of heart rate (HR). Fractal-like correlation properties of HR can be assessed with detrended fluctuation analysis (DFA). A short-time scaling exponent (α_s) value of 1 is associated with healthy HR dynamics, whereas values that deviate away from 1, in either direction, indicate fractal collapse. The purpose of this study was to examine the effect of resistance exercise training (RT) on fractal correlation properties of HR dynamics. Resting ECG was collected at baseline, following a 4-wk time control period and 6 wk of RT (3 days per wk) in 34 men (23 ± 1 years of age). Fractal properties of HR were assessed with DFA. There was no change in α₂ following either the time control period or RT (1.01 ± 0.06 to 0.98 ± 0.06 to 0.93 ± 0.04, P > 0.05). Given the potential bidirectional nature of fractal collapse, subjects were retrospectively separated into two groups (higher α_s and lower α_s) on the basis of the initial α_s by using cluster analysis. An interaction was detected for α_s following RT (P < 0.05). There was no change in α_s in either group following the time control, but α_s increased following RT in the lower α_s group (n = 18; 0.73 ± 0.04 to 0.69 ± 0.04 to 0.88 ± 0.04) and α_s decreased following RT in the higher α_s group (n = 16; 1.20 ± 0.04 to 1.24 ± 0.04 to 0.98 ± 0.04). In conclusion, RT improves fractal properties of HR dynamics.