A Personalized Low Intensity Exercise Prescription based on an Index of Non-Linear Heart Rate Variability: A Case Report
The potential of an index of cardiac interbeat fractal complexity (DFA a1) to demarcate low intensity training was undertaken in a recreational athlete. The influence of absolute heart rate elevation versus work rate as factors responsible for loss of interbeat complexity was also examined via the usage of beta adrenergic blockade. Incremental cycling ramps were performed with and without the beta adrenergic blocking agent Atenolol 25 mg measuring DFA a1 during the last 2 minutes of each stage. No difference was seen between control and Atenolol trials for lactate thresholds, ventilation rates, rectus femoris muscle O2 desaturation and DFA a1 despite a 15 to 20 beat decrease in heart rate across all stages in the Atenolol trial. In both studies, DFA a1 progressively declined with cycling power reaching a value consistent with white noise at 25 Watts above the first ventilatory threshold. In conclusion, DFA a1 transition to an uncorrelated low complexity state occurred just above the VT1. In addition, the complexity index was related to cycling power, ventilation and presumably VO2 rather than the absolute heart rate. Longer constant power intervals near VT1 did not show additional or progressive complexity loss. DFA a1 may be a promising guide for low intensity training zone demarcation.