Autonomic nervous system changes during 21 half-marathon in 21 days: a case report

Introduction. Heart Rate Variability (HRV) has recently been proposed to detect the Autonomic Nervous System (ANS) adaptations to endurance training and performance. However, the effect of training on HRV indexes remains inconclusive. To assess whether the morning short-term HR recording in clinostatic position may be useful in evaluating the training-induced autonomic changes, detecting fatigue and predict the possible overtraining. Methods. A 42 yrs old woman (height 162 cm; body mass 52 kg; fat mass 16,6%; VO2max 45,9 mlkg-1min-1), voluntarily perform 21 half-marathons in 21 days, one half-marathon each day. Every day before the half marathon, the subject performed RR series were recorded after awakening supine (10 min) positions. The HRV parameters considered were LogRMSSD, LogpNN50, LogHF, [0.15-0.40 Hz]. Frequencies: absolute powers, LF/HF ratio, and the non-linear index Sample Entropy (SampEn) and Detrended Fluctuations (1). In addition the velocity (kmh-1) of each half-marathon was reported. Results. ANS changed in relation to half-marathon performances. The mean velocity of all half-marathons was 11.00.5 kmh-1. A positively correlation between RMSSD (r=0.63), pNN50 (r=0.53), HF (r=0.57), SampEn (r=0.42) index and the number of half-marathon (p<0.05), while negative correlation in LF/HF ratio (r=-0.57) and DFA1 (r=-0.57) (p=ns), was found. In particular, HRV indexes of cardiac vagal tone comparing the 21th half-marathon with the 1st tended to increase (LogRMSSD: 2.18 vs 1.95, +11.8%; LogpNN50: 1.72 vs 1.65, + 4.2%; LogHF: 3.65 vs 3.20, +14.1%; SampEn: 1.60 vs 1.42, +12.7%). The cardiac sympathovagal balance tended to decrease (LF/HF ratio: 1.19 vs 5.74, -79.3%; 1: 0.92 vs 1.30, -29.3%). A positively correlation between RMSSD (r=0.45, p<0.05), pNN50 (r=0.58, p<0.05), HF (r=0.52, p<0.05), SampEn (r=0.45, p=0.057) index and the velocity during half-marathon, while negative correlations in LF/HF ratio (-0.57, p<0.05) and 1 (-0.57, p