Ski mountaineering is held at moderate altitude and harsh atmospheric conditions. Acute hypoxia increases sympathetic activity. However, this speciic autonomic modulation, combined with very-high-intensity exercise has not been investigated. We aimed to investigate the efects of a competitive micro-cycle on heart rate variability (HRV). Nine male (23.0 ± 1.5 years) and four female (23.5 ± 1.3 years) elite ski mountaineers participated in the Spanish Championships, composed by sprint, individual and vertical races. RR intervals were recorded every morning in supine (8 min) and orthostatic (6 min) positions before, during, and after the Championships. HRV time-domain, frequency-domain, and non-linear indices were calculated using 5-min windows. Total score of fatigue (TSF), session rate of perceived exertion (s-RPE), and individual racing performances were assessed. Lower supine mean RR in the vertical race evidenced HRV imbalance (p = 0.032), whereas decreased supine LF spectral power showed vagal tone recovery within a 24-h interval (p = 0.013). A parasympathetic rebound was also revealed by the mean RR 48 h after the vertical race (p = 0.017). Performance correlated positively with supine mean HR (p = 0.013), and inversely with the standing-supine diference of heart rate (HRor-su) (p = 0.005). TSF was related positively with overall performance (p = 0.015), and negatively with supine mean RR (p = 0.036). Intense exercise at moderate altitude exacerbates the physiological stress leading to parasympathetic withdrawal and increased sympathetic activity that can be restored after 24-48-h recovery. HRV analysis and perceptual measurements can be valuable in monitoring stress-recovery balance during multi-race events.Keywords Autonomic heart modulation · Ski mountaineering · Performance · Elite athletes · Altitude · Hypox
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