J Syst Sci Complex (2013) 26: 104–116 HEART RATE VARIABILITY DURING HIGH-INTENSITY EXERCISE SARMIENTO Samuel · GARC ´ IA-MANSO Juan Manuel · MART ´ IN-GONZ ´ ALEZ Juan Manuel · VAAMONDE Diana · CALDER ´ ON Javier · DA SILVA-GRIGOLETTO Marzo Edir DOI: 10.1007/s11424-013-2287-y Received: 10 January 2012 c The Editorial Office of JSSC & Springer-Verlag Berlin Heidelberg 2013 Abstract The aim of this paper is to describe and analyse the behaviour of heart rate variability (HRV) during constant-load, high-intensity exercise using a time frequency analysis (Wavelet Transform). Eleven elite cyclists took part in the study (age: 18.6±3.0 years; VO2 max: 4.88±0.61 litres·min -1 ). Ini- tially, all subjects performed an incremental cycloergometer test to determine load power in a constant load-test (379.55±36.02 W; 89.0%). HRV declined dramatically from the start of testing (p<0.05). The behaviour of power spectral density within the LF band mirrored that of total energy, recording a significant decrease from the outset LF peaks fell rapidly thereafter, remaining stable until the end of the test. HF-VHF fell sharply in the first 20 to 30 seconds. The relative weighting (%) of HF-VHF was inverted with the onset of fatigue, [1.6% at the start, 7.1 (p<0.05) at the end of the first phase, and 43.1% (p<0.05) at the end of the test]. HF-VHF peak displayed three phases: a moderate initial increase, followed by a slight fall, thereafter increasing to the end of the test. The LF/HF-VHF ratio increased at the start, later falling progressively until the end of the first phase and remaining around minimal values until the end of the test. Key words Cycling, heart rate variability, wavelet. 1 Introduction The cardiovascular system at rest is mostly controlled by higher brain centres and cardio- vascular control areas in the brain through the sympathetic and parasympathetic branches of the autonomic nervous system (ANS). Efferent sympathetic and vagal activities directed to the sinus node are characterised by discharges largely synchronous with each cardiac cycle, which can be modulated by central and peripheral oscillators. These mechanisms generate rhythmic SARMIENTO Samuel · GARC ´ IA-MANSO Juan Manuel Department of Physical Education, University of Las Palmas de Gran Canaria, Spain. MART ´ IN-GONZ ´ ALEZ Juan Manuel Physics Department, University of Las Palmas de Gran Canaria, Spain. VAAMONDE Diana Department of Morphological Sciences, School of Medicine, University of C´ ordoba, Spain. CALDER ´ ON Javier Physical Activity Sciences Faculty, Polytechnic University of Madrid, Spain. DA SILVA-GRIGOLETTO Marzo Edir Andalusian Centre of Sports Medicine, C´ ordoba, Spain. Email: pit researcher@yahoo.es. ⋄ This paper was recommended for publication by Editors FENG Dexing and HAN Jing.