title: Heart rate variability indexes as a marker of chronic adaptation in athletes: a systematic review.
abstract: BACKGROUND
  Regular exercise promotes functional and structural changes in the central and peripheral mechanisms of the cardiovascular system. Heart rate variability (HRV) measurement provides a sensitive indicator of the autonomic balance. However, because of the diversity of methods and variables used, the results are difficult to compare in the sports sciences. Since the protocol (supine, sitting, or standing position) and measure (time or frequency domain) are not well defined, the aim of this study is to investigate the HRV measures that better indicates the chronic adaptations of physical exercise in athletes.
  
  
  METHODS
  PubMed (MEDLINE), Web of Science, SciELO (Scientific Electronic Library), and Scopus databases were consulted. Original complete articles in English with short-term signals evaluating young and adult athletes, between 17 and 40 years old, with a control group, published up to 2013 were included.
  
  
  RESULTS
  Selected 19 of 1369 studies, for a total sample pool of 333 male and female athletes who practice different sports. The main protocols observed were the supine or standing positions in free or controlled breathing conditions. The main statistical results found in this study were the higher mean RR, standard deviation of RR intervals, and high frequency in athletes group. In addition, the analyses of Cohen's effect size showed that factors as modality of sport, protocol used and unit of measure selected could influence this expected results.
  
  
  CONCLUSIONS
  Our findings indicate that time domain measures are more consistent than frequency domain to describe the chronic cardiovascular autonomic adaptations in athletes.
authors.affiliation: Psychiatric Institute, Federal University of Rio de Janeiro (IPUB/UFRJ), Brazil; Department of Physical Education and Sports, , Centro de Instrução Almirante Alexandrino-Brazilian Navy, Rio de Janeiro, Brazil; Exercise Neuroscience Laboratory-LaNEx, Rio de Janeiro, Brazil.
authors.forename: Vanessa Pereira
authors.initials: VP
authors.lastname: da Silva
authors.forename: Natacha Alves
authors.initials: NA
authors.lastname: de Oliveira
authors.forename: Heitor
authors.initials: H
authors.lastname: Silveira
authors.forename: Roger Gomes Tavares
authors.initials: RG
authors.lastname: Mello
authors.forename: Andrea Camaz
authors.initials: AC
authors.lastname: Deslandes
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D056352:Athletes / Q000706:statistics & numerical data*; D004562:Electrocardiography; D015444:Exercise / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D011187:Posture; D055070:Resistance Training; D055815:Young Adult
keywords: autonomic nervous system; electrocardiogram; physical exercises; resistance training; sports