title: Is cardiac autonomic function associated with cardiorespiratory fitness and physical activity in children and adolescents? A systematic review of cross-sectional studies.
abstract: BACKGROUND
  Heart rate variability (HRV) is considered to explain improvements in cardiovascular health accrued by physical activity (PA) and cardiorespiratory fitness (CRF) over and above traditional cardiovascular risk factors.
  
  
  OBJECTIVE
  To systematically address associations between HRV, PA and CRF in children and adolescents.
  
  
  METHODS
  Medline, EMBASE, SportDISCUS and CINAHL Plus were searched on 5th September 2015 and updated on 4th August 2016.
  
  
  METHODS
  Observational studies comparing HRV in different groups of PA and CRF, and/or studies investigating associations between PA, CRF and HRV. Sports practices and PA intensities were also included. The square root of the mean of the sum of the squares of differences between adjacent RR intervals (RMSSD), the spectral density in the high (HF) and low (LF) frequency, and the LF/HF ratio were included. Risk of bias was assessed using the adapted Newcastle-Ottawa Scale (NOS).
  
  
  RESULTS
  Heterogeneity exists in the assessment of the exposures and outcomes, and sample characteristics. Risk of bias (NOS) was observed in most of the studies. Studies with low risk of bias showed positive associations between moderate-to-vigorous PA and RMSSD. The evidence for the associations between PA and frequency indices is weak. Similarly, the evidence for the association between CRF and HRV is weak.
  
  
  CONCLUSIONS
  Despite the heterogeneity in the studies, moderate-to-vigorous PA is positively associated with RMSSD, but less clear are the associations between CRF and HRV, as well as other PA intensities. Further research is needed to clarify the role of PA and CRF on HRV in children and adolescents.
authors.affiliation: Children's Health and Exercise Research Centre, Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK.
authors.forename: Ricardo Santos
authors.initials: RS
authors.lastname: Oliveira
authors.affiliation: Children's Health and Exercise Research Centre, Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK.
authors.forename: Alan Robert
authors.initials: AR
authors.lastname: Barker
authors.affiliation: Children's Health and Exercise Research Centre, Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK.
authors.forename: Kelly Michelle
authors.initials: KM
authors.lastname: Wilkinson
authors.affiliation: Evidence Synthesis and Modelling for Health Improvement (ESMI), University of Exeter Medical School, University of Exeter, Exeter, UK.
authors.forename: Rebecca Anne
authors.initials: RA
authors.lastname: Abbott
authors.affiliation: Children's Health and Exercise Research Centre, Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, Exeter, UK. Electronic address: C.A.Williams@exeter.ac.uk.
authors.forename: Craig Anthony
authors.initials: CA
authors.lastname: Williams
mesh_terms: D000293:Adolescent; D000367:Age Factors; D000072599:Cardiorespiratory Fitness / Q000502:physiology*; D002648:Child; D003430:Cross-Sectional Studies; D015444:Exercise / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D064887:Observational Studies as Topic / Q000379:methods; D010809:Physical Fitness / Q000502:physiology
keywords: Heart rate variability; Moderate-to-vigorous; Risk factor gap