title: Effect of aerobic training on orthostatic tolerance, circulatory response, and heart rate dynamics.
abstract: BACKGROUND
  This study was performed to investigate the effects of aerobic training on orthostatic tolerance and to quantify the post-training changes in cardiovascular response and heart rate variability (HRV).
  
  
  METHODS
  Tolerance and circulatory responses to two types of lower body negative pressure (LBNP) were examined and compared in a group of healthy male students before and after 6 mo of aerobic training, and the results were further compared with a group of athletes (runners). Changes in HRV associated with training were analyzed by conventional and time-varying autoregressive spectral analysis, as well as by approximate entropy measurement (ApEn)--a statistic quantifying heart rate "complexity" derived from non-linear dynamics.
  
  
  RESULTS
  After aerobic training, there was an initial transient hypotension during the supine -50 mmHg LBNP testing and a significant decrease in tolerance to upright graded LBNP in most of the student-subjects. Moreover, after training, there was a significant decrease in ApEn value of the HRV time series during both supine control and LBNP testing, and the rate of cardiac vagal withdrawal and sympathetic activation during the onset of LBNP was faster than that before training.
  
  
  CONCLUSIONS
  The present study has provided further evidence that certain types of aerobic training may affect orthostatic tolerance and may be associated with a loss of complexity of HRV during supine resting and orthostatic stress.
authors.affiliation: Department of Aerospace Physiology, The Fourth Military Medical University, Xi'an, P.R. China. Zhanglf@fmmu.edu.cn
authors.forename: L F
authors.initials: LF
authors.lastname: Zhang
authors.forename: J
authors.initials: J
authors.lastname: Zheng
authors.forename: S Y
authors.initials: SY
authors.lastname: Wang
authors.forename: Z Y
authors.initials: ZY
authors.lastname: Zhang
authors.forename: C
authors.initials: C
authors.lastname: Liu
mesh_terms: D000222:Adaptation, Physiological; D000293:Adolescent; D000328:Adult; D000337:Aerospace Medicine; D001775:Blood Circulation; D005081:Exercise Therapy / Q000379:methods*; D006339:Heart Rate; D006801:Humans; D007024:Hypotension, Orthostatic / Q000209:etiology / Q000503:physiopathology* / Q000517:prevention & control*; D008150:Lower Body Negative Pressure / Q000009:adverse effects*; D008297:Male; D008889:Military Personnel; D008991:Monitoring, Physiologic; D017711:Nonlinear Dynamics; D010807:Physical Endurance; D012044:Regression Analysis; D012420:Running; D012815:Signal Processing, Computer-Assisted; D013334:Students; D016683:Supine Position