Rapid Adjustments to Autonomic Control of Cardiac Rhythm at the Onset of Exercise in Healthy Young Adults

Abstract

Changes in sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) influences on cardiac rhythm during exercise can be reflected in time‐ and frequency‐based heart rate variability (HRV) analysis and differ depending on exercise intensity. However, the SNS and PNS contributions to cardiac rhythm at the onset of exercise has not been investigated due to the complexity of applying HRV analysis during short durations of exercise. Continuous wavelet transform (CWT) combines time‐ and frequency‐based analyses to allow autonomic influences on beat‐to‐beat fluctuations in R‐R intervals (RRI) to be captured in rapidly changing signals, such as at the onset of exercise. Therefore, the objective of this study was to characterize the rapid autonomic adjustments influencing cardiac rhythm at the onset of exercise in healthy young adults. This study was a secondary analysis of data previously collected from 14 healthy young adults (26 ± 2 years, 6 male/8 female) during two trials: a 5‐minute baseline followed by performance of isometric, right‐calf exercise at 70% maximal voluntary contraction (MVC) for 30 seconds (70% MVC) or continued rest (0% trial). Heart rate was continuously measured using a three‐lead electrocardiogram from which RRIs were derived. Analysis involved applying CWT to the RRI data using low‐frequency (LF; 0.04‐0.15 Hz) and high‐frequency (HF; 0.15‐0.4 Hz) bands for HRV analysis. Log‐transformed absolute (aLF, aHF) and normalized (nLF, nHF) LF and HF time‐frequency bands and their ratio (LF/HF) were calculated for each time period (one baseline time period – 0‐30 seconds, and six rest/exercise time periods – 300‐305, 305‐310, 310‐315, 315‐320, 320‐325, and 325‐330 seconds) in each trial (0% and 70% MVC). Group means and standard deviations for all five variables were calculated for each time period in each trial. Statistical analysis involved two‐way repeated measures analysis of variance with post hoc analysis involving paired t tests and a Holm‐Bonferroni correction with statistical significance set at P < 0.05. It was hypothesized that the SNS influence on cardiac rhythm, measured by LF and LF/HF, would increase while the PNS influence on cardiac rhythm, measured by HF, would decrease at the onset of exercise in healthy young adults. There was a significant trial‐time interaction for aHF, aLF, and LF/HF (P < 0.001, P = 0.003, and P = 0.027, respectively). aHF was lower during 5‐30 seconds (all P ≤ 0.025), aLF was lower during 20‐30 seconds (all P ≤ 0.011), and LF/HF was higher during 5‐20 seconds (all P ≤ 0.045) following the start of exercise in the 70% MVC trial compared to the 0% trial. There were significant effects of trial for aHF, nLF, nHF, and LF/HF (all P ≤ 0.014) and time for aHF and LF/HF (all P ≤ 0.011). These results indicate that the reduction of the PNS influence on cardiac rhythm begins sooner than the augmentation of the SNS influence on cardiac rhythm at the onset of exercise in healthy young adults.