Monitoring non-invasive indices of cardiac autonomic activity during acute post-exercise recovery

Abstract

Background: Consistent with the ‘reactivity hypothesis’, monitoring autonomic responses to exercise might be of value in clinical and high-performance settings. Post-exercise autonomic recovery may be non-invasively assessed using heart rate variability indices of cardiac parasympathetic neural activity (cPNA-HRV) and systolic time intervals reflecting cardiac sympathetic neural activity (cSNA-STI). However, the dose of exercise may confound autonomic recovery, independent of underlying pathology or function. This thesis investigated how these indices respond to variations of the factors constituting an acute exercise dose, i.e. intensity, duration, and modality. Methods: Following a literature review, three studies investigated the effect of exercise dose on post-exercise cPNA-HRV and cSNA-STI. Firstly, participants performed three 8-min bouts of different intensities (below, between, and above the ventilation thresholds). Secondly, two durations (8-min vs. 32-min) of moderate-intensity exercise were performed. Finally, participants performed two 8-min bouts of different modes (arm-cranking vs. leg cycling) of heart rate-matched submaximal exercise. Results: Prior research on cPNA-HRV responses to exercise was methodologically diverse, while cSNA-STI responses had rarely been examined. The recovery profiles of cPNA-HRV and cSNA-STI were slower following progressively higher intensity exercise. Longer duration exercise delayed the recovery of cPNA-HRV, but not cSNA-STI. Arm-cranking elicited a slower recovery of cSNA-STI compared with heart rate-matched leg cycling, while cPNA-HRV recovery was similar between modes. Conclusion: The intensity, duration, and modality of exercise can each independently influence the recovery of cPNA-HRV and cSNA-STI. Thus, the exercise dose needs to be considered when interpreting these indices. Acute post-exercise recovery may be a valuable period during which to concurrently monitor autonomic reactivity using cPNA-HRV and cSNA-STI.