The role of high-intensity interval training (HIIT) on cardiac autonomic function in physically inactive males

Purpose: Cardiac autonomic dysfunction and physical inactivity are associated with increased risk of cardiovascular disease. Short duration high intensity interval training (HIIT) has been shown to improve aerobic capacity; however, adaptations of the cardiac autonomic system are less clear. Therefore, the aim of the present study was to assess cardiac autonomic adaptations to HIIT.

Methods: In a randomised crossover controlled trial, 40 physically inactive males (aged 21±1.7 years) completed 2-weeks of HIIT and control period. The HIIT protocol consisted of 3 x 30-second maximal cycle ergometer sprints against a resistance of 7.5% body weight, interspersed with 2-minutes of active recovery. In total, 6-sessions were performed. Cardiac autonomic function was measured using a plethysmographic device (Task Force Monitor). Total spectral density (PSD) and associated low-frequency (LF) and high-frequency (HF) power spectral components were recorded in absolute (ms2) and normalised units (nu).

Results: Each participant completed 18-maximal cycle ergometer sprints over a 2-week period. The HIIT intervention produced significant improvements in HR (62.2±8.6 to 57.7±8.3 b·min-1; p<0.001), HRV (R-R PSD ln) (3.53±0.27 to 3.67±0.26; p<0.005), R-R HF (ln) (2.96±0.37 to 3.05±0.33; p<0.005) and R-R LF/HF ratio (2.00±1.04 to 1.47±0.77; p<0.001) compared to the control period. The present study also produced statistically significant improvements in peak VO2 (ml·min-1) (3535.6±487.9 to 3744.6±581.7 ml·min-1; p<0.001), peak VO2 (ml·min-1·kg-1) (43.17±5.2 to 45.29±5.2 ml·min-1·kg-1; p<0.001), systolic blood pressure (116.1±4.9 to 111.3±8.8 mmHg; p<0.001) and rate pressure product (7385.6±1177.5 to 6387.8±908.7 mmHg; p<0.001) post HIIT compared to the control period.

Conclusion: A short-term programme of HIIT was associated with a significant increase in cardiac autonomic modulation, demonstrated by a residual increase in cardiac vagal activity. HIIT also produced significant improvements in functional capacity and reductions in systolic blood pressure.