Purpose: Effects of short-term high-intensity interval training (HIT) on cardiac autonomic function are unclear. The present study assessed cardiac autonomic adaptations to short-term HIT in comparison with aerobic endurance training (AET).
Methods: Twenty-six healthy middle-aged sedentary men were randomized into HIT (n=13, 4-6x30 s of all-out cycling efforts with 4-min recovery) and AET (n=13, 40-60 min at 60 % of peak workload) groups, performing six sessions within two weeks. The participants underwent a 24-h ECG recording before and after the intervention and, additionally, recorded R-R interval data in supine position (5 min) at home every morning during the intervention. Mean heart rate (HR) and low-(LF) and high-frequency (HF) power of R-R interval oscillation were analyzed from these recordings.
Results: Peak oxygen consumption (VO2peak) increased in both groups (p<0.001). Compared with AET (n=11), HIT (n=13) increased 24-h LF power (p=0.024), tended to increase 24-h HF power (p=0.068), and increased daytime HF power (p=0.038). In home-based measurements, supine HF power decreased on the days following the HIT (p=0.006, n=12), but not AET (p=0.80, n=9) session. The acute response of HF power to HIT session did not change during the intervention.
Conclusions: In conclusion, HIT was more effective short-term strategy to increase R-R interval variability than aerobic training, most probably by inducing larger increases in cardiac vagal activity. The acute autonomic responses to the single HIT session were not modified by short-term training.
(C) 2014 American College of Sports Medicine