Resting and exercise-related heart rate responses to high intensity interval training in women: A pilot study

Purpose: We aimed to 1) explore associations between changes in supine and standing heart rate variability (HRV) and submaximal exercising HR (HRex) and post-exercise HR recovery (HRR) variables 1 day post-high intensity interval training (HIIT), and 2) determine how baseline characteristics (e.g., aerobic fitness, HRV, etc.) associate with HRV responses in women. Methods:After baseline assessment of maximal oxygen uptake (V̇ O_2max) and post-waking supine and standing HRV (5 days natural logarithm of the root-mean square of successive differences, LnRMSSD), healthy women (n = 10, age = 23.3 ± 3.4 years, height = 163.1 ± 8.9 cm, weight = 61.4 ± 9.7 kg) performed HIIT on a treadmill (7×3 min at 90% of the velocity attained at V̇ O_2max, interspersed with 2min recovery at 4 km^-h^-1). LnRMSSD was obtained the subsequent morning. Immediately before and 24 h post-HIIT, a submaximal exercise test was performed to obtain HRex at 60, 70, 80, and 90% of velocity attained at V̇ O_2max and post-test HRR. Changes (∆) from pre-to 1 day post-HIIT were calculated. Results:A large but non-significant relationship was observed between ∆LnRMSSD standing and ∆HRex at 60% (r=-0.55, p=0.10). All other associations between ∆LnRMSSD, ∆HRex and ∆HRR ranged from trivial– moderate (all p>0.05). V̇ O_2max, HR recovery at 2 min post-V̇ O_2max, baseline LnRMSSD (supine and standing), and standing LnRMSSD coefficient of variation were each associated with ∆LnRMSSD standing (all large, p<0.05). Conclusions: These findings suggest that ∆LnRMSSD does not predict ∆HRex in healthy women at moderate-high intensities. Women with lower and less stable baseline LnRMSSD may require a longer recovery duration from HIIT, impacting training intensity and frequency prescription considerations.