The Effects of Heart Rate Versus Speed-Based High-Intensity Interval Training on Heart Rate Variability in Young Females

Introduction: The aim of this study was to compare the effects of high-intensity interval training (HIT) prescription by heart rate (HR-based) and running speed (speed-based) methods on natural logarithm of the square root of the mean of the sum of the squares of differences between adjacent normal R-R intervals (Ln rMSSD) as a measure of heart rate variability (HRV) in young female student athletes. Methods: Seventeen female student athletes participated in this study and were divided into HR-based (n=9, age: 16.7 years) and speed-based (n=8, age: 16.9 years) HIT groups. 30-15 Intermittent Fitness Test was used for the speed-based group to detect the reference maximum speed (VIFT) for prescribing the HIT intensity accordingly. Age predicted maximal HR was used for the HR-based group as the reference value. All subjects performed similar training protocol for 5 weeks, except the method of individualizing HIT sessions (2 weekly sessions of HIT=3 sets of 3 minutes work interspersed with 3 minutes passive recovery with the 15-15 seconds format during each working set); either according to 90%-95% of maximal HR or VIFT. Results: HR- and speed-based HIT groups showed the most likely large improvements in Ln rMSSD of +7.9%, 90% confidence limits [CL] (5.9; 10.0); standardized change: +1.75 (1.32; 2.19) and +5.5%, (2.8; 8.3); +1.41 (0.72; 2.09), respectively. In between group analyses, HR-based HIT produced likely a small greater improvement in Ln rMSSD than speed-based HIT (+1.9%, [-5.0; 4.4]; +0.50 [-0.14; 1.14], chances for greater/similar/lower values of 79/17/4). Conclusion: It is concluded that both HIT prescription strategies were effective in Ln rMSSD elevation, but using maximal HR as a reference may elicit higher parasympathetic dominance with small effect in young female student athletes.