Heart rate variability is related to training load variables in interval running exercises

Overload principle of training states that training load (TL) must be sufficient to threaten the homeostasis of cells, tissues, organs and/or body. However, there is no golden standard'' for TL measurement. The aim of the present study was to investigate if post-exercise heart rate variability (HRV) could be used to evaluate TL of interval running exercises with different intensities and durations. Thirteen endurance-trained men (35 +/- 5 years) performed MO250 [moderate intensity, 2 x 6 x 250 m/rec 30 s/5 min at 85% of the maximal velocity of the graded maximal test (V-max)], MO500 (2 x 3 x 500 m/rec 1 min/5 min at 85% V-max) and HI250 (high intensity, 2 x 6 x 250 m/rec 30 s/5 min at 105% V-max) interval exercises on a treadmill. HRV was analyzed during rest, exercise and immediate 15 min recovery. Fast recovery of LFP (P < 0.001), HFP (P < 0.01) and TP (P < 0.01) occurred during the first two recovery minutes after each exercise. Strong negative correlations (P < 0.01) were found between post-exercise HRV and perceived exertion as well as excess post-exercise oxygen consumption. Post-exercise HRV differentiated interval exercises of equal work, but varying intensity or distance of running bout. The results of the present study suggest that immediate post-exercise HRV may offer objective information on TL of interval exercises with different bout durations and intensities.