Estimation of Runners' Lactate Threshold Heart Rate and Speed by Heart Rate Variability.

Accurate measurement of training intensity is crucial for optimizing endurance running performance. Lactate threshold heart rate and lactate threshold speed are commonly used but require invasive, costly methods. Heart rate variability metrics offer a non-invasive alternative by assessing autonomic nervous system activity. This study evaluated the validity of various heart rate variability metrics in predicting the lactate threshold heart rate and lactate threshold speed during indoor and outdoor graded exercise tests in 19 recreational runners. Heart rate variability thresholds were determined using SD1, SD2, detrended fluctuation analysis scaling exponent for short-term fluctuations, detrended fluctuation analysis scaling exponent for long-term fluctuations, standard deviation of NN intervals, and root mean square of successive RR intervals via the Dmax method, and are referred to as HRVT1 to HRVT6. Indoor graded exercise test results showed that heart rate at heart rate variability threshold often underestimated the lactate threshold heart rate, with all heart rate variability thresholds, except speed at heart rate variability threshold (detrended fluctuation analysis scaling exponent for long-term fluctuations, p=0.06), showing significant differences from lactate threshold. Moderate correlations were observed for HRVT5HR (standard deviation of NN intervals, r=0.66), HRVT2speed (SD2, r=0.56), and HRVT3speed (detrended fluctuation analysis scaling exponent for short-term fluctuations, r=0.70). Outdoor graded exercise tests showed no significant differences between most heart rate variability thresholds and lactate threshold, except HRVT6HR (root mean square of successive RR intervals, p=0.03). Speed at heart rate variability threshold demonstrated moderate correlations (r=0.54-0.7) with the lactate threshold speed. While the heart rate variability threshold and lactate threshold may reflect different physiological thresholds, heart rate variability thresholds, particularly those based on detrended fluctuation analysis scaling exponent for long-term fluctuations, showed promise as non-invasive predictors of lactate threshold in recreational runners.