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 (LT_HR) and Speed (LT_speed) are commonly used but require invasive, costly methods. Heart Rate Variability (HRV) metrics offer a non-invasive alternative by assessing autonomic nervous system activity. This study evaluated the validity of various HRV metrics in predicting LT_HR and LT_speed during indoor and outdoor graded exercise tests (GXTs) in 19 recreational runners. HRV thresholds (HRVTs) were determined using SD1, SD2, DFA-α1, DFA-α2, SDNN, and RMSSD via the Dmax method, and are referred to as HRVT₁ to HRVT₆. Indoor GXT results showed HRVT_HR often underestimated LT_HR, with all HRVTs, except HRVT_4speed (DFA-α2, p = 0.06), showing significant differences from LT. Moderate correlations were observed for HRVT_5HR (SDNN, r = 0.66), HRVT_2speed (SD2, r = 0.56), and HRVT_3speed (DFA-α1, r = 0.70). Outdoor GXTs showed no significant differences between most HRVTs and LT, except HRVT_6HR (RMSSD, p = 0.03). HRVT_speed demonstrated moderate correlations (r = 0.54-0.7) with LT_speed. While HRVT and LT may reflect different physiological thresholds, HRVTs, particularly those based on DFA-α2, show promise as non-invasive predictors of LT in recreational runners.