Physiological associations with heart rate-speed decoupling during a half-marathon in adolescent endurance runners

From a process-oriented perspective of endurance durability, this exploratory study aimed to characterize the dynamic evolution of heart rate-running speed decoupling during a simulated half-marathon in adolescent endurance athletes and to examine phase-specific associations between decoupling characteristics and physiological regulation.

Thirteen adolescent endurance runners with systematic endurance training completed a 21.1-km simulated half-marathon under self-paced conditions. Heart rate and running speed were recorded continuously and aggregated into consecutive 2-km distance segments. Internal and external loads were normalized to individual maximal heart rate and running speed at VO_2peak, respectively, and used to derive a relative heart rate-running speed decoupling index (DM_i). Decoupling characteristics included the distance of decoupling onset (Onset), maximal decoupling magnitude (DM_max), and mean decoupling during the late phase of exercise (15-20 km). Core body temperature, interstitial glucose concentration, and sweat-derived estimates of fluid and sodium loss were monitored concurrently. Associations were examined using Spearman correlation analysis.

Heart rate-running speed decoupling increased progressively throughout the run and showed substantial inter-individual variability. The median decoupling onset occurred at 10.0 km (interquartile range: 7.0-12.0 km). An earlier onset was associated with greater concurrent increases in core body temperature (ρ = -0.579, p = 0.038) and sodium loss per unit distance (ρ = -0.605, p = 0.037). The maximal decoupling magnitude (DM_max) was positively correlated with changes in core body temperature (ρ = 0.632, p = 0.021) and sodium loss per unit distance (ρ = 0.643, p = 0.024). In contrast, the change in decoupling during the final stage of the run (15-20 km) was not associated with thermoregulatory or fluid-electrolyte indices but was positively correlated with changes in glucose concentration (ρ = 0.599, p = 0.031).

During a simulated half-marathon, heart rate-running speed decoupling evolved progressively in adolescent endurance athletes and demonstrated marked inter-individual variability. Decoupling onset and maximal magnitude were primarily associated with thermoregulatory and electrolyte-related physiological loads during the early to middle stages of exercise, whereas decoupling dynamics in the latter stage were more closely aligned with physiological responses related to energy supply regulation. These findings support heart rate-running speed decoupling as a durability-related indicator with phase-specific physiological characteristics during prolonged endurance exercise.