An intensity-dependent slow component of HR interferes with accurate exercise implementation in postmenopausal women

Heart rate (HR) targets are commonly used to administer exercise intensity in sport and clinical practice. Yet, as exercise protracts, a time-dependent dissociation between HR and metabolism can lead to a mis-prescription of the intensity ingredient of the exercise dose. Purpose: we tested the hypothesis that a slow component of HR (i.e. scHR) occurs in all intensity domains, greater than the slow component of oxygen uptake (scV[Combining Dot Above]O2), and we developed an equation to predict it across exercise intensities. Method: 18 healthy, postmenopausal women (54 ± 4 years) performed on a cycle-ergometer: i) a ramp incremental test for thresholds and V[Combining Dot Above]O2max detection; ii) 30-min constant-work exercise at 40, 50, 60, 70, and 80 %V[Combining Dot Above]O2max for the measurement of scHR, scV[Combining Dot Above]O2, stroke volume (SV) and body temperature (T°). scHR and scV[Combining Dot Above]O2 were compared by two-way RM-ANOVA (intensity and variable); Pearson correlation was calculated between the slow component of all variables, relative intensity, and domain; scHR (b·min-2) was predicted with a linear model based on exercise intensity relative to the respiratory compensation point (RCP). Results: A positive scHR was present in all domains, twice the size of scV[Combining Dot Above]O2 (p < 0.001) and significantly correlated with the slow components of V[Combining Dot Above]O2 (r2 = 0.46), T° (r2 = 0.52) and with relative intensity (r2 = 0.66). A linear equation accurately predicts scHR based on %RCP (r2 = 0.66, SEE = 0.15). Discussion: A mismatch exists between the slow components of HR and metabolic intensity. Whenever exercise is prescribed based on HR, target values should be adjusted over time to grant that the desired metabolic stimulus is maintained throughout the exercise session.