Identifying time-varying dynamics of heart rate and oxygen uptake from single ramp incremental running tests.

OBJECTIVE: The fact that ramp incremental exercise yields quasi-linear responses for pulmonary oxygen uptake (V̇O2) and heart rate (HR) seems contradictory to the well-known non-linear behavior of underlying physiological processes. Prior research highlights this issue and demonstrates how a balancing of system gain and response time parameters causes linear V̇O2 responses during ramp tests. This study builds upon this knowledge and extracts the time-varying dynamics directly from HR and V̇O2 data of single ramp incremental running tests. APPROACH: A large-scale open access dataset of 735 ramp incremental running tests is analysed. The dynamics are obtained by means of 1st order ARX models with time-variant parameters. This allows for the estimates of time constant (τ) and steady state gain (SSG) to vary with work rate. MAIN RESULTS: As the work rate increases, τ-values increase on average from 38 to 132 seconds for HR, and from 27 to 35 seconds for V̇O2. Both increases are statistically significant (p < 0.01). Further, SSG-values decrease on average from 14 to 9 bpm/(km.h-1) for HR, and from 218 to 144 mL.min-1 for V̇O2 (p < 0.01 for decrease parameters of HR and V̇O2). The results of this modelling approach are line with literature reporting on cardiorespiratory dynamics obtained using standard procedures. SIGNIFICANCE: We show that time-variant modelling is able to determine the time-varying dynamics HR and V̇O2 responses to ramp incremental running directly from individual tests. The proposed method allows for gaining insights into the cardiorespiratory response characteristics when no repeated measurements are available.