engleski

Estimation of VO2 kinetics from graded exercise tests in endurance runners

Introduction The kinetics of oxygen uptake are determined by computation of the time constant, or ’τ’ (’tau’), from breath-by-breath VO2 data during square-wave work. In an incremental (ramp) test, presuming a linear increase of VO2 with increasing intensity (for the intensity range below the anerobic threshold), tau can be estimated from the time delay (τ=Δt) needed to reach the VO2 that corresponds to the steady-state VO2 (VO2ss) at the same intensity (Whipp et al., 1981). We hypothesized that in treadmill tests with graded increases of intensity (step tests, no steady-state stages with short duration) the same estimation of tau can be performed from: 1) a single step test, if VO2ss is known, or 2) two step tests with different speed increments. Methods 23 endurance runners completed 3 incremental treadmill tests to exhaustion with increase in running speed of: 1 km/h each min (T1), 1 km/h each ½ min (T05), and 2 km/h each 4 min (SS). Breathby- breath VO2 data were averaged to 30 s, and centered to the corresponding running speeds for T1 and T05. The VO2ss values at 8, 10 and 12 km/h were defined as the average VO2 values in the 4th minute of respective stages in the SS test, and were corrected to account for the VO2 difference between ramp and step protocols. The time constants (Δt) were determined from linear regression parameters derived from the VO2/running speed relationship in SS, T1 and T05. The analysis was performed within the aerobic speed range (8-12 km/h). The significance of differences between Δt were determined from 1) SS–T1, 2) SS–T05, and 3) T1–T05 test data (p<.05). Results and discussion Similar VO2 kinetics were found for Δt determined from SS–T1, SS–T05 and T1–T05 data (32.5±26.6 s, 34.0±45.4 s, and 31.1±18.8 s, respectively ; p>0.05), and were consistent within the analyzed speed range. The increase of intensity in T05 (1kmh/30s) is exactly twice the increase in T1 (1kmh/60s). Therefore, for a certain VO2, the Δt between T1 and T05 tests should be equal to Δt between T1 and SS, enabling estimation of τ and VO2ss. Indeed, the average error of VO2ss and Δt estimated from the time delay between T1 and T05 was practically irrelevant (0.1-0.15 ml/min/kg, or 2-3 s). The Δt values of endurance runners in this study proved to be very variable (many of them outside of the physiological range for τ), reflecting accumulated effects of high biological variability and technical errors for repeated measurements. Also, Δt values were significantly higher compared to tau values measured directly from square-wave tests in previous studies. This difference may be explained by the delay time needed for transportation of blood from working muscles to the lungs (~ 10-20 s). An alternative explanation is that incremental exercise, unlike constant (square-wave) exercise, has more complex and slower VO2 kinetics.

VO2 ; exercise test ; runners

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