Characterisation, asymmetry and reproducibility of on- and off-transient pulmonary oxygen uptake kinetics in endurance-trained runners

The purpose of this study was three-fold: (1) to characterise both the on- and off-transient oxygen uptake (V̇O₂) kinetics in endurance runners during moderate-intensity treadmill running; (2) to determine the degree of symmetry between on- and off-transients; and (3) to determine the reproducibility of V̇O₂ kinetic parameters in endurance runners. Twelve endurance-trained runners [mean (SD) age 25.2 (4.7) years, body mass 70.1 (9.7) kg, height 179.5 (7.5) cm, ventilatory threshold (V_T), 3,429 (389) ml.min⁻¹, maximal V̇O₂ (V̇O_2max) 4,138 (625) ml.min⁻¹] performed two multiple square-wave transition protocols on separate days. The protocol consisted of six (three transitions, 15 min rest, three transitions) square-wave transitions from walking at 4 km.h⁻¹ to running at a speed equivalent to 80% of the V̇O₂ at the V_T (80%V_T). To determine the reproducibility, the protocol was repeated on a separate day (i.e. a test-retest design). Pulmonary gas-exchange was measured breath-by-breath. The V̇O₂ data were modelled [from 20 s post-onset (or offset) of exercise] using non-linear least squares regression by a mono-exponential model, incorporating a time delay. The on- and off-transient time constants (τ_on and τ_off), mean response times (MRT_on and MRT_off) and amplitudes (A_on and A_off) were obtained from the model fit. On- and off transient kinetics were compared using paired t-tests. The reproducibility of each kinetic parameter was explored using statistical (paired t-tests) and non-statistical techniques [95% limits of agreement (LOA, including measurement error and systematic bias) and coefficient of variation (CV)]. It was found that the τ_on [12.4 (1.9)] was significantly (P<0.001) shorter than τ_off [24.5 (2.3) s]. Similarly, MRT_on [27.1 (1.9) s] was shorter than MRT_off [33.4 (2.2) s]. With respect to the reproducibility of the parameters, paired t-tests did not reveal significant differences between test 1 and test 2 for any on- or off-transient V̇O₂ kinetic parameter (P>0.05). The LOA for τ_on (1.9 s), τ_off (2.3 s), MRT_on (1.2 s), MRT_off (3.2 s), A_on (204 ml.min⁻¹) and A_off (198 ml.min⁻¹) were narrow and acceptable. Furthermore, the measurement error (range, 4.3 to 15.1%) and CV (1.3 to 4.8%) all indicated good reproducibility. There was a tendency for τ_off to be more reproducible than τ_on. However, MRT_on was the most reproducible kinetic parameter. Overall, the results suggest that: (1) a multiple square-wave transition protocol can be used to characterise, reproducibly, both on- and off-transient V̇O₂ kinetic parameters during treadmill running in runners; (2) the phase II time constant is independent of V̇O_{2 max}, and (3) asymmetry exists between on- and off transient V̇O₂ kinetic parameters.