The reliability of back-extrapolation in estimating in different swimming performances at the severe-intensity domain

The amount of anerobic energy released during exercise might modify the initial phase of oxygen recovery (fast-O_2debt) post-exercise. Therefore, the present study aimed to analyze the reliability of peak oxygen uptake $$ estimate by back-extrapolation $$ under different swimming conditions in the severe-intensity domain, verifying how the alterations of the $$ recovery profile and anerobic energy demand might affect $$ values. Twenty swimmers (16.7 ± 2.4 years, 173.5 ± 10.2 cm, and 66.4 ± 10.6 kg) performed an incremental intermittent step protocol (IIST: 6 × 250 plus 1 × 200 m, IIST_v200m) for the assessment of $$ . The $$ off-kinetics used a bi-exponential model to discriminate primary amplitude, time delay, and time constant (A_1off, TD_1off, and τ_off) for assessment of fast-O_2debt post IIST_v200m, 200-m single-trial (v200 m), and rest-to-work transition at 90% delta (v90%Δ) tests. The linear regression estimated $$ and the rate of $$ recovery (BE-slope) post each swimming performance. The ANOVA (Sidak as post hoc) compared $$ to the estimates of $$ in v200 m, IIST_v200 m, and v90%Δ, and the coefficient of dispersion (R²) analyzed the association between tests. The values of $$ during IIST did not differ from $$ in v200 m, IIST_v200 m, and v90%Δ (55.7 ± 7.1 vs. 53.7 ± 8.2 vs. 56.3 ± 8.2 vs. 54.1 ± 9.1 ml kg^-1 min^-1, p > 0.05, respectively). However, the $$ variance is moderately explained by $$ only in IIST_v200 m and v90%Δ (R_Adj ² = 0.44 and R_Adj ² = 0.43, p < 0.01). The TD_1off and τ_off responses post IIST_v200 m were considerably lower than those in both v200 m (6.1 ± 3.8 and 33.0 ± 9.5 s vs. 10.9 ± 3.5 and 47.7 ± 7.9 s; p < 0.05) and v90%Δ ( 10.1 ± 3.8 and 44.3 ± 6.3 s, p < 0.05). The BE-slope post IIST_v200m was faster than in v200 m and v90%Δ (-47.9 ± 14.6 vs. -33.0 ± 10.4 vs. -33.6 ± 13.8 ml kg^-1, p < 0.01), and the total anerobic (Anaer_Total) demand was lower in IIST_v200 m (37.4 ± 9.4 ml kg^-1) than in 200 m and 90%Δ (51.4 ± 9.4 and 46.2 ± 7.7 ml kg^-1, p < 0.01). Finally, the τ_1off was related to Anaer_Total in IIST_v200m, v200 m, and v90%Δ (r = 0.64, r = 0.61, and r = 0.64, p < 0.01). The initial phase of the $$ recovery profile provided different (although reliable) conditions for the estimate of $$ with BE procedures, which accounted for the moderate effect of anerobic release on $$ off-kinetics, but compromised exceptionally the $$ estimate in the 200-m single trial.