Time limit and V̇O₂ kinetics at maximal aerobic velocity: Continuous vs. intermittent swimming trials

The time sustained during exercise with oxygen uptake (V̇O₂) reaching maximal rates (V̇O_2peak) or near peak responses (i.e., above second ventilatory threshold [t@VT₂) or 90% V̇O_2peak (t@90%V̇O_2peak)] is recognized as the training pace required to enhance aerobic power and exercise tolerance in the severe domain (time-limit, t_Lim). This study compared physiological and performance indexes during continuous and intermittent trials at maximal aerobic velocity (MAV) to analyze each exercise schedule, supporting their roles in conditioning planning. Twenty-two well-trained swimmers completed a discontinuous incremental step-test for V̇O_2peak, VT₂, and MAV assessments. Two other tests were performed in randomized order, to compare continuous (CT) vs. intermittent trials (IT₁₀₀) at MAV until exhaustion, to determine peak oxygen uptake (Peak-V̇O₂) and V̇O₂ kinetics (V̇O₂K). Distance and time variables were registered to determine the t_Lim, t@VT₂, and t@90%V̇O_2peak tests. Blood lactate concentration ([La^-]) was analyzed, and rate of perceived exertion (RPE) was recorded. The tests were conducted using a breath-by-breath apparatus connected to a snorkel for pulmonary gas sampling, with pacing controlled by an underwater visual pacer. V̇O_2peak (55.2 ± 5.6 ml·kg·min^-1) was only reached in CT (100.7 ± 3.1 %V̇O_2peak). In addition, high V̇O₂ values were reached at IT₁₀₀ (96.4 ± 4.2 %V̇O_2peak). V̇O_2peak was highly correlated with Peak-V̇O₂ during CT (r = 0.95, p < 0.01) and IT₁₀₀ (r = 0.91, p < 0.01). Compared with CT, the IT₁₀₀ presented significantly higher values for t_Lim (1,013.6 ± 496.6 vs. 256.2 ± 60.3 s), distance (1,277.3 ± 638.1 vs. 315.9 ± 63.3 m), t@VT₂ (448.1 ± 211.1 vs. 144.1 ± 78.8 s), and t@90%V̇O_2peak (321.9 ± 208.7 vs. 127.5 ± 77.1 s). V̇O₂K time constants (IT₁₀₀: 25.9 ± 9.4 vs. CT: 26.5 ± 7.5 s) were correlated between tests (r = 0.76, p < 0.01). Between CT and IT₁₀₀, t_Lim were not related, and RPE (8.9 ± 0.9 vs. 9.4 ± 0.8) and [La^-] (7.8 ± 2.7 vs. 7.8 ± 2.8 mmol·l^-1) did not differ between tests. MAV is suitable for planning swimming intensities requiring V̇O_2peak rates, whatever the exercise schedule (continuous or intermittent). Therefore, the results suggest IT₁₀₀ as a preferable training schedule rather than the CT for aerobic capacity training since IT₁₀₀ presented a significantly higher t_Lim, t@VT₂, and t@90%V̇O_2peak (∼757, ∼304, and ∼194 s more, respectively), without differing regards to [La^-] and RPE. The V̇O₂K seemed not to influence t_Lim and times spent near V̇O_2peak in both workout modes.