The aim of the present study was to assess whether the critical speed calculated by the slope of the distance–time relationship (Sd–t) represents the boundary between the heavy and severe intensity domains in swimming and would be sustainable during intermittent exercise. Nine competitive male swimmers (mean ± SD: aged 21.2 ± 2.6 yrs; peak of 3866 ± 529 mL min−1) performed, (a) four fixed-distance (100–200–400–800 m) all-out efforts to determine Sd–t and peak ; (b) three constant-speed efforts to exhaustion (TTE) at and 5% above and below Sd–t ( and , respectively); (c) a set of 10 × 400 m at Sd–t with 40-s recovery in between. Capillary blood lactate concentration ([La]B), oxygen uptake (), and RPE remained stable at (TTE = 48.9 ± 14.1 min) with end values of 3.8 ± 1.9 mmol L−1, 87 ± 14% peak , and 4.7 ± 1.3. TTE decreased at (8.6 ± 3.1 min), with end [La]B of 10.2 ± 1.9 mmol L−1. Peak was reached at exhaustion. Similarly, Sd–t could only be maintained for 24.3 ± 7.7 min with an increase in RPE and [La]B, reaching its peak (95 ± 5% peak ). RPE increased but [La]B remained stable throughout the ten 400 m blocks performed at Sd–t (overall time of 53.9 ± 2.7 min). The physiological responses when swimming 5% below and 5% above Sd–t are those characterising the heavy and severe intensity domain, respectively. While Sd–t lies within the severe intensity domain, intermittent swims at this intensity induce [La]B steady state alongside high rates of perceived exertion.
- critical speed
- 2 parameter model