Characterising the slope of the distance–time relationship in swimming

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