Energy cost of swimming of elite long-distance swimmers

The aim of this study was: (1) to assess the energy cost of swimming (C_s, kJ km⁻¹) in a group of male (n=5) and female (n=5) elite swimmers specialised in long-distance competitions; (2) to evaluate the possible effect of a 2-km trial on the absolute value of C_s. C_s was assessed during three consecutive 400-m trials covered in a 50-m pool at increasing speeds (v₁, v₂, v₃). After these experiments the subjects swam a 2-km trial at the 10-km race speed (v_2km) after which the three 400-m trials were repeated at the same speed as before (v₅=v₁, v₆=v₂, v₇=v₃). C_s was calculated by dividing the net oxygen uptake at steady state \(({\dot{V}}{\text{O}}_{2{\text{ss}}})\) by the corresponding average speed (v, m s⁻¹). \({\dot{V}}{\text{O}}_{2{\text{ss}}}\) was estimated by using back extrapolation technique from breath-to-breath \({\dot{V}}{\text{O}}_2\) recorded during the first 30 s of recovery after each test. C_s increased (from 0.69 kJ m⁻¹ to 1.27 kJ m⁻¹) as a function of v (from 1.29 m s⁻¹ to 1.50 m s⁻¹), its values being comparable to those measured in elite short distance swimmers at similar speeds. In both groups of subjects the speed maintained during the 2-km trial (v_2km) was on the average only 1.2% faster than of v₂ and v₆ (P>0.05), whereas C_s assessed at the end of the 2-km trial (v_2km) turned out to be 21±26% larger than that assessed at v₂ and v₆ (P<0.05); the average stroke frequency (SF, cycles min⁻¹) during the 2-km trial turned to be about 6% (P<0.05) faster than that assessed at v₂ and v₆. At v₅, C_s turned out to be 19±9% (P<0.05) and 22±27% (0.1<P=0.05) larger than at v₁ in male and female subjects (respectively). SF was significantly faster (P<0.05, in male subjects) and the distance per stroke (Ds=v/SF) significantly shorter (P<0.05) in female subjects at v₅ and v₆ than at v₁ and v₂. These data suggest that the increase of C_s found after the 2-km trial was likely related to a decrease in propelling efficiency, since the latter is related to the distance per stroke.