The critical velocity in swimming

In supra-maximal exercise to exhaustion, the critical velocity (cv) is conventionally calculated from the slope of the distance (d) versus time (t) relationship: d = I + St. I is assumed to be the distance covered at the expense of the anaerobic capacity, S the speed maintained on the basis of the subject’s maximal O2 uptake $$(\dot{V}\hbox{O}_{\rm 2max}).$$ This approach is based on two assumptions: (1) the energy cost of locomotion per unit distance (C) is constant and (2) $$\dot{V}\hbox{O}_{2\rm{max}}$$ is attained at the onset of exercise. Here we show that cv and the anaerobic distance (d anaer) can be calculated also in swimming, where C increases with the velocity, provided that $$\dot{V}\hbox{O}_{2\rm{max}},$$ its on-response, and the C versus v relationship are known. d anaer and cv were calculated from published data on maximal swims for the four strokes over 45.7, 91.4 and 182.9 m, on 20 elite male swimmers (18.9 ± 0.9 years, 75.9 ± 6.4 kg), whose $${\dot{V}}\hbox{O}_{2\rm{max}}$$ and C versus speed relationship were determined, and compared to I and S obtained from the conventional approach. cv was lower than S (4, 16, 7 and 11% in butterfly, backstroke, breaststroke and front crawl) and I (=11.6 m on average in the four strokes) was lower than d anaer. The latter increased with the distance: average, for all strokes: 38.1, 60.6 and 81.3 m over 45.7, 91.4 and 182.9 m. It is concluded that the d versus t relationship should be utilised with some caution when evaluating performance in swimmers.