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 O(2) uptake (VO2max) This approach is based on two assumptions: (1) the energy cost of locomotion per unit distance (C) is constant and (2) VO2max 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 VO2max 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 VO2max 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.