Comparison between passive drag and drag during leg kicking of the crawl stroke in top level swimmers

A body moving through water encounters a resistance called drag. Sixteen crawl swimmers (ten females and six males) and 11 breaststrokers (seven females and four males) were subjected to measurements of the force resisting towing in the prone position (Ft). Towing was performed in a 25-m swimming pool with a new device (Ben Hur, APLab, Rome) that measures Ft at a predetermined constant speed. Each crawl swimmer performed: I) six passive tows at constant speed (from 1.2 to 2.2 ms-1) in order to assess Ft1, which corresponds to passive drag; II) four tows at constant speed (from 1.6 to 2.2 ms-1) during leg kicking of the crawl stroke performed at maximum intensity for 8-12 s in order to assess Ft2. In breaststrokers, Ft1 was measured at the water surface and at a depth of 0.5 m. In eight swimmers (five females and three males), Ft1 was measured at the water surface twice to evaluate the reliability of the Ben Hur device. Ft1 was significantly higher in males than in females (p<0.01 at all speeds). In breaststrokers, Ft1 was significantly higher at the water surface than at a depth of 0.5 m (p<0.05 at 1.6 ms-1; p<0.001 above 1.8 ms-1). In the eight swimmers who performed passive towing at the water surface twice, there was no difference between the two Ft1 values. In male crawl swimmers, Ft2 was lower than Ft1 at all speeds (p<0.01), while in females Ft2 was lower than Ft1 up to 2.0 ms-1 (p<0.05). In two female swimmers, leg kicking during towing increased drag (Ft2 > Ft1). We conclude that Ben Hur is a simple and effective measurement device for the evaluation of the individual passive drag in a common swimming pool. The effect of leg kicking on drag is very different among swimmers of the same technical level, especially in females. The individual comparison between Ft2 and Ft1 provides information about the effectiveness of leg kicking at race speed.