Correlation between two propulsion efficiency indices in front crawl swimming

In front crawl, stroke length (SL) and stroke rate (SR) are two important technical determinants of performance. The relationship between these parameters has been investigated by several authors and two propulsion efficiency indices have been proposed to assess improvements of the front crawl stroke effectiveness in swimmers. The stroke index (SI), proposed by Costill in 1985, relates the velocity (v) and the SL as shown in the following equation: SI=SL v; while the propelling efficiency of the arm stroke, proposed by Zamparo et al. in 2005, relates v, SR and the shoulder-to-hand distance of the arm (l) following the equation: ηp=((0.9 v)/2π SF l) 2/π. The aim of this study was to assess whether the two indices were correlated in two groups (M, males; F, females) of young expert front crawl swimmers, in order to utilise both of them as interchangeable tools when evaluating improvements in propulsion efficacy. The participants performed a 400-m trial, and analyses were carried out for the overall performance as well as for each 100-m section of the 400 m (S1, S2, S3, S4), underlining also a possible gender difference. All the correlations found were significant (p<0.01). The correlation coefficients were lower in M than in F for the overall 400-m trial (R=0.74; R=0.83, respectively) and for each 100-m section (M, R=0.76, R=0.73, R=0.67, R=0.71 for S1, S2, S3 and S4, respectively; F, R=0.84, R=0.84, R=0.79, R=0.83 for S1, S2, S3 and S4, respectively). Since the correlations are not constant throughout the trial, it seems that fatigue affects the two indices differently as well as males and females. Particularly, female swimmers kept more homogeneous stroke parameters, although their performances were lower than the male counterpart. In conclusion, both indices can be used as a device for monitoring the propulsion efficiency in front crawl in our groups of young swimmers, but further studies are needed to better explain how fatigue affects both SI and ηp, especially in the last phases of the performance.