Introduction: The logic of the butterfly co-ordination for expert swimmers is associated with two undulations of legs for each complete arm cycle. Costill et al. (1992) and Maglischo (1993) showed that (1) the backward phase of the first undulation of legs was synchronised with the arm catch phase, and (2) the upward phase of the second undulation was synchronised with the arm pull phase. It seems nevertheless that this synchronisation depends on the increase of the velocity (Chollet & Boulesteix, 2001). The aim of this study was to evaluate the effects of the velocity increase on the arm and leg co-ordination but also to study this change of co-ordination through one tool of propulsion quantification during a swimming stroke. As Chollet et al. (2000) used the index of co-ordination to study the time gap between one arm and the other one based on injury times between the propulsive phases of every arm, we use the index of butterfly propulsion (IBP) to evaluate the time gap between the propulsive phases of the arms and legs for expert swimmers. Methods: The study on co-ordination for these 11 international swimmers was established with the data of 3 above and underwater video cameras (face and profile views) connected to a video - timer. The protocol used is the one described by Chollet et al. (1998). Each subject swam 12.5m at four different swimming velocities - 400m (V400), 200m (V200), 100m (V100) and 50m (V50). The appearance of the co-ordination in duration was studied together with the time gap analysis (T) separating the change of phase in the arm and leg movements. The index of butterfly propulsion (IBP) was calculated with a specific computer program based on the time gap (T) variations and the evolution of arm and leg phases, expressed in percentage of a total arm stroke. This index was calculated by addition of the relative values of propulsive times of the legs (PT1) and arms (PT2) during a complete swimming stroke. Results: The analysis of arm and leg co-ordination for the expert swimmers showed the steady space-time relations between arm and leg movements for T3 and T4. The mean values were 3.45 ± 2.90% and 2.18 ± 1.93% respectively. In contrast, the space-time relation for T1 and T2 evolved with the increase of velocity, ranging from 5.31 ± 2.52% (V400) to 3.65 ± 4.09% (V50) for T1 (the penetration of the hands in the water and the high break even point of the 1st undulation) from 15.18 ± 8.73% (V400) to 6.88 ± 4.34% (V50) for T2 (the beginning of the backward movements of the hands and the low break even point of the 1st undulation). With the decrease of these space-time relations, a significant increase of the index of butterfly propulsion was noted, ranging from 57.79 ± 6.34 % V400) to 66.22 ± 4.23 % (V50). Discussion: The increase of swimming velocity was accompanied by an increase in stroke rate and a decrease in stroke length (Keskinen & Komi, 1993) and also an increase of the drag (Kolmogorov & Duplisheva, 1992), with the progressive gap of the arm and leg co-ordination. The regular evolution of times T1 and T2 and the constancy of the others (T3 and T4) demonstrated the capacity of the experts to develop an accuracy aspect of their co-ordination. The outcome of this evolution is an increase of the butterfly propulsion index and a decrease of the propulsive time out-placed between the arm and leg propulsive actions. For the expert swimmer, the IBP at slow velocity (V400) showed that propulsion represented more than half of the total arm stroke and increased with velocity. The evolution of arm and leg co-ordination with the increase of velocity directly influenced the IBP, first in quantitative term, second in qualitative term: both propulsive phases, which allowed the calculation of the IBP, increase. Moreover, the space which separated them tends to decrease with the velocity. This study showed the capacity of the expert swimmer in butterfly to develop an accuracy aspect of their coordination to optimise their propulsion. The Index of Butterfly Propulsion is an evaluation tool calculated as a percentage of a complete cycle of arm. It may be used to compare populations of different skill levels in order to determine the technical level of a butterfly swimmer.
© Copyright 2002 Expertise in Elite sport. 2nd International Days of Sport Sciences, 12.-15. November 2002, INSEP, Paris (France). Veröffentlicht von INSEP. Alle Rechte vorbehalten.