In a previous article, data from 62 high level swimmers were analysed to provide an indication of how variables relating the amount of undulation were correlated with the fluctuations in centre of mass velocity and performance (Soons 2001 (10), Silva et al. 2003 (9), Soons et al. 2005 (12)). The findings indicated that for the sample as a whole, and for the women separately, the more undulation the smaller the centre of mass velocity fluctuations and the better the performance. In the five most undulating style variants (all women) of this sample, 27.7% less velocity fluctuation was found than in the five flattest (all men) (Colman et al. 1998 (2)). Nevertheless, as emphasised in another previous article, one must take into account that, although women are generally physically more suited to an undulating style and men to a flat style, this cannot be generalised (Colman et al. 2005 (3)). Earlier, a similar sample was divided into five style variants, based on as many as seven instants in the stroke cycle (related to body waving and trunk rotations and cambering). This grouping was precise but too time demanding for diagnosing. Moreover, some of the groups were too small for a statistical analysis (Persyn & Colman 1999 (5)). To allow a statistical analysis, in this article, the 62 high-level swimmers were divided into four style groups with an almost equal number of swimmers (about 16). This classification was based on two instants most typical for the amount of undulation (body waving and cambering). This classification can be obtained very quickly for a qualitative diagnosis. Based on specific movement variables and the centre of mass velocity fluctuation in breaststroke style groups, ship-like or marine animal-like propulsion concepts can be derived. Even in homogeneous style groups, a number of movement variables appear, in addition, to be performance relevant. In the flat style group, screw-like propulsion could be derived from extreme velocity increases during lateral sweeps of the feet and hands. Even in the flat style group, a deep kick (typical for undulation) correlates with performance, as well as a less backward arm pull and an earlier arm recovery. Because flat swimmers have 19% more velocity fluctuation than undulating swimmers, they lose more energy. However, thanks to high scores in specific flexibility in the lower limbs and strength in the upper limbs, they can generate a powerful kick and pull to attain a high velocity. In the undulating style group, most movement variables correlating with performance are typical for undulation. In this group propulsion concepts are applied similar to those used by marine animals, such as dolphins, eels, penguins, … During slow phases in the flat style (from the end of the leg kick to the beginning of the arm pull), the whole undulating body passes through a small drag section. A critical increase and decrease in velocity in the flat style (from the end of the pull to the beginning of the recovery) are smoothed in the undulating style, thanks to back-forward trunk rotations, causing consecutively additional drag and propulsion. After classifying any swimmer being diagnosed in his gender and in one of four style groups, every deviation in a performance relevant movement variable should be confronted with deviating velocity fluctuations. In a subsequent article, interactions between the water and the body will be visualised, using colours, and interactions between the body parts below and above the water surface will be emphasised.