The maximum speed of swim depends on the equilibrium between the drag and the propulsive energy. The drag is directly related to the body frontal area contrasting the forward motion and represents a limiting factor of elite performances (Zamparo et al., 2012). It grows exponentially with increasing velocity (Toussaint and Hollander, 1994) and can be affected by several factors, some of which have never been considered until recently (i.e. the characteristics of the swim cup) (Gatta et al., 2013). One of the most effective strategy to reduce drag is maintaining the body position as much hydrodynamic as possible during the propulsive action (Clarys, 1979).
During swimming, also non-propulsive forces are generated. Those forces produce a series of rotations around the reference spatial axes (Yanai, 2003) that are substantially ineffective to the forward motion of the swimmer and are mainly due to the feature of the technique components, included breathing frequency (Seifert et al., 2004), and to the fatigued condition of the swimmer (Aujouannet et al., 2006). These rotations often affect the swimmer's attitude, generating a non-optimal hydrodynamic position due to an increase of the body frontal area and thus of the active drag (Maglischo, 2003). The body movements ineffective to the forward motion lead to an energy wastage that is related to the amplitude of the oscillations around the ideal trajectory, with predictable negative consequences on performance. Therefore, one can hypothesize that the lower the technical skill, the more the swimmers are expected to diverge from the faster and more favorable progress line, thus increasing both distance swam and race time. For this reason, during the training sessions coaches usually correct the swimmer movements in order to maintain the linearity during the forward motion. However, it is difficult to define the value above which it is necessary to give indications to the swimmer and to which extent is possible to correct these oscillations. The deviations of the swimmer attitude, which can occur around 3 axes, are commonly classified in: rolls, pitches and yaws. Besides body roll and pitch angles, which are typical of either asymmetric or symmetric strokes, respectively, the body yaw angles occur in both symmetric and asymmetric strokes and can largely affect the path linearity of swim. In a pilot study, the instantaneous lateral-medial displacements, away from or toward of the midline of the lane (namely lateral fluctuations, LF), and the overall drift from a straight path, were assessed for the first time in 25 m front crawl trials at different velocities, analyzing the swimmers head trajectory by means of stereo-photogrammetry (Gatta et al., 2008). Briefly, stereo-photogrammetry allows the reconstruction of the three-dimensional position of a marker by means of triangulation, which can be performed when, after the calibration of the field of view, two cameras identify the same marker. The results showed a lower energy dissipation for LF in higher level athletes and a lower overall drift with training velocity between 80% and 90% of maximal speed, compared to lower level athletes. As hypothesized, LF were found to contribute to increase drag because the swimmer body take up more space and disrupt laminar flow. However, the overall drift may be considered negligible when considering short-distance swimming events (Gatta et al., 2008). To the best of our knowledge, nobody has investigated the body yaw angles of elite athletes during long-distance, high level, official competitions, thus in the actual conditions of the race.
The aim of the present work was twofold: firstly, to obtain the model of the path linearity of the center of the head in elite athletes during an official competition of 400 m front crawl (Italian Swimming Championship); secondly, to give useful information regarding path linearity of swimming to the coaches of high level athletes.
The present study evaluated 10 elite male swimmers (age: 23 [+ or -] 4.2 years; body mass: 78.0 [+ or -] 3.8 kg; height: 1.85 [+ or -] 0.05 m) at the 2009 Italian Swimming Championship. All subjects had experience in International swimming competitions and their race times (400 m front-crawl) ranged between 3'47.45" and 3'57.56". The Italian Swimming Federation gave a written permission to film all the athletes during the competition. The project was approved by the local Ethics committee and all participants signed an informed consent form to participate in the study before the competition.
In order to evaluate swimmers trajectories, the races were video-recorded and later analysis were performed on the row material recorded.
The 3rd and 4th lanes of 400 m front crawl stroke were analyzed. The number of lanes acquired was restricted to 2 due to the limited space available to situate the cameras. All the races were filmed by 2 synchronized digital cameras (Sony Dcr-Hc1000, Tokyo, Japan). The sampling rate of video recordings was 50 Hz. Cameras were placed beside the 50 m swimming pool at 6 m high with respect to the water surface (Figure 1). The central part of the swimming pool, corresponding to 40 m, was analyzed excluding the underwater phases (starting, flip turning and pushing from the wall) and considered in the present study as the length. The underwater phases were not taken into account, as they are not related to the stroke technique. A reference system was defined as follows: the origin at the beginning of volume of acquisition near the 4th lane, the X-axis corresponds to the medio-lateral fluc tuations, the Y-axis corresponds to the direction of swim, and the Z-axis corresponds to the vertical axis (Figure 1). The calibration of the acquisition field was performed by means of a virtual parallelepiped, 5 m wide (X-axis), 40 m long (Y-axis), and 1 m high (Z-axis). The parallelepiped was defined by 2 virtual panels, at the beginning and at the end of the central part of the lane, and by 12 uniformly distributed buoys with a diameter of 0.08 m. The calibration volume, visible from both cameras, was acquired before the competition. The consistency of the calibration procedure showed a difference between...