(2005): �� 0.4, �� 0.5 and �� 0.4 mm, for the x, y and z axes, respectively. The results of this study indicated Src Bosutinib that the reconstruction errors were higher in underwater than above water environment. However, in both conditions, the magnitude of the reconstruction errors may be considered suitable for 3D swimming kinematic analysis. Complementarily, in spite of a lower resultant RMS error of the calibration volume #2, the choice of the number of control points and corresponding location should consider the specificity of the aquatic activity; for instance, calibration volume #3 could be used for synchronised swimming since its actions are mostly in y and z axes, in which the volume #3 presented low RMS error values.

Acknowledgments This investigation was supported by grants of Portuguese Science and Technology Foundation: SFRH/BD/38462/2007 and PTDC/DES/101224/2008 (FCOMP-01-0124-FEDER-009577).
Competitive swimmers should have two aims to improve speed and thus enhance performance. They should: (i) maximize the propulsive forces produced by the propelling segments and; (ii) minimize the hydrodynamic drag resisting forward motion (Callaway et al., 2009; Marinho et al., 2009). Regarding the latter aim, a substantial energy is wasted to the water in order to overcome the resistance (Toussaint & Beek, 1992; Kolmogorov et al., 1997). Thus, expert swimmers seem to improve technique due to an increase in propulsive force, as well as, minimizing hydrodynamic drag (Seifert et al., 2007). Efforts to minimize hydrodynamic drag should be carried-out during all swimming phases.

However, decreasing drag during the gliding after starts and turns should be a main concern for swimmers and their coaches, especially nowadays, when the underwater gliding plays a major role to the overall swimming performance (Vilas-Boas et al., 2010). Thus, swimmers must adopt the most hydrodynamic position during gliding. Several studies (Vilas-Boas et al., 2000; Cossor & Mason, 2001) suggested that rather than the start technique used by the swimmer, it is his/her body position after immersion that mostly determines the success of the start. Indeed, there are several postures that the swimmers can assume during the underwater gliding, although experimental results were not conclusive concerning the best body position to perform this phase (Jiskoot & Clarys, 1975; Maglischo, 2003).

Some swimmers prefer to glide in a lateral position and others in a prone one. In addition, during gliding swimmers can change their body posture. Moreover, in some techniques swimmers must change their limb positions. For instance, in breaststroke, the gliding is initially performed with the arms fully extended at the Cilengitide front. But then, swimmers perform a second gliding with the arms aside the trunk. It can be thought that these different postures might lead to differences in the intensity of the drag forces experienced by the swimmers.