, 2009c). However, it raises the question if different swimmers would present the same tendency as the one studied in this paper. Moreover, this study only analyzed a passive drag situation, when the swimmer is passively gliding after starts and turns. In the future, the development of this methodology must consider the body movements in the CFD domain, novel analysing, for instance, the second part of the gliding when the swimmer is kicking, allowing to study the total underwater phase. As a conclusion, one can state that the water depth seems to have a positive effect on reducing hydrodynamic drag during the gliding. Although increasing depth position could contribute to a decrease in hydrodynamic drag, this reduction seems to be lower with depth, especially after 0.

75 m depth, thus suggesting that performing the underwater gliding (and the underwater dolphin kicking) more than 0.75 m depth will not be to the benefit of the swimmer. Nevertheless, a commitment between decreasing drag (by increasing water depth) and gliding distance should be the main concern of swimmers and an important goal to be addressed in future investigations. Acknowledgments The Portuguese Government supported this work by a grant of the Science and Technology Foundation (PTDC/DES/098532/2008, FCOMP-01-0124-FEDER-009569; SFRH/BD/39511/2007).
Postural control is the ability to control the position of the body��s centre of mass (COM) over its base of support (BOS) to prevent the body from falling and to achieve specific functional tasks (Winter, 1995).

The process by which humans maintain the integrity of their postural control is referred to as balancing (Westcott et al., 1997). Stability exists when the vertical line of gravity from the COM falls within the BOS and stability improves with a larger BOS, a lower COM, and/or a more central COM within the same BOS (Bell, 1998). Postural control is a complex process requiring integration of sensory information (somatosensory, visual and vestibular feedback) and execution of appropriate postural responses (Maurer et al., 2006). Biomechanically, the high COM of the standing human together with the correspondingly small BOS results in unstable posture as compared with quadrupedal animals. Hence, the natural consequence is spontaneous sway requiring a dynamic postural stability control system (Winter et al., 1998).

Gender differences exist in postural stability of children that vary depending on their age. Several papers have noted that girls exhibit less postural sway than boys of similar ages (Demura et al., 2006; Geldhof et al., 2006; Lee and Lin, Batimastat 2007; Nolan et al., 2005; Odenrick and Sandstedt, 1984; Peterson et al., 2006; Steindl et al., 2006). Specifically, Demura and colleagues noted that boys aged 3�C4 years demonstrate significantly more sway than girls even though there were no significant differences in their anthropometrics (Demura et al., 2006).