Sex differences in the centre of buoyancy location of competitive swimmers.

The aims of this study were to identify differences in the centre of buoyancy (CB) and centre of mass (CM) locations of male and female collegiate swimmers, and to assess the influence that buoyancy has on freestyle kicking performance. Sixteen female collegiate swimmers (mean +/- s: age 19.1 +/- 1.2 years) had significantly more adipose tissue (20.2 +/- 4.4%) than 15 male collegiate swimmers (19.9 +/- 1.0 years, 12.6 +/- 3.8%). The ratio of the sum of abdominal and suprailiac skinfolds to the thigh skinfold was significantly greater for the males (2.07 +/- 0.37) than the females (1.31 +/- 0.32), implying that females had proportionately more fatty tissue caudally than males. The distance d between the centres of buoyancy and mass was significantly larger for the males (0.79 +/- 0.43 cm) than the females (0.16 +/- 0.34 cm). Both points were more caudal in the female subjects (59.9 +/- 0.7% and 59.8 +/- 0.7% of body height respectively) than in the male subjects (61.7 +/- 0.8% and 61.2 +/- 0.9% respectively). These data suggest that the difference in d may be attributed to the difference in the location of the centre of buoyancy, because the centre of mass difference was not significant and was characterized by a smaller effect size. The amount and distribution of adipose tissue accounted for a significant proportion of variance in d (R2 = 0.25 and 0.29 respectively). Males had a significantly higher proportional kick time, defined as the ratio of times to complete a 22.9 m sprint when kicking and swimming respectively, than females (1.57 +/- 0.09 and 1.51 +/- 0.13 respectively). This shows that the male swimmers kicked proportionally more slowly than the female swimmers. However, the distance d did not account for a significant proportion of variance in the proportional kick time. Therefore, our results do not support the notion that skilled male swimmers are at a performance disadvantage in terms of natural buoyancy characteristics.