Biomechanics of front-crawl swimming: Buoyancy as a measure of anthropometric quantity or a motion-dependent quantity?

The swimmer's body is acted upon by time- and position-dependent fluid forces which control linear and angular motions of the swimmer's body. In this presentation, researches on the buoyancy of human body and its influence on the front-crawl swimming are reviewed, and the current approaches for quantifying the buoyancy of a swimmer are re-examined. An early study on buoyancy of human body was found in 1757. In this study, the primary question was whether or not motionless human bodies can float in water. Similar studies were conducted in early 1900's, in which the buoyancy of human body was measured as anthropometric quantities such as the buoyant force acting on the human body submerged in water, the specific gravity of the body and the apparent body weight under water. These studies revealed that (a) not all human can float motionless in fresh water, (b) in general, women are more buoyant than men, (c) the volume of air in body cavities affects one's buoyancy, (d) buoyancy characteristics changes as children get older, and (e) the legs tend to sink from a horizontal motionless floating position. Similar approach was adopted to study if the buoyancy of a swimmer influences swimming performance. On the basis of the findings from these studies, the buoyancy was postulated to influence the swimming performances with the following mechanism: The greater the buoyancy and the lesser the leg sinking effect of buoyancy help the body to float horizontally and to reduce the immersed body cross-sectional area, both of which reduce hydrodynamic resistance, and thereby, the swimming economy would increase for better performances. In a recent study, the leg-sinking effect of buoyancy was measured as motion-dependent quantities and demonstrated that the buoyant force generated the moment that directed primarily to raise the legs and lower the head during the front-crawl swimming - the buoyancy made the leg-raising effect, rather than leg-sinking effect, during the front-crawl swimming. This counter-intuitive finding on the rotational effect of buoyancy raised a question regarding the aforementioned postulated mechanism; how the leg-sinking effect of buoyancy in a horizontal motionless floating could help the swimmer's body align horizontally during front-crawl swimming, given that the buoyancy results in the leg-raising effect on the human body during the performance of front-crawl swimming? To seek an answer for this question, the mechanism of horizontal floating of the swimmers during the front-crawl swimming needs to be re-examined in the future studies. In such studies, the buoyancy of a swimmer should not only be measured as an anthropometric quantity, but it should also be measured as a motion-dependent quantity.