Academic journal article Journal of Physical Education and Sport

Human Body Flotation and Organic Responses to Water Immersion

Academic journal article Journal of Physical Education and Sport

Human Body Flotation and Organic Responses to Water Immersion

Article excerpt

Introduction

James Counsilman (1921-2004), one of the most renowned swimming scientists and trainers, wrote in his last book: "We are 65 percent water, so they say, but when humans enter the water it is a foreign element in which we are poorly designed for efficient locomotion ... We can only attain top speeds of about six miles per hour, whereas dolphins and some fish reach speeds five times that fast" (Counsilman & Counsilman, 1994). This idea summarizes the situation of human when are immersed in water.

Human locomotion in water is rather inefficient due to, on the one hand, the specific properties of water: a dense, viscous fluid in which it is difficult to apply forces of propulsion and where the hydrodynamic resistance forces are very important. Despite these special properties, animals that have evolved in aquatic media, such as fish or cetaceans are capable of moving efficiently in this medium. On the other hand, animals such as humans that have evolved on land, while able to move inside the aquatic medium, have very low levels of efficiency. The reason for the different aquatic movement efficiency lies on their different morphology. Aquatic animals have bodies shaped for moving under water that make them very hydrodynamically streamlined. They also have flat fins which enable them to create propulsion. Conversely, human beings are shaped with little hydrodynamic streamlining, and moreover their propulsion surface is small and rounded. Thus, human beings' mechanical efficiency (mechanical work done/net energy consumed) when swimming can barely reach 8 % (Di Prampero, Pendergast, Wilson, & Rennie, 1972), whereas whilst running the human mechanical efficiency varies between 25 % to 35 % (Kaneko, 1990).

To understand human locomotion on water, it is necessary to know the forces that are applied to the swimmer. Figure 1 shows the four forces that dictate how a human being swims: weight force and buoyancy (or flotation force) determine mainly the swimmer's floatability, while propulsion and drag forces determine the swimming speed.

Even though there are numerous studies that explain the basics of propulsion and drag forces in water, scientific literature is still scarce about the effects of water immersion and buoyancy on the human body. Therefore, the aim of this review was to establish a theoretical background and to highlight the dearth of scientific knowledge addressing this matter.

Method

For this systematic review, an extensive literature search was conducted using the MEDLINE (PubMed) and SportDiscus electronic databases with no year, gender, age or type of article restriction. The key words used in the online search included "swimming" "aquatic exercise", "locomotion", "flotation", "buoyancy", "microgravity", "hydrostatic pressure" and "water recovery".

Our search yielded 56 articles, and a final selection of 39 articles which were considered relevant was done to fit the objectives of the study. Among the 39 studies, 33 were research articles and the 6 remaining papers were reviews.

Results

WHAT IS FLOTATION?

The flotation of a body in water depends on the vertical forces that are applied at any given moment. At rest, flotation is determined by Archimedes' principle (3rd cent. B.C), according to which "any object wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object". This force is called buoyancy (B) or flotation force. As a consequence, when somebody enters the water and remains at rest, their flotation depends on their weight/buoyancy relationship: when buoyancy is greater than weight the person will float and, in the same way, when weight is greater than buoyancy, the person will sink. In the latter situation where the body tends to sink, a person could remain in the water surface only by creating upwards forces equal or greater than the weight force through the movement of their body segments (Cureton Jr, 1930), what is known as active (or dynamic) flotation. …

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