Air-displacement plethysmography (i.e. BOD POD) has gained popularity among body composition researchers since its introduction in 1995. This is mainly attributable to the non-invasive test-procedure and the lack of technical expertise required compared to the traditional hydrostatic weighing procedure.
The BOD POD is a single fiberglass unit composed of two chambers. The test chamber accommodates the subject during testing and the reference chamber contains instrumentation for measuring changes in pressure between the two chambers . The operating principles of the BOD POD are detailed elsewhere [1–3]. Briefly, the volume of the test chamber is determined by pressure changes precipitated between the test chamber and reference chamber by a moving diaphragm mounted on the common wall between the chambers. The pressure ratio relationships between the chambers are inversely related and are characterized by Boyle's Law:
P1/P2 = V2/V1
where V1 and P1 are the volume and pressure prior to subject entry into the test chamber and V2 and P2 are the volume and pressure while the subject is in the test chamber. Therefore, subject body volume will equal the volume of the test chamber before subject entry less the test chamber volume with the subject present.
Because of difficulty maintaining isothermal conditions in the enclosed environment of the test chamber, the BOD POD functions under adiabatic conditions (i.e. air temperature is gaining/loosing heat), thus Poisson's Law more accurately characterizes the pressure volume relationship in the testing chamber:
P1/P2 = (V2/V1)γ
where V1 and P1 are the volume and pressure prior to subject entry into the test chamber, V2 and P2 are the volume and pressure while the subject is in the test chamber, and γ is the ratio of the specific heat of a gas at constant pressure to constant volume (1.4 for air) [4, 5]. Moreover, isothermal air present in the test chamber during a body volume measurement will result in an underestimation of body volume because isothermal air is more easily compressed (40%) than an equivalent volume of adiabatic air, resulting in a lower pressure output signal for a given body volume . There is one source of isothermal air (i.e. air in the lungs) and several sources that are "isothermal-like" (air trapped within the fabric of clothing and air trapped within hair on the head and body). Instructions and procedures have been recommended by the manufactures to correct and control for these sources of error [1, 6].
To avoid erroneous data the BOD POD manufacturers recommend that testing be conducted prior to exercise, that the subject be dry, and that the testing environments temperature remain stable . Strict adherence to these conditions can sometimes prove difficult when testing large numbers of subjects in a short period of time and when testing people who are perspiring or have an elevated temperature due to illness. In one study, BOD POD measurements were performed following hydrostatic weighing, which resulted in a regression that significantly deviated from the line of identity . However, other studies have reported significant differences when the BOD POD preceded hydrostatic weighing [9, 10]. Thus, the specific effect of elevated body temperature and body moisture (resulting from hydrostatic weighing) on BOD POD measurements needs clarification. An increase in body temperature and moisture could increase the quantity of isothermal-like air surrounding the skin. Therefore, the purpose of this study was to determine the effect of increased body temperature and moisture on BOD POD estimates of %fat, body volume, and body density. We hypothesized that an increase in body temperature and moisture would result in an underestimation of %fat. We also speculated that the increase in temperature, the increase in moisture, and total body surface area (BSA) would be positively associated with the magnitude of this underestimation.