AIR-DISPLACEMENT AIR-DISPLACEMENT PLETHYSMOGRAPHYPLETHYSMOGRAPHY

(i.e. Bod Pod)(i.e. Bod Pod)

Densitometry PlethysmographyPlethysmography

Known as the “Bod Pod”Known as the “Bod Pod”Based on volume differences between fat Based on volume differences between fat

mass and fat-free massmass and fat-free massLeaner individuals have less volume per Leaner individuals have less volume per

mass than fatter individualsmass than fatter individualsTechnology allows volume (air Technology allows volume (air

displacement) to be measured by pressure displacement) to be measured by pressure changes in a closed chamberchanges in a closed chamber

220 kg vs. 191cm

AIR-DISPLACEMENT PLETHYSMOGRAPHY

““Plethys” = old Greek word for pressurePlethys” = old Greek word for pressure

ADP determines body volume by ADP determines body volume by measuring the the volume of air-displaced measuring the the volume of air-displaced from a chamber by an objectfrom a chamber by an object

Archimedes’ PrincipleArchimedes’ Principle states, “An object states, “An object immersed in water is immersed in water is buoyed up by a force buoyed up by a force equal to the weight of the water displaced.”equal to the weight of the water displaced.”

Introduction to Physical Principles Calculation of volume of air in empty Calculation of volume of air in empty

chamberchamber

Calculation of volume of air in chamber Calculation of volume of air in chamber with subject presentwith subject present

Volume Volume emptyempty – Volume – Volume subjectsubject

= Subject body = Subject body volumevolume

Introduction to Bod Pod Physical Principles

BV is corrected for thoracic gas volume (TGV)BV is corrected for thoracic gas volume (TGV) Body surface area estimated from height and Body surface area estimated from height and

weightweight

Boyles Law: PBoyles Law: P11VV1 1 = P= P22VV22

P P test test V V test test = P = P ref ref V V refref

V V ref ref is known, if measure P is known, if measure P ref ref and P and P test test then then V V testtest can be calculatedcan be calculated

ADP Technique

Faster and easier than UWWFaster and easier than UWW

Follow pretest guidelinesFollow pretest guidelines

Minimal, tight clothing and swim cap are Minimal, tight clothing and swim cap are essential for accuracyessential for accuracy

Closely follow calibration procedures and Closely follow calibration procedures and computer prompts during testingcomputer prompts during testing