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Sept 24 - body comp assessment 2.doc

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Physical Education and Sport
Scott Forbes

Sept 24 -Body Composition Assessment #2 – Selecting the Best tool Methods we’ll look at.. • Underwater weighing(Hydrostatic)* • Dual energy xray absorption(DEXA)* • Air displacement plethysmography(Bod Pod)* • Magnetic resonance imaging(MRI)* • skinfold.* • bioelectrical impedance(BIA)* • Body mass index (is an indicator of health status but doesn’t determine % body fat) *Used to determine % body fat Errors predicting % body fat? What is an “acceptable “ error?? • Lower is better (obvious) • 3-4% standard errors of estimate (SEE) with BIA and a athropometry • 2% DEXA, 2-3 % underwater weighing • 4-5% SEE with NIR Remember • If changes in body comp are less than SEE of the technique (often the case), they will be very difficult to detect • For example, how do you detect a “real” 4% change in body fat with NIR that has a SEE of + or - 5%? Underwater (hydrostatic) weighing • Based on archimdes principle – if you put something into water, the weight of the water displaced is equal to the buoyant force • The buoyant force is equal to the weight of the displaced water • We can figure out body density and then calculate body fat from this method Measuring the compartment of interest • Fat mass is made up of lipids • Fat Free Mass is made up of water, protein, glycogen, and minerals • Hydrostatic weighing will only measure fat mass and fat free mass – it wont analyze glycogen Detailed assumptions for 2C model (2 compartments of fat; fat mass and fat free mass) • Fat mass is 0.901 g/cm^3 and FFM density is 1.10g/cm^3 • Densities of FM and FFM compartments are the same for all individuals • Proportion of the FFM molecular components (water, protein, mineral) are constant within an individual • These assumptions aren’t always true Density of FFM is variable and dependent on: • age(children vs. Elderly) • gender • ethnicity (ex. Caucasians vs. Blacks) • level of body fatness • physical activity level • health status (ex. Osteoporosis) Due mainly to variations in the relative proportion of water and minerals within the FFM Note slide 9 An infant at birth has a FFM mass compartment mass of water at 84.4% Depending on the age, we see that the assumptions of the 2C model aren’t always true Other Lab Methods Alternatives are needed: • People unwilling/unable to submerge in water • Develop method that doesn’t rely on assumed density or constants – we know these assumptions can be misleading Air displacement plethysmography (bod pod) Dual energy x-ray absorptiometry (DXA) With DEXA we can measure bone mineral density We can see if someone is oteoporotic Dual Energy X-Ray Absorptiometry (DXA) • Strong promise (total and regional body composition) • 3 compartment method (FM, FFM, bone) • Short measurement time, improved resolution • Helpful for longitudinal studies (growth, aging, nutritional or activity program evaluation) Air Displacement Plethysmography (Bod Pod) • Measure body volume by air displacement • Measures pressure changes from when you are outside the bod pod and when you are in the bod pod • Based on the same principles as underwater weighing but is much easier • Calculate body density (D = M/V) Several Advantages of the Bod Pod • Portable, easy to operate • Short measurement time (good for children, elderly, obese, disabilities)
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