when submerged under water, a person is pushed up toward the surface with a force equivalent to the weight of the volume of water displaced, causing underwater weight to be less than TBM
errors: volume of air in lungs cant be measured or estimated, many people cannot exhale all the air (appears as a higher % Fat), siri- assumes density of FM and FFM are constant but there is some variablitliy
- compare a person with high % fat
- fat (adipose tissue) has a density that is greater than water, therefore sinks.
- Fat is less dense than FFM, the individual with a higher % fat will have a greater body volume, and therefore a lower underwater weight