Underwater weighing assumes standard densities for which of the following?

Underwater weighing, also known as hydrostatic weighing, relies on the principles of buoyancy to estimate body composition, specifically the proportions of fat mass and lean mass (which includes muscle and bone). The assumption of standard densities for bone, muscle, and fat is crucial here because these tissues have different densities that affect how much someone weighs underwater.

Bone is denser than water, muscle is denser than fat, and fat is less dense than water. When an individual is submerged, the buoyancy force measured provides information that, when combined with a known density for each tissue type, allows for the calculation of body fat percentage. This technique assumes that the proportions of those tissues remain consistent with established standard densities to derive an accurate assessment of body composition.

In contrast, factors like cardiovascular and muscular endurance, height and body width, or attributes like skin and hair density do not directly influence the physical principles involved in underwater weighing. They do not provide relevant information for calculating body composition through this method. Therefore, the correct answer accurately reflects the necessary assumptions made within the context of underwater weighing and body composition analysis.