Underwater Weighing: Understanding Body Composition, Procedure, and Results

What is under water weighing?

Underwater weighing, also known as hydrostatic weighing or hydrodensitometry, is a highly accurate method for determining body composition by measuring body density based on Archimedes' Principle.

Understanding Body Composition

Body composition refers to the proportions of fat and fat-free mass (muscle, bone, water, organs) in the body. While body weight is a simple measure, understanding body composition provides a more insightful picture of health, fitness, and disease risk. Excess body fat, rather than just excess weight, is linked to numerous health issues. Accurate assessment of body composition is crucial for tracking progress in fitness programs, managing weight, and conducting research.

The Principle Behind Underwater Weighing

Hydrostatic weighing operates on Archimedes' Principle, which states that an object submerged in water experiences an upward buoyant force equal to the weight of the water displaced by the object. The core idea is that:

• Fat mass is less dense than water, meaning it floats or has greater buoyancy.
• Fat-free mass (muscle, bone) is denser than water, meaning it sinks.

By measuring an individual's weight on land and their weight while fully submerged in water, along with the water's temperature (to determine its density), their body volume can be calculated. From body mass and body volume, body density is determined using the formula:

Body Density = Body Mass / Body Volume

Once body density is known, established equations (such as the Siri or Brozek equations) are used to convert this density into a percentage of body fat.

How Underwater Weighing is Performed

The procedure for underwater weighing requires specialized equipment and a trained technician:

• Equipment: A large tank or pool of water, a specialized scale suspended over or within the water, and a chair or platform for the individual to sit on. A scale to measure residual lung volume (the air remaining in the lungs after maximal exhalation) is also often used.
• Procedure:
  • The individual is first weighed accurately on dry land.
  • They then sit on a specialized chair or platform and are slowly lowered into the water until fully submerged.
  • While submerged, the individual is instructed to exhale as much air as possible from their lungs and hold their breath completely. This is critical to minimize the influence of air in the lungs, which can affect buoyancy and skew results.
  • Their submerged weight is then recorded.
  • This process is typically repeated multiple times (e.g., 3-7 trials) to ensure consistency and reliability, with the average of the most consistent readings used for calculation.
  • The temperature of the water is also recorded, as water density varies with temperature.
  • Measurement of residual lung volume (RV) is paramount. If not directly measured (e.g., via helium dilution or nitrogen washout), it must be estimated, which can introduce error. RV accounts for the air remaining in the lungs even after maximal exhalation, which contributes to buoyancy but is not part of the body's tissue mass.

What the Results Mean

The calculated body density is then plugged into a specific formula to estimate body fat percentage. The two most common equations are:

• Siri Equation: Body Fat % = (495 / Body Density) - 450
• Brozek Equation: Body Fat % = (457 / Body Density) - 414.2

These equations are based on assumptions about the density of fat mass (0.900 g/cm³) and fat-free mass (1.100 g/cm³). The result provides a highly accurate estimate of an individual's percentage of body fat and lean body mass.

Advantages of Underwater Weighing

• High Accuracy: Historically considered the "gold standard" for body composition assessment due to its precision. It is highly reliable when performed correctly with proper control for residual lung volume.
• Direct Measurement of Volume: Unlike many other methods that estimate volume or density, hydrostatic weighing directly measures body volume through water displacement.
• Established Science: Based on a fundamental scientific principle (Archimedes' Principle) with extensive research backing its validity.

Limitations and Considerations

Despite its accuracy, underwater weighing has several practical limitations:

• Accessibility: Requires specialized equipment and facilities, making it unavailable to the general public. It's typically found in research labs, universities, or high-performance athletic centers.
• Subject Cooperation: Requires the individual to be comfortable with full submersion in water and capable of exhaling maximally and holding their breath for several seconds. This can be challenging for some individuals, including those with claustrophobia, respiratory issues, or young children.
• Cost: The equipment and professional administration can make it an expensive assessment.
• Measurement of Residual Lung Volume (RV): Accurate RV measurement is crucial. If RV is estimated rather than directly measured, it can introduce significant error into the body fat percentage calculation.
• Not Suitable for All Populations: Not ideal for individuals who cannot fully submerge, exhale completely, or hold their breath (e.g., very young children, elderly, individuals with severe respiratory conditions).

Is Underwater Weighing Right for You?

For most individuals seeking to track their fitness progress, more accessible and less invasive methods like bioelectrical impedance analysis (BIA), skinfold measurements, or DEXA scans (Dual-energy X-ray Absorptiometry, now often considered the new "gold standard" due to its ability to differentiate bone mineral content) are often sufficient and more practical.

However, underwater weighing remains a valuable tool for:

• Research studies: Where high precision in body composition is critical.
• Elite athletes: Who require extremely accurate body composition data for performance optimization.
• Clinical settings: Where precise monitoring of body composition changes is necessary for specific health conditions.

Conclusion

Underwater weighing stands as a foundational method in exercise science for assessing body composition. While its practical limitations make it less common for everyday use, its scientific rigor and high accuracy have cemented its place as a benchmark against which other body composition assessment techniques are often compared. Understanding its principles provides valuable insight into the complexities of human body composition analysis.