As the person is being submerged, the air in the lungs must be exhaled completely. The air remaining in the small pockets of the lungs following a maximal expiration is referred to as the residual lung volume. The residual lung volume may be determined using a number of laboratory techniques or it is often estimated using age, height, and gender-specific equations.

Once your body weight, the underwater weight, and the residual lung volume are known, total body density may be calculated. From the total body density, the percent body fat can be estimated using the appropriate age-gender equation.

One limitation of hydrostatic weighing is that it is based on the two- component model fat and fat-free mass which assumes when calculating total body density that the relative amounts and densities of bone, muscle, and water comprising the fat-free mass are essentially the same for all individuals, regardless of age, gender, race or fitness level.

It is now known that this is not the case. For instance, the fat-free body density of young Black men is greater than that of white men.

Because of this, the lean body mass is overestimated and the body fat is underestimated for many Blacks. Also, after age 45 to 50, substantial changes in bone density, especially in women, invalidate the use of an assumed constant value for fat-free body density when converting total body density to percentage of body fat.

This is why age and gender specific equations need to be used for estimating body fat. As researchers learn more about age-related changes in bone mineral, hydrostatic weighing will eventually provide a more accurate prediction of body fat for older men and women.

Bioelectrical impedance analysis BIA is based on the fact that the body contains intracellular and extracellular fluids capable of electrical conduction.

A non-detectable, safe, low-level current flows through these intracellular and extracellular fluids. Since your fat-free body weight contains much of your body's water and electrolytes, it is a better conductor of the electrical current than the fat, which contains very little water.

So this technique is essentially an index of total body water, from which fat-free mass is estimated. The popularity of the BIA method has grown significantly over the last few years because it is painless, quick, and easy to administer the test.

To take the test, you lie on a testing table or floor and electrodes are attached to your hands and feet. You do not feel a thing as the current passes through your body. Average time for administering this test is about 10 minutes. No eating or drinking within 4 hours of the test 2. No exercise within 12 hours of the test 3.

Urinate within 30 minutes of the test 4. No alcohol consumption within 48 hours of the test 5. There is a tendency for BIA to overestimate percent body fat in very lean clients and underestimate body fat in obese clients.

All in all, if the guidelines for testing are followed, the BIA method is a satisfactory method for assessing body composition of most people. Skinfold Method The skinfold method of measuring body fat is a practical, economical, and administratively feasible field technique for body composition analysis.

It involves measuring the skinfold subcutaneous fat thickness at specific sites of the body. Most equations use the sum of at least three skinfolds to estimate body density from which body fat may be calculated.

Skinfold measurement does not require expensive equipment and it can be routinely incorporated into many health promotion settings. Skinfold technicians can be trained rather easily, but must practice on at least clients before the skinfold technique is mastered.

When using the skinfold method, it is assumed that the distribution of subcutaneous fat and internal fat is similar for all individuals. This assumption is not fully supported. It is now known that older subjects of the same body density and gender have proportionately less subcutaneous fat than their younger counterparts.

There is considerable biological variation in the distribution of subcutaneous, intermuscular, intramuscular, and internal organ fat due to age, gender, and degree of fatness Heyward, Accuracy of Skinfold Measurements The accuracy of the skinfold method is dependent on the technician's skill as well as the type of caliper and the skinfold prediction equation used.

Reasonably priced plastic calipers have a less precise measuring scale, and often provide variable pressure and a smaller range of measurement.

Despite this, a number of researchers have reported only small differences between skinfolds measured with high quality calipers and plastic calipers for highly skilled technicians refer to Guide to Skinfold Caliper for more information on where to purchase calipers.

However, plastic calipers are not recommended for use by untrained technicians. To assure accuracy, the skinfold technician must follow standardized testing procedures: 1. Take all skinfold measurements on the right side of the body. Carefully identify and mark the skinfold sites.

Place the thumb and index finger approximately 3 inches 8 cm perpendicular to the skinfold, following the natural cleavage lines of the skin.

Do not release the skinfold during the measurement. Always release the caliper jaw pressure slowly. The skinfold measurement should be taken 4 seconds after the pressure is released. You should take a minimum of two measurements at each site. It is advisable to take measurements in a rotational order rather than consecutive readings at the same site.

If your values differ by more than 1 mm, take additional measurements. The client's skin should be dry and free of any oils and lotions. Skinfold measurements should not be done immediately after exercise due to the shift of body fluid to the skin.

Fortunately, the time of day or the phase of the menstrual cycle will have little effect on the skinfold measurements. As with many skills, the more you practice the better you will become at measuring skinfolds.

It always helps if you have another trained technician to compare your results. One alternative for obese clients would be to use fat-specific equations Segal et al. Measuring Sites for Women Triceps: Take a vertical fold on the posterior midline of the upper arm halfway between the top of the shoulder and the elbow joint.

Keep the elbow extended and relaxed. Thigh: Take a vertical fold on the front aspect of the thigh, midway between the top of the knee cap and the hip. Suprailium: Take a diagonal fold above the crest of the ilium hip- bone , at the spot where an imaginary line comes down from the anterior line of the armpit anterior axillary line.

Measuring Sites for Men Chest: Take a diagonal fold half the distance between the anterior axillary line line of armpit and nipple. Target Body Fat Keep a record of each client's estimated body fat.

This is very useful information for you to track the success of the exercise program you have prescribed. It will also give you insight about changes you feel may be necessary.

Knowing a person's body fat will help you determine a more realistic target body fat and body weight for them. The actual fat weight for this person is X. Therefore, to reach this new body fat level she needs to lose 7 lb of body fat. The following table summarizes the steps for calculating target body fat and target body weight.

Target Body Fat Calculation 1. Client's weight in lb lb 2. Desired weight 97 lb ÷. Fat lb to lose 7 lb This information is particularly valuable if a client is going on a weight loss diet. Oftentimes diets will result in loss of mostly lean body tissue and water.

By tracking a client's body fat you can closely monitor what body composition changes are actually happening. Just knowing how many pounds a person has lost is insufficient. The goal of any weight control program is going to involve adjusting a person's exercise and dietary habits to reach that ideal percentage of body fat.

Body composition is an integral component of total health and physical fitness. By lessening obesity in our society, we will also have a positive impact in reducing the risk of a number of diseases including coronary heart disease, diabetes, hypertension and musculo-skeletal problems. Knowing what percentage of your weight is fat and lean muscle mass can help you tailor diet and exercise to meet your unique health goals.

Underwater weighing, also known as hydrostatic weighing, is a method of calculating body fat percentage based off body density. The test is usually only available at universities, research facilities, and some fitness centers. Before the underwater weighing takes place, the individual being measured will be weighed outside the water.

Then, wearing a bathing suit, the individual will enter the pool or tank and sit in a chair that is submerged in the water. The chair will either be lowered further into the water or the individual will be asked to lower their head into the water until their entire body is submerged.

Once submerged, the individual exhales as much air from their lungs as possible by blowing bubbles. The test administrator records the weight when no more air bubbles appear in the water and the individual can then resurface. The test is typically repeated several times and the average of the weights used to minimize errors.

The recorded weight will be plugged into equations that account for any leftover air in the lungs and gastrointestinal tract to calculate body density in water and total body fat percentage. Underwater weighing is considered one of the most accurate forms of measuring body composition.

Underwater weighing was once considered the "gold standard" for comparing accuracy and reliability of other body composition measurement methods. However, with newer technologies available, it's being used less often since it does require special equipment. The individual should also be capable and comfortable expelling air from their lungs and keeping still while submerged underwater.

Underwater weighing is one method for calculating percentages of body fat and lean mass. Although highly accurate, it is not widely available, and therefore, may not be convenient for taking regular weight measurements. Other methods, such as bioelectrical impedance or skinfold measurements, may be more convenient for tracking changes in weight and body composition.

Ana Reisdorf, MS, RD is a freelance health writer and dietitian with 12 years of experience in the field of nutrition. You can learn more about her at anareisdorf. Body Fat Testing Through Underwater Weighing 2 Minutes Read. Underwater weighing is a traditional and reliable method of measuring body composition.

Underwater Weighing Underwater weighing, also known as hydrostatic weighing, is a method of calculating body fat percentage based off body density. How does underwater weighing work?