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Hydrostatic Weighing The gold standard for ascertaining body composition analysis is hydrostatic or hydro-densitometry weighing. In point of fact, underwater weighing is the standard or criterion for comparing different methods of estimating body composition. If done correctly, hydrostatic weighing is the most accurate and valid method for determining body composition. Briefly, hydrostatic weighing involves determining an individual’s residual lung volume and land weight calculations. Underwater weighing compares the weight of the subject in air to the weight of the subject completely submerged in water. Because fat is less dense than water, a person with a high amount of fat will weigh less underwater than a person of similar weight who has less fat. Measurements must consider the amount of air in the lungs during measurement (i.e., corrected for residual volume—the amount of air left in the lungs after complete exhalation) and for gastrointestinal air. Underwater weighing, in the hands of skilled technicians and subjects who are comfortable underwater, provides a good estimate of the density of the body, which can then be converted to an estimate of fat percentage. Objective: to measure body composition. Age level: age 16 through college level. Reliability: Reliability coefficients range from .92 to .96. Objectivity: An objectivity coefficient of .89 was reported by Fields, Hunter and Goran (2001). Validity: Face validity is generally accepted for this test. Equipment required: Hydrostatic weighing tank, including scale, weighted belt, and a nose clip. Procedure: The dry weight of the subject is first determined by weighing him on a scale. While dressed in a swimsuit, the participant is seated on a submerged chair that is suspended in shoulder deep water from a weighing scale above. The participants exhale completely and then instructed to immerse themselves under the water briefly, while their underwater weight is measured. Several trials are performed and recorded. Body density is then calculated and used to estimate body composition Scoring: Body density = Wa / (((Wa - Ww) / Dw) - (RV + 100cc)), where Wa = body weight in air (kg), Ww = body weight in water (kg), Dw = density of water, RV = residual lung volume, and 100cc is the correction for air trapped in the gastrointestinal tract. The body density (D) can be converted to percent bodyfat (%BF) using the Siri equation. Siri Equation: Many body composition equations derive their measure of percent body fat from first determining body density. Once body density is determined, percent bodyfat (%BF) can be calculated using the Siri equation below : % Body Fat = (495 / Body Density) - 450. The Siri Equation is based on the two compartment model, that is the body is made up of essentially two components: fat mass (the total fat of an individual) and fat-free mass (everything else: bone, water, lean tissue etc). Studies have determined that the densities of these two components are:

fat mass = 0.9 grams per cc fat-free mass = 1.10 grams per cc. As density = mass / volume, and the mass of a human is made up of the total of fat mass and fat-free mass, therefore density = (fat mass + fat-free mass) / volume. Following this through and substituting mass/density for volume, and using the values for density above, eventually you get to the Siri Equation as aforementioned. Advantages: Underwater weighing is the most widely used test of body density and is the criterion measure for other indirect measures. Other considerations: Residual lung volume is required for the calculations of body composition using hydrostatic weighing. For more accuracy it should be measured directly, there are however methods that can be used to estimate residual volume. One rough calculation of residual volume is one third of forced vital capacity (FVC). As indicated hydrostatic weighing can be very accurate under laboratory conditions, there are however, many methodological requirements associated with hydrostatic weighing that limit its usefulness and widespread application: the equipment required to do underwater weighing is expensive. The tanks are mostly located at larger universities or other research institutions, and there is generally not easy access for the general population. The method is also time consuming, cumbersome, and complicated, consequently, many physiologists turn to other types of measurement as an alternative means of body composition assessment. Also, its application to specific populations such as the obese, elderly, infants, or cardiac patients is often trying due to strict compliance issues. Because of the aforementioned disadvantages it is an impractical method for most physical education programs.

The BOD POD Composition Analysis The BOD POD composition system uses whole body densitometry to determine body composition…body fat and lean body mass. As mentioned historically, hydrostatic underwater weighing has been the gold standard for the determination of body composition. As also mentioned, while hydrostatic weighing is very accurate under laboratory conditions, there are many methodological requirements associated with hydrostatic weighing that limit its usefulness and widespread application. The BOD POD uses air instead of water to measure body volume. It is based on the application of Boyle's Law, which states volume and pressure vary inversely with one another, i.e., as volume increases, pressure decreases and vice versa. (P1VI = P2V2 = Constant) (P = Pressure, V = Volume)

Objective: to measure body composition. Age level: age 16 through college level. Reliability: Reliability coefficients range from .92 to .96. Validity: Face validity is generally accepted for this test. It might also be noted that validity coefficients ranging from .89 to .94 were found when the BOD POD was compared to hydrostatic weighing. Equipment: The BOD POD actually consists of two chambers; the front or test chamber, and the rear or reference chamber. The seat inside the BOD POD divides the unit into front and rear chambers and provides a common wall between these two chambers. There is a diaphragm mounted on the common wall, which is oscillated during testing by computer control. During a measurement, the diaphragm effectively moves back and forth between the two chambers. It may be useful to think of this as though it were a piston in a cylinder. You do know what a piston in a cylinder is‌the thingy that goes in and out of the cylinder. Of course you do. When the volume is increased in one of the chambers, it is decreased by the same amount in the other chamber, and vice versa. The pressure in each of the two chambers responds immediately to this volume change, and the magnitude of the pressure changes indicates the relative size of each of the chambers (the pressure response is less in a large space and greater in a small space). The pressure changes are very small, less than 2 cm H20. Procedure: All subjects must wear a swimsuit and swim cap during the BOD POD test. This minimizes errors caused by air trapped in clothing and hair. Subjects are weighed prior to entering the BOD POD, and then sit comfortably in the measurement chamber. The door is closed and the run begins. Each subject’s run takes less than one minute. The test is then repeated as a reliability check. If the two subjects’ measurements do not agree, a third run is done. The two closest runs are then averaged. Once the body volume phase of the procedure is completed, the lung volume measurement begins. The subject remains seated in the BOD POD and is instructed to begin breathing normally through a disposable tube when given a cue. After 2 to 3 breaths, the tube airway is closed and the subject gently puffs against the closed airway (2 to 3 times, about once per second). Pressure in the breathing tube changes as the subject's diaphragm contracts and expands. Airway and chamber pressures are monitored and lung volume can be calculated. This completes the measurement procedure, and results for percent fat and fat free mass are displayed. During the body volume measurement, the subject breathes in a normal fashion (no differently than you are doing right now as you read this manual). This is known as relaxed tidal breathing. This is unlike underwater weighing, which typically requires maximal exhalation to residual volume. Thus, the relevant measurement of lung volume for the BOD POD is not residual volume, but the average lung volume during normal tidal breathing (average thoracic gas volume). This is a much easier measurement to obtain and no difficult maneuvers are required. Scoring: The BOD POD produces very small volume changes inside the chamber and measures the pressure response to these small volume changes. This method is first used to

determine the interior volume of the empty BOD POD chamber. Then it is used to determine the interior volume of the chamber when the subject is seated inside. By subtraction, the volume of the person may be determined. For example, if the interior air volume of the empty chamber is measured at 400 liters, and the interior air volume of the chamber is reduced to 350 liters with the subject inside, then the body volume of the subject is 50 liters. The BOD POD allows you to either: directly measure the lung volume or use standard prediction equations based on gender, age and height or enter a previously determined lung volume. Other considerations: Testing is highly accurate, safe, and quick, with a complete analysis in about 5 minutes. The test is easy and suitable for all types of populations including the obese, disabled, elderly, and children. More importantly the error rate is only 2%. Unfortunately, the equipment is rather expensive, and a trained administrator is needed to administer the test; consequently, it may not be practical for most schools and universities.

Bioelectrical Impedance Here we go again, well, kind-of-sort-of‌bioelectrical impedance (BIA) devices. We talked about bioelectrical impedance scales, but now we are moving on to the big boys. Bioelectrical impedance devices were developed on the basis that water conducts electricity better then fat. The use of BIA was first documented in 1880 (Kalvin), as a potentially safe, convenient and accurate technique to measure conductivity in the body. Because muscle has high water content and fat has very low water content, the rate at which your body conducts electricity can be used to estimate body fat analysis. These two compartments have, therefore, very different impedance (or resistance) values to a high frequency electrical signal. The signal impedance measurement reflects the degree of resistance to the flow of current in the body, water being a good conductor but fat a bad conductor. In brief, the physical principle behind the BIA technique is that the body’s lean compartment, comprising approximately 60% to 75% electrolytic water, conducts electricity far better than the body’s fat compartment which is very low in body water content (between 5% to 10%). Consequently, this technique is essentially an index of total body water, from which fat free mass is estimated. Objective: to measure body composition. Age level: age 6 through college level. The BIA method is risk free! It sends an extremely weak electrical current (50 kHz) that is painless, safe, and non-detectable. However subjects who have a pacemaker or other internal device should not undergo BIA. The weak electrical signal associated with BIA measurement may cause such devices to malfunction. Reliability: Reliability coefficients range from .79 to .86. In addition, recent scientific reviews of BIA methods indicate that they are in general unreliable. Objectivity: An objectivity coefficient of .91 was reported by Hamby and Hamby. (2004). Validity: the validity of BIA instruments is questionable. Validity coefficients ranging from as low as .72 to as high as .84 have been reported when BIA was compared to hydrostatic weighing or hydro-densitometry weighting. As mentioned, body fat scales in which people stand on, or devices where the person grasps the handles of the device, are NOT accurate, and could lead to inappropriate treatment. In subjects who have varying body water contents, BIA may not give an accurate body fat measurement. A review of the literature indicates that bio-impedance units which utilize these linear regression equations tend to be somewhat valid for a "normal" population, but under predict body fat for obese subjects and over predict body fat of lean subjects. The standard

errors of estimate for these equations are +/-5% to +/- 6.4% in normal populations when compared to the hydrostatic tank. (Jackson et al 1988) (Segal et al 1988) Equipment required: Many bio-electrical impedance devices have been developed. There are even hand held versions. Generally the equipment consists of electrodes, clips, and a small portable BIA instrument. Procedure: A BIA meter is attached to the body, at the extremities, and a small 500 to 800 micro-amp, 50 kilohertz, signal measures the body's ability to conduct the current. The more lean tissue present in the body the greater the conductive potential, measured in ohms. (Thomasett, 1963) In order to avoid increases or decreases in body water content that could result in a less accurate measurement the subject is required to adhere to the following instructions prior to testing: Avoid vigorous exercise, preferably 8-12 hours prior to the test. Avoid alcohol, preferably within 24-48 hours of the test. Avoid eating or drinking, at least two hours prior to the test. The subject must remove any metallic objects that he may be wearing. They will also have to remove his shoes and socks. While in a lying position, self-adhesive disposable electrodes are placed on the subject’s right hand and two on his right foot. Clips are attached to the electrodes, a battery generated signal is passed through the body and an impedance value is produced. This value, together with other details of age, height, weight, and gender are used to analyze the data and within three seconds produces a comprehensive personal body composition statistical analysis. There are different equations used to calculate body fat from your body density. These are usually preprogrammed into the machine. Other Considerations: As long as the BIA testing conditions are maintained, BIA is moderately accurate and fairly reproducible. This is unlike the skinfold caliper where the skill of the technician, the accuracy of the calipers, the placement of the caliper during measurement, and the individual’s fat patterns can cause errors greater than +/-6%. We will discuss this momentarily. In addition, body fat scales in which people stand on, or devices where the person grasps the handles of the device, are not very accurate. In subjects who have varying body water contents, BIA may not give an accurate body fat measurement. However, BIA is still a reasonable method for monitoring changes in body fat.

Skinfold Thickness An estimate of total body fatness is made by measuring subcutaneous fat in various areas of the body. As approximately 50% of the body fat is subcutaneous (under the skin), it follows that the thicker the skin folds the greater the amount of fat a person is carrying. Specially designed calipers measure the thickness of representative sites throughout the body. These measurements are applied to mathematical equations to estimate the body's density. The measurements are then converted into a body fat percentage. Objective: to measure body composition. Age level: age 10 and older. Reliability: Reliability coefficients range from .86 to .89. Objectivity: An objectivity coefficients range from .64 to .74.

Validity: Validity coefficients range from as low as .61 to .89 with Hydrostatic weighing. According to the American College of Sports Medicine, when performed by a trained, skilled tester, skinfold measurements of body fat are up to 94% accurate. The key words here are obviously trained, skilled tester. For this reason, it's important to find a qualified expert if you have a skinfold measurement done. The accuracy of these tests may also depend upon the type of calipers being used and a person's level of hydration at the time of the test. There are also problems opening skin fold calipers wide enough for very obese persons, resulting in underestimation. Equipment: Skin Calipers Procedure: If you are right handed grasp the caliper in your right hand and with your left hand, pull out the fold of skin with its underlying layer of fat. Grasp the skin and the underlying layer of fat between the thumb and fingers of your left hand. Pull it out in the appropriate direction and continue to hold the skinfold as you apply the caliper. Place the teeth of the caliper onto the skinfold. The teeth should be about 1/4" (quarter of an inch) from the fingers of your left hand, which continues to hold the fold of the skin. Completely release the trigger of the calipers so the entire force of the jaws is on the skinfold. Do not release the fingers of the left hand while taking the readings. The teeth will move a little to a lower reading then when first applied. This occurs because of subcutaneous water and will cease after a few seconds. This is when the reading should be taken. (Reverse the process if you are left-handed) Skinfold measurements are generally taken at specific sites on the right side of the body. Two measurements are recorded and averaged. The measurement sites vary depending upon the specific skinfold testing protocol being used, but typically include the following seven locations on the body: 1. 2. 3. 4. 5. 6. 7.

Triceps - The back of the upper arm Pectoral - The mid-chest, just forward of the armpit Subscapula - Beneath the edge of the shoulder blade Midaxilla - Midline of the side of the torso Abdomen - Next to the belly button Suprailiac - Just above the iliac crest of the hip bone Quadriceps - Middle of the upper thigh

Once you have taken skinfold measurements you'll need to convert these numbers into a percent of body fat. The easiest way to calculate the percent of body fat is to use a software program. There are as many different formulas and calculations as there are ways to measure skinfold thickness, but some that have held up over time include those published by Jackson and Pollock. You can find these being used in the following online body fat calculators:

Other Considerations: Skin caliper methods utilizes a "Skin fold Caliper" to pinch the skin at several predetermined sites and measure the thickness of that pinch. The results are then summarized and evaluated via several published methods or equations. These methods are usually based on large population studies, which took skin fold and hydrostatic weighing measurements and then attempt to find equations that manipulate the skin fold measurements so that they match the hydrostatic weighing results. As noted the reported validity of skin fold measurements is low +/- 7% at the very best and may be off much more due to several problems with measurement accuracy. Just a few of the problems associated with the use of skin calipers to measure body fat are as follows: 1. Measurements vary widely from technician to technician. 2. Technique errors, such as failure to identify the proper measurement site and failure to pinch only fat and no other tissue. 3. The underlying assumption that subcutaneous fat represents the same percentage of total fat in every person. 4. There are also problems opening skin fold calipers wide enough for very obese persons, resulting in underestimation. 5. There are many site measurements where skin fold measurements can be taken. Currently over 100 different equations are available to estimate body fat with the use of skin fold calipers. The wide variety of equations reflects the problem with the accuracy of this methodology. 6. The assumption that 50% of human body fat is located in subcutaneal tissues and the remaining 50% is found in intra-muscular and essential fat (around organs) is not universally valid. Body fat distribution and health risk varies depending on genetics, exercise and nutritional patterns. (Cooper, et al, 1978) Obviously there are many limitations with the Skin Fold measurement technique. Again, the validity of skin fold measurements is at best Âą 7% compared to the hydrostatic tank. Because of the inaccuracy associated with skin fold calipers, many credible organizations such as the United States Army and the Los Angeles Police Department have abandoned the use of skin calipers to measure body fat.

Height and Weight Tables In 1953 the Metropolitan Life Insurance Company developed the first height and weight tables to calculate the degree of individuals over or under weight status. The data was based on "averages" from its client base for both men and women. In 1983 the tables were revised based on updated data.

Frame size is an important, subjective factor utilized in the development of the tables with small, medium and large frame determinations changing the "ideal weight" recommendation. Improvement on frame size determinations were implemented in 1986 with the elbow breadth or wrist circumference measurements used to classify frame size. Objective: to measure body composition. Age level: age 16 through college level. Reliability: Reliability coefficients range from .91 to .97. Objectivity: An objectivity coefficients range from .91 to .96. Validity: The validity of estimation of percent body fat and density by height and weight measurements when compared to the Hydrostatic tank is very poor with correlation coefficients ranging from .31 to .53. This just goes to show you that a test could be very reliable, have a high objectivity rating and have absolutely no validity. Equipment required: Scale, and tape measure. Procedure: Understood Other considerations: The use of the Metropolitan height and weight table gives no indication as to the degree of either obesity or leanness on an individual basis. In the individual clinical setting, height and weight tables can provide grossly inaccurate conclusions about an individual's health risk. As mentioned the validity of estimation of percent body fat and density by height and weight measurements when compared to the Hydrostatic tank is very poor.

Body Mass Index (BMI) Body Mass Index has recently been used to quantify an individual's obesity level. BMI is derived from a ratio equation of height squared divided by weight. Here again, only an individual's height and weight are used and no indication of actual lean or fat mass can be determined. Thus, BMI offers little advantage over the existing Metropolitan tables. Age level: age 12 through college level. Reliability: Reliability coefficients range from .91 to .97. Objectivity: An objectivity coefficients range from .91 to .94. Validity: Although the use of the Body Mass Index provides a reasonably reproducible value and gives a topographical assessment of an individual, the established accuracy for the prediction of body fat is at least Âą7% compared to the hydrostatic tank. Equipment required: standard calibrated scale. Procedure: Use a weight scale on a hard, flat, uncarpeted surface. Wear very little clothing and no shoes. Weigh yourself to the nearest pound. With your eyes facing forward and your heels together, stand very straight against a wall. Your buttocks, shoulders and the back of your head should be touching the wall. Mark your height at the highest point of your head. Then measure your height in feet and inches to the nearest 1/4 inch. Also figure your height in inches only. Find your height in feet and inches in the first column of the Body Mass Index Risk Levels table. To calculate your exact BMI value, multiply your weight in pounds by 703, divide by your height in inches, then divide again by your height in inches. You can calculate your Body mass index by using the following online calculator: The ranges of weight that correspond to minimal risk, moderate risk (overweight) and high risk (obese) are shown in the three columns for each height.

Height Minimal risk Moderate risk (BMI under (BMI 25– 25) 29.9) Overweight

High (BMI above) Obese


118 lbs. or less

119–142 lbs.

143 lbs. or more


123 or less


148 or more


127 or less


153 or more


131 or less


158 or more


135 or less


164 or more


140 or less


169 or more


144 or less


174 or more


149 or less


180 or more


154 or less


186 or more


158 or less


191 or more


163 or less


197 or more


168 or less


203 or more


173 or less


209 or more


178 or less


215 or more


183 or less


221 or more


188 or less


227 or more


193 or less


233 or more


199 or less


240 or more


204 or less


246 or more


risk and

Other considerations: The body mass index is a general measure of body weight based on a person's weight and height. Though it does not actually measure the percentage of body fat, it is used to estimate a healthy body weight based on a person's height. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify weight problems within a population, usually whether individuals are underweight overweight or obese. Some argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health. The medical establishment has generally acknowledged some shortcomings of BMI. Because the BMI is dependent only upon weight and height, it makes simplistic assumptions about distribution of muscle and bone mass, and thus may overestimate fat on those with more lean body mass (e.g. athletes) while underestimating fat on those with less lean body mass (e.g. the elderly). A further limitation relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight.


% Body Fat = (495 / Body Density) - 450. The Siri Equation is based on the two compartment model, that is the body is made up of essentially...