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Provide an estimate of total body volume with hydrostatic weighing

This is an excerpt from Advanced Fitness Assessment and Exercise Prescription, Seventh Edition With Online Video by Vivian H. Heyward, PhD, and Ann L. Gibson, PhD.

Hydrostatic Weighing

Hydrostatic weighing (HW) is a valid, reliable, and widely used laboratory method for assessing total Db. Hydrostatic weighing provides an estimate of total body volume (BV) from the water displaced by the body’s volume. According to Archimedes’ principle, weight of a body under water is directly proportional to the volume of water displaced by the body’s volume. For calculating Db, body mass is divided by body volume. The total Db is a function of the amounts of muscle, bone, water, and fat in the body.

 

Using Hydrostatic Weighing

Determine BV by totally submerging the body in an underwater weighing tank or pool and measuring the underwater weight (UWW) of the body. To measure UWW, you can use either a chair attached to an HW scale (see figure 8.1) or a platform attached to load cells (see figure 8.2). Given that the weight loss under water is directly proportional to the volume of water displaced by the body’s volume, the BV is equal to the body mass (BM) minus the UWW (see figure 8.3). The net UWW is the difference between the UWW and the weight of the chair or platform and its supporting equipment (i.e., tare weight). The BV must be corrected for the volume of air remaining in the lungs after a maximal expiration (i.e., residual volume or RV), as well as the volume of air in the gastrointestinal tract (GV). The GV is assumed to be 100 ml.

Hydrostatic weighing using scale and chair.

Hydrostatic weighing using load cells and platform.

The RV is commonly measured using helium dilution, nitrogen washout, or oxygen dilution techniques. The RV is measured in liters and must be converted to kilograms (kg) in order to correct UWW. This is easy to do because 1 L of water weighs approximately 1 kg; therefore, the water weight per liter of RV is 1 kg. To correct the BV, you subtract the equivalent weight of the RV and the GV (100 ml or 0.1 kg). Since water density varies with water temperature, the BV is corrected for water density (see figure 8.3). Under normal circumstances, the water temperature of the underwater weighing tank or swimming pool will be between 34° and 36° C. The resulting equation for BV is

BV = [(BM - net UWW) / density of water] - (RV + GV)

Calculate body density (Db in g·cc-1) by dividing BM by BV: Db = BM / BV. After you calculate Db, you can convert it into percent body fat (%BF) by using the appropriate population-specific conversion formula (see table 8.2).

You should adhere to "Guidelines for Hydrostatic Weighing" when using the HW technique.

In addition to the HW testing guidelines, following the suggestions in "Tips for Minimizing Error in Hydrostatic Weighing" may improve the accuracy of your underwater weighing measurements.

 

Guidelines for Hydrostatic Weighing

Pretest Guidelines for Clients

  • Do not eat or engage in strenuous exercise for at least 4 hr before your scheduled appointment.
  • Avoid ingesting any gas-producing foods or beverages (e.g., baked beans, diet soda) for at least 12 hr before your test.
  • Bring a towel and a tight-fitting, lightweight swimsuit.

Testing Procedure Guidelines

  • Carefully calibrate the body weight scale and underwater weighing scale. To determine the accuracy of the autopsy scale, hang calibrated weights from the scale and check the corresponding scale values. To calibrate a load cell system, place weights on the platform and check the recorded values.
  • Measure the underwater weight of the chair or platform and of the supporting equipment and weight belt; the total is the tare weight.
  • Measure your client’s dry weight (weight in air) to the nearest 50 g.
  • Check and record the water temperature of the tank just before the test; it should range between 34° and 36° C. Use the constant values in figure 8.3 to determine the density of the water at that temperature.
  • Instruct your client to enter the tank slowly, so that the water stays calm. Have the client gently submerge without touching the chair or weighing platform and rub hands over the body to eliminate air bubbles from the swimsuit, skin, and hair.
  • Have the client kneel on the underwater weighing platform or sit in the chair. Your client may need to wear a scuba diving weight belt to facilitate the kneeling or sitting position. If RV is being measured simultaneously, insert the mouthpiece at this time. If RV is measured outside of the tank, administer the RV test before the client changes clothes and showers.
  • Have the client take a few normal breaths and then exhale maximally while slowly bending forward to submerge the head. Check to make certain that the client’s head and back are completely underwater and that the arms and feet are not touching the sides or bottom of the tank. Instruct the client to continue exhaling until RV is reached. The client needs to remain as still as possible during this procedure. A relaxed and motionless state under water will aid in an accurate reading of UWW.
  • Record the highest stable weight with the client fully submerged at RV, then signal to the client that the trial is completed.
  • Administer as many trials as needed to obtain three readings within ±100 g. Most clients achieve a consistent and maximal UWW in four or five trials (Bonge and Donnelly 1989). Average the three highest trials and record this value as the gross UWW.
  • Determine the net UWW by subtracting the tare weight from the gross UWW. The net UWW is used to calculate body volume (see figure 8.3).

Tips for Minimizing Error in Hydrostatic Weighing

  • Make sure that your clients adhere to all pretesting guidelines.
  • Before each test session, check the calibration of the BW and UWW scales or load cells and carefully calibrate the gas analyzers used to measure RV.
  • Precisely measure BW to ±50 g, UWW to ±100 g, and RV to ±100 ml.
  • Coach the client to maximally exhale and remain motionless under the water.
  • Steady the underwater weighing apparatus as the client submerges, but remove your hand from the scale before actually reading the UWW.
  • If possible, use a load cell system and measure RV and UWW simultaneously.
  • Carry the calculated Db value out to five decimal places. Rounding off a Db of 1.07499 g·cc-1 to 1.07 g·cc-1 corresponds to a difference of 2.2% BF when converted with the Siri (1961) two-component model formula.
  • If you are estimating %BF from Db with a two-component model, use the appropriate population-specific conversion formula (see table 8.2).

Special Considerations

Some clients may have difficulty performing the HW test using these standardized procedures. Accurate test results are highly dependent on the client’s skill, cooperation, and motivation. The following section addresses the use of modified HW procedures, as well as other questions and concerns about the use of this method.

What should I do when my client is unable to blow out all of the air from the lungs or remain still while under water?

 

You will likely come across clients who are uncomfortable expelling all of the air from their lungs during HW. In such cases, you can weigh these individuals at functional residual capacity (FRC) or total lung capacity (TLC) instead of RV. Thomas and Etheridge (1980) underwater-weighed 43 males, comparing the body densities measured at FRC (taken at the end of normal expiration while the person was submerged) and at RV (at the end of maximal expiration). The two methods yielded similar results. Similarly, Timson and Coffman (1984) reported that Db measured by HW at TLC (vital capacity + RV) was similar (less than 0.3% BF difference) to that measured at RV if TLC was measured in the water. However, when the TLC was measured out of the water, the method significantly overestimated Db. When using these modifications of the HW method, you must still measure RV in order to calculate the FRC or TLC of your client. Also, be certain to substitute the appropriate lung volume (FRC or TLC) for RV in the calculation of BV.

 

People uncomfortable under water tend to have difficulty being still while fully submerged. Your client’s movement under water causes the arm of the scale to move. In addition to prolonging the time your client is under water, it may preclude your ability to confidently determine your client’s underwater weight. The damping technique as described by Moon and colleagues (2011) reduces the magnitude of the swings in the scale arm until the client and chair become stable under water. Damping is performed by temporarily holding the moving part of the scale (where the chair attaches) to apply an upward force that counters the motion associated with submersion or movement in the chair. Gently releasing the hold prior to the end of the maximal exhalation maneuver allows the scale arm to stabilize for a more accurate measurement. The damping technique produced similar underwater weights compared to hydrodensitometric assessments made via load cell and without damping (Moon et al. 2011).

 

Because of their lower Db, clients with greater amounts of body fat are more buoyant than leaner individuals; therefore, they have more difficulty remaining motionless while under the water. To correct this problem, place a weighted scuba belt around the client’s waist. Be certain to include the weight of the scuba belt when measuring and subtracting the tare weight of the HW system.

 

What should I do when my clients are afraid to put their face in the water or are not flexible enough to get their backs and heads completely submerged?

Occasionally, you will encounter clients who are extremely fearful of being submerged, who dislike facial contact with water, or who are unable to bend forward to assume the proper body position for HW. In such cases, a satisfactory alternative would be to weigh your clients at TLC while their heads remain above water level. Donnelly and colleagues (1988) compared this measure (i.e., TLCNS or total lung capacity with head not submerged) to the criterion Db obtained from HW at RV for 75 men and 67 women. Vital capacity was measured with the subject submerged in the water to shoulder level. Regression analysis yielded the following equations for predicting Db at RV, using the Db determined at TLCNS as the predictor:

 

Males

Db at RV = 0.5829(Db at TLCNS) + 0.4059

r = 0.88, SEE = 0.0067 g·cc-1

Females

Db at RV = 0.4745(Db at TLCNS) + 0.5173

r = 0.85, SEE = 0.0061 g·cc-1

 

The correlations (r) between the actual Db at RV and the predicted Db at RV were high, and the standard errors of estimate (SEE) were within acceptable limits. These equations were cross-validated for an independent sample of 20 men and 20 women. The differences between the Db from HW at RV and the predicted Db from weighing at TLCNS were quite small (less than 0.0014 g·cc-1 or 0.7% BF). This method may be especially useful for HW of older adults, obese individuals with limited flexibility, and people with physical disabilities.

 

Will the accuracy of the HW test be affected if I estimate RV instead of measuring it?

Several prediction equations have been developed to estimate RV based on the individual’s age, height, gender, and smoking status (see appendix D.1, "Prediction Equations for Residual Volume"). However, these RV prediction equations have large prediction errors (SEE = 400 - 500 ml). When RV is measured, the precision of the HW method is excellent (\lte\1% BF). However, this precision error increases substantially (±2.8 - 3.7% BF) when RV is estimated (Morrow et al. 1986). Therefore, always measure RV when you are using the HW method.

 

When is the best time during the menstrual cycle to hydrostatically weigh my female clients?

Some women, particularly those whose body weight fluctuates widely during their menstrual cycles, may have significantly different estimates of Db and %BF when weighed hydrostatically at different times in their cycles. Bunt, Lohman, and Boileau (1989) reported that changes in total body water values due to water retention during the menstrual cycle partly explain the differences in body weight and Db during a menstrual cycle. On the average, the relative body fat of the women was 24.8% BF at their lowest body weights, compared to an average of 27.6% BF at their peak body weights during their menstrual cycles. Because their low and peak body weights occurred at different times during the menstrual cycle (varied from 0 to 14 days prior to the onset of the next menses), the effect of total body water fluctuations cannot be routinely controlled by using the same day of the menstrual cycle for all women. However, when you are monitoring changes in body composition over time or establishing healthy body weight for a female client, it is recommended that you hydrostatically weigh her at the same time within her menstrual cycle and outside of the period of her perceived peak body weight.


This is an excerpt from Advanced Fitness Assessment and Exercise Prescription, Seventh Edition With Online Video by Vivian H. Heyward, PhD, and Ann L. Gibson, PhD.

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