To measure UWW, you can use either a chair attached to an HW scale see figure 8. 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.

The net UWW is the difference between the UWW and the weight of the chair or platform and its supporting equipment i. The BV must be corrected for the volume of air remaining in the lungs after a maximal expiration i. The GV is assumed to be ml. 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 ml or 0. Since water density varies with water temperature, the BV is corrected for water density see figure 8. The resulting equation for BV is. 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.

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 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. 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 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. 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 compared this measure i. 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:.

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. This method may be especially useful for HW of older adults, obese individuals with limited flexibility, and people with physical disabilities.

Several prediction equations have been developed to estimate RV based on the individual's age, height, gender, and smoking status see appendix D. Therefore, always measure RV when you are using the HW method. Bunt, Lohman, and Boileau 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 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. Again, if your measurement device provided you with mass instead of weight, then calculate the object weight. Explain your reasoning. Calculate the mass of the water displaced.

Now you know the body volume of the object, so if you know its body mass then you can calculate its body density. Calculate the body mass from body weight you found earlier, and then calculate body density. Show all work.

The work you just did recreated the complicated formula for shown at the front of the lab except for the residual body gasses part! Now that we know how it works, from now on we can just use the formula instead of going through the extra steps. Just to make sure, use the formula to calculate density of your object, only leaving out the part accounting for residual body gasses.

Use the body fat percentage formula at the start of the lab to calculate the body fat percentage of your object. Does the result seem reasonable? If not, any thoughts on why?

mathematical explanation of the relation between measured values that is used for making predictions. Body Physics: Motion to Metabolism Copyright © by Lawrence Davis is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.

Skip to content Hydrostatic Weighing Materials: graduated cylinder required for displacement method, but not required for hydrostatic weighing method plastic bin to contain spills object to submerge calculator scale with at least 0.

Formulas for calculating body fat percentage from body density and other body measurements. Image Credit: MattVerlinich via Instructables [1] Body density is calculated from body weight , under water weight , water density and residual lung volume.

Outcome Use your recorded mass and volume to calculate the density of your object. Based on your measured weight and apparent weight, what must be the size of the buoyant force?

Body Fat Percentage Use the body fat percentage formula at the start of the lab to calculate the body fat percentage of your object.

definition mathematical explanation of the relation between measured values that is used for making predictions.