Underwater Weighing Using the Hubbard Tank vs the Standard Tank

Transcription

1 Underwater Weighing Using the Hubbard Tank vs the Standard Tank DUANE WILLIAMS, TIM ANDERSON, and DEAN CURRIER The purpose of this study was to determine whether underwater weighing in the Hubbard tank is a valid and reliable method for estimating body composition. Thirty healthy subjects were weighed underwater for four separate trials; each trial consisted of five weighings. The first trial was completed in a standardized sit-in underwater weighing tank, and the last three trials were completed in a Hubbard tank. Validity was determined by calculating Pearson's correlation coefficient across the four underwater trials, which resulted in r =.077 or higher; The coefficient of variation was found to be 1.22 percent for the sit-in tank and 1.23 percent for the Hubbard tank. A paired t test between the two methods was significant at the.001 level. Reliability for the Hubbard tank method was determined by the repeated measures analysis of variance across the three trials and found not to be significant. The intraclass correlation for the three Hubbard tank trials resulted in R =.997. In addition, the interrater reliability between the two researchers who made independent observations of the weight scale resulted in r =.999 for the standardized sit-in tank and r =.998 for the Hubbard tank. Therefore, the Hubbard tank appears to be valid, reliable, and equally effective as the standardized sit-in underwater weighing tank for determining body composition. Because the paired t test was significant, however, the two methods should not be interchanged; one or the other method should be used, but not both. Key Words: Body weight, Obesity, Physical therapy, Skinfold thickness. A renewed interest in studying body composition, especially fat content as it relates to fitness, nutrition, and weight control, has surfaced in the literature in recent years. 1-4 Although lean bodies are generally equated with fitness, a minimal amount of fat is essential for good health. For men, a minimal level of 3 to 5 percent body fat is advocated, and for women, 7 to 12 percent is recommended. 1-3 College-aged men average 12.5 percent body fat; college-aged women average about 25 percent body fat. 1-3 Obesity is defined as more than 20 percent fat for men and more than 30 percent fat for women. 1, 3 The optimal level for each individual lies somewhere between the extreme ranges (3-20% for men, 7-30% for women). Essential fat, the minimal amount of fat needed for health and normal function, is stored in the brain, spinal cord, bone marrow, fat pads of synovial joints, certain organs, mammary tissues of women, and in the subcutaneous tissues. 2, 4 Nonessential fat, excessive fat not needed for health or function, can be detrimental to human performance, cardiovascular function, overall health, and general esthetic well-being. 1 " 4 Mr. Williams was Associate Professor, Department of Physical Therapy, HP 500, University of Kentucky Medical Center, Lexington KY 40536, at the time of this study. He is currently with Physical Therapy Rehabilitation Services, 125 W McKinney Ave, Rogersville, TN (USA). Mr. Anderson is Assistant Professor, Department of Physical Education, Rm 102, California State University, Fresno, CA Dr. Currier is Professor, Department of Physical Therapy, HP 500, University of Kentucky Medical Center, Lexington, KY This article was submitted April 25, 1983; was with the authors for revision 27 weeks; and was accepted December 20, Various methods are used to estimate the percentage of body fat in an individual. The least suitable method is using standardized weight and height tables for estimating percentage of body fat. 1, 3 These tables are misleading because they are constructed on a theoretical basis that assumes human stature and somatotype to be similar when viewed cross sectionally and longitudinally. Another commonly used method is that of measuring several body sites with a skinfold caliper. Although this method is convenient, it only measures the distribution of subcutaneous fat and does not measure internal fat; however, the caliper method becomes more reliable when multiple sites are sampled and the results are 1, 3, 5 combined with selected anthropometric measurements. For increased accuracy, investigators concerned with human body composition have relied on the hydrostatic underwater weighing method as the standard for determining relative body density or specific gravity The hydrostatic method of underwater weighing is based on Archimedes' principle, which states that the reduction of the body weight underwater is equal to the weight of the water displaced by the body. Body density is the ratio of body mass to body volume, whereas specific gravity is the ratio of body density to the density of a standard substance, which is water for solids and liquids. Calculating body density or specific gravity allows estimating the percentage of body fat, which is useful for clinicians working with cardiac patients, obese patients, and clients concerned about general fitness and sports medicine. 658 PHYSICAL THERAPY

2 RESEARCH Standard tanks used for hydrostatic underwater weighing are expensive and not commonly available to clinical physical therapists. The Hubbard tank, however, is often available in physical therapy clinics in hospitals. The purpose of this study was to determine whether the Hubbard tank, if used as an alternative to special underwater weighing tanks, is a valid and reliable method for underwater weighing. If so, this measurement procedure would be more readily available, and physical therapists could provide this service for specific patient groups or for the general public. METHOD TABLE 1 Means and Standard Deviations of Demographic Information of Subjects s Range Variable Women (n = 16) Age (yr) Height (cm) Weight (kg) Men (n = 14) Age (yr) Height (cm) Weight (kg) Subjects Thirty healthy subjects, 14 men and 16 women, volunteered for this experiment. Table 1 shows demographic information about the subjects. All subjects were familiar with the technique of underwater weighing and had been weighed underwater at least once during the six months before the study. We obtained written consent after all subjects were informed of the nature of the study. Procedure Subjects reported for weighing at midafternoon after they had followed their routine dietary patterns. After donning a bathing suit, subjects were measured for height (cm) while standing barefooted with their backs against a wall where a tape measure was secured. Their body weight (lb) was measured on a standard calibrated balance scale.* The protocol required each subject to be weighed first by one trial in the underwater weighing tank (a standard tank at the University of Kentucky), then dry off, and walk approximately 220 m to another building where the Hubbard tank was located. We did only one trial of underwater weighing in the standard tank because the reliability and validity of this method has been established.6"10 Then, three trials of underwater weighing were performed in the Hubbard tank. A trial consisted of five individual weighings without the subject leaving the tank. Each weighing consisted of a single submersion with independent readings of the spring scale by two investigators. The heaviest weight repeatedly observed by both investigators was recorded as the underwater weight for that trial and was used for later calculations. The water temperature for both tanks was maintained at 35 to 36 C ( F) to approximate the body temperature of the subjects to minimize further corrections and to maintain a standard density of the water.8 We measured the water temperature with a portable thermometer. Standard Tank Procedure The standard tank used for underwater weighing in this study was a custom-made sunken tank, 165 cm (66 in) deep, 165 cm (66 in) wide, and cm (69 in) long. The tank was designed and built specifically for underwater weighing with subjects sitting upright (Fig. 1). Before entering the water, each subject donned a 3.8-kg (8.25-lb) weight belt to maintain negative buoyancy underwater. The combined weight of the chair and weight belt was measured and found to be 6.57 kg (14.45 lb) underwater. Each subject, trying not to agitate the water, slowly descended into the tank on a ladder, and sat on a plastic chair. Before each weighing, an investigator assisted in wiping away any observed air bubbles from the chair or subject. The chair was suspended by chains from a suspension scale (10 lb x 1 oz, weighs to 30 lb, triple-turn dial) which was, in turn, supported by a hydraulic lift. For each weighing, Fig. 1. Standard sit-in tank. Subject is submersed motionless in a sitting position while an observer allows damping of the needle oscillations on the weight scale before recording WW. Volume 64 / Number 5, May 1984 * Health-O-Meter, 7400 W 100 Place, Bridgeview, IL Yellow Springs Instrument, Box 279, Yellow Springs, OH Toledo Scale, Div of Reliance Electric Co, Toledo, OH Ted Hoyer & Co, Inc, Oshkosh, WI (rated capacity 450 lb, Serial #L37531). 659

3 head electric hoist# assembly. The submersed body stretcher was weighed underwater before each weighing period; drainage holes in the perimeter tubing of the stretcher were covered with waterproof tape to prevent water catchment. The weight of the stretcher ranged from a low of 8.05 kg (17.75 lb) to a high of 8.24 kg (18.16 lb) for the weighings made on different test days. For the actual weighings, we asked each subject to enter the tank slowly and to assume a prone position on the stretcher with minimum disturbance of the water. The subjects did not need a weight belt to maintain negative buoyance. An investigator assisted in wiping away any bubbles observed on the subject or stretcher. Each subject then submersed slowly onto the stretcher making sure no body parts touched the bottom of the tank. Each subject remained submersed in a prone position, exhaled until RV was attained, and then remained motionless underwater as long as possible to allow damping of the needle oscillations on the weight scale. Two investigators then made independent readings of the scale. Each subject completed three trials of five weighings each in the Hubbard tank. Subjects took turns being weighed; their trials were not consecutive to reduce the probability of investigators remembering the results from the previous trial. Data Processing Fig. 2. Hubbard tank. Subject is submersed motionless in a prone position on a stretcher while the scale is steadied by one observer and weight is recorded by another observer. the subject submersed slowly from standing to a sitting position in the chair and then gently lifted his or her feet off the bottom of the tank while gradually exhaling and bending the trunk forward until residual volume (RV) was attained. The subject then sat motionless underwater as long as possible to allow damping of the needle oscillations on the weight scale and ensure accurate readings. The five submersions constituting a trial were completed while the two investigators made independent readings of the scale on each submersion. The human body can be considered in two parts: fat and fat-free mass. Fat-free mass consists of water, organic matter, and minerals. Water has a density (mass/volume) of approximately 1.00 kg/l at 4 C (39.2 F). Because fat has a lower density, approximately 0.90 kg/l, it willfloatin water.6, 10 On the other hand, fat-free mass of the human body has a higher density, approximately 1.10 kg/l, so it will sink in water.710 Thus, lean people who have small percentage of body fat will sink more easily and weigh heavier underwater than fat people who have a high percentage of body fat. Fat peoplefloatmore easily and weigh lighter underwater than lean people. When determining body density (mass/volume), the mass is considered equivalent to the weight of the body in air (WA). The volume of the body is determined by calculating the loss of body weight when weighed underwater (WW) and corrected for by the density of water (DW) corresponding to the water temperature at the time of the weighing. For this experiment, the DW was.994 kg/l for the water temperature range of 35 to 36 C ( F). Thus, we derived the volume (V) from the following equation:7 (1) Hubbard Tank Procedure The Hubbard Tank used in this study is commonly found in physical therapy departments (Fig. 2). The depth of the tank is 56 cm (22 in), the overall length is 267 cm (105 in), the overall width at the upper end is cm (77 in), the inset width is 89 cm (35 in), and the overall width at the lower end is 135 cm (53 in). We placed waterproof tape over the overflow drain outlets and filled the tank with water to within 5.1 cm (2 in) of the tank's brim. The actual depth of the water was 51 cm (20 in). We suspended a standard body stretcher with an aluminum frame and nylon cover from the same scale used in the standard tank, which was, in turn suspended from the over Ille Hydromassage Subaqua Therapy, Ille Electric Corp, Williamsport, PA (Model 801, Serial # ). 660 The underwater body weight is determined by suspending a subject below the surface of the water on a suspension apparatus attached to a line leading to a weight scale as performed in this experiment. Behnke et al devised the basic formula for body density (D):7 (2) The WW, however, is influenced by two extraneous volumes, air trapped in the lungs and air trapped in the gastrointestinal (GIV) tract. Furthermore, the WW has to be corrected # Chisolm Moore, Tonawanda, NY PHYSICAL THERAPY

4 RESEARCH for the weight of the suspension device and for any weight belts used to aid a subject in staying submerged below the surface of the water, ie, maintaining negative buoyancy. These two air volumes need to be subtracted from the volume of the body underwater. Behnke et al 7 and Buskirk 8 devised this basic formula for determining the density of the body with these factors: Various methods for measuring or predicting RV include indirect measuring by nitrogen or oxygen dilution, predicting a percentage of vital capacity, assuming RV to be 1,300 ml for women or 1,500 ml for men, or predicting from a regression equation. 11, 12 Wilmore calculated the densities of a group of 197 subjects (69 men and 128 women), using all of the above methods for assessing RV. 11 He found that the mean within-subject difference in calculated density was 1 percent and recommended that when high degrees of accuracy are necessary, measurements of RV should be used. In situations where systematic errors are of no consequence (such as, comparing two methods of weighing), however, predictions can be used. Current study. We used the following regression equations devised by Grimby and Soderholm to predict RV for men and women, respectively: 12 (3) RV M = [.022 (age in yr) (height in m) (4) (wt in kg)- 1.54] RV F = [.007 (age in yr) (height in m) ] (5) Buskirk found the average GIV to be negligible, 100 ml. 8 The GIV is not generally considered for the standardized sitin method commonly performed at the University of Kentucky, so we did not use it in the formula for this study. In addition, any extraneous weight caused by the suspension apparatus or weight belts used for subjects to maintain negative buoyancy was subtracted from the measured WW. As mentioned previously, a weight belt was not necessary for the subjects tested using the Hubbard tank procedure. Most likely, the weight belt was not necessary for the same subjects tested in the standard tank but was used because it was a standard procedure in our Health, Physical Education, and Recreation department. After accounting for the extraneous volumes and the extraneous weights, we computed the equation of Goldman and Buskirk for D: 13 (6) Equations that convert D of humans to percentage of fat have been developed by several investigations. 6, 7,10 We chose the most recent and widely used of these equations, which was developed by Brozek et al: 10 To attain optimal accuracy with this particular equation, body weight should be stable and the individual should be normally hydrated. 10 Furthermore, this equation does not provide good (7) estimation of body fat for children and older persons or for groups outside the range of 10 to 30 percent fat In summary, the protocol for this study specified the following steps to be used to calculate the percentage of body fat for each individual for each trial of underwater weighing: 1. Subject dons a bathing suit, stands barefooted, his or her height is measured in centimeters, and then recorded in meters. 2. The subject's WA and WW is measured in pounds and recorded in kilograms. 3. The net WW is determined by subtracting the weight of the suspension system and any other additional weights (weight belt) if used to retain negative buoyance. 4. The RV is determined by using regression equation 4 for men and regression equation 5 for women. 5. The body density is calculated using equation 6, which accounts for the water density and the RV. 6. The percentage of body fat is estimated by using equation 7. The Appendix gives a sample calculation for percentage of body weight for one subject. Analysis of Data We calculated D (kg/l) with equation 6 and percentage of body fat with equation 7 for each subject for each of the four underwater weighing trials. The descriptive mean, standard deviation, and range of this data were determined for each of the statistical tests to determine validity and reliability. Validity was determined by three methods. First, we calculated the Pearson's correlation coefficient (r) across the four underwater weighing trials. Second, we calculated the paired t test for the mean value of the sit-in tank and the mean value of the three trials of the Hubbard tank. Third, we determined the coefficient of variation, the ratio of the standard deviation to the mean, for the mean value of the sit-in tank and the mean value of the three trials in the Hubbard tank. An intraclass correlation coefficient (ICC) was used to determine reliability by using one-way repeated measures for analysis of variance (ANOVA) to compare mean scores for the three Hubbard tank trials.** The ICC, the degree of association between multiple measures and the coefficient, represents the variance common in repeated measures Interrater reliability was determined by calculating a Pearson correlation coefficient. Results Table 2 shows measures of body fat (%) and body density (kg/l). Pearson's correlation coefficient calculated across the four underwater weighing trials for determining validity ranged from to as depicted in Table 3. The calculated t value equaled 5.35 for the paired test, which was highly significant at the.001 level. The coefficient of variation was calculated to be 1.22 percent for the mean value of the sit-in tank and 1.23 percent for the mean of the three trials for the Hubbard tank. Repeated measures for ANOVA for the three Hubbard tank trials resulted in the F ratio = 0.42, which was not significant. The intraclass correlation revealed R =.997 across ** SASBMDP computer program, SAS Institute, Inc, Cary, NC Volume 64 / Number 5, May

5 TABLE 2 Body Fat (%) and Body Density (kg/l) for Four Underwater Weighing Trials (N = 30) Method Body fat Standard sit-in tank Hubbard tank (ail 3 trials) Hubbard tank (trial 1) Hubbard tank (trial 2) Hubbard tank (trial 3) Body density Standard sit-in tank Hubbard tank (all 3 trials) Hubbard tank (trial 1) Hubbard tank (trial 2) Hubbard tank (trial 3) s Range the three trials in the Hubbard tank. Interrater reliability between the two researchers making independent observations of the weight scale resulted in r =.999 for the standardized sit-in tank and r =.998 for the Hubbard tank. DISCUSSION Validity Analysis revealed that underwater weighing in the Hubbard tank is a valid approach for estimating D and the percentage of body fat when compared with the standardized sit-in tank. Although the correlations were very high between the measures of the four underwater weighing trials, and the difference of the coefficient of variation between the two methods was only 0.01 percent, the significant t test indicates that the two methods should not be interchanged; use one or the other but not both. Furthermore, Table 2 reveals that the percentage of body fat calculated was slightly higher for all three trials in the Hubbard tank compared with the standardized sit-in tank. Individual data revealed that 24 of the 30 subjects demonstrated a higher percentage of fat values from the Hubbard tank than from the standardized sit-in tank. Perhaps, the prone position used in the Hubbard tank produced slightly greater RVs (with resulting greater buoyancy) than those achieved in the standardized sit-in tank because the subjects bent forward slightly from the erect sitting position. Several of the subjects commented during the Hubbard tank trials that they felt it was more difficult to expire air in the prone position compared with the bent forward sitting position. A search of the literature, however, revealed no published values for RV in the prone position. We found values for supine 20 and for "lying," 21 and these values were less than for sitting. The authors speculate that RV would be greater in the prone position than in the supine position, because a slight hyperextension of the upper back and neck in the prone position occurs, resulting in a slight elevation of theribcage. Further research is needed to clarify the influence of the prone posture on RV and to answer the question of whether the differences in results from the two tanks are the result of differences in RV. The technique described for underwater weighing in the Hubbard tank, therefore, appears to be valid for the purpose intended. For optimal accuracy, however, to do repeated measures for research compared with screening programs to categorize subjects, RV should be measured during the underwater weighing. Reliability The multiple trials in the Hubbard tank showed high repeatability, suggesting that the technique is very reliable. Thus, different physical therapists performing the technique on the same person would be expected to produce results in close agreement. The quality of the subjects used in the study may have contributed to the high reliability. All subjects were familiar with the procedure and were comfortable in water. Such a high degree of reliability may not be achieved with naive subjects or with persons uncomfortable in water. TABLE 3 Pearson's Correlation Coefficient, r, Across the Four Underwater Weighing Trials Sit-in tank H1 H2 H1 a a H = Hubbard Tank. Potential Errors H H H Although the results of this research give credibility to using the Hubbard tank for underwater weighing, the procedure needs to be approached with an awareness of potential errors. Potential errors in underwater weighing are both biological and methodological. Biologically, the fat-free body varies in composition among different groups. Water, skeletal content, and muscle mass are the major variables. 3 The density of the different tissues varies as well and is affected by age, sex, race, physical training, recent large fluctuations in body weight, and fluid imbalances. 3 " 10 Generally, density of tissues is greater in the young, 914 " 16 greater in men than in women, 4-7, 11, greyer in blacks than in whites, 617 and greater in persons who have participated in heavy physical training programs. 4, 9, 15,16 Density of tissue greater than those on which the formulas were developed will produce underestimations of percentage of body fat. Additionally, recent large fluctuations in body weight or abnormal states of hydration may 662 PHYSICAL THERAPY

6 RESEARCH produce errors, as the composition or density of the tissues or 6, 7, 10 both may be altered. Estimations of RV will usually introduce additional error. Residual volume is affected by variables, such as age, sex, exercise training, posture, smoking habits, and presence of pulmonary disorders. Generally, women have a lower RV than men, and RV tends to increase in both sexes with increasing age and with deconditioning. 6,8,22,23 Smokers generally have larger RVs than do nonsmokers. Underestimation of RV results in underestimation of the buoyancy force and thus overestimation of percentage of body fat. Overestimation of RV has the opposite effect. Persons who are uncomfortable in water may require extensive instruction and practice to attain true RV underwater. As an alternative, these persons may be weighed at total lung capacity rather than RV. When this is done, vital capacity must be measured and added to the estimation or measurement of RV to calculate total lung capacity (TLC). The buoyancy force of TLC is then calculated (divide TLC in milliliters by 454, then multiply by density of water) and added to WW. Weltman and Katch have demonstrated that no differences in results occur when weighings are done with TLC versus RV. 24 CONCLUSIONS The results of this investigation suggest that using the Hubbard tank for underwater weighing and subsequent determination of body composition produces valid and reliable results. Therefore, to estimate body density, percentage of body fat and lean body weight for classification purpose, the Hubbard tank is a viable alternative to standard, specifically designed sit-in tanks. Use of the Hubbard tank for underwater weighing would make the procedure available to many more clients than the standard procedure that requires specialized equipment not commonly found in hospitals. During application of underwater weighing techniques, physical therapists should keep in mind the sources of error for the test and realize that the formulas and protocol used in this study have been developed for group-specific subjects with stable weight who are hydrated and have normal pulmonary function. REFERENCES 1. Sharkey B: Physiology of Fitness. Champaign, IL, Human Kinetics Publishers Inc, 1979, pp Katch Fl, Katch VL, Behnke AR: The underweight female. The Physician and Sportsmedicine 8:55-60, Lohman TG: Body composition methodology in sports medicine. The Physician and Sportsmedicine 10:47-58, Wilmore JH: Body composition in sport and exercise: Directions for future research. Med Sci Sports Exerc 15:21-31, Durnin, JV, Womersley J: Body fat assessed from total body density and its estimation from skinfold thickness: Measurements on 418 men and women aged from 16 to 72 years. J Nutr 32:77-97, Behnke AR: Comment on the determination of whole body density and a resume of body composition data. In Brosek J and Henschel A (eds): Techniques for Measuring Body Composition. Washington, DC, National Academy of Sciences, National Research Council, 1961, pp Behnke AR, Wilmore JH: Evaluation and Regulation of Body Build and Composition. Englewood Cliffs, NJ, Prentice-Hall Inc, 1974, pp Buskirk ER: Underwater weighing and body density: A review of procedures. In Brozek J and Henschel A (eds): Techniques for Measuring Body Composition. Washington, DC, National Academy of Sciences, National Research Council, 1961, pp Flint MM, Drinkwater BL, Wells CL, et al: Validity of estimating body fat of females: Effect of age and fitness. Hum Biol 49: , Brozek J, Grande F, Anderson JT, et al: Densiometric analysis of body composition: Revision of some quantitative assumptions. Ann NY Acad Sci 110: , Wilmore JH: The use of actual, predicted and constant residual volumes in the assessment of body composition by underwater weighing. Med Sci Sports Exerc 1:87-90, Grimby G, Soderholm B: Spirometric studies in normal subjects. III. Static lung volumes and maximum voluntary ventilation in adults with a note on physical fitness. Acta Med Scand 173: , Goldman RF, Buskirk ER: Body volume measurement by underwater weighing: Description of a method. In Brozek J and Henschel A (eds): Techniques for Measuring Body Composition. Washington, DC, National Academy of Sciences, National Research Council, 1961, pp Jackson AS, Pollock ML: Generalized equations for predicting body density of men. Br J Nutr 40: , Jackson AS, Pollock ML, Ward A: Generalized equations for predicting body density of women. Med Sci Sport Exerc 12: , Womersley J, Durnin JV, Boddy K, et al: Influence of muscular development, obesity, and age on the fat-free mass of adults. J Appl Physiol 41: , Harsha DW, Frerichs RR, Berenson GS: Densiometry and anthropometry of black and white children. Hum Biol 50: , Safrit, MJ: Evaluation in Physical Education, ed 2. Englewood Cliffs, NJ, Prentice-Hall Inc, 1981, pp Shrout PE, Fleiss JL: Intraclass correlations: Uses in assessing rater reliability. Psychol Bull 86: , Comroe, JH, Forster RE, Dubois AB, et al: The Lung. Chicago, IL, Year Book Medical Publishers Inc, 1962, p Dittmer DS, Grebe RM (eds): Handbook of Respiration. Philadelphia, PA, WB Saunders Co, 1958, p Brozek J: Age differences in residual volume and vital capacity of normal individuals. J Gerontol 15: , Whitfield AG, Waterhouse JA, Arnott WM: The total lung volume and its subdivision: A study of physiological norms: II. The effect of posture. Brit J Soc Med 4:86-97, Weltman A, Katch V: Comparison of hydrostatic weighing of residual volume and total lung capacity. Med Sci Sports Exerc 13: , 1981 Volume 64 / Number 5, May

7 APPENDIX Sample Calculation of Percentage of Body Fat Using the Standard Tank Procedure for One Subject 1. Height in meters = cm x 100 = m 5. Body density (D) = mass (kg) volume (L) 2. Body weight in air (WA) = 160 lb x.4536 = 72.6 kg Body weight underwater (WW) = lb x.4536 = kg 3. Body weight underwater (WW) = kg -Suspension system and weight belt = kg Net underwater weight = 3.70 kg 4. Residual volume for man (equation 4) RV M = [.022 (35) (1.715) (72.6) ] age ht in m wt in kg RV M = RV M = 1.537L 664 PHYSICAL THERAPY