Fields et al. In the current study, residual lung volume was measured simultaneously in water and TGV was measured during testing.

This is significant because only two other studies have been able to simultaneously measured residual lung volume while underwater weighing in children [ 13 , 14 ]. Interestingly, the coefficient of variance CV for repeated measures over two days in a subset of the children in this study for ADP and HW was 3.

reported an ADP CV of 8. Consequently, the high CV may be playing a role in the true relationship between ADP and HW. Due to the ease in the testing procedure and high subject compliance for all ages, ADP has quickly begun to emerge as a popular body composition method to use in children and adults.

In the current study, we found an overall poor agreement between ADP and HW with the study inadequately powered to make any definitive statements concerning potential gender differences between the techniques.

In conclusion, we recommend more studies validating ADP and HW in children be performed utilizing a larger sample size. Hedley AA, Ogden CL, Johnson CL, Carroll MD, Curtin LR, Flegal KM: Prevalence of overweight and obesity among US children, adolescents, and adults, Article CAS PubMed Google Scholar.

Dempster P, Aitkens S: A new air displacement method for the determination of human body composition. Med Sci Sports Exerc. Biaggi RR, Vollman MW, Nies MA, Brener CE, Flakoll PJ, Levenhagen DK, Sun M, Karabulut Z, Chen KY: Comparison of air-displacement plethysmography with hydrostatic weighing and bioelectrical impedance analysis for the assessment of body composition in healthy adults.

Am J Clin Nutr. CAS PubMed Google Scholar. Fields DA, Wilson GD, Gladden LB, Hunter GR, Pascoe DD, Goran MI: Comparison of the BOD POD with the four-compartment model in adult females. Levenhagen DK, Borel MJ, Welch DC, Piasecki JH, Piasecki DP, Chen KY, Flakoll PJ: A comparison of air displacement plethysmography with three other techniques to determine body fat in healthy adults.

J Parenteral Enteral Nutr. Article CAS Google Scholar. Nunez C, Kovera AJ, Pietrobelli A, Heshka S, Horlick M, Kehayias JJ, Wang Z, Heymsfield SB: Body composition in children and adults by air displacement plethysmography. Eur J Clin Nutr. Vescovi JD, Hildebrandt L, Miller W, Hammer R, Spiller A: Evaluation of the BOD POD for estimating percent fat in female college athletes.

J Strength Cond Res. PubMed Google Scholar. Vescovi JD, Zimmerman SL, Miller WC, Hildebrandt L, Hammer RL, Fernhall B: Evaluation of the BOD POD for estimating percentage body fat in a heterogeneous group of adult humans.

Eur J Appl Physiol. McCrory MA, Gomez TD, Bernauer EM, Mole PA: Evaluation of a new air displacement plethysmography for measuring human body composition. Collins MA, Millard-Stafford ML, Sparling PB, Snow TK, Rosskopf LB, Webb SA, Omer J: Evaluation of the BOD POD for assessing body fat in collegiate football players.

Lockner DW, Heyward VH, Baumgartner RN, Jenkins KA: Comparison of air-displacement plethysmography, hydrodensitometry, and dual X-ray absorptiometry for assessing body composition of children 10 to 18 years of age.

Ann N Y Acad Sci. Demerath EW, Guo SS, Chumlea WC, Towne B, Roche AF, Siervogel RM: Comparison of percent body fat estimates using air displacement plethysmography and hydrodensitometry in adults and children. Int J Obes Relat Metab Disord. Fields DA, Goran MI: Body composition techniques and the four-compartment model in children.

J Appl Physiol. Dewit O, Fuller NJ, Fewtrell MS, Elia M, Wells JC: Whole body air displacement plethysmography compared with hydrodensitometry for body composition analysis. Arch Dis Child. Article CAS PubMed PubMed Central Google Scholar. Wells JC, Douros I, Fuller NJ, Elia M, Dekker L: Assessment of body volume using three-dimensional photonic scanning.

Fields DA, Hunter GR, Higgins PB: Assessment of body composition by air displacement plethysmography: Influence of body temperature and moisture. Dynamic Medicine. Google Scholar. Lohman TG: Assessment of body composition in children. Pediatr Exerc Sci. Ruppell G: Manual of pulmonary function testing.

Louis, Mosby, Bland JM, Altman DG: Statistical methods for assessing agreement between two methods of clinical measurement. Fields DA, Goran MI, McCrory MA: Body-composition assessment via air-displacement plethysmography in adults and children: a review.

Fields DA, Hull HR, Cheline AJ, Yao M, Higgins PB: Child-specific thoracic gas volume prediction equations for air-displacement plethysmography. Obes Res. Article PubMed Google Scholar. Hull HR, Fields DA: Effect of Short Schemes on Body Composition Measurements using Air-Displacement Plethysmography.

Dyn Med. Article PubMed PubMed Central Google Scholar. Vescovi JD, Zimmerman SL, Miller WC, Fernhall B: Effects of clothing on accuracy and reliability of air displacement plethysmography. Fields DA, Hunter GR, Goran MI: Validation of the BOD POD with hydrostatic weighing: influence of body clothing.

Crapo RO, Morris AH, Clayton PD, Nixon CR: Lung volumes in healthy nonsmoking adults. Int Europ Physiopath Resp. CAS Google Scholar. Download references.

We would like to acknowledge Drs. Mike Bemben and Andrew Gardner for their involvement in the project. Department of Health and Exercise Science, University of Oklahoma, Norman, OK, USA.

Department of Pediatrics, University of Oklahoma Health Science Center, Oklahoma City, OK, USA. Children's Medical Research Institute's Metabolic Research Center, University of Oklahoma Health Science Center, OUCP Diabetes and Endocrinology, Oklahoma City, OK, USA.

You can also search for this author in PubMed Google Scholar. Correspondence to David A Fields. David A. Fields has received funding from Life Measurement Incorporated for past studies though Life Measurement Incorporated did not fund this study.

None of the authors have non-financial competing interests or own stock or are applying for patents that may represent a conflict of interest.

GC carried out the hydrostatic weighing and ADP studies, participated in the design of the study, coordinated the study, and helped draft the manuscript. HH provided critical evaluation on ADP testing and participated in writing of the manuscript.

DF conceived the study, performed the statistical analysis, and drafted the manuscript. All authors read and approved the final manuscript. Open Access This article is published under license to BioMed Central Ltd.

Reprints and permissions. Claros, G. Comparison of air displacement plethysmography to hydrostatic weighing for estimating total body density in children. BMC Pediatr 5 , 37 Download citation. Received : 18 April Accepted : 09 September Published : 09 September Anyone you share the following link with will be able to read this content:.

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Skip to main content. Search all BMC articles Search. Download PDF. Methods Sixty-six male and female subjects 40 males: Background Recent results obtained from the National Health and Nutrition Examination Survey NHANES found increases in overweight and obesity not only in adults but also in children [ 1 ].

Methods Subjects A total of 77 subjects were recruited through the Norman Youth Soccer Association. BOD POD instrumentation The BOD POD ® Body Composition System Life Measurement Instruments, Concord, CA was used to assess body volume and total body density with the operating procedures previously described elsewhere [ 2 ].

Hydrostatic weighing Each child's total body density was measured by underwater weighing, with a simultaneous measurement of residual lung volume by using the closed-circuit oxygen dilution technique measurement system EXERTECH, Dresbach, MN.

Data analysis Accuracy, precision, and bias were examined in ADP with HW serving as the criterion method. Results The purpose of this study was to compare ADP with HW in male and female children and adolescents between the ages of 10—15 years old.

Figure 1. Full size image. Table 1 Physical Characteristics of subjects Full size table. Figure 2. Figure 3. Figure 4. Table 2 Summary of regression for density Full size table. Figure 5. Figure 6. Figure 7.

Table 4 Summary of residual volume and thoracic gas volumes for the total group and for both genders Full size table. Discussion The purpose of this study was to validate ADP with HW in a group of children of varying degrees of fatness.

Conclusion Due to the ease in the testing procedure and high subject compliance for all ages, ADP has quickly begun to emerge as a popular body composition method to use in children and adults. References Hedley AA, Ogden CL, Johnson CL, Carroll MD, Curtin LR, Flegal KM: Prevalence of overweight and obesity among US children, adolescents, and adults, Article CAS PubMed Google Scholar Dempster P, Aitkens S: A new air displacement method for the determination of human body composition.

Article CAS PubMed Google Scholar Biaggi RR, Vollman MW, Nies MA, Brener CE, Flakoll PJ, Levenhagen DK, Sun M, Karabulut Z, Chen KY: Comparison of air-displacement plethysmography with hydrostatic weighing and bioelectrical impedance analysis for the assessment of body composition in healthy adults.

CAS PubMed Google Scholar Fields DA, Wilson GD, Gladden LB, Hunter GR, Pascoe DD, Goran MI: Comparison of the BOD POD with the four-compartment model in adult females. Article CAS PubMed Google Scholar Levenhagen DK, Borel MJ, Welch DC, Piasecki JH, Piasecki DP, Chen KY, Flakoll PJ: A comparison of air displacement plethysmography with three other techniques to determine body fat in healthy adults.

Article CAS Google Scholar Nunez C, Kovera AJ, Pietrobelli A, Heshka S, Horlick M, Kehayias JJ, Wang Z, Heymsfield SB: Body composition in children and adults by air displacement plethysmography.

Article CAS PubMed Google Scholar Vescovi JD, Hildebrandt L, Miller W, Hammer R, Spiller A: Evaluation of the BOD POD for estimating percent fat in female college athletes. PubMed Google Scholar Vescovi JD, Zimmerman SL, Miller WC, Hildebrandt L, Hammer RL, Fernhall B: Evaluation of the BOD POD for estimating percentage body fat in a heterogeneous group of adult humans.

Article CAS PubMed Google Scholar McCrory MA, Gomez TD, Bernauer EM, Mole PA: Evaluation of a new air displacement plethysmography for measuring human body composition. Article CAS PubMed Google Scholar Collins MA, Millard-Stafford ML, Sparling PB, Snow TK, Rosskopf LB, Webb SA, Omer J: Evaluation of the BOD POD for assessing body fat in collegiate football players.

Article CAS PubMed Google Scholar Lockner DW, Heyward VH, Baumgartner RN, Jenkins KA: Comparison of air-displacement plethysmography, hydrodensitometry, and dual X-ray absorptiometry for assessing body composition of children 10 to 18 years of age.

Article CAS PubMed Google Scholar Demerath EW, Guo SS, Chumlea WC, Towne B, Roche AF, Siervogel RM: Comparison of percent body fat estimates using air displacement plethysmography and hydrodensitometry in adults and children.

Article CAS PubMed Google Scholar Fields DA, Goran MI: Body composition techniques and the four-compartment model in children. CAS PubMed Google Scholar Dewit O, Fuller NJ, Fewtrell MS, Elia M, Wells JC: Whole body air displacement plethysmography compared with hydrodensitometry for body composition analysis.

Article CAS PubMed PubMed Central Google Scholar Wells JC, Douros I, Fuller NJ, Elia M, Dekker L: Assessment of body volume using three-dimensional photonic scanning. An individual may be on the lower end of the obesity spectrum in terms of total weight, but still possess an enormous risk of cardiovascular diseases due to having too much body fat.

For that reason, BMI is more commonly used despite the lower confidence in this data. Air Displacement Plethysmography is an emerging technology that utilizes air perturbations that occur when a subject enters a confined space in order to determine their body fat levels.

Please click here to view figures collected from a US patent filed for the BodPod: an air plethysmographic apparatus manufactured by Life Measurements Instruments, a medical device company based in Concord, California.

The Bod Pod consists of an air circulation system represented by item 60 on figure 2 linked to a plethysmographic measurement chamber pointed out by item 50 on figure 2.

The air circulation system embodied in greater detail by Fig 3 of the patent , comprised of one or more pumps, acts as both a source of circulation and filtration within the chamber by using ambient air air that is derived from a temperature-enclosed environment.

Clean air is pumped into the chamber via an inlet tube represented by item 86 while contaminated air is moved out of the chamber through an outlet tube represented by item 88 , where it is later filtered and recycled.

In order to gather accurate data, it is imperative that the volume of air in the chamber is recorded before a subject enters the chamber.

Once all data has been collected, it is wirelessly transmitted to a computer for further analysis using software provided by Life Instruments. Dempster Phillip, Michael Homer, and Mark Lowe

For comparison, data from 24 subjects who had undergone hydrostatic weighing were evaluated for the validity of using predicted vs. measured residual lung volume VRpred vs. VRmeas, respectively.

Furthermore, although the use of Vtgpred has some application, determining Vtgmeas is relatively simple in most cases. Therefore, we recommend that the use of Vtgmeas remain as standard experimental and clinical practice. The study findings and the operational and physical characteristics of the system indicate that the PEA POD has the potential to provide clinicians and researchers with a diagnostic and research tool that is accurate, easily used by operators, and comfortable for subjects.

Manoja P. Herath, Jeffrey M. Beckett, … Andrew P. Ameyalli M. Rodríguez-Cano, Omar Piña-Ramírez, … Otilia Perichart-Perera. Dana F. Yumani, Dide de Jongh, … Mirjam M.

van Weissenbruch. The prevalence of obesity in adults and children continues to rise 1 , 2. Strong links also exist between early infant development and childhood obesity 4. In addition to its great affect on the quality of life, obesity is associated with serious health risks 5 , 6.

The assessment of body composition throughout life is therefore an important diagnostic and research tool. In recent years, novel technologies have resulted in the development of new body composition methods 7. Some of these methods have been applied to the infant population 8.

However, technological and practical limitations have hampered the success of these new methods to the point that body composition assessment in infants is still rarely performed. All body composition methods used to test live subjects are indirect.

As a result, each method is based on a theoretical model, which is used to assess body composition from indirect measurements.

Therefore, the accuracy of a body composition method is dependent on the soundness of its theoretical model and the assumptions surrounding the said model.

In light of this, when selecting a method to assess body composition in infants, its accuracy in other populations must be considered because it is representative of the method's theoretical soundness.

Densitometry is considered to be among the most accurate indirect body composition method 9. A densitometric approach to infant body composition assessment could therefore have the potential for high measurement accuracy. Body composition assessment by densitometry involves the measurement of the density of the whole body.

Body density is then used in a two-compartment model to calculate percentage of fat, fat mass, and fat free mass 10 , By definition, the density of the whole body is body mass divided by body volume.

Body mass is easily measured using an accurate weighing device. Body volume is a more difficult measurement and is commonly determined either by hydrodensitometry HD or air displacement plethysmography ADP. HD measurement procedures are performed in water. ADP measurement procedures are performed in air.

This difference is the result of the different operating principles used by the two methods. HD uses Archimedes' principle to determine body volume. ADP uses gas laws to determine body volume.

Because HD requires subjects to be totally submerged during a test, compliance and safety issues prevent the implementation of this technique in the infant population. Conversely, the use of ADP in children and the elderly 9 , 12 has demonstrated that its measurement procedures are easily tolerated in these populations and would probably be tolerated by infants.

The only commercially available ADP system is the BOD POD Body Composition System Life Measurement Inc. The success of the BOD POD has prompted the development of the PEA POD Infant Body Composition System Life Measurement Inc. The PEA POD is intended to provide researchers and clinicians with an infant body composition system that is accurate and easily used by operators as well as comfortable for the subjects.

The physical characteristics of the system allow the testing of infants between birth and 6 mo of age. This article first introduces the PEA POD by giving an overview of its theory, physical design, operating principle, and test procedure.

The precision, reliability, accuracy, and linearity of the PEA POD mass and volume measurements are then assessed by testing National Institute of Standards and Technology NIST -traceable weights and aluminum cylinders.

The relationships between pressure and volume expressed by Boyle's Law and Poisson's Law are the basis of the operating principles used by the PEA POD to measure body volume.

Boyle's Law describes the behavior of air compressed under isothermal conditions constant temperature as follows: where P 1 and V 1 are pressure and volume at an initial condition and P 2 and V 2 are pressure and volume at a final condition. When air is allowed to change temperature in response to volume changes adiabatic conditions , Poisson's Law expresses its behavior as follows:.

For air, γ is 1. A consequence of Equations 1 and 2 is that equal volume changes result in different pressure changes for air under isothermal and adiabatic conditions. The PEA POD basic components are housed inside or mounted on a movable cart Fig.

The movable cart houses the reference chamber, calibration volume, electronic components, printer, and central processing unit CPU. The test chamber, scale, and monitor are mounted on the cart's top surface.

A volume-perturbing diaphragm is mounted between the test and reference chambers. A pneumatic valve calibration valve allows the test chamber to be connected to the calibration volume. Pressure transducers are connected to the two chambers. The prototype version of the PEA POD, Body Composition System, used in this study.

Numbers refer to the test chamber 1 , calibration valve 2 , diaphragm 3 , calibration volume 4 , reference chamber 5 , electronics 6 , sliding tray 7 , and scale 8. The test chamber contains a clear plastic tray on a slide mechanism. The slide mechanism is secured to the inside of a clear acrylic plastic cylinder.

A clear sheet of acrylic plastic seals the back of the cylinder. An aluminum door is mounted to the front of the cylinder. During a test, the door is kept closed by an electromagnet. The test chamber is connected to the reference chamber by an acrylic plastic manifold.

However, the two chambers are not in direct contact with each other. The volume-perturbing diaphragm is mounted between them in the manifold. The same design and materials of the test chamber, with the exclusion of the door system, are used for the reference chamber.

Furthermore, the two chambers are equal in volume 37 L. The test chamber is connected to the calibration volume by an aluminum manifold. The calibration valve is mounted between the test chamber and the aluminum manifold. When the calibration valve is open, the test chamber and calibration volume are in direct contact with each other.

The calibration volume consists of a 5 L aluminum sphere. The PEA POD scale measures mass using strain gauge technology.

The materials used in the scale's strain gauge were selected for their stability. The scale has a capacity of 12 kg and a resolution of 0. Extensive testing in the scale's weight range for noise, drift, and hysteresis has confirmed the scale's stability. The CPU and electronic components control the diaphragm, calibration valve, pressure transducers, and scale.

Both control and analysis software programs are written in C Borland Scott's Valley, CA, U. When in operation, the diaphragm's oscillations create sinusoidal volume perturbations in the two chambers that are equal in magnitude but opposite in sign. The precision of the diaphragm position is maintained by an electronic servo system.

The magnitude and frequency of the volume perturbations are 35 mL and 6 Hz, respectively. Pressure changes resulting from the volume perturbations are below ±0. The magnitude of the pressure changes in the two chambers is purposely maintained low atmospheric pressure is approximately cm H 2 O , for comfort, and because, for small pressure changes, the power relationship expressed by Equation 2 is closely approximated by a linear relationship.

This means that the ratio of the pressure perturbations in the two chambers is equal to the inverse ratio of the chambers' volumes. Because volumes are assessed by a ratiometric approach, the repeatability of the volume perturbations is not critical as long as their magnitudes are small with respect to the chambers' volumes, thus ensuring that a linear relationship exists between pressure and volume.

Therefore, for a known reference chamber volume, and assuming adiabatic conditions, varying test chamber volumes are a linear function of the ratios of the pressure perturbations in the two chambers. However, to ensure measurement accuracy, these volumes must be corrected for the impact of small quantities of air at isothermal conditions.

When a subject is tested, air close to the subject's surface and in the subject's lungs behaves isothermally. Assuming that all of the air in the test chamber is under adiabatic conditions results in an underestimation of the volume being measured.

The PEA POD volume measurements are therefore automatically corrected for the impact of the isothermal behavior of air close to the subject's surface and in the subject's lungs.

The impact of air behaving isothermally as a result of its proximity to a surface was investigated by testing aluminum sheets with known volumes and areas. This investigation resulted in the derivation of a constant k. This constant was derived so that the product of its multiplication by the surface area of the object being tested would equal the difference between the volume measured by the PEA POD and the object's actual volume, a negative value.

This adjustment was defined as the Surface area artifact SAA and used to correct PEA POD volume measurements. When live subjects are tested, the PEA POD automatically corrects for the surface area effect by first computing the subject's body surface area BSA and then multiplying BSA by k to obtain the SAA.

The following two equations describe this process: MATH. The PEA POD uses the Boyd formula 15 to determined BSA Equation 3. This formula has been shown to be the most accurate in estimating the surface area of infants As previously stated, this effect is the result of air being under isothermal conditions in the subject's lungs.

The PEA POD uses a predicted value for V TG because its direct measurement would be too invasive. During PEA POD testing, subjects breathe normally. This state represents average V TG during tidal breathing. V TG is therefore equal to functional residual capacity FRC plus approximately half of tidal volume V T.

Historically, FRC has been measured using either helium dilution or plethysmographic assessment. Helium dilution assessment of FRC in infants routinely gives lower values than those measured by plethysmography This difference results from trapped air in the lungs that cannot be detected by helium dilution The PEA POD uses a FRC prediction equation derived by plethysmographic assessment because any air in the lungs free or trapped is not part of the subject's body volume.

The following equation is used to predict FRC because it was derived recently using data from multiple centers 18 : MATH. V T values used by the PEA POD were also consolidated from multiple studies Interpolation is used when V T values need to be calculated at ages other than those presented.

After V T is determined, half of it is added to FRC to obtain V TG. The PEA POD uses the SAA and V TG expressed in liters to correct directly measured raw body volume V br.

Body volume V b is computed as follows: MATH. Note that SAA, a negative value by definition, is subtracted from V br and V TG is added to V br to arrive at V b.

To illustrate with a typical example, a 1. Existing equations are not used because they were derived using fat-free mass density values specific to the adult population 10 , Equation 7 is solved for M f.

To solve Equation 7 for M f , D f and D ffm must be known. D f is constant throughout life and equal to 0. D ffm , however, changes during growth Age- and sex-specific D ffm values are used in Equation 7 and are obtained by extrapolating D ffm values presented in the literature for boys and girls at 0.

For boys, these values are 1. For girls, these values are 1. A complete PEA POD test is performed according to the following protocol. The subject's mass is measured on the PEA POD electronic scale. Each time the PEA POD is moved and every 2 wk, the mass measurement is preceded by a scale calibration procedure to account for the unlikely possibility of drift in the system and changes in the acceleration as a result of gravity at different geographical locations.

A The scale output is then adjusted in case of a discrepancy. Simultaneous to the mass measurement, an automated volume calibration is performed. With the test chamber empty, pressure changes are collected while the calibration valve is closed and open.

The closing and opening of the calibration valve allows the system to perform a two-point calibration giving a linear relationship between the inverse ratio of the pressure perturbations in the two chambers and varying test chamber volumes.

The calibration procedure lasts 50 s, at the end of which the door automatically opens. Next, the tray is pulled out of the test chamber so that the subject can be placed in it. To start a test, the tray and subject are pushed back into the chamber, and the door is closed.

During the first 15 s of the test, a valve connecting the test chamber to the outside environment is opened, allowing the exchange of air between the test chamber and the outside environment.

This equilibration procedure is performed to avoid temperature-dependent deformations in the test chamber's walls resulting from heat generated by the subject. Pressure changes are then collected in the two chambers for 25 s.

Data collection ends with the automatic opening of the test chamber door. This procedure is performed a second time to test for consistency.

If the two volume measurements are within 5 mL of each other or 0. If this level of agreement is not reached, then a third measurement is performed.

If the specified level of agreement is not reached after three measurements, then the entire testing procedure is repeated. First, the ability of the PEA POD to measure mass was determined by testing NIST-traceable weights with the following masses: 1, These weights had NIST Class F tolerances ± The weights tested during this study were representative of infants between birth and 6 mo of age.

During a single session, the mass of each weight was measured five times. Second, the ability of the PEA POD to measure volume was determined by testing aluminum cylinders after they had been added together to obtain the following volumes: 1, The accuracy of the linear dimensions of the cylinders tested was ±0.

This level of accuracy was obtained using instruments traceable to the NIST and in accordance with MIL-I and MIL-STDA standards. The range of volumes tested 1, Four sets of data were collected over 2 d.

Each day was divided into a morning and an afternoon session. All four sessions were performed with the same procedure. During each session, the above volumes were tested five times. Volume testing was performed in four separate sessions so that the reliability of the system could be assessed.

Results from sessions performed during the same day gave a measure of within-day reliability. Results from sessions performed during different days gave a measure of between-day reliability.

With the exception of the mass measurement, each test followed the procedure described in the previous section. The BSA values used for the SAA calculations were computed using the cylinders' known dimensions. No correction was performed for V TG because it applies only to in vivo testing.

Mean values, as well as SD and coefficient of variation CV values for repeated mass measurements, are presented in Table 1. The largest SD was 0. Table 2 presents the precision of the volume measurements. The SD and CV for repeated volume measurements within each session were 1.

The mean values for both SD and CV were within very narrow ranges 1. Table 3 presents within- and between-day reliability of the volume measurements. Within-day reliability is presented for day 1 sessions 1 and 2 and day 2 sessions 3 and 4. Between-day reliability is presented for session 1 of day 1 and session 3 of day 2, as well as session 2 of day 1 and session 4 of day 2.