Measurement in Physical Education and Exercise Science
ISSN: 1091-367X (Print) 1532-7841 (Online) Journal homepage: https://www.tandfonline.com/loi/hmpe20
A comparison of the agreement, internal
consistency, and 2-day test stability of the InBody
®
720, GE iDXA, and BOD POD gold standard for
assessing body composition
Bruce W Bailey, Gabrielle LeCheminant, Timothy Hope, Mathew Bell & Larry
A Tucker
To cite this article: Bruce W Bailey, Gabrielle LeCheminant, Timothy Hope, Mathew Bell
& Larry A Tucker (2018) A comparison of the agreement, internal consistency, and 2-day
®
test stability of the InBody 720, GE iDXA, and BOD POD gold standard for assessing body
composition, Measurement in Physical Education and Exercise Science, 22:3, 231-238, DOI:
10.1080/1091367X.2017.1422129
To link to this article: https://doi.org/10.1080/1091367X.2017.1422129
Published online: 05 Jan 2018.
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MEASUREMENT IN PHYSICAL EDUCATION AND EXERCISE SCIENCE
2018, VOL. 22, NO. 3, 231–238
https://doi.org/10.1080/1091367X.2017.1422129
A comparison of the agreement, internal consistency, and 2-day test stability of
the InBody 720, GE iDXA, and BOD POD® gold standard for assessing body
composition
Bruce W Bailey, Gabrielle LeCheminant, Timothy Hope, Mathew Bell, and Larry A Tucker
Department of Exercise Sciences, Brigham Young University, Provo, UT
ABSTRACT
ARTICLE HISTORY
The study compared the agreement, internal consistency, and measurement stability of the GE iDXA,
BOD POD, and InBody 720. Body composition of 43 men and 37 women (31.4 ± 10.7 years; 90%
Caucasian and 10% other) was assessed in triplicate using each method over two different days. Mean
percent body fat (% BF) of the participants was different for all three machines (27.6 ± 10.0% [GE iDXA)]
25.6 ± 10.4% [BOD POD], and 21.3 ± 10.6% [InBody 720]; p < .05). The coefficient of variation for same
day repeated measures was 1.06% (GE iDXA), 3.29% (BOD POD), and 2.97% (InBody 720). The coefficient
of variation for 2-day repeated measures was 1.81% (GE iDXA), 4.61% (BOD POD), and 4.24% (InBody
720). The difference between the GE iDXA and BOD POD was within acceptable variability, while the
InBody 720 significantly underestimated % BF. The internal consistency was highest for the GE iDXA,
followed by the InBody 720, and then the BOD POD.
Received 20 June 2017
Revised 22 December 2017
Accepted 23 December 2017
Research Highlights
●
●
●
●
●
The InBody 720 significantly underestimated percent body fat (% BF) compared to the GE iDXA
and BOD POD.
Device differences were similar for men and
women.
Agreement between machines was better at higher
BMIs.
Order of internal consistency strength was GE
iDXA (strongest), InBody, and BOD POD.
Two-day test stability was best for the GE iDXA
and similar for InBody and BOD POD.
Introduction
Assessing body composition accurately is important for
determining the health status of an individual. Measuring
body composition rather than body mass index (BMI) is
more salient for determining disease risk. This is specifically true for those in the intermediate range for BMI
(Bigaard et al., 2004; Dervaux, Wubuli, Megnien,
Chironi, & Simon, 2008; Flegal et al., 2009; Frankenfield,
Rowe, Cooney, Smith, & Becker, 2001; Gomez-Ambrosi
et al., 2012; Muller et al., 2012; Okorodudu et al., 2010).
KEYWORDS
Percent body fat;
agreement; reliability; dual
energy X-ray
absorptiometry; air
displacement
plethysmography;
bioimpedance; adiposity
However, cost, accessibility, as well as concerns regarding
reliability, have prohibited body composition from being
used more frequently. In addition, there are concerns
about the lack of agreement between different methods
of measuring body composition and between different
manufacturers using the same method.
There are currently a variety of methods of indirectly
measuring body composition. Three common methods
include dual energy X-ray absorptiometry (DXA), air displacement plethysmography, and bioelectrical impedance.
DXA allows for the assessment of body composition into
three compartments, accounting for variation in bone
mineral content, lean body mass, and fat mass (Laskey,
1996). The DXA has been validated against a four-compartment model and is used frequently as a point of reference or Gold Standard (Ballard, Fafara, & Vukovich, 2004;
Sardinha, Lohman, Teixeira, Guedes, & Going, 1998;
Sopher, Horlick, Wang, Pierson, & Heymsfield, 2002).
In contrast to the DXA, the BOD POD uses air
displacement plethysmography to determine body
composition. The BOD POD was first used over
20 years ago and is often compared to hydrostatic
weighing in its validity and reliability (McCrory,
Gomez, Bernauer, & Mole, 1995; Wagner, Heyward,
& Gibson, 2000). Although the reliability of the BOD
POD has been evaluated in several studies, the results
CONTACT Bruce W Bailey
bruce.bailey@byu.edu
Department of Exercise Sciences, Brigham Young University, Provo, UT 267 SFH.
All authors are from Department of Exercise Sciences at Brigham Young University, 106 SFH, Brigham Young University, Provo, UT, 84602-2216
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/hmpe.
© 2018 Taylor & Francis
232
B. W. BAILEY ET AL.
vary widely (coefficients of variations between 1.7% and
4.5% for same-day test–retest), making a direct comparison of this method of measuring body composition
to other methods difficult (Biaggi et al., 1999; Iwaoka
et al., 1998; McCrory et al., 1995; Miyatake, Nonaka, &
Fujii, 1999; Noreen & Lemon, 2006; Sardinha et al.,
1998; Tucker, Lecheminant, & Bailey, 2014; Vescovi
et al., 2001).
Bioelectrical impedance estimates body composition
by measuring the voltage drop initiated from a current
as it passes between electrodes. Historically, results from
bioelectrical impedance have been viewed as less consistent than other measures of body composition, but
recently there have been some changes to bioelectrical
impedance that are thought to have improved the outcomes. First, segmental assessment allows the estimation
of lean and fat mass for arms, legs, and trunk separately,
recognizing that each segment has a different resistance.
Additionally, multifrequency bioelectrical impedance
uses a large range of electrical frequencies that allows for
better assessment of extracellular water (Anderson, Erceg,
& Schroeder, 2012; Benton, Swan, Schlairet, & Sanderson,
2011; Mally & Dittmar, 2012). One machine that takes
advantage of these improvements in bioelectrical impedance is the InBody 720.
There have been a couple of studies that have compared
the measurements of DXA, air-displacement plethymsmography, and bioelectrical impedance (Biaggi et al., 1999;
Lazzer et al., 2008). However, these studies have exclusively
looked at agreement of the measurements and have not
directly compared the internal consistency (same-day test–
retest) or test stability of these measurements between days.
In addition, previous studies have evaluated older models,
different manufacturers, or single-frequency bioelectrical
impedance. These studies have also failed to examine how
body size influences agreement, internal consistency, and
test stability. The purpose of this study was to directly
compare the agreement, internal consistency, and test stability of the GE iDXA, and BOD POD and InBody 720 for
measuring percent body fat (% BF).
Materials and methods
To accomplish the purpose of the study, a repeated measures design was used with randomized test order on three
different methods of measuring body composition. Eighty
men and women were recruited to participate in the study.
Each participant had body composition measured on two
separate days over a 3-day period. On the first visit to the
lab, height and weight were determined as well as body
composition from each of three different methods (GE
iDXA, BOD POD, and InBody 720). Each of these methods
of assessing body composition was performed twice on
each individual during the first assessment day. The second
assessment day was 2 days later and, again, weight and
body composition, using each of the three methods, were
assessed; however, during this appointment, assessments
were performed only once for each method.
Procedures
Once participants were screened and eligibility determined, they were scheduled for two appointments.
During the first visit, two DXA scans, two BOD POD
tests, and two segmental multifrequency bioelectircal
impedances were performed. Participants arrived at the
lab having fasted for a minimum of 3 h, not exercised for
12 h, not consumed caffeine for 6 h, and not consumed
alcohol for 12 h. While participants were asked to abstain
from caffeinated drinks and alcohol, they were also asked
to maintain normal hydration per manufacturer’s recommendation. Beyond asking participants to maintain normal hydration, there was no effort to standardize
hydration status between days. The participants’ heights
were then measured. After this assessment, the participants were measured using each of the three body composition methods. After all three tests were completed, the
measurements were performed again.
The second visit involved only one measurement on
each of the three machines. These measurements were
taken at the same time of the day as the first assessments. All other procedures and instructions were
identical to the first visit.
Participants
Participants included 43 men and 37 women between
the ages of 18 and 55 years. Participants were excluded
if pregnant or lactating. There was no restriction on the
fitness level of the participants. Recruitment was performed by word of mouth, fliers, and posters. The study
was approved by the university Institutional Review
Board and written informed consent was given prior
to testing.
Measurements
Anthropometrics
Body weight was measured using a digital scale (Tanita
Corp., Inc., Japan) accurate to ±.1 kg with participants
barefoot and wearing a standardized one-piece swimsuit
or lycra shorts. Height was obtained using a digital stadiometer accurate to ±.1 cm (SECA, Chino, CA). Body
weight and height were used to calculate BMI.
MEASUREMENT IN PHYSICAL EDUCATION AND EXERCISE SCIENCE
Body composition
All body composition assessments were performed per
manufacturer’s instructions. Assessments were done in
random order. The machines were calibrated prior to
any assessment per manufacturer’s recommendations.
Tests were conducted in a temperature controlled room
(22–24°C).
Dual energy X-ray absorptiometry. The participants’
body composition was measured using the GE iDXA
(GE, Fairfield, CT). The GE iDXA measured adipose
tissue, bone tissue, and nonbone lean mass. % BF was
calculated by dividing the adipose tissue by the total
body mass that included bone tissue, nonbone lean
tissue, and adipose tissue. The machine was calibrated
daily using a manufacturer-provided calibration block.
Scans were analyzed using Encore software version
13.20.033.
233
the strength of the relationship between machines.
Absolute mean differences were calculated for the sameday test–retest and the 2-day test–retest on each machine.
This was done to better preserve the magnitude of the
difference between test–retests, which can be lost when
using only the mean of the difference. Internal consistency
and test stability of the machines were analyzed using both
the intraclass correlation coefficient and the coefficient of
variation. The coefficient of variation (%) was calculated
for each machine using the following equation:
CV = ±√((∑x1 − x2)2/2n)/((∑x1 + x2)/2n) × 100. Analysis
of variance was used to evaluate how the coefficient of
variation for % BF differed between machines for both
same-day test–retest and between-day test–retest.
Significance was set at p < .05. All analyses were done
using SAS version 9.4.
Results
BOD POD. The participants’ % BF was assessed using
the BOD POD. Participants wore a lycra cap over their
hair. We used the BOD POD software version 5.2.0 and
body fatness was calculated using the general equation
of Siri. To eliminate error from the measurement of
thoracic volume, we used predicted values and the same
value was used for each assessment (McCrory, Mole,
Gomez, Dewey, & Bernauer, 1998; Otterstetter et al.,
2015).
Segmental multifrequency bioelectrical impedance.
The InBody 720 (segmental multifrequency bioelectrical impedance) was used to predict % BF. The InBody
720 calculates 30 impedance measurements by using 6
different frequencies (1, 5, 50, 250, 500, and 1,000 kHz)
at each of 5 different segments, which include the right
arm, left arm, trunk, right leg, and left leg.
Statistical analysis
The primary dependent variable of interest for this study
was % BF as measured by each machine. Means and
standard deviations were calculated for all variables of
interest (age, height, weight, BMI, and % BF for each
machine). Mean difference in % BF between machines
was analyzed using a mixed effects model. The impact of
measurement order and gender were evaluated using partial correlation. Participants were stratified into BMI categories of less than 20, 20 to 24.9, 25 to 29.9, and 30 or
greater. A mixed model was used to evaluate the interactive
effects of machine and BMI category on % BF. The least
squares means technique with a Tukey adjustment was
used to analyze differences between machines by BMI
category. Pearson correlations were calculated to evaluate
Eighty participants (37 women, 43 men) completed all
aspects of the study. Demographic information for the
participants is presented in Table 1. The sample was 90%
Caucasian and 10% African American, Asian, and
Hispanic. The mean age was 30.8 ± 10.5 years, and the
range was 18 to 55 years. The mean BMI was 25.8 ± 5.7,
with a range from 16.1 to 46.5. Of the participants, 14%
had a BMI of less than 20, 37% had a BMI between 20
and 24.9, 33% had a BMI between 25 and 29.9, and 16%
had a BMI greater than 30.
Agreement
Figure 1 displays the Bland–Altman plots comparing
the three methods of assessing body composition. The
Pearson correlation was strongest between GE iDXA
and BOD POD (r = .98, p < .001), followed by the
correlation between the InBody 720 and GE iDXA
(r = .95, p < .001) and the correlation between the
Table 1. Demographic information for both men and women
who participated in the study.
Men (N = 43)
Mean
Age (yrs)
31.4
Height (cm)*
179.6
Weight (kg)*
86.7
BMI*
26.9
Body Fat GE iDXA (%)*
22.9a
Body Fat BOD POD (%)*
21.2b
Body Fat InBody 720 (%)* 16.8c
Women
(N = 37)
Total (N = 80)
SD
Mean
SD
Mean
±10.7 30.0 ±10.2 30.8
±7.5 167.5 ±8.0 174.0
±17.7 68.9 ±18.3 78.5
±5.3
24.4 ±5.9
25.8
±8.8
32.9a ±8.7
27.5a
b
±9.0
30.9 ±9.4
25.7b
±8.4
26.5c ±10.6 21.3c
SD
±10.5
±9.8
±20.0
±5.7
±10.0
±10.4
±10.6
BMI: Body mass index.
*Means in the same row are significantly different between men and
women (p < .05).
a,b,c
Means in the same column with different superscript letters are significantly different (p < .05).
234
B. W. BAILEY ET AL.
b Bland-Altman Plot for Percent Body fat: InBody 720 vs. GE iDXA
25
20
20
15
15
Diff: InBody 720 minus GE iDXA
Diff: GE iDXA minus BodPod
a Bland-Altman Plot for Percent Body fat: BOD POD vs. GE iDXA
25
10
5
0
-5
0
10
20
30
40
50
60
-10
-15
10
5
0
-5
0
10
20
30
40
50
60
-10
-15
-20
-20
-25
-25
c Bland-Altman Plot for Percent Body fat: BOD POD® vs. InBody 720
25
Diff: InBody 720 minus BodPod
20
15
10
5
0
-5
0
10
20
30
40
50
60
-10
-15
-20
-25
Figure 1. (a) Bland–Altman plot for percent body fat: BOD POD vs. GE iDXA. (b) Bland–Altman plot for percent body fat: InBody 720
vs. GE iDXA. (c) Bland–Altman plot for percent body fat: BOD POD® vs. InBody 720.
Note: The x-axis for all three plots represents the mean percent body fat for the two measures of body composition being expressed in the
plot. The line of best fit was added to visually express how the agreement between the two measures of body fat change as percent body
fat increases or decreases.
InBody 720 and BOD POD (r = .93, p < .001).
However, both the BOD POD and InBody 720 produced significantly lower body fat results than the GE
iDXA (see Table 1). The mean difference between GE
iDXA and BOD POD was 1.80 ± 2.23 percentage
points, between the GE iDXA and the InBody 720
was 6.24 ± 3.37 percentage points, and between the
BOD POD and the InBody 720 was 4.42 ± 4.06 percentage points. The intraclass correlation between GE
iDXA and BOD POD was .975 (p < .001), between
GE iDXA and InBody 720 .877 (p < .001), and between
BOD POD and InBody 720 was .914 (p < .001).
The differences between the three different measures
of body fat were similar for men and women. In addition, the differences between the measures of body fat
did not change based on the order of the assessments.
The differences between machines were greater for those
with lower BMIs and became progressively smaller as
BMIs increased (F = 5.05, p = .004; see Figure 2).
Internal consistency
The absolute mean difference for same-day test–retest
was .29 ± .30 percentage points for GE iDXA, .92 ± .76
for the BOD POD, and .83 ± 1.87 for the InBody 720. For
same-day test–retest, 88% of tests were within .5 percentage points for GE iDXA, 33% for BOD POD, and 69% for
the InBody 720. Similarly, 98% of test–retests were within
1 percentage point for GE iDXA, 59% for BOD POD, and
87% for InBody 720. There were no paired tests that
differed by more than two percentage points for GE
iDXA; however, 8% and 13% of same-day tests–retests
differed by greater than 2 percentage points for BOD
POD and the InBody 720, respectively. For the InBody
720, there were four tests that differed by more than 5
percentage points.
The coefficient of variation for repeated measures was
1.06%, 3.29%, and 6.70% for the GE iDXA, BOD POD, and
InBody 720, respectively. When outliers were removed
(measurements > 5 percentage points apart, N = 4), the
coefficient of variation for the InBody was reduced to
2.97%. With outliers removed (N = 4), the coefficient of
variation for BOD POD was significantly higher than both
the GE iDXA (p < .001) and InBody 720 (p = .018). The
same-day test–retest intraclass correlation for GE iDXA,
BOD POD, and InBody 720 was .999 (p < .001), .990
(p < .001), and .996 (p < .001), respectively.
The same-day test–retest differences between measured
% BF for GE iDXA and BOD POD were not different
between genders. There was a difference between genders
for the InBody 720 (p = .015); however, once outliers
(N = 4) were removed, the difference was nonsignificant.
When comparing how internal consistency differs by BMI,
the same-day test–retest was not different for any of the
MEASUREMENT IN PHYSICAL EDUCATION AND EXERCISE SCIENCE
12
235
*
Difference in Percent Body Fat
*
10
*
*
8
*
*
*
6
*
4
*
2
0
GE iDXA vs. BOD POD
GE iDXA vs. InBody 720
Underweight n = 9
Normal n = 32
BOD POD vs. InBody 720
Overweight n = 27
Obese n = 12
Figure 2. Differences in percent body fat between GE iDXA, BOD POD, and InBody 720 by BMI Category.
*Difference between machines is significantly different than zero (p < .05).Error bars represent the standard deviation.
machines; however, there was a significant relationship for
BOD POD (r value = − .23, p = .049) with better agreement
for individuals with higher BMIs compared to lower BMIs.
Two-day test stability
The absolute mean difference for test–retest separated by
2 days was .51 ± .49 percentage points for GE iDXA,
1.22 ± 1.14 for the BOD POD, and .96 ± 2.54 for the
InBody 720. For the 2-day test–retest, 66% of tests were
within .5 percentage points for GE iDXA, 30% for BOD
POD, and 41% for the InBody 720. In addition, 91% of
tests were within 1 percentage point for the GE iDXA,
61% for the BOD POD, and 70% for the InBody 720. Only
3% of the 2-day test–retests differed by more than 2% for
GE iDXA, while 14% differed by more than 2 percentage
points for the BOD POD and 11% for the InBody 720.
The coefficient of variation for 2-day repeated measures was 1.81%, 4.61%, and 6.73% for the GE iDXA,
BOD POD, and InBody 720, respectively. When outliers
were removed (measurements > 5 percentage points
apart, N = 3), the coefficient of variation for the BOD
POD was reduced to 4.24, and the coefficient of variation
for the InBody 720 was reduced to 4.36%. Once outliers
(N = 3) were removed, the coefficient of variation for the
GE iDXA was significantly lower than either the InBody
720 (p < .001) or the BOD POD (p < .001); however, there
was no difference between the InBody 720 and the BOD
POD. The 2-day test–retest intraclass correlation for the
GE iDXA, BOD POD, and InBody 720 was .997
(p < .001), .987 (p < .001), and .992 (p < .001), respectively.
Discussion
The accurate measurement of adiposity is important to
help individuals understand weight-related disease risk.
All three measures that were tested in this study were
significantly different with the greatest disagreement
between the GE iDXA and InBody 720 (6.2 ± 3.4%).
However, agreement was better at higher BMIs, although
the mean difference between machines remained significant. While the accurate assessment of body fat is important at all BMIs, the better agreement at high BMIs
between the three different methods is encouraging
since these are the individuals that would benefit from
body fat testing to confirm that they have excess body fat
and are at risk of weight related diseases.
Since the three machines use different methods to
estimate % BF, they are subject to different assumptions
and limitations that could explain some of the discrepancies between machines. For example, based on the
attenuation properties of X-rays, water will appear to
have about 8% fat content (Kohrt, 1995). In contrast,
because of the conducting properties of water, increased
water volume in the body would favor a lower fat content
using bioelectircal impedance. However, the large difference between the InBody 720 and GE iDXA suggests that
the reason for the difference goes beyond participant
differences and the assumptions made by each measurement method. This was shown in another study that also
reported the InBody 720 underestimating % BF (JenskySquires et al., 2008). Given the relatively high correlation
coefficients between the measurements of body composition, it seems that agreement could be improved.
Previous research demonstrates that agreement
between machines may be manufacturer and model
dependent. In previous studies, the BOD POD has
shown good agreement to DXA, but those studies
have generally used Hologic machines. To date, we
are not aware of studies that have compared the BOD
POD to GE machines and specifically to the GE iDXA.
Internal consistency is a reliability measure evaluating
the error inherent within the instrument and is performed
by taking multiple measurements back-to-back on the
same day. This same-day test–retest measure was good
236
B. W. BAILEY ET AL.
for all three measures of body composition. While the
internal consistency was reasonable for all three measures
of body composition, the GE iDXA was far superior to
either the BOD POD or the InBody 720 (coefficient of
variations of 1.06%, 3.29%, and 2.97%, respectively).
However, the internal consistency of the InBody 720
was very good and slightly better than the BOD POD.
One concern about the InBody 720 is that a small percentage (4 scans out of the 240 performed) of the scans was
drastically off (>5 percentage points). The occurrence of
this measurement error seems to be random and can be
easily caught by performing two measurements back-toback to ensure consistency of results and to catch the
occasional erroneous measurement. Since this measurement takes less than 2 min to perform, it would be easy to
perform repeated measures on this device.
For the BOD POD, over 40% of the same-day test–
retests differed by more than 1 percentage point compared
to 2% and 13% for the GE iDXA and InBody 720, respectively. Unlike the InBody 720, the same-day test–retest for
the BOD POD did not result in any tests that were extremely different (>5 percentage points). However, other studies that have looked at the internal consistency of the
BOD POD have reported tests that differed by as much
as 12.3% (Noreen & Lemon, 2006; Wells & Fuller, 2001).
Our findings are similar to those of Tucker et al. who did
test–retest on over 200 women (Tucker et al., 2014). The
results from this study demonstrated that one in three
measurements were greater than 1 percentage point
apart. This study also showed that some of the limitations
in the internal consistency of the BOD POD can be overcome by performing measurements in duplicate and then
performing a third measurement if the first two are greater
than 1 percentage point apart. The mean of the two measurements that are in agreement is then used.
The measure of test stability evaluates the error
inherent in the machine and the relative ability of the
machine to reproduce results over time and withstand
changes that occur in the environment and within the
individual. All three measures of % BF were relatively
stable. However, the InBody 720 lost some of the
advantage it had in terms of reliability over the BOD
POD, and while the absolute mean difference was still
lower than the BOD POD, the coefficient of variation
for repeated measures was no longer statistically different from the BOD POD.
There are very few studies that have evaluated the
reliability of DXA for assessing % BF and none, to
our knowledge, that have been performed using the
GE iDXA. Coefficients of variation ranging from .6
and 1.6% have been reported for bone mineral content, bone mineral density, and lean tissue mass
(Cordero-MacIntyre et al., 2002, 2000; Heymsfield,
Lohman, Wang, & Going, 2005). The coefficient of
variation observed in our study for % BF is within
this range of coefficients, but it is hard to say if this is
better or worse than what has been observed previously, since none of these previous observations
have evaluated % BF. One study looked at the stability of the measurement over several days and found a
coefficient of variation of 1.89% for body fat, which
was similar to our study (Kiebzak, Leamy, Pierson,
Nord, & Zhang, 2000).
Several studies have evaluated the reliability of the
BOD POD for measuring adiposity. The results from
these studies are similar to the results from our study.
Coefficients of variation between 1.7 and 4.5 for same-day
test–retest have been observed. The results of our study
are in the middle of this range (Biaggi et al., 1999; Iwaoka
et al., 1998; McCrory et al., 1995; Miyatake et al., 1999;
Noreen & Lemon, 2006; Sardinha et al., 1998; Tucker
et al., 2014; Vescovi et al., 2001). The between-day coefficient of variation for the BOD POD has been shown to be
between 2.0% and 2.3%, which is lower than what we
observed (Levenhagen et al., 1999; Miyatake et al., 1999;
Nunez et al., 1999). However, it is unclear if the coefficient
of variation for these studies was calculated in the same
way. For studies that clearly reported how they calculated
the coefficient of variation, our results compared well.
Vescovi et al. found a coefficient of variation of 3.4%
and Noreen et al. found a coefficient of variation of
3.1% (Noreen & Lemon, 2006; Vescovi et al., 2001). Our
study also supports previous research that has indicated
that the BOD POD is more precise at higher BMIs
(Collins & McCarthy, 2003; Wells & Fuller, 2001).
Few studies have been conducted evaluating the reliability of the InBody 720. Jensky-Squires et al. (2008) evaluated the internal consistency of the InBody 320 and found
a coefficient of variation of 1.83, which was lower than
what was observed in our study. However, how the coefficient of variation was calculated was not clear in this study.
While our study has strengths, there are a few limitations that should be pointed out. First, although participants were asked to maintain normal hydration, there was
no effort to standardize hydration between days. This
could be a problem since hydration status adds to the
error seen by each machine between days of assessment.
This is especially true for bioelectrical impedance and the
InBody 720, which can be very sensitive to alterations in
hydration status. Dehydration increases the body’s electrical resistance and can result in overestimating fat mass
and underestimating fat free mass. In addition, since we
rotated through the machines, there may have been some
shifts in fluid distribution throughout the body. Shifts in
fluid distribution in the body could have an impact on
bioelectrical impedance (InBody 720) test results
MEASUREMENT IN PHYSICAL EDUCATION AND EXERCISE SCIENCE
specifically. However, the order of the tests was randomized and there were no order effects observed. Finally,
we did not control for menstrual cycle, which also could
affect hydration status and may have a small impact on
the results for women.
Despite these limitations, the study adds significance
to the current literature and can help when choosing a
body composition assessment for tracking changes in %
BF. To our knowledge, the GE iDXA has not been
evaluated specifically for reliability, and there is little
information, in general on the precision of DXA for
measuring % BF. The GE iDXA is very precise; however, widespread use is limited given the high cost of
the assessment and it uses radiation to measure body
fat (although the radiation is minimal, similar to cross
country flight). Reliability of the BOD POD has been
evaluated, but the range of coefficient of variations is
relatively large (1.7–4.5). This makes it difficult to
compare the consistency of the BOD POD measurements to other methods of measuring body fat in
different studies. Finally, the InBody 720 is a newer
method of assessing body fat and there is limited information on the reliability of this instrument. The ease of
the measurement makes it attractive for measuring
body fat and, based on this study, it is reliable, comparing favorably to the BOD POD for reliability.
Conclusion
The GE iDXA, BOD POD, and InBody 720 each demonstrates good reliability for measuring adiposity. While the
GE iDXA had a coefficient of variation that was three times
smaller than either the BOD POD or InBody 720, it is not
practical for all situations. Choosing either the BOD POD
or the InBody 720 would result in similar internal consistency and test stability; however, there is a high percentage
of tests that are separated by more than 1 percentage point
and there is also the possibility of erroneous measurements.
Because of this, we recommend that the BOD POD and
InBody 720 be repeated until two tests are within 1 percentage point of each other and the mean of the two closest
tests be used. Finally, while all the machines produced
mean values that differed significantly, the difference
between the GE iDXA and BOD POD was within acceptable variability, while the InBody 720 significantly underestimated % BF.
Acknowledgments
This research did not receive any grant from funding agencies in the public, commercial, or not-for-profit sectors.
237
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