Validity of the Caltrac Accelerometer in Estimating Energy

Pediatric Exercise Science, 1991, 3, 141-151
Validity of the Caltrac Accelerometer
in Estimating Energy Expenditure and Activity
in Children and Adults
Ann F. Maliszewski, Patty S. Freedson, Chris J. Ebbeling,
Jill Crussemeyer, and Kari B. Kastango
The Caltrac accelerometer functions as either an activity monitor that
provides activity counts based on vertical acceleration as the individual
moves about, or as a calorie counter in which the acceleration units are used
in conjunction with body size, age, and sex to estimate energy expenditure.
This study compared V02 based energy expenditure with Caltrac estimated
energy expenditureduring three speeds of treadmill walking in children and
adults. It also tested the validity of the Caltrac to differentiatebetween high
and low levels of walking activity (activity counts). Ten boys and 10 men
completed three randomly assigned walks while oxygen consumption was
monitored and Caltrac estimates were obtained. The results indicate that the
Caltrac does not accurately predict energy expenditure for boys and men
across the three speeds of walking. Although there were no significant
differences between actual and predicted energy expenditure values, the
standard errors of estimate were high (17-25%) and the only significant
correlation was found for men at the fastest walking speed (r=.81).
However, the 95% confidence intervals of the activity counts and energy
expenditure estimates from the Caltrac support its use as an activity monitor
during walking.
Physical activity is an area of growing interest since it is considered a factor
in the prevention of coronary risk factors and disease (4). However, assessing
physical activity has been problematic for epidemiologists since most testing
methods lack precision and validity. The search for a reliable, objective testing
method has led to the use of such devices as heart rate monitors, motion sensors,
and accelerometers (3).
The Caltrac accelerometer (Hemokinetics, Madison, WI) may be a
promising device for physical activity monitoring. It is designed to accumulate
counts that reflect total vertical acceleration of the body. These counts are used
The authors are with the Department of Exercise Science, University of Massachusetts, Amherst, MA 01003.
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The Caltrac Accelerometer
- 143
Table 1
Descriptive Data
Men
Age (Yrs)
Height (cm)
Mass (kg)
BSA (mq
Boys
M
SD
M
SD
20.0
179.2
76.3
1.9
1.99
7.73
6.35
0.11
9.5
137.6
33.5
1.1
0.07
6.64
6.14
0.11
Subjects reported to the exercise laboratory; they were introduced to the
safety factors involved in treadmill use and practiced for 5 to 15 minutes at the
different walking speeds. During testing, subjects walked on the treadmill for 8
minutes at three speeds. The men walked at 3.70,5.45, and 7.39 kilometers per
hour (kph) and the boys walked at 3.35,5.03, and 6.7 kph. These speeds represented slow, medium, and fast paces, as reported in Kline et al. (7) for adults
and in Freedson et al. (5) for children, and were assigned randomly among
subjects to minimize an order effect.
A Caltrac was programmed with each individual's height (in.), weight
(lbs), age (yrs), and sex and was secured to the right hip. Vertical displacement
of the Caltrac causes an accumulation of movement units that are used with
individual data to compute an estimate of energy expenditure in kilocalories
(kcal). A cumulative kcal over the 8 minutes was used for analysis (CAL) for
each of the three speed conditions. A second Caltrac secured to the left hip
was programmed to eliminate metabolic factors and cause the accelerometer to
accumulate activity counts (ACTS) based on vertical acceleration over the 8
minutes.
Open-circuit spirometry was used to determine oxygen consumptionduring
exercise. Inspired air volume was measured by a Rayfield dry gas meter. Expired
air was analyzed continuously, and minute to minute values of oxygen consumption (Ametek S3AI) and carbon dioxide production (Arnetek CD3A) were determined. An &bit AID converter was used to convert analog data from the
instruments to digital output using the Vista computer software (Vacumed, Ventura, CA). The system was calibrated before and after each testing session with
gas samples of known concentration (16% 02,4 % C02).
The mean oxygen consumption for Minutes 6, 7, and 8 were used for
analysis of energy expenditure. V02 was converted to caloric expenditure using
the RER caloric equivalent conversion. Values of kcallmin were multiplied by
8 to obtain a total energy expenditure value (TICAL).
Analysis of variance (ANOVA) was used to test for differences between
the energy expenditure estimates by the Caltrac accelerometer and oxygen consumption between adults and children. Relationships between the VO, estimated
and the Caltrac estimated caloric cost of walking for each speed, as well as for
all speeds combined, were examined for the adults and children.
144
- Maliszewski, Freedson, Ebbeling, C~sserneyer,and Kastango
Table 2
Means and Standard Deviations of Activity Counts (ACTS),
Estimated Caloric Expenditure (CAL), and Actual Caloric
Expenditure (TKAL) for all Walking Speeds
ACTS
Speed
Adults
Slow
Medium
Fast
Children
Slow
Medium
Fast
CAL
TKAL
M
SO
M
SO
M
SD
2.4
3.5
6.4
0.84
0.85
1.43
38.3
50.6
79.2
11.13
10.19
13.39
32.1
44.4
74.6
2.40
4.15
9.08
2.2
3.4
7.6
0.83
0.73
1.51
17.5
24.6
34.9
6.46
4.81
8.76
17.9
24.7
38.3
4.01
4.90
9.94
Results
Means and standard deviations for Caltrac estimated caloric cost (CAL) and
activity counts (ACTS), and total kilocalories from oxygen consumption
(TKAL), are presented in Table 2. Analysis of variance for repeated measures
indicated that each variable was significantly higher for each successive speed
of walking for the men and the boys.
ANOVA for repeated measures was used to test for a significant difference
between the actual caloric cost (TKAL) and the caloric estimate by the Caltrac
(CAL). There was no significant difference between CAL and TKAL for the
men at any walking speed (F=.21, p=.8145). However, the individual data
shown in Table 3 indicated that the difference between the two measures ranged
from -37.9 to +83.7% at the slow speed, from -8.9 to +56.5% at the medium
speed, and from - 15.6 to + 18.4% at the fast speed. The mean percent differences showed that the Caltrac consistently overestimated caloric expenditure,
with differences more exaggerated at the slower speeds. There was no significant
difference between the measure of caloric cost by oxygen consumption and estimation by the Caltrac at any speed for the boys (F=1.63, p = .2310). Differences
ranged from - 18.5 to +46.7 % at the slow speed, from -40.4 to +43.9% at
the medium speed, and from -42.0 to 14.3% at the fast speed. The mean
differences indicated that the Caltrac slightly underestimated energy expenditure
at the slow and fast speed while it slightly overestimated energy expenditure at
the medium speed for the children.
Table 4 presents the correlation analysis of actual caloric cost (TKAL) with
Caltrac estimation (CAL) and activity counts (ACTS) for all speeds combined.
All correlations were significant, but the SEE indicates a large amount of variability in the boys (25-27%) in comparison to the men (16.7-17.1 %). Removing
the effect of body surface area (partial correlation, Table 5) did not change these
correlations. When speed was partialled out, however, the correlations were no
+
The Caltrac Accelerometer
- 145
Table 3
Individual Data and Percent Differences (Error) Between Actual (TKAL)
and Predicted (CAL) Measures of Energy Expenditure
Subject TKAL
Adults
01
02
03
04
05
06
07
08
09
10
M
SD
Children
11
12
13
14
15
16
17
18
19
20
M
SD
Slow
CAL
34.3
32.2
27.8
28.5
33.4
33.8
32.7
30.3
34.0
34.0
63.0
40.0
31.0
33.0
40.0
21.0
43.0
32.0
35.0
45.0
18.1
18.4
16.1
15.4
22.5
24.4
14.4
18.9
12.8
18.4
15.0
17.0
17.0
33.0
14.0
13.0
17.0
14.0
20.0
Error
TKAL
Medium
CAL
Error
Fast
TKAL CAL
Error
X
X indicates missing data.
Table 4
Correlations Between Actual Caloric Cost (TKAL)
and Caltrac Estimated Caloric Cost (CAL) and Activity Counts (ACTS)
Adults
CAL
ACTS
Children
SEE
r
t2
SEE
O
h
r
P
.896*
.883*
.803
.780
8.33
8.80
16.6
17.6
.759*
.715*
.576
.511
*r=602 for significance at p<.05.
6.76
7.25
O
h
25.0
26.9
146
- Maliszewski, Freedson, Ebbeling, C m e m e y e r , and Kastango
Table 5
Partial Correlations Between Actual Caloric Cost (TKAL)
and Caltrac Estimated Caloric Cost (CAL) and Activity Counts (ACTS)
With the Effects of BSA and Speed Removed
Adults
BSA
CAL
ACTS
Children
speed
r
P
.895*
.899*
.801
.808
r
.622*
.526
BSA
Speed
P
r
rZ
r
P
.387
.277
.759*
.715*
576
511
438
.058
.I92
,003
*r= .602for significance at 6.05.
longer significant for the children for either CAL or ACTS. For the men, the
correlation between TKAL and CAL remained significant but the correlation
between TKAL and ACTS was no longer significant.
When correlations between caloric cost (TKAL) and Caltrac estimations
of kcal (CAL) and activity (ACTS) were computed for each speed, they were
only significant at the fastest walking speed for the men (r= .81 with CAL and
r = .65 with ACTS). For the boys, none of the correlations between TKAL and
either of the Caltrac estimates were significant at any speed. The correlations
between actual and estimated caloric cost are illustrated in Figures 1 and 2 for
the boys and men, respectively.
In summary, the fmdings of this study were as follows: (a) No significant
difference was found between actual (TKAL) and predicted (CAL) measures of
energy expenditure by an analysis of variance. (b) With combined speed data,
correlations between the Caltrac estimates and actual measures of caloric expenditure by oxygen consumption are statistically significant. (c) When data were
correlated at individual walking speeds, significance was observed only at the
fastest walking speed for the men. None of the correlations were significant for
the boys.
Discussion
There was a significant difference in caloric cost (TKAL) between walking
speeds, indicating that the walking activity levels could be clearly differentiated
by this testing regime. Although there were no significant differences between
measurement methods of energy expenditure, and significant correlations were
observed with combined speed data, closer observation revealed that the dispersion of actual and predicted kcal values is quite large. The individual data for
differences between measures clearly illustrate this spread (Table 3). When BSA
was partialled out, there was little effect (Table 4). When speed was partialled
out, however, the correlations between TKAL and ACTS in both groups and
TKAL and CAL in the boys decreased and were no longer significant. The CAL
appears to reflect the increases in energy cost for the men. These findings suggest
that the increase in activity counts is primarily a function of the increased speed
of movement. In fact, speed was significantly correlated with ACTS (r= .83 and
The Caltrac Accelerometer
- 147
FAST
(b.527)
MEDIUM
(I= .286)
SLOW
(r= .460)
CAL: Estimated caloric cost (kcal)
Figure 1 - Actual (TKAL) versus estimated (CAL) energy expenditure across
speeds for boys.
90
FAST
(r- .809)
Overall
rs.896
80
70
-
60
-
50
-
40
-
MEDIUM
(b.263)
-
m
30
--
20
--
I
t
I
20
40
60
80
SLOW
(ra.402)
I
I
.
100
CAL: Estimated caloric cost (kcal)
Figure 2 - Actual (TKAL) versus estimated (CAL)energy expenditure across
speeds for men.
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The Caltrac Accelerometer
- 149
For the boys in this study, energy expenditure was slightly underestimated
at the slow and fast speeds but overestimated at the medium speed. The mean
percent differences between TKAL and CAL are quite small (-7 to +5 %), but
the range of the individual differences is quite large, resulting in a standard
deviation of 20 to 25 % .
Sallis et al. (11) reported a significant correlation (r=.82) between
kcal-kg-' and activity counts across treadmill walking and running speeds (3,
4, 5 mph) for 44 children. Accompanying this was a 23% SEE. In the present
study a correlation of r= .72 was found between activity counts and total caloric
cost in the 10 children at walking speed of 2.1,3.1, and 4.1 mph, with a SEE of
26.9 % . When correlations were examined for each walking speed, no statistically
significant relationship was found between energy cost and ACTS, suggesting
that the Caltrac reflected changes in activity level (increased walking speeds) but
not individual differences in the energy cost of walking.
In this study the correlation between TKAL and CAL was also significant
for the boys across all speeds (r= .76). When the effect of speed was removed
from the correlation, the correlations were no longer significant. Additionally,
the correlations between TKAL and CAL at any given walking speed were not
significant. These data suggest that the differences are due to changes in activity
level at different walking speeds but do not reflect individual differences in energy expenditure.
In order to be valid as an activity monitor, different levels of activity must
be distinguishable. Computation of the 95 % confidence intervals supports the
use of the Caltrac to distinguish between low, medium, and high walking speeds
when the Caltrac is used as an activity monitor (ACTS) for both the men and the
boys (Figure 3). Although the slow (boys = 1.71-2.69; men = 1.88-2.92) and
medium (boys = 3.08-4.12; men = 2.97-4.03) levels were close, there is clear
differentiation between these lower levels and the fast walking speed (boys =
6.57-8.54; men = 5.51-7.29). This would support the use of the Caltrac as an
activity monitor to categorize individuals into low and high physically active
groups for walking.
FAST
MED
-
-
SLOW.
0
*-----3
men
I
I
I
I
i
2
4
6
8
10
ACTS
Figure 3
boys
13
- 95%confidence intervals for Caltrac activity counts (ACTS).
150
- Maliszewski, Freedson, Ebbeling, Crossemeyer, and Kastango
MED
-- -
DQ
boys
men
CAL
Figure 4
(CALI.
- 95% confidence intervals for Caltrac energy expenditure estimates
The 95% confidence intervals for the caloric expenditure (CAL) values are
presented in Figure 4. There was no overlap between the levels for the boys,
suggesting that the Caltrac estimates of caloric expenditure can be used to distinguish between the three levels (slow = 13.8-21.8 kcal, medium = 24.2-27.8
kcal, and fast = 29.2-39.9 kcal). However, there was overlap between the slow
(31.4-45.2 kcal) and medium (44.3-56.9 kcal) speeds for the men. There was a
clear distinction between the medium and fast (70.9-87.5 kcal) speeds for the
men, which supports the use of the Caltrac estimates of caloric expenditure
to distinguish between low and high groups. When used in this manner, the
accumulated Caltrac estimate of energy expenditure reflects walking activity
level and not actual energy expenditure. In fact, the findings in this study suggest
that the only case in which Caltrac energy expenditure accurately reflected energy
cost was for the men at fast walking speeds. Furthermore, these results are based
only on walking and cannot be extrapolated to other types of activities.
In conclusion, the Caltrac may be a useful tool in grouping individuals into
low and high physically active levels for walking, and in monitoring changes
based on these groupings. However, the validity of the tool for comparing between individuals within a narrow walking speed range remains questionable,
and thus it may not be possible to distinguish energy expenditure differences.
These findings are limited to controlled treadmill walking, and further research
is needed to determine whether the instrument will be effective for distinguishing
activity levels across a wide range of activity types in everyday situations.
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