The purpose of this study was to compute Actical cut-points for moderate and vigorous intensity exercise in overweight and obese adults. All subjects completed one progressive submaximal exercise session. Oxygen uptake was measured using indirect calorimetry as the criterion measure. The data from the current study was compared to previously collected data from this laboratory using the same methodology in normal weight adults. All data were collected in the Applied Physiology Laboratory on the campus of the University of North Carolina at Chapel Hill and in the school gymnasium at Haw River Elementary School located in Burlington, North Carolina.
Subjects
Thirty overweight or obese (BMI >25 kg/m²) male and female adults aged 18 to 50 years were tested in this study. In addition, one normal weight (BMI <25 kg/m²) female was tested in an attempt to balance sample size between groups. Participants were obtained through the ongoing UNC Family Partners for Health study (Principal
Investigator: Diane Berry, grant number 1RO1NRO10254-03) and from the Chapel Hill, NC area. Each potential subject was informed of the possible risks of the exercise
protocol and signed an informed consent, previously approved by the UNC Institutional Review Board, before they participated in this study. All subjects had received medical clearance to participate in physical activity from a UNC Family Partners for Health study
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related nurse, or their personal physician. Subjects were required to have the ability to walk and jog on a motorized treadmill to be included in this study. Those individuals with a musculoskeletal injury, uncontrolled high blood pressure or diabetes, history of
myocardial infarction, who were claustrophobic, or were pregnant were excluded from participating.
Instrumentation
The subjects‟ characteristics were measured using a portable stadiometer (Seca, Hamburg, Germany) for height and an electronic scale (Tanita, Arlington Heights, IL) for body mass when exercise took place at Haw River Elementary School. When exercise took place in the Applied Physiology Laboratory, height was measured using a portable stadiometer (Perspectives Enterprises, Portage, MI) and a mechanical scale (Detecto, Webb City, MO) for body mass. Body mass index (kg/m²) was calculated from measured height (m) and mass (kg) obtained before their participation in the exercise protocol. Heart rate was measured using a Polar heart rate monitor (Polar Electro, Inc., Lake Success, NY) and rating of perceived exertion (RPE) was measured using Borg‟s original 6-20 RPE scale (Borg, 1970).
Activity counts during subjects‟ treadmill exercise were measured by the Actical accelerometer (Respironics Inc., Bend, OR). The Actical accelerometer is a small (28 x 27 x 10 cm), lightweight (17 g) device which measures accelerations in multiple planes (Crouter & Bassett, 2008). The Actical was initialized to measure activity counts in one minute epochs. Activity count data was uploaded using the Actical reader and Actical software version 2.0 (Repironics, Inc., Bend, OR). Breath by breath pulmonary gases collected were analyzed by the K4b² portable metabolic system (COSMED, Rome, Italy)
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during testing in the field and by the Parvo Medics TrueMax 2400 Metabolic Measurement System (Parvo Medics, Salt Lake City, UT) during testing in the
laboratory. The use of the COSMED K4b² system for measuring oxygen uptake over a range of exercise intensities during cycling has been validated (McLaughlin et al., 2001). Although the K4b² system measured VO₂ significantly higher than the Douglas bag method between power outputs of 50-200 W, differences were < 100 ml/min
(McLaughlin et al., 2001). Measurements of VCO₂ with the K4b² were significantly different only at 200 and 250 W (McLaughlin et al., 2001). An attempt was made during this study to ensure that the COSMED and Parvo Medics metabolic systems measured oxygen uptake consistently. Standard gases (16% O₂, 4% CO₂) and ventilatory volumes (1L) were used to compare VO₂ measurements between the two instruments. It was found that the COSMED measured VO₂ 0.3% higher than the Parvo Medics metabolic system, which was deemed an acceptable error rate; therefore, both instruments were used for data collection in this study.
All exercise took place on a calibrated motorized treadmill in either the
gymnasium of Haw River Elementary School in Burlington, North Carolina (Marquette, United States) or on a Quinton treadmill in the Applied Physiology Laboratory on the campus of the University of North Carolina- Chapel Hill (Cardiac Science Corporation, Bothell, WA). Both treadmills were calibrated for the range of speeds used in the exercise protocol according to manufacturer‟s directions (Table 1).
Protocol
Subjects were asked to complete only one exercise session which lasted approximately 45 min. Informed consent was obtained from all subjects before testing
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began. All study exercise sessions took place at the same time of day (during the late afternoon to evening), regardless of field or laboratory location. Subjects received prior approval from their personal physician to participate in physical activity or were screened upon arrival to the Applied Physiology Laboratory to ensure they were ready to
participate in physical activity.
Before subjects arrived to the exercise session, the Actical accelerometer was initialized with the subject‟s information and the COSMED or Parvo Medics system was calibrated using standard gases. Upon arrival, height and body mass of subjects were measured. Subjects were fitted with the heart rate monitor and asked to rest seated for five minutes in order to obtain an accurate resting heart rate value. After the rest period was completed, the Robergs and Landwher (2002) equation [208.754 – (0.734 * age)] was used to calculate predicted maximal heart rate of the participant. Heart rate reserve was then calculated using the Karvonen formula (Karvonen et al., 1957). The termination point of the exercise session was 75% of the subject‟s heart rate reserve. Subjects then had the opportunity to familiarize themselves with the motorized treadmill and ask any questions they had of the researcher. They then learned how to report ratings of perceived exertion using the Borg scale. Next, subjects were fitted with the Actical accelerometer on the right hip aligned with the midline of the right thigh. The portable COSMED system was then strapped to the subject‟s back or the subject was then fitted with the mouthpiece, depending on the location of testing. All COSMED straps were adjusted so the pack containing the COSMED system was fitted snug against the subject, with all wires held in place, but remained comfortable. The COSMED headpiece containing the
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mouthpiece for the subject to breathe into was the fitted and adjusted to ensure no air leaks occurred.
The subject then moved to the treadmill and was asked to stand still for a two minute recording of resting oxygen uptake. The exercise protocol (Table 1) involved walking, and possibly jogging, for four minute stages at a constant (0% grade), with speed increasing 1.0 mile per hour (mph) at the end of every four minute stage. The test terminated when 75% of heart rate reserve was reached. Heart rate and RPE were monitored throughout the test, with recordings of each taken during the last 10 s of each stage. When the subject reached the termination heart rate value, the test was ended, and final heart rate and RPE rating were recorded. If the subject reached this heart rate value during the first minute of the stage, the subject was encouraged to continue exercising until minute two, producing only two minutes of data for that stage. If the subject reached this heart rate value in the second or third minute, the researchers encouraged the subject to continue and complete the end of the stage, producing four minutes of data. The subject had the option to end the test at any point during the protocol.
Table 1. Exercise session protocol, constant grade (0%). Stage (4 min) Treadmill Speed (mph) 1 2.0 2 3.0 3 4.0 4 5.0 5 6.0
Subjects engaged in an active walking cool down at 2.0 mph until their heart rate slowed to at least 120 beats per minute (bpm). When subjects‟ heart rates sufficiently slowed, they were helped off the treadmill and encouraged to drink water and lightly
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stretch to avoid any possible post-exercise muscular pain. Subjects then sat in a chair until their heart rate returned to resting values while the researcher went over their heart rate response to the session with them. Subjects who desired were provided with a personalized exercise plan based on their heart rate training zones. At this point, subjects rejoined the Family Partners for Health exercise class or were released from the APL. Data Management
Oxygen uptake (VO₂) data from minutes one and two of each stage were eliminated in order to obtain an accurate representation of steady state exercise at that speed. The average of the oxygen uptake of minutes three and four were used to represent steady state metabolic demands of that stage. For subjects who did not complete the final stage of the protocol, the last minute of data was used. To reduce activity count data, minutes one and four of each stage were eliminated in order to remove any changes in acceleration due to changes in treadmill speed between stages. The average of minutes two and three were used to represent the counts for the stage. For subjects who did not complete four minutes of the final stage, the average of the last two minutes was used. However, for those subjects who had a wide variation in the last two minutes of data, the first of the final two minutes was used to represent the speed.
Individual regression equations were computed for each subject to determine activity counts at 3 (VO₂ = 14 ml/kg/min) and 6 METS (VO₂ = 24.5 ml/kg/min) (Pate et al., 1995). The association between activity counts and exercise intensity was calculated using METs as the measure of intensity. All VO₂ used in calculations were verified for the attainment of steady state responses. To determine the association between activity counts and VO₂ for each individual subject, maximum oxygen uptake (VO₂max) was first
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estimated for each subject (Margaria et al., 1965). This procedure has been validated for use in various subject populations with an established variability of ±7% (Margaria et al., 1965). Relative VO₂ at two different workloads (speeds) was used to predict VO₂max for each individual subject. Only speeds during which the subject completed all four minutes were used in the calculation. Therefore, all subjects used 2.0 mph as the low workload and their highest speed during which four minutes were completed as the high workload. The following prediction equation was then used: VO₂max=[HRmax*(VO₂”-VO₂’) + HR”VO₂‟ - HR‟VO₂”]/[HR”-HR‟] where HRmax represented predicted maximum heart rate, VO₂” represented the oxygen uptake at the high speed, VO₂‟ the oxygen uptake at the low speed, HR” the heart rate at the high speed, andHR‟ the heart rate at the low speed. A linear regression equation with VO₂ as the independent variable and activity counts as the dependent variable was then computed for each subject and used in activity count extrapolation at 3 and 6 METS.
Statistical Analysis
Receiver Operation Characteristic curves (ROC curves) were run to determine the counts per minute threshold at moderate and vigorous intensity for the normal and
overweight groups (purpose 1). The cut-point selected for each intensity had the optimal value of sensitivity and specificity, and simultaneously had the largest area under the curve. A standard approach was utilized in choosing the lower threshold for activity counts corresponding to moderate intensity physical activity (Trueth et al., 2004). True and false positives classifications were also computed to determine the percent of
subjects who were misclassified at each intensity for both groups. In addition, variation in counts per minute at 3 and 6 METS was also computed.
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Three 2 (normal weight vs. overweight) by 3 (speed) mixed model ANOVAs (General Linear Model) were run to compare differences between heart rate, VO₂, and counts per minute at the chosen speeds of 2, 3, and 4 mph (purpose 2). The Dunnett‟s Test was run as a post hoc to determine where the statistically significant difference occurred within groups for main effects. All data analysis was executed using SPSS version 17.0 (Chicago, IL) with an accepted significance level (set apriori) at α = 0.05.
CHAPTER IV RESULTS
The purpose of this study was to determine cut-points using the Actical accelerometer which correspond to moderate and vigorous intensity exercise in
overweight or obese adults using the metabolic equivalent (METS) method of classifying intensity. In addition, this study aimed to compare cut-points that have been calculated using normal weight subjects in previously completed research with the cut-points computed in the overweight and obese subjects of this study (Diaz, 2009). The first portion of this section describes the physical characteristics of the subjects in this study and presents the physiological differences in data between groups. The second segment of this section presents and compares the cut-points for both the overweight and normal weight subjects. A comparison of cut-points in overweight subjects between the two methods of measuring intensity is also offered. Finally, an attempt to compare the cut- points between groups is presented.
Subject Characteristics
Thirty overweight or obese subjects completed this study protocol. Six
overweight subjects were eliminated from final statistical analysis because oxygen uptake (VO₂) data were lower than expected, and therefore, invalid at the treadmill speeds. Four overweight subjects were added to the group from previous research at the University of North Carolina- Chapel Hill (Diaz, 2009). Therefore, the overweight or obese group
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ended with a total n = 28. One normal weight subject completed testing during this research study and was added to the existing data set that was previously tested resulting in a total n = 22. The final analysis included all 50 subjects. The overweight or obese group (OW) contained 17 females and 11 males. The normal weight group (NW) contained 12 females and 10 males. Sample sizes were too small and the distribution of subjects unequal; therefore, analysis by gender were not computed. The physical characteristics of the subjects are presented in table 2. The overweight subjects were significantly older, heavier, with a greater BMI and a lower relative VO₂max. Height and absolute VO₂max were similar between groups.
Table 2. Physical characteristics of the subjects (mean ± SD).
Variable Normal weight Overweight
Subject number (N) 22 28 Age (years)* 24.0±4.3 32.8±9.6 Height (cm) 173.5±7.3 169.6±8.7 Body mass (kg)* 67.5±7.5 101.5±24.3 BMI (kg/m²)* 22.4±1.9 35.3±7.9 Maximum VO₂ (L O₂/min) 2.8±0.8 2.8±1.2 Maximum VO₂ (ml O₂/kg/min)* 42.2±9.3 28.0±11.4 *Significantly different between groups, p < 0.0005.
The number of overweight and normal subjects who completed the stages of the exercise protocol is presented in table 3. The distribution of subjects that completed the stages of the protocol was different between groups.
Table 3. Total number of subjects that finished the protocol speeds.
Speed (mph)
2 3 4 5 6 7 8
Normal weight 22 22 22 22 19 10 2
Overweight 28 28 24 12 3 2 1
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A greater amount of normal weight subjects were able to complete the faster speeds of the protocol. In addition, the majority of subjects completed 2 mph, 3 mph, and 4 mph. Therefore, these were the speeds which were chosen for analysis between groups. Table 4. Comparison of exercise data between normal weight (NW) and overweight (OW) groups (mean ± SD).
Speed (mph) NW OW Heart Rate (bpm)† 2 89±11 103±15 3 97±12 118±18 4 116±14 146±23 VO₂ (ml/kg/min)* 2 9.49±1.33 9.72±2.12 3 12.01±1.55 12.85±2.13 4 17.02±2.20 18.62±2.93* Activity counts (Counts per minute) ª 2 1264±403 1357±536 3 2529±493 2604±760 4 4382±915 4773±1522 †Significant interaction effect, p<0.0005.
*Significant between group difference at 4mph, p=0.021.
ªSignificant difference within group, between all speeds, p<0.01.
The differences between groups in exercise data are shown in table 4. Heart rate and VO₂ were observed to increase at a faster rate in the overweight group compared to the normal weight group (Table 4). Heart rates were significantly greater at every speed in the overweight group compared to the normal weight group. The mean difference was 14 beats per minute (bpm) at 2 mph, 21 bpm at 3 mph, and 30 bpm at 4 mph. As speed increased, the oxygen uptake of the overweight group was greater than the of the normal weight group; however, this difference was only significant at 4 mph where the
difference was 1.60 ml/kg/min (Table 4). Actical activity counts (cpm) were not significantly different between groups (p = 0.365) (Table 4). In summary, the exercise was more physiologically demanding for the overweight group to complete, but there was no statistical significance in Actical activity counts.
50 ROC Curves
The Receiver Operating Characteristic (ROC) curves used in the determination of the thresholds for moderate intensity exercise are shown in figure 1. The ROC curves used in the determination of the thresholds for vigorous intensity exercise are shown in figure 2. Separate Receiver Operating Characteristic (ROC) curves were run for each group at each intensity threshold, for a total of four curves. The diagonal line on the graphs represents the segment produced by ties, when the value for sensitivity was equal to the value for 1-specificity. The area under the curve was different for each ROC curve. For the normal weight group curves, area under the curve was 0.958 and 0.985 for the 3 MET and 6 MET intensities, respectively. For the overweight group curves, area under the curve was 0.875 and 0.968 for the 3 MET and 6 MET intensities, respectively. The area under the curves was smaller for the overweight group at both intensities, with a greater 95% confidence interval surrounding the value.
Figure 1. ROC curves comparing the normal weight (NW) and overweight (OW) counts per minute threshold for moderate intensity exercise.
OW NW
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Figure 2. ROC curves comparing the normal weight (NW) and overweight (OW) counts per minute threshold for vigorous intensity exercise.
Cut-points
Individual regression equations were used to calculate activity counts at 3 and 6 METS for each subject. The cut-point chosen to represent the threshold for moderate (3 METS) and vigorous (6 METS) intensity exercise was the inflection point in the curve which maximized sensitivity and minimized specificity. The cut-points selected for the normal weight and overweight groups are presented in table 5.
Table 5. Counts per minute cut-points by group.
Normal weight Overweight
3 METS 6 METS 3 METS 6 METS
Cut-point (CPM) 1994 4381 1839 3900
95% Confidence Interval
Area under the curve 0.941-0.975 0.975-0.994 0.835-0.916 0.947-0.989
Sensitivity 0.928 0.985 0.918 0.948
Specificity 0.192 0.182 0.299 0.214
Average (±SD) counts 3442 (1118) 8014 (1992) 3425 (1447) 7292 (2661)
The cut-point chosen to represent the threshold for moderate intensity exercise in the normal weight group was 155 CPM higher than the cut-point chosen for the
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overweight group. The cut-point chosen as the threshold for vigorous intensity exercise was 481 CPM higher in the normal weight group compared to the overweight group. Several misclassifications occurred in the subjects in this study. Within the normal weight group at moderate intensity, one subject (4 %) was a false negative because CPM did not reach the definition of moderate intensity (1994 CPM). At the vigorous intensity, all normal weight subjects reached the threshold of 4381 CPM. Within the overweight group at moderate intensity, three subjects (11 %) would have been false negatives because CPM did not reach the definition of moderate intensity (1839 CPM). At vigorous intensity, three overweight subjects (14 %) would have been false positives because CPM did not reach the 3900 CPM threshold. Several misclassifications at the moderate
intensity threshold also would have occurred using the cut-points developed in this study. Three normal weight subjects who were classified as moderate intensity at 3 METS actually had CPM which placed them in the vigorous intensity category. Within the overweight group, seven subjects were misclassified as exercising at moderate intensity when their CPM actually placed them in the vigorous intensity category. The average (± SD) activity count value at 3 METS was 3442 (1118) CPM for the normal weight group and 3425 (1447) CPM for the overweight group. The average (±SD) activity count value