Maximal oxygen consumption (V˙O2max) represents the ability of the body to transport and use oxygen during strenuous exercise and provides an indication of an individual’s aerobic fitness level. Laboratory-based aerobic fitness assessments that push participants to the functional limits of the cardiovascular and pulmonary systems are typically used to measure V˙O2max. In the pediatric population, the criteria used to determine a maximal effort are not clearly defined, and a low percentage of children exhibit a plateau in oxygen uptake (19). Therefore, the term V˙O2peak is used to represent the peak oxygen uptake in children, compared with V˙O2max for adults (1,8).
In a laboratory setting, V˙O2peak is determined using indirect calorimetry by measuring the highest V˙O2 during the final stages of a maximal, graded exercise test (GXT) (23,24). However, this method is not feasible outside of the laboratory or for testing a large population. Therefore, field tests have been developed to estimate aerobic fitness in a variety of settings, including schools and community programs.
Field tests of aerobic fitness requiring a maximal effort to estimate V˙O2peak include the One-Mile Run Test and the FITNESSGRAM’s Progressive Aerobic Cardiovascular Endurance Run (PACER) test. The One-Mile Run Test estimates the V˙O2peak on the basis of the time needed to complete 1 mile. This test requires a maximal effort (27), motivation, and an appropriate self-pacing throughout the duration of the test (7). The PACER is a progressive exercise test in which participants run 20-m shuttles in sync with audio cues that get faster each minute (7). Much like a laboratory GXT (9), the intensity is submaximal in earlier stages, increases throughout the test, and only requires a maximal effort in the participant’s final stage. V˙O2peak is estimated based upon the highest speed attained, corresponding to the number of completed stages, and the participant’s age.
The PACER has been validated as a field test of aerobic fitness (7) in the youth. V˙O2peak estimated from the PACER is highly correlated (r = 0.61 to 0.90) with V˙O2peak measured in the laboratory during a treadmill GXT (2,5,13,14). However, few studies have actually measured physiological variables using indirect calorimetry during the PACER. Voss and Sandercock (28) reported that the PACER elicits a maximal effort in 11- to 16-yr olds, evidenced by HRpeak. McVeigh et al. (16) compared the HRpeak achieved during the PACER (boys = 203 beats·min−1; girls = 201 beats·min−1) to a treadmill GXT (boys = 200 beats·min−1; girls = 204 beats·min−1) in 13- to 14-yr-old youths.
Currently, only three studies (15,17,24) involving the youth have used portable gas analyzers to obtain measured V˙O2peak values during the PACER. In the studies conducted by Ruiz et al. (24) and Melo et al. (17), the V˙O2peak was measured while the participants performed the PACER test for comparison against predicted values; however, these assessments were not compared with the values from a treadmill GXT. Mahar et al. (15) investigated the criterion-related validity of the PACER prediction equation in 12- to 16-yr-old adolescents. They reported a high correlation (r = 0.75) between the V˙O2peak measured using indirect calorimetry during both a treadmill GXT and PACER test, and no significant differences for any performance variables were found between the two tests. However, the agreement between the maximal exercise responses measured during a treadmill GXT and PACER test has not been examined in children.
Therefore, the purpose of this study was to compare the V˙O2peak, associated peak physiological variables (HRpeak and RERpeak), and RPEpeak during a treadmill GXT and PACER test to determine the ability of the PACER to elicit a peak exercise response in 10- to 15-yr-old youths.
Participants were recruited from the Knoxville community via flyers, word of mouth, and discussion forums. The sample included 45 youths who were “apparently healthy” (without diagnosed disease or illness) and free of any orthopedic issues that might limit running performance. Written informed consent and participant assent were obtained before data collection. The study was approved by the Institutional Review Board at the University of Tennessee, Knoxville, TN.
Each participant visited the Applied Physiology Laboratory on two separate occasions, with one aerobic fitness assessment completed at each visit. All participants were required to fast at least 4 h before testing. The treadmill GXT and the PACER test were administered in random order, separated by a minimum of 24 h. At the initial visit, height (to the nearest 0.1 cm) and weight (to the nearest 0.1 kg) were measured, and the body mass index (BMI) was calculated (kg·m−2) and used to determine a BMI percentile based on the Centers for Disease Control and Prevention age- and sex-specific growth charts (12). Blood pressure was measured with an automatic blood pressure monitor (OMRON, Vernon Hills, IL) before the aerobic fitness assessment. Body composition was assessed using air displacement plethysmography (BODPOD; COSMED USA, Inc., Concord, CA), and child-specific equations were used to estimate thoracic gas volume (10). Height, weight, and blood pressure assessments were repeated on the second visit.
During both the treadmill GXT and PACER test, heart rate was monitored continuously via telemetry (Polar T31 coded transmitter, Lake Success, NY), and expired gases were analyzed breath by breath with the Oxycon Mobile respiratory exchange gas analysis system (CareFusion, Inc., San Diego, CA). Before each test, the flow meter and gas analyzers were calibrated using a certified calibration gas mixture (4% CO2, 16% O2; CareFusion, Inc.). The subjects were fitted with a Hans Rudolph 8900 series nasal and mouth breathing mask (Hans Rudolph Inc., Kansas City, MO) and a vest containing the portable gas analyzer. To determine whether a peak effort was achieved, at least two of the following criteria had to be fulfilled: 1) HRpeak greater than or equal to 195 beats·min−1, 2) RERpeak greater than or equal to 1.05, and 3) subjective signs of fatigue (excessive sweating, dyspnea, unsteady gait, etc.) (21).
Before testing, the participants were offered a habituation period if they were uncomfortable or unfamiliar with treadmill walking or running. Once comfortable on the treadmill, the participants completed a GXT on a motorized treadmill (Quinton MedTrack T55, Bothell, WA) using the Children’s Mercy Hospital Maximal (CMH Max) protocol (25). The CMH Max is a continuous protocol that progressively becomes more difficult by increasing speed or grade in 1-min increments after the first 3 min (Table 1). The participants were instructed to refrain from using the handrails. Two researchers and a spotter ensured safe testing conditions during the trial. The test was terminated when the subject reached volitional fatigue, indicated he or she could no longer continue, or exhibited unsteady changes in gait. Using the Children’s OMNI Scale (26), participants were asked to report their RPE at the end of each minute and upon completion of the final stage (RPEpeak).
The PACER test was administered according to the guidelines in the Fitnessgram & Activitygram Test Administration Manual (18). The participants ran 20-m shuttles back and forth between cones. The initial speed began at 8.5 km·h−1 and progressively increased by 0.5 km·h−1 each minute. The running pace was kept in time with the signals and background music played from the audio compact disc provided with FITNESSGRAM materials. The participants were encouraged to run as long as possible, and testing was stopped when the participant reached volitional fatigue or could not maintain the required pace for two consecutive shuttles. Upon completion, the participants were asked to report their RPEpeak. The test was conducted in a recreational gymnasium on a wooden floor or in a spacious hallway on a smooth tile surface. If needed, a researcher ran alongside the participants to aid with pacing. All the participants received verbal encouragement from the researchers.
Statistical analyses were performed using SPSS version 20.0 (IBM Corp., Armonk, NY). Means and SD were analyzed for all the variables. HRpeak was taken as the highest value achieved during the last 30 s of the final stage for each test. V˙O2peak and RERpeak values were averaged over the last 30 s of the final stage from the PACER and treadmill GXT. The peak values for the boys and girls were compared using t-tests. Paired samples t-tests and regression analyses were used to compare the outcome variables (HRpeak, RERpeak, V˙O2peak, and RPEpeak) between the PACER and treadmill GXT. An alpha value of P < 0.05 was assumed to show statistical significance. Bland–Altman (4) plots were used to examine the variability in V˙O2peak between the treadmill GXT and the PACER test in the boys and girls.
Participant characteristics are presented in Table 2. There was a significant difference between the boys and girls in the percentage body fat (P < 0.01), PACER V˙O2peak, treadmill V˙O2peak, and number of PACER laps completed (P < 0.05). The PACER data are presented by sex and age in Table 3. Because of the small samples sizes in each age/sex group, the statistical analyses were only run to compare the differences between the boys and girls. The average treadmill GXT time to exhaustion was 556.3 ± 126.0 s.
The means and SD of the treadmill GXT and PACER HRpeak, RERpeak, V˙O2peak, and RPEpeak for the boys, the girls, and the total group are shown in Table 4. No significant differences were found between the peak variables (HRpeak, RERpeak, V˙O2peak, and RPEpeak) measured during the treadmill GXT and those measured during the PACER. Paired samples t-tests revealed a mean difference of −0.9 ± 5.1 mL·kg−1·min−1 (P = 0.232) between the GXT V˙O2peak and PACER V˙O2peak, and the SE of the measurement (SEM) was 1.4 mL·kg−1·min−1. In addition, significant correlations (P < 0.05) were found for peak variables obtained from the treadmill GXT and PACER: HRpeak (r = 0.60), RERpeak (r = 0.55), V˙O2peak (r = 0.87), and RPEpeak (r = 0.44).
Bland–Altman plots (Fig. 1A, B) were used to examine the variability of V˙O2peak in the boys and girls between the treadmill GXT and the PACER. The mean difference and the 95% prediction intervals are displayed. The prediction intervals indicate minimal variability in the outcome measure between the treadmill GXT and PACER for all the participants. Using data from the Bland–Altman plots, there were no significant regression relationships between the V˙O2peak differences and means for the treadmill GXT and PACER for the boys (r2 = 0.025, P = 0.509) and girls (r2 = 0.038, P = 0.349). These values indicate a random dispersion of scores without systematic bias, no heteroscedasity.
There were no significant differences in HRpeak, RERpeak, V˙O2peak, or RPEpeak between the treadmill GXT and PACER tests. These findings indicate that the PACER test elicits a similar V˙O2peak response compared with a treadmill GXT in 10- to 15-yr-old youths. This finding is validated by similar peak responses in HRpeak, RERpeak, and RPEpeak during the treadmill GXT and PACER tests and supports findings reported in previous research studies (15,16,24). This is the first study to provide measured V˙O2peak data during the PACER test and to validate the use of the PACER as a field assessment of aerobic fitness in the 10- to 15-yr-old age group.
The comparable HRpeak values achieved during the PACER test and treadmill GXT have been widely documented throughout the literature (5,14,16). On the basis of the heart rate response during these tests, it appears that the PACER is capable of eliciting a peak exercise response. In the current study, the mean HRpeak achieved was 197 ± 9 beats·min−1 during the PACER and 197 ± 9 beats·min−1 during the treadmill GXT; both of which were within the range of heart rate values indicating a peak response that are typically reported in the youth (195–210 beats·min−1) (29).
Few studies (11,15) have measured the V˙O2peak using indirect calorimetry during the PACER test for comparison against a treadmill GXT. In adults, Gadoury and Leger (11) used the Douglas bag method to determine the V˙O2max at the end of the 20-m shuttle run and a treadmill GXT and reported values of 49.3 ± 10.1 and 48.8 ± 9.3 mL·kg−1·min−1, respectively. More recently, Mahar et al. (15) conducted a study in which 15- and 16-yr-old adolescents completed both a treadmill GXT and PACER while wearing a portable metabolic system. They reported a mean difference of 1.4 mL·kg−1·min−1 between the V˙O2peak values measured during the tests. These results are similar to the 1.6 mL·kg−1·min−1 mean difference found in the current study with 10- to 15-yr-old youths.
In this investigation, the verification of peak effort was achieving at least two of the following criteria: 1) HRpeak greater than or equal to 195 beats·min−1, 2) RERpeak greater than or equal to 1.05, and 3) subjective signs of fatigue (excessive sweating, dyspnea, unsteady gait, etc.) (21). The maximal effort using the HRpeak criteria was achieved in 63.6% of the participants during the treadmill GXT and 62.2% of the participants during the PACER. RERpeak greater than 1.05 was present in 82.2% and 91.1% of the participants during the treadmill GXT and PACER test, respectively. The primary investigator conducted all of the fitness testing and reported that subjective signs of fatigue were evident in the final stages of both fitness tests for all the participants. In addition, the RPEpeak values, shown in Table 4, indicate that a participant effort was similar for both tests and support the observation of subjective symptoms. All the participants achieved at least two of the three criteria on both tests. Although V˙O2 plateau is a common criterion for determining the maximal effort in adults, only 30%–50% of children achieve plateau (19). A study by Peyer et al. (20) indicated that V˙O2peak values in children were not different in those who plateaued (34.3 ± 9.9 mL·kg−1·min−1) compared with those who did not (34.0 ± 6.6 mL·kg−1·min−1). Similarly, studies in adults (22) demonstrate that a plateau in V˙O2 is not a good criterion for verifying whether maximal oxygen uptake is attained at the end of a continuous, GXT. Therefore, V˙O2 plateau was not used as a criterion for peak effort in this study.
The strengths of the present study include the moderate sample size with a percentage of overweight and obese participants (27%) similar to that of the general population (31.7%). The order of fitness testing, treadmill GXT or PACER, was determined randomly. However, both tests were completed within 1 wk, separated by a minimum of 24 h. The primary investigator conducted all the testing and provided the same verbal encouragement to all the participants during both tests. A unique aspect of the study included the measurement of HRpeak, RER peak, V˙O2peak, and RPE peak during both the treadmill GXT and PACER tests.
For nine participants, a researcher ran beside the youth (pacer) to assist with pacing. A “pacer” has been used in other studies with the participants in the same age group (3). This may influence participant effort; however, it was necessary to ensure adherence to the protocol. Whether or not a “pacer” was used, participant motivation is a potential limitation to the study; however, all the children received the same level of verbal encouragement throughout both tests.
Although the PACER has been used as a maximal field test in children, the current study is one of the first to document this using a criterion method (indirect calorimetry) and comparing outcome variables to a treadmill GXT in this age group. In conclusion, the findings of this study determined that the PACER does indeed elicit peak exercise responses in 10- to 15-yr-old youths that are comparable with those seen with inclined treadmill running protocols. There was no significant difference between the GXT and PACER V˙O2peak measurements (SEM = 1.4 mL·kg−1·min−1). Although both tests progress gradually, maximal effort is required only in the final stages of the tests. There are several advantages to using the PACER instead of a treadmill GXT as an aerobic fitness assessment. The short duration and background music may hold interest and improve motivation, both of which greatly factor into the children’s performance (24). In addition, the PACER mimics the sporadic start–stop activity patterns typically seen in the youth, and it likely increases the comfort level of the children for those accustomed to running on the floor versus on a treadmill. Lastly, the PACER test can be administered in a variety of settings including gymnasiums or spacious hallways and has recently been added as an option to assess aerobic capacity in the Presidential Youth Fitness program. As demonstrated in the current study, the PACER can be used to assess V˙O2peak and peak exercise response. These findings support the possible use of the PACER as a maximal exercise test in a field-based, clinical, or laboratory setting.
The authors would like to thank Dr. David Bassett Jr. for his contributions to the manuscript; Pam Andrews, Jessica Chandler, Scott Conger, Jenny Flynn, Antonio Spates, Andy Van Grinsven, Whitney Welch, Brittney Wiseman, and Dana Wolff for their assistance with data collection; and Cary Springer for her assistance with statistical analysis and interpretation.
The authors did not receive external funding to conduct this study.
The authors report no conflict of interest.
The results of the current study do not constitute endorsement of any products by the American College of Sports Medicine.
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