The cardiovascular (CV) fitness component measured aerobic capacity through a submaximal treadmill test. The goals of this component are to provide: 1) nationally representative data on cardiovascular fitness for youth ages 12-15 years; 2) prevalence estimates of youth ages 12-15 years at risk due to poor physical fitness; and 3) data to study the association between cardiovascular fitness and other health conditions and risk factors, such as obesity, physical activity and dietary patterns.
Participants aged 12-15 years, who did not meet any of the exclusion criteria, were eligible for this component.
Participants are excluded from this component based on medical conditions, medications, physical limitations and safety limitations.
Protocol and Procedure
The treadmill protocol for the National Health and Nutritional Examination Survey National Youth Fitness Survey (NNYFS) for ages 12-15 years was conducted previously in NHANES 1999-2006 to measure aerobic capacity. The protocol specifically followed the NHANES 2004-2006 protocol.1 The protocol is a submaximal treadmill exercise test from which maximal oxygen consumption (VO2max) was estimated. Participants were assigned to one of five treadmill test protocols varying in grade and speed based on gender, age, BMI, and self-reported level of physical activity. Each of the protocols was designed so that the participant walks at all times. The test consists of a 2-minute warm up, two 3-minute exercise periods, and a 2-minute recovery period. The goal of each protocol is to elicit a heart rate that is approximately 75 percent of the predicted maximum (220 minus age) by the end of the test.
Heart rate was monitored continuously using an automated monitor with four electrodes connected to thorax and abdomen of the participant and was recorded at the end of warm-up, each exercise stage, and each minute of recovery. At the end of warm-up and each exercise stage, participants were asked to rate their perceived exertion using the Borg scale.
The main outcome of this component is the estimated VO2max, which is estimated using the heart rate at the end of each exercise period. Based on gender and age specific criteria, the estimated VO2max is categorized using Fitnessgram® criterion-referenced standards.2
Detailed descriptions of the protocol are provided in the NNYFS Treadmill Examination Manual3 available on the NNYFS website at: https://www.cdc.gov/nchs/data/nnyfs/treadmill.pdf
Quality Assurance & Quality Control
Examiners were regularly monitored by NCHS staff and field supervisors. Retraining sessions were conducted periodically with the examiners to reinforce the proper protocols and techniques. Inspection, calibration, and maintenance of the equipment and supplies were performed on a regular basis.
As part of on-going quality control practices, all data were reviewed systematically for logical or operational inconsistencies and examiner errors.
Data Processing and Editing
During the data processing, edits were made to ensure the logical consistency and analytic usefulness of the data. Extreme values were reviewed and cross-checked with other available data. When there was insufficient information to conclude that the values were invalid, they were retained in the data set.
Variable specific editing:
CVDEXSTS (CV fitness exam status)
This derived variable indicates the following for each participants:
1. VO2max estimated.
2. Tested but VO2max estimate missing - data obtained during the treadmill test were insufficient in calculating VO2max.
3. Did not participate in the treadmill test.
CVDEXCMT (Comment code for the exam status)
This derived variable denotes the reason the treadmill test was not done or data are missing. About 8% of participants who were eligible were excluded.
“0” – None: the exam was completed with no missing data point present.
“1” – Met exclusion criteria: exam was not done due to the participant met the exclusion criteria of the component other than pregnancy.
“3” – Refusal: participant refused to undergo or continue the test.
“4” – No time: the exam was not done because there was not enough time.
“5” – Technical problem: problems that occurred during the test with equipment, software applications, or technician error.
“6” – Met priority 1 stopping criteria: participant discomfort or distress observed by the health technician or reported by the participant that warranted an emergency stop of the protocol.
“7” – Met priority 2 stopping criteria: safety concerns arose during the test that warranted an early stop of the protocol.
“8” – Not able to calculate VO2max: heart rate obtained at the end of stage 2 lower than what obtained at the end of stage 1, or the difference between stages 1 and 2 heart rates is 5 beat/minute or less.
“90” – Other.
CVXPARC (Physical activity level)
This variable summarizes the responses from a series of questions and describes the participant’s typical physical activity level using the NASA/JSC physical activity scale.4 See the NHANES National Youth Fitness Survey (NNYFS) Treadmill Examination Manual Appendix D for further details.
CVDVOMAX (Predicted VO2max)
This derived variable designates predicted VO2max value calculated with the equation developed by Jackson et al.5 The prediction model is based on participant’s gender, age, body mass index, and self-reported level of physical activity. Predicted VO2max value is used in the determination of the exercise protocol for the treadmill test. See the NHANES National Youth Fitness Survey (NNYFS) Treadmill Examination Manual Appendix F for further details.
CVAPROT and CVDPROT (Assigned exercise protocol and the actual protocol used in stages 1 and 2)
Each participant was assigned to one of five protocols based on the predicted VO2max. The original protocol used in NHANES 1999-2004 had 8 different protocols. In NHANES 2003-2004, exercise protocols 1, 2 and 8 were eliminated for all participants 12-19 years. This procedure was followed by NNYFS. Participants who were assigned to exercise protocol 1 or 2 were re-assigned to protocol 3, and persons who used to be assigned to exercise protocol 8 were re-assigned to protocol 7. According to the procedure, if the heart rate in warm-up is greater than 60% of the predicted maximal heart rate (PMHR), the assigned exercise protocol is decreased by 1 for the remainder of the treadmill test (the exception being if the protocol was already at the lowest possible one). When the heart rate in warm-up is less than 50% of the PMHR, the assigned protocol is increased by 1 for the remainder of the test (the exception bring if the protocol is already at the highest possible protocol). The variable CVAPROT denotes the exercise protocol that was originally assigned to the case while variable CVDPROT reflects the actual exercise protocol used in stage 1 and stage 2 after the adjustment during warm-up. See the NHANES National Youth Fitness Survey (NNYFS) Treadmill Examination Manual , Sections 18.104.22.168 and 22.214.171.124 for further details.
CVDESVO2 (Estimated maximal oxygen uptake)
This derived variable designates the main outcome of the component: VO2max (ml/kg/min). It is estimated by extrapolation using measured heart rate responses to known levels of exercise workloads, assuming the relation between heart rate and oxygen consumption is linear during exercise.6 See the NHANES National Youth Fitness Survey (NNYFS) Treadmill Examination Manual , Appendix G for further details.
CVDFITLV (Cardiovascular fitness level) - change
The level of cardiovascular fitness is categorized based on gender-age specific cut-points of estimated VO2max. The reference standards used for adolescents ages 12-15 years are based on the criteria used in the FITNESSGRAM program.2 See Treadmill Examination Manual , Appendix H for further details.
The NNYFS examination sample weights should be used to analyze the cardiovascular fitness data. Please refer to the NNYFS Analytic Guidelines for details on the use of sample weights and other analytic issues.
Due to the nature of the submaximal test, some estimates obtained in this component may appear extreme compared to data obtained from more direct measures. Analysts need to carefully examine the data distribution and consider whether or not it is appropriate to include or exclude extreme values in a given analysis.