Obesity Epidemiology

130 STUDY DESIGNS AND MEASUREMENTSflex heart rate (Flex HR) method, 59 percent heart rate reserve (% HRR), or % VO 2 maxreserve. 60 In general, validation studies have demonstrated a moderate to high degree ofaccuracy of both Flex HR and % HRR methods in estimating energy expenditure measuredby DLW and indirect calorimetry (r = .54 to .98). 58After correcting for age and fitness level, Strath et al. 60 reported a strong correlationbetween heart rate and energy expenditure (r = .87) measured by indirect calorimetryduring field- and laboratory-based moderate intensity activities. However, the reliabilityand validity of heart rate monitors for measuring low-intensity physical activityare known to be considerably lower because of nonlinearity of heart rate and VO 2 maxduring resting and light activity. 58 Thus, heart rate monitors have been combined withaccelerometry-based technology to assess sedentary and light activity. In addition, otherfactors (e.g., psychological stress or changes in body temperature) can influence heartrate. Extended periods of monitoring can cause skin irritation and discomfort, consideringthat multiple-day monitoring (including both weekdays and weekends) is required toreflect usual activity patterns.There is some evidence that combining heart rate monitors with movement sensorscan improve the precision and accuracy of physical activity measurements. Strathet al. 61 showed a higher accuracy of energy expenditure estimated by the simultaneousheart rate-motion sensor technique (CSA accelerometers and Yamax pedometer) thanthat estimated by heart rate and motion sensors used independently among 16 men and14 women. The combined technique also provided more accurate estimates of energyexpenditure at different intensities of activity. 62Physical FitnessAlthough physical fitness and physical activity are closely related concepts, they are notsynonymous and should not be used interchangeably. As already mentioned, physicalfitness is a functional construct, while physical activity consists of behaviors. Bouchardand Shephard 63 refined the concept of health-related fitness by dividing it into fourmajor components: cardiorespiratory endurance (or fitness), body composition, muscularstrength, and flexibility. Among these components, cardiorespiratory fitness is the mostextensively studied in clinical and epidemiologic settings.Cardiorespiratory fitness, also termed aerobic fitness or endurance, is defined as “ahealth-related component of physical fitness that relates to the ability of the circulatoryand respiratory systems to supply oxygen during sustained physical activity.” 3 Habitualphysical activity status is one of the major modifiable determinants of cardiorespiratoryfitness. Other influencing factors include age, sex, genetics, and medical conditions. 64 Inepidemiologic studies, assessments of cardiorespiratory fitness have been used as objectiveand surrogate measures of physical activity to validate self-reported physical activityquestionnaires.VO 2 max, the gold standard or criterion measure of cardiorespiratory fitness and exercisecapacity, 4 is the maximum volume of oxygen that muscles can use while performingdynamic exercises, such as walking, running, or cycling. It is commonly expressed asmultiples of METs. VO 2 maxis typically estimated according to a protocol that involvesthe performance of an activity, usually running or cycling, until exhaustion. The test canbe performed on a treadmill or bicycle ergometer, and oxygen uptake can be accuratelyestimated from the rate of work (speed, grade, and resistance). 64 Cardiorespiratory fitnessmeasures have been obtained in several relatively large epidemiologic studies, notablythe Aerobics Center Longitudinal Study (ACLS), 65 Lipid Research Clinics Mortality