Heart rate monitoring can be used as an objective measure of total energy expenditure in laboratory and free living conditions. Contemporary devices are small and unobtrusive, cost between $200 and $500, and consist of a chest strap transmitter and a receiver watch (Freedson and Miller, 2000). Its use is based on the premise that heart rate and oxygen consumption are linearly related, particularly between 110 and 150 bpm (Freedson and Miller, 2000; Livingstone, 1997). Using a regression equation, exercise heart rate can be used to estimate VO2. Monitors that provide minute-to-minute recordings of heart rate enable the evaluation of day-to-day variability of energy expenditure and provide information about the frequency, duration, and intensity of activity (Freedson and Miller, 2000).
Although there are several different methods to analyze heart rate data, a frequently used method is the FLEX heart rate method (Ceesay etal, 1989; Livingstone etal, 1990). In the FLEX
method, heart rate and oxygen consumption are monitored at different exercise intensities and different postures (i.e., sitting, supine, and standing). Basal metabolic rate is also estimated for each subject. These data are used to construct a calibration curve to estimate energy expenditure (Freedson and Miller, 2000). The FLEX heart rate is the threshold heart rate used to differentiate between resting and exercise heart rate (Livingstone, 1997). Satisfactory estimates of total energy expenditure have been reported using this method (Ceesay et al, 1989; Spurr etal, 1988).
There are several potential limitations associated with the use of heart rate monitors as a proxy for physical activity. First, extraneous factors other than physical activity can affect heart rate, which may lead to potentially biased estimates of energy expenditure. Factors such as temperature, humidity, emotional states, food intake, and body position may affect heart rate independent of physical activity (Livingstone, 1997). Second, the cost of heart rate monitors may be prohibitive for large, epidemiologic studies. Third, energy expenditure data derived from calibration curves may be invalid if heart rate data are not representative or poorly discriminate between resting and exercise heart rate (Livingstone, 1997).
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