Comparative study of flex heart rate in three samples of Nepali boys

Flex heart rate (HR, beats per minute) has assumed increasing importance in studies of energy expenditure and physical activity. Flex HR is defined as the mean of maximum rest and minimum exercise HR recorded during a standard test. This report examines methodological and substantive issues regarding the measurement and interpretation of population variation in flex HR values. Flex HR was determined for 80 Nepali 10-14-year-old boys living in contrasting physical and socioeconomic environments (31 village boys, 24 urban middle-class schoolboys, and 25 homeless street boys). The three populations exhibited significant differences in flex HR, with villagers averaging lower values than school or homeless boys (91, 100, and 103 bpm, respectively, P < 0.0001). They also differed in mean resting HR (P < 0.0002), but not in the percentage increase of flex over resting HR. To evaluate reliability, flex HR measurements were repeated on 14 school boys after a 3-month interval. Mean initial and repeat values were not significantly different, but discrepancies were large for individual subjects (-15 to +24 bpm). This suggests that flex HR is reliable at the population level, but not necessarily stable for individuals over time. The range of factors contributing to variation in flex HR between and within populations have implications for the use of HR monitoring to estimate levels of physical activity. © 1996 Wiley-Liss, Inc.