Comparative study of flex heart rate in colombian children and in pregnant, lactating, and non-pregnant, nonlactating women.

The heart rate (HR) value employed to separate resting and active portions of the calibration curves used to estimate energy expenditure (EE) from minute-by-minute HR recordings is called the flex-HR. The present study has characterized it, the resting HR and the average daily HR during the awake portion of the day (12-14 h) by comparing age, gender and nutritional status effects related to measured maximum oxygen consumption (V̇ O 2 max; ml/min/kg body weight)in school-aged Colombian children (145 boys and 132 girls), 6-16 years of age. The same HR variables have been individually measured in nutritionally normal, nonpregnant, nonlactating (NPNL; n = 48), pregnant (n = 26), and lactating (n = 18) women, 19-43 years of age, on three occasions separated by 3 months. In general, the flex-HR followed the differences observed in resting and average daily HRs. All three values decreased with age in children, were higher in girls than boys, and did not exhibit differences between nutritionally normal and undernourished children. All three HRs had a statistically significant negative relationship with V̇ O 2 max in boys but not in girls. NPNL and lactating women showed no significant change in the mean values of the repeated HR measurements but exhibited maximum individual differences of flex-HR of -56 to +42 beats/min. Pregnant women had higher HRs in all 3 rounds of measurement compared to NPNL subjects. The data support the generalization that the flex-HR method of estimating EE is appropriate in groups of subjects but not in individuals, and that individual calibration of subjects close to the time of application to the making of EE measurements is an important feature of its use. Am. J. Hum. Biol. 9:647-657, 1997. © Wiley-Liss, Inc.