Heart rate indicators for assessing physical activity level in the field.

The reliability and validity of heart rate (HR) indicators for assessing physical activity level (PAL) was examined in 16 male and female farmers in northeast Thailand. For each subject, daytime HRs were monitored for 12 days, 4 days each in three agricultural seasons in a year. At the group level, the percentage of time when HR exceeded flex-HR (%Flex) had the highest reliability as an indicator of PAL even in the season of lowest activity (r > 0.91 in males and r > 0.89 in females). Regarding the reliability of each subject's %Flex for assessing PAL, regression analyses revealed coefficients of determination of 0.73-0.97 in males and 0.62-0.97 in all females except one. Analyses of covariance (ANCOVA), with %Flex as a covariate, showed that the difference among the slopes of PAL of different subjects was not significant in females, but was in males (P = 0.003). Comparison of measured PAL values and corresponding PAL values estimated by a regression equation for all subjects showed that 75% were within +/-0.2; further, PAL values tended to be underestimated in cases when they exceeded 2.0 for males. The results for these Thai-Lao subjects are consistent with previous studies on other populations; thus, it may be concluded that %Flex is reliable and valid for assessing PAL for a group of subjects and for assessing intraindividual variability of PAL. Am. J. Hum. Biol. 11:647-657, 1999. Copyright 1999 Wiley-Liss, Inc.