Heart rate indicators for assessing physical activity level in the field

The reliability and validity of heart rate (HR) indicators for assessing ph ysical activity level (PAL) was examined in 16 male and female farmers in n ortheast 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-MR (%Flex) had the highest rel iability 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 s ubject'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 th at the difference among the slopes of PAL of different subjects was not sig nificant in females, but was in males (P = 0.003). Comparison of measured P AL values and corresponding PAL values estimated by a regression equation f or all subjects showed that 75% were within +/-0.2; further, PAL values ten ded to be underestimated in cases when they exceeded 2.0 for males. The res ults for these Thai-Lao subjects are consistent with previous studies on ot her populations; thus, it may be concluded that %Flex is reliable and valid for assessing PAL for a group of subjects and for assessing intraindividua l variability of PAL. (C) 1999 Wiley-Liss, Inc.