Time constant of heart rate recovery after low level exercise as a useful measure of cardiovascular fitness.

In this study we aimed to establish the usefulness of the time constant of heart rate recovery (Tr) in the evaluation of cardiovascular fitness. 15 male subjects exercised on recumbent bicycle at three different workloads (75 W, 100 W 125 W) where R-R intervals were monitored to determine Tr. In order to find the maximal oxygen uptake (VO2max) of each subject, oxygen consumption rate (VO2) was recorded throughout the treadmill exercise (10 km/h). Based on VO2max, we classified the subjects into two groups: the "fit" group and the "unfit" group. We found a significant difference in Tr between these two groups only existed when the workload was 75 W (<or=0.01) and only at this workload did the R-R intervals achieve stability during the 5 minutes of exercise. Furthermore, we found the cut-off value for predicting cardiovascular fitness at this workload was 55 seconds, with an associated sensitivity of 85.7% and specificity of 87.5%