The time constant (T) obtained by fitting post-exercise heart rate (HR) recovery to a first order exponential decay curve has been promoted as an index of parasympathetic activity. However, acceptance has been limited because reported data are inadequate to assess goodness of fit for the model, determine the best exercise protocol, or optimize the duration of post exercise monitoring. Consequently, we evaluated T for nine healthy volunteers (age 24-46) following treadmill exercise at maximal (max) and two stages sub- max exercise (Bruce protocol). T stabilized only after 3 min of post-exercise monitoring. With max exercise, T varied unacceptably with small changes in onset of monitoring, e.g. -16.7 ± 16.6 (-13.2%) in the first 5 s, and residuals of the fitted curve were non-random. In contrast, sub-max exercise produced consistent T values, e.g. -1.9 ± 3.2 (-4.2%) in the first 5 s, and residuals were more nearly random. In conclusion, first order decay is an inadequate model for HR recovery following max exercise, but may be reasonable for sub-max levels. (C) 2000 Elsevier Science B.V.
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