Fast “ON” and “OFF” Heart Rate Transients at Different Bicycle Exercise Levels

Abstract

Rapid heart rate (HR) changes take place at the onset and at the end of exercise. The aim of this study was to analyze these changes at different exercise levels. A total of 17 healthy men (23 ± 4yrs), who without being told the aim of the study pedaled an electrically braked bicycle at 60rpm “ON” transients, were studied during changes in work rates from 0 to 1, from 1 to 2, and from 2 to 4 W.kg^-1. “OFF” transients were measured from 4 and from maximal work rate to 0 W.kg^-1. The ECG was recorded starting from 5 s before until 15 s after each work rate change. The mean HR was calculated at 2.5 s intervals as Δ% HR - the heart rate change as percent of the final pretransition value. Linear regressions between Δ% HR and time were obtained for each transient and their angular coefficients compared. Of the ON transients only the 2nd and 3rd suggested differences, while the OFF transients did not differ significantly. All of the transients were reproducible for a given subject (P > 0.05). The parasympathetic blockade performed in two subjects produced slower transients (except for the Ist ON). V̇O₂max was determined at the end of a continuous progressive test, performed after the Ist OFF. No correlation was found between the transients and V̇O₂max (P > 0.05). In conclusion, within 15 s, the ON and OFF transients showed changes of 10% and 5%, respectively, in HR. They were primarily mediated by the vagus. The transients reported here are slower than those found by others, and this probably reflects the lack of anticipation by the subjects or previous inactivation of the joint receptors, due to the fact that the ON transients were always measured during cycling, starting at 20 W and 60rpm.

Key words

exercise prescription - autonomic system - heart rate control - sudden exercise