Endurance training slows down the kinetics of heart rate increase in the transition from moderate to heavier submaximal exercise intensities

Summary To find out whether endurance training influences the kinetics of the increases in heart rate (f_c) during exercise driven by the sympathetic nervous system, the changes in the rate off_c adjustment to step increments in exercise intensities from 100 to 150 W were followed in seven healthy, previously sedentary men, subjected to 10-week training. The training programme consisted of 30-min cycle exercise at 50%–70% of maximal oxygen uptake (O_2max) three times a week. Every week during the first 5 weeks of training, and then after the 10th week the subjects underwent the submaximal three-stage exercise test (50, 100 and 150 W) with continuousf_c recording. At the completion of the training programme, the subjects'O_2max had increased significantly(39.2 ml·min^–1·kg^–1, SD 4.7 vs 46 ml·min^–1·kg^–1, SD 5.6) and the steady-statef_c at rest and at all submaximal intensities were significantly reduced. The greatest decrease in steady-statef_c was found at 150 W (146 beats·min^–1, SD 10 vs 169 beats·min^–1, SD 9) but the difference between the steady-statef_c at 150 W and that at 100 W (f_c) did not decrease significantly (26 beats·min^–1, SD 7 vs 32 beats·min^–1, SD 6). The time constant () of thef_c increase from the steady-state at 100 W to steady-state at 150 W increased during training from 99.4 s, SD 6.6 to 123.7 s, SD 22.7 (P<0.01) and the acceleration index (A=0.63·f_c·^–1) decreased from 0.20 beats·min^–1·s^–1, SD 0.05 to 0.14 beats·min^–1·s^–1, SD 0.04 (P<0.02). The major part of the changes in and A occurred during the first 4 weeks of training. It was concluded that heart acceleration following incremental exercise intensities slowed down in the early phase of endurance training, most probably due to diminished sympathetic activation.