Effect of Exercise Intensity on Autonomic Nervous System Activity during And After Acute Exercises

A study was conducted to investigate the effect of exercise intensity on the recovery of autonomic nervous activity after exercise. Ten subjects performed four kinds of 10–min cycle exercise with target heart rates of 100, 120, 140, and 160beats/min (THR 100, THR 120, THR 140 and THR 160, respectively) following 5 min of exercise to increase the heart rate to the target level. The beat-by-beat variability of the R–R interval was recorded throughout the experiment including the 5-min pre-exercise control period and the 30-min recovery period. Spectral analysis (fast Fourier transform) was applied to every 5-min R-R interval data set before, during (5 —10 min) and after exercise at the target heart rate. The low- (0.05 ∼ 0.15 Hz : P₁) and high- (0.15 ∼ 1.0 Hz : P_h) frequency areas were calculated to evaluate sympathetic (SNS) and parasympathetic (PNS) nervous activities as P₁/P_h and P_h, respectively. During exercise, SNS of THR 160 was significantly higher, and PNS of THR 140 and THR 160 was significantly lower than the respective pre-exercise values (p<0.05). Although all indicators recovered to, or overshot the pre-exercise values at 20∼30 min after THR 100 and THR 120, heart rate and SNS were still higher and PNS was still lower than the pre-exercise value after THR 160. These results suggest that the recovery of cardiac autonomic nervous activity is slower after high-intensity exercise than after low-intensity exercise, and that the recovery of autonomic nervous activity after acute exercise does not always corrrespond linearly on the exercise intensity.