[Heart rate response and its variability during different phases of maximal graded exercise]. / Comportamento da freqüência cardíaca e da sua variabilidade durante as diferentes fases do exercício físico progressivo máximo.

PURPOSE: Heart rate variability (HRV) has been studied at rest as a non-invasive tool for the assessment of cardiac autonomic control and, its attenuation is related to cardiovascular risk. However, during exercise, when important neural changes take place, HRV behaviour is not well established. The aim of this investigation was to study the heart rate (HR) and HRV responses during the different metabolic phases of maximal graded exercise in young men. METHODS: Seventeen men (age 28 +/- 6 years) were submitted to a graded cardiopulmonary exercise test in a cycloergometer (30W/3 min). To study HR and HRV (standard-deviation), the electrocardiographic signal was amplified and acquired beat-to-beat in a computer at a frequency of 125 Hz (AT/Codas). RESULTS: The HR increases concomitantly to the increase in exercise intensity. The HRV was significantly lower than rest values after exercise intensities of 60% of peak exercise oxygen uptake, 45-60% of maximal power and, after anaerobic threshold intensity. CONCLUSION: This results suggest that HRV measured by standard deviation of HR decreases during exercise phases when HR increment is determined mainly due to vagal withdrawal.