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PORTIER, H., F. LOUISY, D. LAUDE, M. BERTHELOT, and C. Y. GUÉZENNEC. Intense endurance training on heart rate and blood pressure variability in runners. Med. Sci. Sports Exerc., Vol. 33, No. 7, 2001, pp. 1120–1125. Physical training with incomplete recovery times can produce significant fatigue. A study of cardiovascular responses showed that there is a sympathetic and a parasympathetic form of fatigue.The purpose of this experimentation was to measure the effects of intense endurance training on autonomic balance through a spectral analysis study of the heart rate (HR) and systolic blood pressure (SBP).Eight elite runners were tested twice: after a relative rest period (RRP) of 3 wk and after an 12-wk intense training period (ITP) for endurance. At the end of each phase, the subjects were tested by means of a O2max test and a tilt-table test.The resting heart rate (HR) variability was lower (P < 0.001) in the intensive training phase. Likewise, there were differences in the low-frequency (0.04–0.150 Hz; LF) and high-frequency (0.150–0.500 Hz; HF) components and the LF/HF ratio of the HR spectral analysis. The LF spectral power was significantly lower in the supine position (P < 0.05) during ITP. Upright tilting was accompanied by a 22.6% reduction in HF values during the rest period, whereas in ITP the HF spectral power rose by 31.2% (P < 0.01) during tilt, characterizing a greater parasympathetic system control. Sympathetic control represented by the LF/HF ratio regressed markedly (P < 0.01) in response to the tilt test in ITP.The spectral analysis of SBP in the high frequencies shows that the changes in cardiac parameters are coupled with a decrease in sympathetic vasomotor control (−18%) and a reduction in diastolic pressure (−3.2%) in the response to the tilt test at the end of ITP. Spectral analysis could be a means of demonstrating impairment of autonomic balance for the purpose of detecting a state of fatigue that could result in overtraining.