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LEE, C. M., R. H. WOOD, and M. A. WELSCH. Influence of Short-Term Endurance Exercise Training on Heart Rate Variability. Med. Sci. Sports Exerc., Vol. 35, No. 6, pp. 961–969, 2003.To examine the influence of 2 wk (eight sessions) of endurance training on cardiac autonomic modulation, as measured by heart rate variability (HRV).Twenty-four males (mean age: 23.1 yr) were randomized to an exercise (EX;N = 12) or control group (CT;N = 12). EX trained for eight sessions (4× wk−1, 40 min, 80–85% HRreserve) on a cycle ergometer. ECG tracings were collected during 5 min of paced breathing (12 breaths·min−1 (PB)), 5 min of spontaneous breathing (SB1), 5 min of 70° head-up tilt (TILT), and a second 5-min period of spontaneous breathing (SB2). Data were collected before (test 1), during (tests 2–4), and 48 h after (test 5) the 2-wk period. HRV was reported as the standard deviation of RR intervals, and as natural logarithm of the normalized units (NU) of high- and low-frequency power (lnHF and lnLF).EX exhibited a significant increase in peak oxygen consumption (8%). During PB and TILT conditions, ANOVA revealed a group × time interaction such that EX exhibited lower lnLFNU and lnLF/lnHF during test 5 compared with test 1.These data suggest that eight endurance exercise-training sessions performed over 2 wk enhance the relative vagal modulation of the heart during PB and TILT, but not during SB.