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Aims: Heart rate variability (HRV) is proposed to be a measure of cardiac autonomic activity.This study was designed to investigate the short- and long-term HRV before, during and aftereight weeks (3x/week) of sprint interval training (SIT), compared to eight weeks (3x/week) ofcontinuously moderate intensity training (CT) in young and healthy women and men, and toexamine associations between HRV and response in maximal oxygen consumption (VO2max).Methods: 22 moderately fit young subjects were divided into the two groups, matched onVO2max, sex and weight. Both groups did three sessions per week, and training were carried outoutside. SIT (four males, eight females) did 5-10, 30-seconds near maximal intensity runs in agraded track, with three minutes of rest (passive walking) between bouts. CT (thee males,seven females) ran at 70-80% of their HRpeak for 30-60 minutes on each session.During the training period, subjects did two short-term recordings of HRV, measured at rest inthe morning (five minutes of sitting, followed by five minutes of standing). Recordings were doneafter training days (AT) and after resting days (AR). Pre-, mid- and post training subjectsperformed a test of VO2max and a 24-hour recording of HRV (long-term). R-R intervals wererecorded using a hear rate monitor Polar RS800CX, Polar Electro Oy, Kempele, Finland) and RRdata were analyzed in Hearts software. HRV indices included are time-domain (mean HR,SDNN), frequency-domain (LF, HF and LF/HF ratio) and non linear analysis (Poincaré Plot; SD1and SD2, α1, APEN). Data are presented as means ± SEM. Short-term data were analyzed forAR and AT in three sub periods (period 1: week 0-2, period 2: week 3-5, period 3: week 6-9).Long-term data were analyzed for the whole 24-hour period and separate for night hours (1-5a.m.).Results: In SIT, HRV was significantly lower on AT days compared to AR days both in sittingand standing posture during the first training weeks. These changes were not observed inperiod 2 and 3. From period 1 to period 3, SIT showed a significant decrease in HRV on ARdays in standing posture (p < 0.05), and tended to decrease also in sitting posture. In CT, nochanges were observed between AT and AR or between periods. We found no increase in 24-hours HRV post training. Both groups increased VO2max; 2.4 ± 0.7 and 2.0 ± 0.9 ml kg-1 min-1 forSIT and CT respectively (p <0.05). We found a significant correlation between changes in shorttermHF power, (ln, ms2) on AR days in sitting posture and training response (ΔVO 2max) for bothgroups together (r=0.531, p=0.011) and for SIT group alone (r=0.615, p=0.033).Conclusion: It can be concluded that during the first weeks of SIT, HRV is lower after trainingdays compared to after resting days. Furthermore, a lower HRV after resting days appearsduring the last weeks of SIT, indicating a withdrawal in cardiac vagal activity and a trainingoverload. On the contrary, during an eight weeks period of CT, HRV do not change after trainingdays compared to after resting days. Although SIT and CT are efficient to increase VO2max, ourdata do not support benefits on HRV in moderately fit young women and men. However, HRVcan be a valuable tool to evaluate the long-term recovery of and for planning a proper trainingload in moderately fit subjects

Effects of Exercise Training on Heart Rate Variability After Coronary Angioplasty