Heart rate variability as a measure of exercise- and training response: a parallel eight weeks experimental training study comparing effects of sprint interval training and continuously moderate intensity training on short- and long-term heart rate variability in moderately fit young women and men
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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 after eight weeks (3x/week) of sprint interval training (SIT), compared to eight weeks (3x/week) of continuously moderate intensity training (CT) in young and healthy women and men, and to examine associations between HRV and response in maximal oxygen consumption (VO2max). Methods: 22 moderately fit young subjects were divided into the two groups, matched on VO2max, sex and weight. Both groups did three sessions per week, and training were carried out outside. SIT (four males, eight females) did 5-10, 30-seconds near maximal intensity runs in a graded 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 in the morning (five minutes of sitting, followed by five minutes of standing). Recordings were done after training days (AT) and after resting days (AR). Pre-, mid- and post training subjects performed a test of VO2max and a 24-hour recording of HRV (long-term). R-R intervals were recorded using a hear rate monitor Polar RS800CX, Polar Electro Oy, Kempele, Finland) and RR data 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; SD1 and SD2, α1, APEN). Data are presented as means ± SEM. Short-term data were analyzed for AR 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 (1-5 a.m.). Results: In SIT, HRV was significantly lower on AT days compared to AR days both in sitting and standing posture during the first training weeks. These changes were not observed in period 2 and 3. From period 1 to period 3, SIT showed a significant decrease in HRV on AR days in standing posture (p < 0.05), and tended to decrease also in sitting posture. In CT, no changes 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 for SIT and CT respectively (p <0.05). We found a significant correlation between changes in shortterm HF power, (ln, ms2) on AR days in sitting posture and training response (ΔVO 2max) for both groups 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 training days compared to after resting days. Furthermore, a lower HRV after resting days appears during the last weeks of SIT, indicating a withdrawal in cardiac vagal activity and a training overload. On the contrary, during an eight weeks period of CT, HRV do not change after training days compared to after resting days. Although SIT and CT are efficient to increase VO2max, our data do not support benefits on HRV in moderately fit young women and men. However, HRV can be a valuable tool to evaluate the long-term recovery of and for planning a proper training load in moderately fit subjects.
Masteroppgave - Norges idrettshøgskole, 2011