CARDIAC AUTONOMIC MODULATION FOLLOWING SETS OF UNILATERAL ISOMETRIC EXERCISE PERFORMED AT TWO DIFFERENT INTENSITIES

Abstract

PURPOSE To examine the kinetics of cardiac autonomic modulation, as measured by heart rate variability (HRV), following a set of isometric contractions and to further determine if the kinetics are intensity-dependent. METHODS Following 10 minutes of rest, EKGs were collected in 18 participants (age=24.3±3 yr) 2.5 minutes before (PRE) and 10 minutes following 2 sets of isometric knee extension exercise. Participants performed a set of contractions with the non-dominant leg (knee at 90 degrees) at both 70 and 85% of their maximal voluntary contraction (MVC). The order of the intensities was randomized and the recovery EKGs were further divided into four consecutive 2.5-minute segments (Rec1-Rec4) in order to evaluate the time course of any possible autonomic changes following exercise. Each 2.5-minute segment was evaluated for the mean heart period (mean RR) and the standard deviation of normal RR intervals (SDNN). Furthermore, low- and high-frequency spectral power were derived via a 512-point FFT and were reported as normalized units (LFNU and HFNU, respectively). Repeated measures ANOVA was used to evaluate differences in HRV during the various time intervals and preplanned contrasts were used to determine which recovery periods differed from PRE. A significance level of 0.05 was used for all tests. RESULTS Compared to PRE, mean RR was lower during Rec1 and Rec3 for both intensities, reflecting an increased heart rate during these periods. Additionally, SDNN was greater during Rec1 for both intensities compared to PRE. Regarding the spectral indices, HFNU was greater (and LFNU lower) than PRE during Rec1 for 70% only. Furthermore, HFNU was lower (and LFNU greater) than PRE during Rec3 and Rec4 for both intensities. CONCLUSION These data suggest that early during the recovery period there is a rapid vagal reactivation, which may be intensity-dependent. This is followed by a subsequent reduction in cardiac vagal modulation, which persists from 5 to at least 10 minutes following exercise and does not differ between sets performed at 70 and 85% of the MVC.