ONE-WEEK STABILITY OF HEART RATE VARIABILITY AT REST AND FOLLOWING A SET OF ISOMETRIC CONTRACTIONS

Abstract

PURPOSE To examine the 1-week stability of heart rate variability (HRV) at rest and during recovery from a set of isometric contractions. METHODS Ten participants (age = 24.8±3 yrs) were tested on 2 visits to the laboratory separated by 1 week. On each visit, 2.5-minute EKGs were collected following 10 minutes of quiet rest in the seated position. EKG tracings were collected during spontaneous breathing (SB), paced breathing (PB; 12 breaths/min), and during PB following a set of 20 isometric contractions of knee extension exercise with the non-dominant leg (REC). Participants contracted at 70% of their maximal voluntary contraction with their knee positioned at 90 degrees. HRV was reported as the standard deviation of normal RR intervals (SDNN), and spectral power in the total (TP; 0.00–0.4 Hz), low- (LF; 0.04–0.15 Hz), and high- (HF; 0.15–0.40 Hz) frequency power ranges was derived via an FFT. LF and HF were further reported as normalized units (NU) to examine cardiac autonomic balance. The reproducibility of HRV under each condition was reported as the intraclass correlation coefficient (ICC), the coefficient of variation, and as the standard error of measurement. Additionally, repeated measures ANOVA was used to examine HRV during the various conditions. RESULTS During both testing days, the mean RR interval was lower and SDNN was greater during REC compared to SB and PB (p < 0.05). Additionally, HFNU was greater during REC and lower during SB compared to PB (p < 0.05), whereas LFNU behaved reciprocally on both testing days. ICCs ranged from 0.56 to 0.81 for all HRV indices measured during SB, PB, and REC. CONCLUSION These data indicate adequate reproducibility of HRV indices under conditions of SB, PB and REC. Furthermore, these data confirm previous studies indicating elevated HF during PB, and a marked vagal reactivation immediately following a set of isometric contractions.