Cardiac Autonomic Modulation Following High-Intensity Static Muscle Contractions

Abstract

The purpose of this investigation was to examine cardiac autonomic modulation, as measured by wavelet analysis of heart rate variability, following a series of high-intensity static muscle contractions. After 10 minutes of rest, electrocardiograms (ECGs) were collected in 17 participants (age, 24.5 +/- 1 yr) 2.5 minutes before (REST) and 10 minutes after a series of static contractions of the knee extensors. Participants performed contractions with the nondominant leg (knee fixed at 90 degrees) at either 70% or 85% (randomized order) of their maximal voluntary contraction (MVC) on two separate visits. The postexercise ECGs were further divided into four consecutive 2.5-minute segments (POST 1-POST 4) to evaluate the time course of any possible autonomic changes after exercise. Each 2.5-minute segment was evaluated for the standard deviation of normal RR intervals (SDNN) and the percentage of successive RR intervals that differed by more than 50 ms (pNN50). Frequency-domain indices were obtained via wavelet transformation giving coefficients that reflected high- and low-frequency fluctuations in heart period (HFw and LFw). SDNN was elevated during POST-1 and pNN50 was reduced during POST-3 and POST-4 (P < 0.05) compared to REST for both intensities. Furthermore, HFw and LFw were increased during POST-1 and reduced during POST-3 and POST-4 compared to REST for both intensities (P < 0.05). These data suggest that complex alterations cardiac autonomic modulation exist for at least 10 minutes following a series of high-intensity static contractions performed at either 70% or 85% of MVC.