Time-frequency analysis of cardiovascular variability during two types of continuous and linearly increasing isometric exercise

To characterize the effect of isometric exercise (IE) intensity on the cardiovascular autonomic variables (CVAV), we assessed the continuous relationships between force and the high frequency power of RR intervals (HF_RR), low to high frequency ratio (LF_RR/HF_RR) and low frequency power of systolic pressure (LF_SP), estimated by a time-frequency distribution (TFD). Thirty five healthy subjects performed continuously increasing static handgrip (HG) and static leg extension (LE) until maximal force (MF). Main findings were: 1) strong correlations between %MF and lnHF_RR; 2) threshold phenomena around 65%MF in %MF vs. LF_SP and LF_RR/HF_RR relations; and 3) greater effects on CVAV of LE than HG (p<;0.001). Combining continuously increasing IE intensity with a TFD allows to obtain continuous relations between CVAV and intensity in the widest range, which show strong correlations, threshold phenomena and greater autonomic responses in LE.