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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 (HFRR), low to high frequency ratio (LFRR/HFRR) and low frequency power of systolic pressure (LFSP), 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 lnHFRR; 2) threshold phenomena around 65%MF in %MF vs. LFSP and LFRR/HFRR 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.