We propose using the low-frequency component (LF) of the maximal first derivative of arterial pressure waveform (dAP/dtmax) as a potential estimator of sympathetic activity (LFdAP/dtmax). This could expand the estimating capability of dAP/dtmax, used as a noninvasive index of left ventricular systolic function and a predictor of mortality in heart failure. We assessed the effects of continuously increasing muscular force (CIMF) on the time courses of systolic pressure (SP), dAP/dtmax, LFdAP/dtmax, and LF of SP (LFSP). 34 healthy volunteers isometrically extended their right leg from 0 to 100% of their maximal force (%MF) at a rate of 0.2 kg.s-1. The spectral indexes were computed using the smoothed pseudo-Wigner-Ville time-frequency distribution. Coherence and correlations between spectral indexes were obtained. The threshold phenomenon was detected by the V-slope method. The indexes' response to CIMF showed patterned dynamics, relative to their baseline. SP increased progressively from the onset to 64.7-8.8 %MF, after which it increased more rapidly. The dAP/dtmax increased gradually from the beginning to 53.5-11.2 %MF, reached a plateau for 21-6 s, and then rose progressively until the end. LFdAP/dtmax and LFSP responses exhibited two phases: initially, they decreased until 65.3-11.3% MF for LFSP and 64.7-13.1% MF for LFdAP/dtmax, followed by an abrupt increase that continued until the end of CIMF. This behavior is considered a threshold phenomenon. The inflection %MF of LFSP, LFdAP/dtmax, and SP were similar. The table shows the mean correlations and coherence between indexes in the post-threshold phase. We found strong correlation and high coherence between our new indicator LFdAP/dtmax and the well-known LFSP sympathetic index. This supports the proper performance of LFdAP/dtmax as a promising cardiac sympathetic activity measure. Furthermore, tracking the time courses of LFdAP/dtmax and LFSP during CIMF enabled us to show a sudden increase in sympathetic outflow, possibly associated with the metaboreflex activation threshold.