The rapid decline in pressure during isovolumic relaxation (IVR) is traditionally fit algebraically via two empiric indexes, , the 'time-constant of IVR', or a logistic time constant (τ_L). Although these indexes are used for in-vivo diastolic function characterization of the same physiologic process, their characterization of IVR in the pressure phase plane is strikingly different and no smooth and continuous transformation between them exists. To avoid the parametric discontinuity between and _Land more fully characterize isovolumic relaxation in mechanistic terms, we modeled ventricular IVR kinematically employing a traditional, lumped relaxation (resistive) and a novel elastic parameter. The model predicts IVR pressure as a function of time as solution of d²P/dt²+1/µdP/dt+E_kP=0, where µ [ms] is a relaxation rate (resistance) similar to or _L, and E_k [1/s²] is an elastic (stiffness) parameter (per unit mass). Validation involved analysis of 310 beats (10 consecutive beats for 31 subjects). This model fit the IVR data as well or better than τ and τ_Lin all cases (mean RMSE dP/dt vs. t 29 mmHg/s for model; and 35 mmHg/s or 65 mmHg/s for and _L respectively). The solution naturally encompasses and _L as parametric limits and good correlation between and 1/µE_k (=1.15/µE_k-11.85 r²=0.96) indicates that isovolumic pressure decline is determined jointly by elastic (E_k) and resistive (1/µ) parameters. We conclude that pressure decline during IVR is incompletely characterized by resistance (i.e. and _L) alone, but is determined jointly by elastic (E_k) and resistive (1/µ) mechanisms.