Left ventricle (LV) performances are determined by the intracellular control of contraction. The muscle power capability, at the cellular level, is derived from the force-velocity relation and relates to the linear dependence of the cross-bridge (the molecular motor) weakening rate (g) on the shortening velocity, .  The time independent parameter g₁ determines the maximal power. The study aims to quantify the whole heart power capability and to define a measurable index for cardiac contractility that will relate to the basic cardiac fiber characteristics. Methods. Swine (n=7) were anesthetized and the heart was exposed by mid-sternotomy. Three orthogonal pairs of sono-crystal were implanted in the LV wall to measure LV diameters. An occluder and a flow-meter were placed around the proximal ascending aorta. A tube connected to a syringe was inserted into the LV cavity via the apex. Programmed step-motor determined the forced ejection velocity (V_EJE) and the time onset of ejection into the syringe. Isovolumic contractions were imposed by inflating the occluder. Forced ejections were imposed by pulling blood through the apical tube while the occluder was inflated. Results. The instantaneous pressure deficit (DP) between the isovolumic pressure (P_ISO) and the ejecting beat pressure was found to be: , where Dt is the ejection duration and G₁= 2.47 +/- 0.693 [%/cc]. The normalized DP  was independent of the time onset of the forced ejection and hence independent of the activation level. Conclusion. A measurable time independent index (G₁) of LV contractility was defined, that links global heart function to the basic properties of the cross-bridges.