Omecamtiv mecarbil (OM) is a novel medicine for systolic heart failure, targeting myosin to enhance cardiomyocyte performance. To assist translation to clinical practice we investigated OMs effect on explanted human failing hearts, specifically; contractile dynamics, interaction with the β1 -adrenoceptor (AR) agonist (-)-noradrenaline and spontaneous contractions. Left and right ventricular trabeculae from 13 explanted failing hearts, and trabeculae from 58 right atrial appendages of non-failing hearts, were incubated with or without a single concentration of OM for 120 min. Time to peak force (TPF) and 50% relaxation (t50% ) were recorded. In other experiments, trabeculae were observed for spontaneous contractions and cumulative concentration-effect curves were established to (-)-noradrenaline at β1 -ARs in the absence or presence of OM. OM prolonged TPF and t50% in ventricular trabeculae (600 nM, 2 µM, p < .001). OM had no significant inotropic effect but reduced time dependent deterioration in contractile strength compared to control (p < .001). OM did not affect the generation of spontaneous contractions. The potency of (-)-noradrenaline (pEC50 6.05 ± 0.10), for inotropic effect, was unchanged in the presence of OM 600 nM or 2 µM. Co-incubation with (-)-noradrenaline reduced TPF and t50% , reversing the negative diastolic effects of OM. OM, at both 600 nM and 2 µM, preserved contractile force in left ventricular trabeculae, but imparted negative diastolic effects in trabeculae from human failing heart. (-)-Noradrenaline reversed the negative diastolic effects, co-administration may limit the titration of inotropes by reducing the threshold for ischemic side effects.