Résumé : Aims. Prolonged immobilization generates cardiac deconditioning, a possible risk factor: efficient countermeasures (CM) are needed to prevent it. We aimed at assessing the effectiveness of high-intensity jump training CM on aortic and mitral flow by Phase-Contrast (PC) MRI during 60 days head-down (-6°) bed-rest (BR). Methods. 23 males (29±6 y, 181±6 cm, 77±7 kg) were enrolled at:envihab research facility in Cologne as part of the European Space Agency BR studies. Participants were randomly allocated to the jump training (JUMP, n = 12) or the control (CTRL, n = 11) groups. Training consisted of 4x10 countermovement jumps and 2x10 hops in a horizontal sledge jump system, with 5-6 sessions/week. PC-MRI images (3T Biograph mMR, Siemens) with interleaved three-directional velocity encoding (VENC: x and y: 80 cm/s; z: 150 cm/s) were obtained (spatial resolution 1.4 x 1.4 mm 2) at the level of the aortic root, and of the mitral plane, before (BCD-4), after 21d (HDT21) and 58d (HDT58), and on the fifth day after BR conclusion (R+4). Velocity images were analysed to compute among others: cardiac output (CO), stroke volume (SV), max blood flow rate (Qpeak), systolic and heart beat duration (RR), rapid filling (Ewave) and early filling peak flow rate (Ef). Results. In CTRL, compared to BCD-4, at HDT58 a significant RR (14%) and systolic (10%) shortening, with a decrease in SV (22%), Qpeak (12%), Ewave (26%), Ef (26%) were observed. In CM, only RR was shortened (8%), together with a decrease in SV (12%), Qpeak (7.5%), Ewave (11%), Ef (15%). Interestingly, at R+4, compared to BCD-4, RR was still decreased by 10% in CTRL, while Ef increased by 18% and by 15%, respectively in CTRL and CM, where also CO (+19%) and SV (+8%) were increased. Conclusions. In this first study addressing aortic and mitral flow using PC-MRI during BR, cardiac deconditioning affected both flows. The applied CM was effective in partially opposing this phenomenon, inducing reduced changes at HDT58.