Final answer:
The cusps of the AV valves are closed during ventricular contraction by the tension generated by the contracting papillary muscles, which pull on the chordae tendineae to prevent backflow of blood into the atria.
Step-by-step explanation:
When the ventricles contract, the pressure within them increases, causing blood to flow towards areas of lower pressure, which is initially towards the atria. To prevent blood from flowing back into the atria, the atrioventricular (AV) valves, specifically the tricuspid and mitral (bicuspid) valves, need to close. This is accomplished by the contraction of the papillary muscles which are connected to the valves by the chordae tendineae. As the ventricular muscles contract, the papillary muscles also contract, increasing the tension in the chordae tendineae, effectively holding the valve cusps in place and ensuring they do not prolapse back into the atria.
During the relaxation phase of the cardiac cycle, or diastole, the papillary muscles are relaxed and the tension on the chordae tendineae is minimal. But as soon as the ventricular contraction, or systole, begins, both the myocardium of the ventricle and the papillary muscles contract. This synchronized contraction and the resulting tension in the chordae tendineae are vital in keeping the AV valves shut during the isovolumetric contraction phase of ventricular systole, thereby preventing the backflow of blood into the atria.