The atrioventricular (av) valves are closed the atrioventricular (av) valves are closed while the atria are contracting when the ventricles are in diastole by the movement of blood from atria to ventricles when the ventricles are in systole

Cuspid Valve

Explanation:

The valves between the atria and ventricles are called atrioventricular valves and are also called Cuspid valves, while those at the bases of the large vessels leaving the ventricles are called Semilunar valves.

The right atrioventricular valve is the tricuspid valve. The left atrioventricular valve is the bicuspid, or mitral, valve. The valve between the right ventricle and pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and the aorta is the aortic semilunar valve.

While the atria are contracting when the ventricles are in diastole by the movement of blood from atria to ventricles when the ventricles are in systole...The ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.