First hea sound occurs during the period of:

First hea sound is a low, slightly prolonged lub caused by vibrations set up by the sudden closure of mitral or tricuspid valves at the sta or ventricular systole and it occurs during the period of Isovolumetric contraction. Ref: Medical Physiology: Principles for Clinical Medicine, 3rd Ed, Page 247 By Rodney Rhoades, David R. Bell, Ph.D