There are a total of four mechanical events of the cardiac cycle, all of which are associated with their own electrical events. Recall that in order for a muscle to contract, it must receive an electrical signal that leads to a depolarization phase. If the heart is to perform a series of contractions, then, for each contraction, the muscle must relax as well. Relaxation of the muscle is correlated with the repolarization of the muscle and indicates a time in which the chambers are filling with blood. Which portion of a normal ECG represents ventricular depolarization