Mechanical contraction of the heart occurs during which of the following phases of the cardiac cycle?

Question

Quizplus · Accepted Answer

During phases 1 and 2,an electrical plateau is created,and during this plateau,mechanical contraction occurs.Because there is no significant electrical change,no waveform appears on the electrocardiogram (ECG).During phase 0 (depolarization),the electrical potential changes rapidly from a baseline of 
2-90 mV to +20 mV and stabilizes at about 0 mV.Because this is a significant electrical change,it appears as a wave on the ECG as the QRS.During phase 3 (repolarization),the electrical potential again changes,this time a little more slowly,from 0 mV back to 
2-90 mV.This is another major electrical event and is reflected on the ECG as a T wave.During phase 4 (resting period),the chemical balance is restored by the sodium pump,but because positively charged ions are exchanged on a one-for-one basis,no electrical activity is generated,and no visible change occurs on the ECG tracing.

Mechanical Contraction of the Heart Occurs During Which of the Following