What is the ECG measuring? The muscles of the heart go through a steady cycle of polarization-depolarization events with each heartbeat. The ECG displays the changes in polarization (electrical charge) as the muscle fibers go from the resting state through complete contraction-relaxation cycle. The electical state of the fiber determines the state of action of the myocardium at that moment. The slope of the line indicates the speed of the signal, the amplitude indicates the number of cells changing as well as the direction of electical conduction. A negative slope indicates the short intervals where conduction actually progresses upward, from apex to base.