What diagnostic tool records the depolarization or contraction of heart muscles?

The electrocardiogram (ECG) is the correct answer because it is specifically designed to measure and record the electrical activity of the heart. During depolarization, heart muscle cells are activated, leading to muscle contraction. The ECG captures this electrical activity as waveforms on a graph, providing vital information about the heart's rhythm and structure.

An echocardiogram, in contrast, uses ultrasound waves to create images of the heart's structure and functionality, which does not directly measure electrical activity. A Holter monitor is a portable ECG device that continuously records the heart's electrical activity over a period, typically 24 hours, but it is still fundamentally an ECG. Cardiac catheterization is an invasive procedure used to visualize coronary arteries and assess cardiac function, primarily through imaging rather than recording electrical impulses. Thus, the ECG is the primary tool used to assess the depolarization or contraction of heart muscles through its electrical activity monitoring.