Abnormal rates of muscle contractions or heart movements: With arrhythmias, a person's heartbeats may be too slow, too rapid, or very irregular.

Rapid arrhythmias (greater than 100 beats per minute) are called "tachycardias" while slow arrhythmias (slower than 60 beats per minute) are called "bradycardias"; and sometimes, irregular heart rhythms are called fibrillations or muscular twitching in the heart that involves individual muscle fibers that move without coordination.