An increase in heart rate. The heart rate is controlled by the opposing actions of the sympathetic nervous system, which accelerates the heartbeat; and the parasympathetic nervous system, which slows it down. The heart rate is normally in the range 60–100 beats per minute (bpm). During exercise there is an increase in sympathetic activity and the heart rate increases to over 100 bpm. Following exercise, at rest, the heart rate returns to normal. However, sustained tachycardia at rest may indicate a form of abnormal heart rhythm (arrhythmia). A slow heart rhythm of less than 60 bpm at rest (i.e. bradycardia) is common in athletes but may also indicate arrhythmia.