Exercise increases heart action.
This increases the heart rate and the intensity of the first heart sound.
At 120 beats per minute the durations of systole and diastole are about equal.
Other conditions, such as stress, elevated temperature and hyperthyroidism can also increase the intensity of the first heart sound. When these conditions are alleviated the first heart sound returns to normal.
The heart sounds are best heard using the diaphragm of the stethoscope.