During a workout the requirement of oxygen to the parts of the body that are being and the entire body in general goes up. Workouts can be of many types which include aerobic and anaerobic.
The target heart rate for aerobic workouts is generally taken to be around 75% of the maximum heart rate. The maximum rate is dependent on age and there are several formulae that have been derived to calculate it, though none of them is universally accepted. The maximum heart rate that a body can withstand decreases with age.
In has been found in studies that the reduction in heart rate once a person stops exercising can give an idea of the health of the heart. The ideal reduction in heart rate one minute after exercise is stopped is 50 beats per minute or higher. If the value is less than 30, the person is at a risk of suffering from cardiac ailments.
Measuring heart rates before and after working out allows the person doing the workout to determine their cardiac health and also the effectiveness of the workout.