The individual's current level of fitness depends on the amount of exercise needed. I would advise my friend to exercise above her/his target heart rate to help improve her/his cardiovascular endurance for an individual has genetically the capacity to adapt to training, his or her fitness goals, and to the component being developed such as improve cardiovascular endurance.
In addition, if my friend is exercising with in or below her/his target heart rate, her/his goal will not be achieve because her/his stamina and strength will not increase. If she/he will like to improve her/his cardiovascular endurance, a gradual increase in her/his workouts must be done. Therefore, she/he must exercise above her/his target heart rate.
