Passage Two It is necessary that most people determine whether or not they should exercise. This is true with anyone over thirty-five, especially if they have been heavy smokers. Likewise, young adults who have been very inactive should proceed slowly and cautiously. They had better get a physical evaluation from a physician, which includes an exercise-stress test. A stress test detects cardiovascular ( 心血管的) problems that might not show up when the body is at rest. This is done by monitoring blood pressure, pulse rate, oxygen consumption, and the heart’s electrical activity while exercising. No matter what kind of exercise they choose, exercisers should start with warm-up periods, which include stretching exercises, jumping jacks ( 原地跳跃运动), or brisk walking. They can help increase heart rate and circulation slowly without placing a sudden strain on the heart or the muscles that are starting to work. Moreover, in the second phase, people have to determine the amount of exercise that is enough to condition the muscles and cardiovascular system without overly straining the body. This involves taking one’s pulse and finding one’s target zone. This target zone, or safe-training pulse rate, is established by subtracting one’s age from 220 and then taking 60 percent to 80 percent of that total. If you are just starting to exercise 60 percent is recommended; 80 percent is recommended if you are already in good condition.