Never allow your heart rate to go beyond an 85 percent TI. A rule of thumb is to train ataround 50 percent TI during the first month of exercise after a long period of inactivity.Cardiorespiratory benefit can be gained by exercising at 50 percent TI. After a fitnesslevel has been reached, the person exercising can work toward 70 percent TI or higher.Competitive athletes’ performing intervals will exceed 85 percent of their TI during hardbouts of training.Remember that the target heart rate (THR) is another measure of cardiorespiratory fit-ness. THRs, based on age and level of fitness, can be calculated or looked up on charts.A THR indicates if the exercise intensity is sufficient to improve health-related fitness. Inplain English, if your heart rate goes up a certain amount during a certain period of exer-cise, you’re either becoming fit or you’re already fit. THR is easy to measure; it simply re-quires taking your pulse and comparing it to a THR chart. We’ve discussed exercise duration in previous units. Not only do you need to push your-self, but you’ve got to get your heart rate up and keep it there for a while for cardio bene-fit. It isn’t enough to get your heart and lungs working for just a minute. Several schools ofthought exist as to the amount of exercise time, or duration, required for cardiorespiratoryhealth. Most fitness professionals seem to agree that cardio health can be improved if theworkout portion of exercise lasts at least 20 to 60 minutes. Duration will vary depending on a person’s fitness level and the intensity of exercise. Theless intense the exercise, the longer it will need to be maintained to get any benefit fromit. If you’re exercising at 85 percent TI, then as little as 20 minutes might be sufficient forimprovement; at 50 percent TI, at least 30 minutes is needed. Some fitness professionalssay you must exercise for at least 20 minutes at a time for cardio benefit. Some researchhas found that more frequent sessions of less time might also provide a cardiorespiratorybenefit, such as three 10-minute workouts per day, separated by four hours, at 70 per-cent TI. Everyone seems to agree that any workout should be preceded by at least a five-minute warm-up and followed by a five-minute cool-down.