Write an article based on this "Recognize your limits. Understand the Rate of Perceived Exertion (RPE). Consider buying a heart rate monitor. Design an interval training session. Warm up before working out. Begin interval training. Adapt your workout to interval training. Finish with a cool down."

Article:
Interval training with moderate to high levels of intensity is generally considered safe for most people. However, each person has their own limits, own bodies, and own histories of exercise. The goal of interval training is to push yourself hard for short periods of time, but be sure that you keep that within reason to avoid injury.Anyone who has been living a long-term sedentary lifestyle or has otherwise been physically inactive for long periods of time should gradually acclimate to a more active lifestyle before attempting to perform interval training. This is a scale you can use to measure the intensity of your exercise, no equipment needed. Use the following to help determine if you are reaching the correct intensity during your intervals.  0 = No exertion — laying down or sitting still 1–2 = Very light intensity — slow walking 3–4 = Light to moderate intensity — brisk walking 4–5 = Moderate to somewhat heavy intensity — jogging 6–8 = Very heavy intensity — running or sprinting 9–10 = Very, very heavy intensity — something you cannot sustain for more than a few seconds. Although it's not necessary to buy a heart rate monitor to perform HIIT, it is fun and you can get more accurate readings of your effort by using one. You will be increasing and decreasing your intensity according to your heart rate, depending on which interval you are on. For this reason, you may wish to buy a heart rate monitor if you intend to perform interval training. Otherwise, you can use the RPE.  Your maximum heart rate is the upper limit of what your heart and lungs (your cardiac system) can safely handle while exercising. You can calculate your maximum heart rate by subtracting your age from 220. So, for example, if you're 20 years old, your maximum heart rate would be 200 beats per minute (bpm). This is an estimate and will vary somewhat from person to person, but is a good approximation for most people. During an intense period of your interval training, you should hit somewhere between 80 and 95% of your maximum heart rate.  Monitor your heart rate closely. Do not attempt to hit 100%, as this may lead to injury or cardiac complications. Many cardio machines have built in heart rate monitors. The biggest factors in designing an interval training program for yourself are the duration, intensity, and frequency of your workout intervals, as well as the duration of your recovery intervals. Interval training sessions are highly personalized. Only you will be able to tell whether you are pushing yourself hard enough (without pushing too hard). This is another reason a heart monitor can be valuable.  An ideal interval training session should push you to feel that you are exercising "hard" to "very hard," depending on your abilities and subjective definitions of difficulty. Try using the talk test as a guide in deciding whether your workout is intense enough or too intense. The talk test involves trying to carry on a conversation while working out, which should be possible but difficult. Because interval training is so strenuous on your heart and muscles, it's best to give yourself an adequate warm up session before starting. A good warm up at your active-recovery intensity (your usual workout level) might last approximately 8 to 10 minutes, but it's important to listen to your body. If you don't feel adequately warmed up after 10 minutes, add another five to seven minutes of warm up time before you begin the actual high-intensity intervals. Once you are warmed up, you're ready to begin your interval training. A good interval training session for someone starting out might last approximately 20 to 30 minutes, and look something like the following:  Warm up Jog at a comfortable or moderate-intensity pace for two minutes. This is about a 5 out of 10 on the RPE. Increase your speed to a sprint for one minute. You should be at about a 7 or 8 on the RPE. Reduce your speed back to the moderate-intensity pace for two minutes. Continue alternating between a two minute jog and a one minute sprint for 7 cycles. Cool down for five to 15 minutes and stretch. Running is the easiest example of an interval training regimen, but you can use interval training with any aerobic workout. It's just a matter of determining how and when to increase your effort and speed in order to maximize your workout.  If you're a swimmer and you're doing eight laps, swim your hardest on laps one, three, four, six, and eight. Use laps two, five, and seven as recovery laps. If you're a cyclist (stationary or outdoor), determine in which gear you can hit 90 to 110 rpm with a moderate degree of effort. Then pedal as hard and fast as you can for 10 seconds in that gear, followed by 20 seconds of recovery pedaling. Interval training is not great on a bicycle outside, however, unless you use hills as intervals. A stationary bike is a better option for intervals unless you have long distances free of traffic. For cycling, an example of interval training would be to repeat the 10 second/20 second rotation for 10 to 15 minutes to complete a single set. The cool down is just as important as the warm up. Having your body go from 90% of your max heart rate down to a total resting state can stress or even damage your heart and cause the valsalva effect in which blood can pool in certain places in your body. Going off the previously-listed sample interval regimen, a good cool down might be to jog for five minutes at a slow and relaxed pace, followed by another five minutes or so of walking at a normal pace (2 to 3 mph or 3 to 4 km/hr, on average).