Technically, the heart rate you should never exceed during cardio exercise is 100 percent of your calculated maximum. In reality, the heart rate you personally shouldn't exceed is much lower than that, as even elite athletes will only want to work at that level for short periods. Even when it's not dangerous, there's a point of diminishing returns.
Maximum Heart Rate
The definition of your maximum heart rate is the highest number of times your heart can beat in a minute. The only really accurate way to learn that number is with a stress test, but if you're a young and with no symptoms of heart disease, your insurance company won't be paying for it. Instead, you can do some calculations. If you don't like complicated equations, go with 220 minus your age. That's acceptable, according to the American College of Sports Medicine, but the farther you are from middle-aged -- on either end of the scale -- the less accurate it is. In 2012, ACSM adopted what it considers a more accurate formula -- 206.9 minus 0.67 times your age.
Your target heart rate is the percentage of your maximum that you want to hit and hold throughout your workout -- usually about 30 minutes. Of course, you're not going the hit the ground running. You have to build to your target during a warm-up period of about five minutes where you start slowly and gradually pick up your speed. If using a cardio machine, you could also increase the incline before you start to cool down for another five minutes. The real measure of your fitness isn't how long you can maintain your heart rate, but how fast it drops during the cool-down. People whose heart rates drop considerably in the first two minutes are very fit.
The maximum you shouldn't exceed is going to depend on your fitness level and goals. If you're a couch potato just venturing out, but are otherwise healthy, you should plan on to start at a target of 50 to 70 percent of your max heart rate. This is considered moderate intensity, and working on the upper end of that range for 150 minutes per week should help you stay healthy and maintain your weight. Vigorous intensity ranges from 70 to 85 percent of your max heart rate. This intensity will allow you to either get the same workout in half the time, or bring your cardio fitness to the next level.
When you get up to the 85 to 90 percent range, you stop burning fat and lactic acid builds more quickly, causing your muscles to tire. Training at these levels isn't going to make you thinner or healthier, so the only reason to do so is if you are training for a sport where you'll need endurance and the ability to work through pain. Some elite athletes will try going into the highest zone -- 90 to 100 percent of max -- but even they will only do it for short periods, known as high-intensity interval training or HIIT.
Rate of Perceived Exertion
Another way to measure your exertion is your rate of perceived exertion (RPE). In this case you rate your exertion level yourself on a scale of one to 10, where one is easy, three to five is moderate and 10 is the absolute maximum you could endure. The benefit of this is that it makes up for variations in maximum heart rate among individuals, and even with the same individual from day to day. It also eliminates having to stop your workout to check you pulse. Of course, it's easy to tell yourself that you've hit your max when you've really barely hit moderate. To force yourself into honesty, do the talk/sing test. At a moderate pace you should be able to talk easily, while singing a song would be difficult. At vigorous intensity, you would have difficulty talking; at maximum ranges you wouldn't be able to talk at all.
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