People riding exercise bikes in a gym

How to do cardio

As explained in Introduction to cardio, cardio is any physical exercise that raises your heart rate for a sustained duration. In its most basic form, known as steady-state training, cardio involves performing an exercise of moderate intensity for a prolonged duration. However, there are different types of cardio, with intensity levels ranging from light to maximal. Training at different intensities for different lengths of time leads to the development of different aspects of fitness.

What are the different levels of exercise intensity?

The American College of Sports Medicine (ACSM) defines five levels of exercise intensity, as presented in Table 1. Each level is defined based on a range of maximum heart rate (HRmax). For example, moderate-intensity exercise is defined as exercise that forces your heart to beat at between 64% and 76% of its maximum rate, whereas vigorous-intensity exercise is defined as exercise that forces your heart to beat at between 77% and 95% of its maximum rate.

ACSM exercise intensity levels

Table 1. Exercise intensity levels as defined by the ACSM, including the range of maximum heart rate (HRmax), range of reserve heart rate (HRreserve), rating of perceived exertion (RPE), and training effects and notes associated with each level. The training effects and notes were added by me.

Instead of HRmax, each level of intensity can also be defined based on either:

  • A range of heart rate reserve (HRreserve)
  • A rating of perceived exertion (RPE)

HRreserve is the difference between your HRmax and your resting heart rate (HRresting). In other words, HRreserve is what’s left after you subtract your HRresting from your HRmax. Since fitter individuals have a lower HRresting, calculating exercise intensity based on HRreserve is more accurate than calculating it based on HRmax because the HRreserve calculation takes the personal level of fitness into account.

The RPE is a numerical rating that you can use to subjectively estimate your level of exercise intensity if you don’t know how to calculate intensity based on HRmax or HRreserve. The RPE scale ranges from 6 to 20 because it is designed to also help you to estimate your heart rate. All you have to do is multiply the rate by 10 to get a very rough estimate. The RPE scale is also useful if you suffer from an abnormal heart rate or take heart-rate-affecting medications (such as beta-blockers), which make your heart rate an unreliable indication of exercise intensity.

Training zones

You can view each level of intensity in Table 1 as a training zone. Training in different zones leads to different training effects. The training effects associated with each training zone were added by me; they were not defined by the ACSM. The training effects that I added give you a general idea of the kinds of effects that you could expect to observe if you were to start cardiorespiratory training as a sedentary beginner and gradually progress through the different levels of intensity.

Note that there is no ‘Fat-Burning Zone’, which is a widely held myth in cardiorespiratory training. As mentioned in Introduction to cardio, your body will burn a higher percentage of fat than carbohydrate at lower intensities of training; however, at higher intensities, it will burn more total kilocalories, which makes training at higher intensities better for weight loss. To learn more about weight loss, see the Weight Loss Guide.

How do you calculate your training zones?

As explained above, you can calculate your training zones in two different ways:

  1. Using your HRmax
  2. Using your HRreserve

The first method is the easiest and is adequate for most gym-goers who are interested in general fitness. However, it is not as accurate as the second method, which also takes your personal level of fitness into account.

Note that your HRmax doesn’t change as you get fitter, whereas your HRresting does change. Therefore, if you use HRreserve to calculate your training zones, you will have to recalculate your training zones every couple of months.

Using HRmax

Your HRmax is the highest number of times your heart can beat per minute during extremely intense (all-out) exercise. It usually decreases as you get older by about one beat per minute, starting from between the ages of 10 to 15.

The best way to get an accurate measure of your HRmax is to undertake a cardiac stress test in a hospital or clinical setting, which, of course, isn’t practical for everyone. Therefore, most people estimate their HRmax using the Fox and Haskell equation:

HRmax = 220 – age in years

However, this method has no scientific basis and has a large margin of error. A more accurate method — albeit one that still has a small margin of error — is to use the Inbar equation:

HRmax = 205.8 − (0.685 x age)

For example, if you’re 25 years old, your equation would look like this:

0.685 x 25 = 17.1

HRmax = 205.8 – 17.1 = 189 beats per minute (bpm)

Once you know your HRmax, you can calculate your personal training zones by multiplying your HRmax by the decimal equivalents of the zone ranges. For example, if you want to train within the moderate-intensity zone (64% to 76% of HRmax), your calculations would look like this:

64% of HRmax = 189 x 0.64 = 121 bpm

76% of HRmax = 189 x 0.76 = 144 bpm

Using HRreserve

In order to use your HRreserve to calculate your training zones, in addition to your HRmax, you will need your HRresting. To measure your HRresting, just take your pulse in the morning, before you get out of bed, a couple of minutes after your alarm clock sounds. Do this for three days in a row and use the average.

To take your pulse, simply place your index finger and middle finger on your wrist, under the base of your thumb, where you will find your radial pulse (Figure 1). Count how many beats you feel for 15 seconds and multiply it by four to get your beats per minute.

Woman taking her radial pulse

Figure 1. How to take your radial pulse.

You can also take your pulse on your neck. Gently press the same two fingers on the side of your neck, in the hollow area just beside your wind pipe, where you will find your carotid pulse (Figure 2).

Woman taking her carotid pulse

Figure 2. How to take your carotid pulse.

Once you have your HRmax and HRresting, plug them into the following equation, known as the Karvonen formula:

Target heart rate = ((HRmax – HRresting) x Intensity) + HRresting

Notice that after you get your HRreserve by subtracting your HRresting from your HRmax, you multiply your HRreserve by the desired level of intensity, before adding your HRresting back on. As such, the level of intensity is only applied to HRreserve. Here’s an example calculation.

If your HRmax is 189, your HRresting is 65, and you want to train in the vigorous-intensity zone (60% to 89% of HRreserve), your equation would look like this:

HRreserve = 189 – 65 = 124 bpm

60% of HRreserve = (124 x 0.60 = 74) + 65 = 139 bpm

89% of HRreserve = (124 x 0.89 = 110) + 65 = 175 bpm

How do you monitor your heart rate during exercise?

Many of the machines that you can use for cardio have sensors built into their handles that can track your heart rate. However, the best way to monitor your heart rate is to use a fitness band. If you don’t have one, you can take your heart rate manually via your radial or carotid pulse as explained above. Simply stop the exercise momentarily, take your pulse for six seconds, and multiply it by 10. This will give you your beats per minute.

In addition to monitoring your heart rate during exercise, you should always consider your perceived effort. The reason is that your heart rate can vary from day to day due to several factors, including diet, temperature, stress, emotions, and level of hydration. Taking into account both your heart rate and perceived exertion will give you a better idea of your performance and progress.

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