Whether you're looking to lose weight, improve aerobic fitness, muscle strength, or endurance, exercise intensity will be an important factor and needs to be considered when trying to reach your health and fitness goals.
Exercise intensity refers to how much energy is expended during exercising. In the most simplistic terms - it’s how hard your body is working.
Intensity can be measured subjectively (based on a “feeling” scale) or objectively (such as through a heart rate response). A talk test may also be used to measure intensity and is a practical application, since it’s relative to an individual and requires no equipment.
I use a combination of all of the above with my clients and what method is dominant depends on who I am working with.
As a trainer and CEP, I know what a heart rate (HR) response means on a physiological level. For this reason, the metric guides me to progress them safely towards their fitness goals.
However, using HR to establish intensity isn’t right for everyone.
If you don’t have a fitness tracker that measures HR or if you are on medications that suppress your vitals (i.e. beta blockers), then using a “feeling” scale or a talk test is a better option.
Measuring exercise intensity by “how you feel”
The original Borg RPE is based off how hard you feel you're working and you're heart rate. The ranges fall between 6-20, with 6 being no exertion and 20 being the most vigorous exertion possible. If you take a zero and plant it behind one of the numbers on the scale it's supposed to represent where your heart rate is for that given activity. This scale makes sense to me, but is not always cognitively easy for clients to translate.
The modified RPE scale has a range from 0-10, with 0 being no exertion and 10 being maximum exertion. This scale corresponds with a feeling of breathlessness.
I always recommend erring on the side of caution when using these scales. After you get clearance from your doctor (important), then I recommend exercising at a low to low-moderate intensity such as a Borg RPE of 9-12.
Once you get stronger and build stamina, you can progress to a Borg RPE of 12-16. Even after building a fitness baseline, I recommend being careful exercising above a RPE of 12-16. In my experience, working at a RPE of 17 or higher isn't necessary to establish and maintain fitness levels. Also, depending on your health status, that high of an exercise intensity may be contraindicated.
Keep in mind, as intensity increases, your risk for injury does as well. Please be careful and listen to your body!
Measuring exercise intensity by heart rate
To measure exercise intensity objectively you need some form of heart rate (HR) device, such as a chest strap or fitness tracker (i.e. Apple Watch, Fitbit, or Garmin). Most fitness devices used for exercise tracking also come with a companion app. Together they categorize your exercise efforts based on your current health status and the health information collected when activating your device (i.e. height and weight).
Last time I researched fitness devices many of them categorized intensity training zones differently and the formulas varied. You can find out how your device derives at your intensity zones by exploring the companion app that comes with the device. If you don’t want to do that, I can give you a simple way to come up with the zone you’re working in based off your heart rate response.
The most simplistic and “user friendly” formula to measure aerobic intensity is based off of your heart rate max (HRmax), which is also known as your age predicted max heart rate (APMHR). Age is the only factor taken into consideration when using this formula.