Exercise intensity is a measure of how hard you are working while performing any exercise task. Intensity applies both to cardiorespiratory ("cardio") exercise and to strength training.
Cardio exercise takes many forms - walking, running, and biking are the most common forms that don't require specialized equipment. Equipment-based cardio includes treadmills, stair-climbers, elliptical trainers, and stationary bikes, such as those found in gyms.
Regardless of how you do your cardio, it is really how much oxygen you consume, and your heart-rate response to the exercise that matters - not the equipment you use. In other words, it's all about the physical stress your body experiences during your exercise session.
Programming for exercise intensity is how you or your trainer should structure your exercise program to meet your personal goals.
There are several ways for measuring intensity. In a clinical setting, you can take a graded exercise test ("GXT"), commonly known as a "stress test." It requires special equipment, and should only be performed by a doctor or a specially trained technician.
But stress tests are expensive, and unless you are an elite athlete or your doctor has ordered a GXT because of a specific concern, there are user-friendly tests which most trainers recommend to their clients which not only measure intensity, but serve to create a baseline against which progress over time can be measured.
Ratings of Perceived Exertion
The most popular of the non-clinical tests is called Rating of Perceived Exertion, or "RPE." RPE is a subjective test, wherein the exerciser self-describes their perceived level of intensity based on a numeric scale.
The original RPE model uses a scale of 6 to 20. Many people have trouble relating to a 15-level scale, especially when the first level is 6, and not 1.
A revised RPE model was developed later, and this uses a 10-level scale. Except the first level is not 1, it is 0, and there is a level greater than 10, which is an asterisk (*).
To compound the difficulty in using these "easy-to-use" models is the fact that a same-degree of difficulty may be used in conjunction with more than one numerical value. For instance, in the revised model, a perceived exertion of "very strong" can be rated as a 7, 8, or 9.
Confused? Me too, and I do this for a living.
My Ratings of Perceived Exertion
I have developed my own RPE, which I think you may find useful. It uses a simple 8-level scale, with straightforward descriptions. It can be used for both cardio work and strength training. (Be sure to scroll down to see the second and third pages, which give appropriate examples.) Click on this link to go to my RPE:
If you are curious about the differences between my Modified RPE and the 6-20 and 0-10(+*) models, click on this link to view a comparison chart.
Using an RPE
Muscles, including the heart muscle, must be worked inefficiently to receive the benefits of exercise. A central theme of exercise is the principle of adaptation. This means that by exercising your muscles at an intensity which is achievable, but which is more than what those muscles are accustomed to, after a while those muscles adapt to the unaccustomed intensity by becoming stronger. Once the muscles have become stronger, they become efficient at exercising at what was once a challenging intensity.
But you don't want your muscles to exercise efficiently. So every time your muscles become stronger by adapting to the present workload, its time to increase the intensity.
If you are self-directing your exercise program, remember this simple rule: increase time before increasing speed (cardio), and increase repetitions before increasing resistance (strength).
Example 1: If you are jogging at an 8-minute-per-mile pace for 20 minutes, increase your time to 30 minutes at the same pace before increasing speed, or before introducing hills, etc. And don't expect to go from 20 minutes directly to 30 minutes. The more likely scenario is that you will be able to increase your time from 20 minutes to 22 minutes, then to 25 minutes, etc.
Example 2: If you are performing, say, a biceps curl, using an 8 to 12 repetition (rep) range, start with a weight that you can correctly curl 8 times, but not nine. Stay with that same weight as you become stronger, and can perform more than 8 reps. Once you can curl that same weight more than 12 times, increase the weight to a resistance that you can only curl 8 times, and begin the process anew.
By practicing this type of overload, adaptation, and progression you will become stronger over time, but you won't actually be working any harder.
That's right, you won't be working any harder. That's because when you increase the workload as part of your progression plan, the new work-
load will stay at the same perceived exertion. For example, on my Modified RPE scale, a productive intensity is between levels 4 ("somewhat hard") and 5 ("hard). So as you get stronger, what had been a level 4 becomes a level 3 ("easy"). At this point, you will just adjust your workload back to an intensity which you perceive as a 4. Get it: more intensity, same effort, stronger you.
Using a Rating of Perceived Exertion scale is an excellent way to make sure that you are exercising in a productive zone. By the way, it doesn't matter which RPE scale you use, so long as you use the same one consistently.