Intensity refers to how hard or intense you put forth an effort in an activity. This is independently relative in that what might be considered extremely hard to one person may be considered moderate to another person.

Listen to your body! Researchers have found the level of your effort during a physical activity is likely to agree with what actual physical measurements would show. In other words, if your body is telling you that the exercise you are doing is moderate, you can probably sense you are challenging yourself but are not near your limit. The Borg Rating Scale is one way to estimate how demanding your physical activity is by listening to your body. As your body adapts to your physical activity program and you become more fit, then you may gradually and in moderation keep making your activities more challenging. The Borg Rating Scale is simple to use. Match your level of effort with the corresponding number on the scale.

Find your comfort level on the scale. Eventually you will be able to work comfortably within the fairly light to somewhat hard range (11-13).

Source: Borg, G.V. (1982). Psychological basis of perceived exertion. Medicine and Science in Sports and Exercise, 14, 377-381. American College of Sports Medicine.