'Intensity refers to how much work is being performed or the magnitude of the effort required to perform an activity or exercise. Intensity can be expressed either in absolute or relative terms.

• Absolute. The absolute intensity of an activity is determined by the rate of work being performed and does not take into account the physiologic capacity of the individual. For aerobic activity, absolute intensity typically is expressed as the rate of energy expenditure (for example, milliliters per kilogram per minute of oxygen being consumed, kilocalories per minute, or METs) or, for some activities, simply as the speed of the activity (for example, walking at 3 miles an hour, jogging at 6 miles an hour), or physiologic response to the intensity (for example, heart rate). For resistance activity or exercise, intensity frequently is expressed as the amount of weight lifted or moved.
• Relative. Relative intensity takes into account or adjusts for a person’s exercise capacity. For aerobic exercise, relative intensity is expressed as a percent of a person’s aerobic capacity (VO2max) or VO2 reserve, or as a percent of a person’s measured or estimated maximum heart rate (heart rate reserve). It also can be expressed as an index of how hard the person feels he or she is exercising (for example, a 0 to 10 scale).'