Exercise experts measure activity in metabolic equivalents, or METs. One MET is defined as the energy it takes to sit quietly. For the average adult, this is about one calorie per every 2.2 pounds of body weight per hour; someone who weighs 160 pounds would burn approximately 70 calories an hour while sitting or sleeping.
Moderate-intensity activities are those that get you moving fast enough or strenuously enough to burn off three to six times as much energy per minute as you do when you are sitting quietly, or exercises that clock in at 3 to 6 METs. Vigorous-intensity activities burn more than 6 METs.
One limitation to this way of measuring exercise intensity is that it does not consider the fact that some people have a higher level of fitness than others. Thus, walking at 3 to 4 miles-per-hour is considered to require 4 METs and to be a moderate-intensity activity, regardless of who is doing the activitya young marathon runner or a 90-year-old grandmother. As you might imagine, a brisk walk would likely be an easy activity for the marathon runner, but a very hard activity for the grandmother.
This table gives examples of light-, moderate-, and vigorous-intensity activity for healthy adults.
Reproduced with permission from The Nutrition Source