I've seen a gazillion articles lately about exercising for "30 minutes at moderate intensity," but they always describe moderate intensity as walking at four miles an hour. I can't walk at four miles an hour because of mechanics; my legs seem to be just the length to have to switch from walking to jogging at about four miles an hour, so I can't do the walk to be able to figure out what "moderate intensity" feels like. Can you give any other measure for what is "moderate" and what is "intense" — percentage of maximum heart rate or METS or anything like that?
The American College of Sports Medicine (ACSM) defines exercise intensity by percentage of maximum heart rate, rate of perceived exertion, and METS (metabolic equivalents) in their Position Stand, Recommended Quantity and Quality of Exercise for Development and Maintenance of Cardiorespiratory and Muscular Fitness and Flexibility in Healthy Adults. Moderate activity has been defined as 55 to 69 percent of maximum heart rate (MHR). ACSM defines "hard" exercise at 70 to 89 percent MHR, and "very hard" at 90 percent and above, with 100 percent being maximal exertion. Check out Minimum and maximum heart rate for aerobic exercise in the Go Ask Alice! archive to learn the calculations and more.
You can also use the "Rate of Perceived Exertion" (RPE), a subjective rating that a person can use to rate her/his exercise intensity. If someone doesn't have any other way to rate workout intensity (i.e., has no watch to use to count heartbeats, or doesn't know how fast s/he is walking or running), RPE is a low-tech method of determining this calculation. For example, a person can consider walking at a leisurely pace a six, and perhaps a mad dash to catch a bus or a flyaway $100 bill a 19; so, rating activity in-between is a way to rate one's exercise intensity. The ACSM Position Stand uses the original scale from 6 to 19 to identify the perceived level of difficulty of physical activity, as follows:
- 6 to 8: very, very light
- 9 to 10: very light
- 11 to 12: fairly light
- 13 to 14: somewhat hard
- 15 to 16: hard
- 17 to 18: very hard
- 19: very, very hard
Moderate intensity, using this scale of a person's self- perception of his or her own exercise difficulty, is 12 to 13, hard exercise is 14 to 16, and very hard activity is at 17 to 19.
The last measure — METS — has nothing to do with baseball players from New York; instead, it refers to metabolic equivalents. One MET is equivalent to your resting metabolic rate; two METS is any activity that requires two times your metabolic rate, etc. This measure is determined by the amount of oxygen consumed, which indicates the level of intensity a person is working. At one MET, an average man would be consuming 250 milliliters (ml) of oxygen per minute; an average woman would be consuming 200 ml of oxygen per minute. For those of you who wish to be even more exact, one MET is equal to 3.5 ml of oxygen per kilogram (kg) of body weight per minute (one kg = 2.2 pounds). Since we aren't going around measuring how much oxygen a person's body is consuming, assigning a MET equivalent can give us an idea as to how intense an activity is. At one MET (resting metabolic rate), a 55 kg female would use about 60 calories per hour, and a 65 kg male would use about 70 calories per hour. Two METS would be double that intensity, or consuming twice the amount of oxygen than at one MET. In other words, two METS means that one is working at twice his or her resting metabolic rate (which is relatively easy or achievable), three METS is three times someone's resting metabolic rate, and so on.
The ACSM rates moderate intensity using METS as decreasing with age. For men, moderate intensity by age is:
AGE (years) # METS (moderate) #METS (hard) # METS (very hard) 20-39 4.8 - 7.1 7.2 - 10.1 >10.2 40-64 4.0 - 5.9 6.0 - 8.4 >8.5 65-70 3.2 - 4.7 4.8 - 6.7 >6.8 80 and over 2.0 - 2.9 3.0 - 4.25 >4.25
For women, mean values are one to two METS lower than for men.
Some examples of how METS are associated with activity are as follows:
METS Activity 1 resting quietly, watching TV, reading 1.5 eating, writing, desk work, driving, showering 2 light moving, strolling, light housework 3 level walking (2.5 mph), cycling (5.5 mph), bowling, golfing using a cart, heavy housework 4 walking (3 mph), cycling (8 mph), raking leaves, doubles tennis 5 walking (4 mph), cycling (10 mph), ice or roller skating, digging in the garden 6 walking (5 mph), cycling (11 mph), singles tennis, splitting wood, shoveling snow 7 jogging (5 mph), cycling (12 mph), basketball 8 running (5.5 mph), cycling (13 mph), vigorous basketball 9 competitive handball or racquetball 10 running (6 mph)
The Centers for Disease Control and Prevention (CDC) standards for moderate activity are more succinct, defining moderate intensity as an activity allowing for sustained, rhythmic movements that are carried out at:
- an RPE of 11 to 14, or
- three to six METS, or
- 3.5 - 7.0 calories expended per minute (The number of calories per minute depends on a person's estimated body weight, fitness level, and intensity. Many charts are on the Internet that calculate energy expenditure for various activities, including the Fitness Partner calculator. An abundance of software, as well as exercise books, are also available for people who want to track this measure.)
Examples of such activity as defined by the CDC include mowing the lawn, dancing, swimming, or biking on a level surface.
Hope these explanations motivate you into moderate activity, so you can reap all its benefits.