This is the fourth of a series related to physical fitness entitled: Think of Exercise as a Pill that Promotes Long Life and a Whole Lot More.
The answer to the question posed here depends entirely on how fit and agile you are. The exercise program you design should be based on health guidelines but tailored to you.
In 2008, the U.S. Department of Health and Human Services (HHS) published its Physical Activity Guidelines for Americans and they are excellently written. They agree substantially with independent recommendations by the CDC, the American Heart Association (AHA), and the American College of Sports Medicine (ACSM). I will summarize the main points but I encourage you to go to www.health.gov and read the chapters that apply to you. They are concise and easy to read, and six years after publication they remain the standard. The footnote below gives instructions on how to access these Guidelines.
Aerobic capacity (endurance) is built by activities like walking, jogging, dancing, swimming and cross-country skiing, where people move their large muscles rhythmically for sustained periods. The 2008 HHS Guidelines state:
“For substantial health benefits, adults should do at least 150 minutes (2 hours and 30 minutes) a week of moderate-intensity, or 75 minutes (1 hour and 15 minutes) a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed in episodes of at least 10 minutes, and preferably, it should be spread throughout the week. … A general rule of thumb is that 2 minutes of moderate-intensity activity counts the same as 1 minute of vigorous-intensity activity.”
Let’s explore what this recommendation means. Physical activity for more than 10 minutes can qualify as aerobic if it’s done at sufficient intensity. Light activity like self care, cooking, or casual walking does not qualify. Moderate-intensity activity will not qualify if it’s done for less than 10 minutes, so walking around the home or office or walking in from a parking lot usually isn’t aerobic. However, some physically-active occupations may count as aerobic and so can active transportation choices like walking or bicycling.
So what is moderate-intensity exercise? For middle-aged adults free of functional limitations, the definition is fairly arbitrary. It’s walking briskly (3 or more miles per hour [mph]) but not race-walking. It’s water aerobics, general gardening, bicycling at less than 10 mph, doubles tennis, hiking, walking as you carry your golf clubs, mowing the lawn with a push mower, or jogging at less than 6 mph. The cardio equipment you find in health clubs (elliptical machines, stair-climbing machines, stationary bicycles, and treadmills) can also provide moderate-effort exercise.
Of course, vigorous exercise is more intense. It’s race-walking or running at more than 6 mph. Racquetball, singles tennis, stair climbing, heavy gardening like continuous digging or hoeing, bicycling at more than 10 mph, rope jumping, calisthenics (jumping jacks, push-ups, pull-ups, sit-ups), and team sports can also be vigorous forms of exercise.
But, adults do differ in their exercise capacities and these differences are most apparent in older adults. Moderate effort is a brisk walk for some and a slow walk for others. So for adults 65 and older and for adults of any age with functional limitations, HHS advises getting away from arbitrary definitions of effort and using an effort scale that relates to an individual’s aerobic capacity. Intensity can be estimated using a scale of 0 to 10, where no effort is 0 and 10 is the highest level of effort possible. On this scale, moderate-intensity activity is a 5 or 6 and vigorous-intensity activity is a 7 or 8. Be careful though because subjective perception of effort is not a precise measure of true effort. If there is uncertainty, a fitness instructor can guide you on the appropriate level of effort. In general, when people expend moderate effort, they are unable to sing and when people exercise vigorously, they are unable to utter more than a few words between breaths.
Notice that, according to HHS, aerobic exercise can be performed for short periods several times a day and integrated into the course of one’s daily schedule; the focus is on total aerobic activity. By making no mention of measuring heart rate, the Guidelines keep things simple. Studies, in fact, have shown that the benefits of exercise can be readily obtained without setting or achieving goal heart rates.
Notice also that the recommendations above are considered minimal – that is, the minimum most adults need to really benefit from exercise. Not all the health benefits of exercise occur at this minimum level, so the HHS Guidelines go on to say:
“For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 minutes (5 hours) a week of moderate-intensity, or 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity.”
Chapter 4 of the Guidelines, under “How Much Total Activity a Week?” lists the additional health benefits achievable with this doubling of aerobic activity.
The upper limit of exercise, beyond which no significant benefits accrue, is not known. Upon making its recommendation for 300 minutes of moderate exercise weekly or 150 minutes of vigorous exercise weekly, HHS goes on to add:
“Additional health benefits are gained by engaging in physical activity beyond this amount.”
One way to achieve the benefits of higher-intensity activity may be with interval exercise and this has become quite popular recently. One example would be high-intensity interval walking, such as repeated three-minute sets of high-intensity walking (basically race-walking) alternating with three-minute sets of low-intensity walking. High-intensity interval training has been found to have several short-term physiologic benefits when compared to steady-tempo training, including improved cardiorespiratory fitness. However, the long-term health effects of high-intensity interval training are not known and there is more potential for injury with high-intensity exercise. The research is still preliminary. Varying the pace of your aerobic workouts may be beneficial, but it is perhaps best to avoid extreme changes in tempo.
Suggestion: Remembering the relative-effort scale, pay attention this week to whether you’re doing any aerobic activity that truly qualifies as moderate or vigorous. Then estimate how many minutes of that activity you do in a typical week.
 Department of Health and Human Services, 2008 Physical Activity Guidelines for Americans. Go to www.health.gov. In the middle of this page, there are 6 boxes arranged 3×3. The lower left box is labeled “Physical Activity Guidelines.” Click on “Learn more” and you’ll be taken to another page with 5 boxes in the middle. The box on the left of the second row has the heading “Guidelines.” Click on that heading and you’ll be taken to the index of the 2008 Guidelines. Chapter 4 on Active Adults and Chapter 5 on Active Older Adults are essential reading. You can also write to: The U.S. Department of Health and Human Services, 200 Independence Avenue, S.W., Washington, D.C. 20201. Toll Free: 1-877-696-6775.
 Douglas P. Exercise and Fitness in the Prevention of Cardiovascular Disease. UpToDate, Wolters Kluwer Health. August 29, 2014.
 Peterson D. Overview of the Risks and Benefits of Exercise. UpToDate, Wolters Kluwer Health. August 20, 2014.