Commentaries on Physical Activity and Health

How Much Physical Activity is Good Enough?

By I-Min Lee, MD, ScD
Professor of Medicine, Harvard Medical School
Professor of Epidemiology, Harvard T.H. Chan School of Public Health

I-Min Lee, MD, ScD

In August, Dr. Ken Powell wrote a compelling article on why physical activity should be regarded as a great bargain – a “best buy” – for public health1. Yet we Americans largely aren’t buying. One of the reasons he gives is that there is a mistaken notion that this best buy requires a large commitment on our part: a significant investment with regard to time spent and physical effort (intensity) required. How did this belief come about? And, indeed, how much physical activity is good enough to qualify as a best buy?

Knowledge on any front, including the health benefits of physical activity, advances over time. As new evidence surfaces, it is best viewed integrated into the totality of evidence. Specifically with regard to the health benefits of physical activity, knowledge related to “how much is good enough” has evolved substantially over the past several decades. We public health professionals probably could have done, and should do, a better job explaining the evolving knowledge base to the public in order to avoid incorrect beliefs about how much and what kinds of physical activity “count” as health enhancing.

The earliest physical activity recommendations in effect from the 1970s and 1980s, such as those from the American College of Sports Medicine and the American Heart Association, asked that we engage in vigorous exercise (e.g., running) for at least 20 minutes continuously, on at least 3 days a week. These recommendations were largely based on studies that demonstrated that cardiorespiratory fitness improved with such an exercise program. However, many of these studies were conducted among young men, who already tend to be physically fit! Thus, to further improve their (already good) fitness, a high intensity program was needed.

By the mid 1990’s, a large body of literature from epidemiologic studies conducted among persons more representative of the general population, showed that (1) even moderate-intensity physical activity, such as brisk walking, is healthful; and (2) bouts of physical activity need not be sustained for long periods; even bouts as short as 8-10 minutes produced health benefits. This led to the current generation of guidelines, including the 2008 Physical Activity Guidelines for Americans.2 For adults, these guidelines ask for at least 150 minutes a week of moderate-intensity physical activity, or at least 75 minutes a week of vigorous-intensity physical activity, or some combination of activities of both intensities that expends equivalent energy, plus muscle-strengthening exercises on 2 days a week. The guidelines ask that activities be conducted in bouts of at least 10 minutes’ duration and that physical activity preferably be spread over the week to minimize the potential for musculoskeletal injury.

These guidelines also go on to state that additional health benefits occur with more physical activity (i.e., 300 minutes a week of moderate-intensity activity, or 150 minutes a week of vigorous-intensity activity, or equivalent combination from both intensities).

The studies that provided the scientific basis for these guidelines largely relied on participants reporting their physical activities. While these reports can be valid, they also are imprecise for activities of lighter intensity, such as light housework, and also imprecise for assessing very short bouts of activity that last perhaps a minute or two. Thus, our current target guidelines do not say anything about such light-intensity activities, nor do they count activities that are shorter than 10 minutes in duration – not because we know such physical activity is not health enhancing, but because existing studies generally do not assess such activities well.

However, a new generation of studies is now taking advantage of technological advances, where small devices (of reasonable cost) can be non-intrusively put on subjects to measure all kinds of movements, including low-intensity physical activities that occur in very short bouts. For example, the National Health and Nutrition Survey, a national survey that measures population levels of physical activity, now has a component that includes participants wearing a movement monitor called an accelerometer for 7 days.3 Studies using such devices will further extend our knowledge on how much is enough, or to state the question more accurately, how little is enough?

Thus, in the coming years, I suspect our guidelines will again evolve, possibly to include light-intensity activities and also to remove the requirement for a 10-minute minimum bout duration. If this comes to pass, there will be much to cheer, in that the guideline will be very achievable for most and make us more inclined to view physical activity as a “best buy”.

Of course, there will also be cause to worry that many people will be content to just reach for the new low bar and not beyond. Should evolving scientific knowledge support a new, “lesser” physical activity guideline, our challenge will be not only to see as many Americans as possible reach this bar, but also to urge all those who are able to go beyond it.


I-Min Lee, MD, MPH, ScD, FASCM is Professor of Medicine at Harvard Medical School and Professor of Epidemiology at the Harvard T.H. Chan School of Public Health.  Her research primarily focuses on the role of physical activity in preventing chronic diseases and enhancing longevity, and in women’s health.  She has served on national expert panels addressing physical activity and health, including the 2008 Physical Activity Guidelines Advisory Committee, and international panels, including the expert panel for the 2010 WHO Global Recommendations on Physical Activity for Health.


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1 Powell, K. (2015). Physical activity is the “best buy” for Americans. National Physical Activity Plan Alliance Commentaries on Physical Activity and Health, 1(1).

2 Physical Activity Guidelines for Americans. (accessed 2015 September 23).

3 Troiano RP, Berrigan D, Dodd KW, et al. Physical activity in the United States measured by accelerometer. Med Sci Sports Exerc 2008;40:181–8.

Suggested Citation:  Lee, I. (2015). How Much Physical Activity is Good Enough?. National Physical Activity Plan Alliance Commentaries on Physical Activity and Health, 1(2).