How much physical activity do I need?

The question of how much physical activity is enough is one that will persist as long as there are physicians. The recommendations that I have heard from the professional organizations and pundits stewarding our health vary as much as the patients asking them. 30 minutes three times a week. 45 minutes daily. Exercise as often as you can. The permutations go on…

Perhaps it is of use to think of the purpose of exercise. Yes, exercise is a way to be healthier – however effluvious and variable the concept is for you. But the purpose of exercise is to recreate what humans – and our ancestors – have been doing millenia: moving! Since the advent of bipedal locomotion 4.4 million years ago1,2, our prehistoric brethren have been walking and, later, running. Some have argued that the development of our wanderlust is rooted in the necessity for being able to travel far and fast to capture prey while hunting3. Since the development of bipedalism precedes our ancestors ability to become effective hunters (at least 40,000 years ago, if not more), it is likely bipedalism evolved to expand the foraging range of early homonins. As our primate-like ancestors became more human, ambulation served other purposes, likes evading dangerous animals and perhaps intra-species foes. It is difficult to truly know the distance our ancestors were traveling since their remnants are fossilized bones and not pedometers. However, we can look at extant hunter-gatherer groups to draw inference.

The best studied hunter-gatherer group of the twentieth century may be the Ju/’hoansi (formerly the Dobe !Kung) of the Dobe area of the Kalahari desert. In the 1960s, the then young Richard Lee, now Professor Emeritus of the University of Toronto, undertook a series of field studies learning about the livelihoods of the Dobe Ju/’hoansi. He found that these people were walking a variable distance daily. The distance they traveled was based on the availability of food sources. After establishing a camp site, they would progressively exhaust their food in a circumferential pattern, initially traveling 5-9 miles4 and later reaching 10-15 miles in search of food – a distance that does not take into account other activities5. Some days they traveled less. Some more, especially when in the pursuit of prey6. Others have found described similar values7.

Into how much time does this translate? Well, that depends on your ability to traverse 5-9 miles daily. Time may not be as important as distance, which may be more representative of our absolute fitness. A recent study has shown that the average American takes 5117 steps, or the distance to cover 2.5 miles assuming there are 2000 steps in a mile – half the value of the minimum 5 miles the Dobe Ju/’hoansi were traversing on an easier day8. On harder days, the Dobe Ju/’hoansi were traveling 15 miles or more. Who is traveling 15 miles or more these days in America today? Using your Subaru or the subway does not count.

The World Health Organization (WHO) recommendation for adults between the ages of 18 and 64 is to get either 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity aerobic exercise per week9. According to the WHO’s lengthy report on physical exercise, moderate intensity is 5-6 on a ten point scale of physical exertion that is solely determined by the one exercising. Vigorous ranks a 7 or 8 on our own personal scale. For the sake of simplicity, assume running an 8 minute mile was a moderate- or vigorous-intensity aerobic activity. All of my readers save for those that are ultra-marathoners, would agree with this. If an 8 minute mile is a vigorous activity for you, then you should be running 1.3 miles a day. If it is a moderate activity for you, then you should be running double. Yet, how does this compare to the Dobe Ju/’hoansi? Well, it may not. The original studies describe the Dobe Ju/’hoansi as walking and, at times, running when in the pursuit of prey or for other reasons. The studies do not describe what their individual ranking of their intensity level or the times spent at varying intensity levels. What the WHO and Dobe Ju/’hoansi do suggest is that if you aren’t walking ten miles regularly, then it would be of benefit to you exert yourself further via moderate- or vigorous-intensity aerobic exercise according to the WHO guidelines.

As living beings, we are self-interested in conserving energy. The advent of combustible engines, electricity, and other labor savor technologies allow us to exert ourselves less and expend less energy. As such, it is vital we continue to find time AND ways to move. We should be moving more. We may not have enough time or knee cartilage to traverse 15 miles every day, but we certainly are more static that our hunter-gatherer comparison, and likely our ancestors.

References:

1 Haile-Selassie, Yohannes. “Late Miocene hominids from the middle Awash, Ethiopia.” Nature 412.6843 (2001): 178-181.

2 Galik, Karol, et al. “External and internal morphology of the BAR 1002’00 Orrorin tugenensis femur.” Science 305.5689 (2004): 1450-1453.

3 Bramble, Dennis M., and Daniel E. Lieberman. “Endurance running and the evolution of Homo.” Nature 432.7015 (2004): 345-352.

4 Lee, Richard. The Dobe Ju/’hoansi. Cengage Learning, 2012.

5 Lee, Richard Barry, and Irven DeVore, eds. Man the hunter. Transaction Publishers, 1973.

6 DeVore, Irven, and Richard Borshay Lee, eds. Kalahari Hunter-gatherers: Studies of the! Kung San and Their Neighbors. Harvard University Press, 1976.

7 Eaton, S. Boyd, SB 3rd Eaton, and M. J. Konner. “Paleolithic nutrition revisited: a twelve-year retrospective on its nature and implications.” European journal of clinical nutrition 51.4 (1997): 207-216.

8 Bassett Jr, David R., et al. “Pedometer-measured physical activity and health behaviors in United States adults.” Medicine and science in sports and exercise 42.10 (2010): 1819.

9 http://www.who.int/dietphysicalactivity/factsheet_adults/en/

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