Why moderate exercise, and for whom?

why do current physical activity recommendations almost universally specify exercise of a moderate intensity? Regular runners and often-vigorous exercisers may be asking, Is it something about moderate exercise per se? What should I be doing to maximize health and fitness gains safely and efficiently?

First, it's important to distinguish between health gains and fitness gains. Health gains such as lower all-cause mortality, lessened cardiovascular disease risk and fewer incidents of type-2 diabetes are associated throughout the literature with moderate and even light exercise. Moderate exercise is as effective as vigorous for reducing blood pressure. Physical activity does not need to result in a training effect to elicit a health benefit. But a previously sedentary person can also expect fitness gains with moderate activity--and that is the crux of the matter. ACSM, CDC and NIH recommendations are for the most part aimed at the sedentary portion of the population, who may not already be sold on the need for regular exercise, and in any case should avoid leaping into a vigorous exercise regimen.

Regular exercisers, for whom physical activity recommendations are arguably moot, tend to find their own paths to vigorous exercise. Runners who compete even infrequently usually want to get faster. Goal setting is a big part of what motivates the already active to add more work to a workout. If you're on a regular exercise plan, you should have already consulted your doctor and gotten the okay to perform intense workouts. As a runner, gradually building up your training to include vigorous intensity workouts will improve your oxygen consumption, strength, power, coordination, and lactic acid threshold--all of which will make you faster, leaner and tougher in the face of fatigue. But other concerns, such as the way our bodies use stored energy and from what sources, are still best addressed with moderate exercise over vigorous. (For a further exploration of which exercise intensities, and in what proportions, may suit your individual running goals, see "Proper Training Pace" below.) There are additional, pragmatic reasons to emphasize moderate exercise.

To be sure, energy expenditure--the linchpin of fat reduction--increases with walking speed, even disproportionately so at higher speeds. Yet, studies have found that when inactive, middle-aged people are asked to "walk briskly," which is firmly in the realm of "moderate," many choose a pace at nearly 60% of maximal oxygen consumption (VO2max). Such a pace is sufficient to improve fitness in addition to health. Yet regardless of self-selected walking intensity, to some extent, exercise duration offsets intensity. While more research, with larger population samples, that directly compares all aspects of these two variables is called for, results so far are promising. In one study, the post-meal storage of fat in subjects was reduced by the same amount with 180 minutes of exercise at 30% VO2max as it was with 90 minutes at 60% VO2max. Another study looked at insulin sensitivity in diabetic women and found a similar trade-off between intensity and duration.

Furthermore, the hazards of physical activity, including injury and, rarely, cardiac events, appear to be associated more with the intensity than with the duration or frequency of the exercise. For example, in the 2002 Aerobics Center Study of over 5,000 men and nearly 1,300 women, injury prevalence in walkers was found to be similar to that of sedentary people. Increased walking duration did not increase injury risk. Not surprisingly, those unaccustomed to intense physical activity are at the greatest risk. So as a practical matter, public health recommendations target sedentary individuals, and stress gradual increases after an exercise regimen is in place. An injury early on may, after all, do much to discourage beginning regular exercise in the future.

Vigorous exercise finds its way to the already active. For the rest of the population, moderate makes the most sense. If someone you know is thinking about getting moving, encourage them by setting regular walking dates. And remember that several 10-minute bouts of walking in a day offer nearly all the health benefits of one 30-minute bout. Work schedule and beginning fitness level may make one plan more feasible than the other, but you needn't worry which. Just urge friends and family to first see their doctor before beginning any regular physical activity. For more information on how to schedule this time and get others up and running in 12 weeks, visit www.americanrunning.org to download the One-on-One, Walk & Run plan. It will tell you how to start a group in your community and give you all the tools you need to make it a success.

Exertion Level   Signs

Light            normal to slightly increased breathing
Moderate         deeper breathing and sweating
Vigorous         gasping, heavy sweating

(Physical Activity and Health: The Evidence Explained by A. Hardman and D. Stensel, Routledge, London, 2003, 289 pp; Med. Sci. Sports & Exerc., 2002, Vol. 34, No. 5, pp. 838-844; Exercise: A Program You Can Live With, Harvard Health Publications, Feb. 2002)