I'm a runner, and I'm always having to take grief from people (especially my wife) who happily point out that you'll get the same benefit walking three miles as you will running the same distance, and you won't risk crippling arthritis of all your major body parts. So the other day I had a flash! Going back to high school auto mechanics and some hot-rod magazines, I explained to my wife that since I'm exerting the same force to move the same mass the same distance, I'm doing the same amount of work — but since I'm doing it twice as fast, I'm exerting two times the power and probably using twice the calories. My wife is pretty bright, though: she pointed out that if I run and exert twice the power in half the time, and then sit on my butt desperately trying to catch my breath for the other half of the time, then average power exerted is a wash. Is she right? Does running have any calorie advantage over walking, or am I really being that dumb (not like it would be the first time)?
Peter Prout, Winchester, Virginia
A complicated question, often answered wrong even by those who ought to know. We’ll take it by steps:
1. The unschooled view, which springs from the ancient instinct that pain = gain, is that running is better than walking because afterward you’re sweating like a horse and gasping for breath.
2. Those with a semester or two of physics under their belts, on the other hand, reason that since the amount of work involved is the same, running a mile has the same impact as walking an equal distance.
3. However, this assumes that running and walking are equally efficient means of locomotion. Generally speaking they’re not — running requires substantially more energy per unit of distance. Several factors contribute to this. Shall we start with entropy and the second law of thermodynamics? Eh, maybe not. How about aerodynamic drag, which increases with the square of the speed? Probably not something you need to worry about unless you’ve really been hitting the steroids. Here’s something a little more relevant: Analyses of the biomechanics of walking vs. running suggest that walking is a more efficient gait except at higher speeds. The crossover point is somewhere around 5 mph, varying with the individual. At that speed, walking and running are equally efficient. Below the crossover point, running is less efficient, apparently because you lose energy absorbing the impact of the ground with your bent knees. Above it, walking falls behind because of the awkwardness of the racewalking gait. Gait is the critical issue, incidentally — running speed is irrelevant. Subtracting out the energy required just to keep you breathing, you’ll use about the same amount of juice finishing the marathon in two hours or four.
4. To return to our main point, running consumes more calories per unit of distance than walking. For a person who weighs 70 kilograms (about 154 pounds), walking at 5 kilometers per hour (3.1 miles per hour) consumes 50 calories per kilometer, whereas running at 10 kph (6.2 mph) consumes 78 calories per kilometer.
5. Aha, you say, running is better than walking! Not necessarily, even if we narrowly define “better” as “consumes more calories.” When you begin a workout your body is metabolizing carbohydrates, but as the minutes tick by you start burning fat — at minute 50 of light exercise, you’re chugging along on a 75/25 fat/carb mix. Sixty percent of your maximum aerobic capacity (reached at roughly 75 percent of maximal heart rate, which is generally calculated as 220 minus your age) is optimal for fat burning; as exercise becomes increasingly strenuous you start burning more carbs. Some have seized on these facts to claim that sustained low- to moderate-intensity exercise is a better way to shed flab than going all out. The point is hotly disputed; I merely note that for the significantly overweight, walking can be easier on the heart, joints, etc.
6. At any rate, calorie consumption isn’t the chief goal of exercise; cardiovascular fitness is. The American College of Sports Medicine says you should exercise three to five times a week in 20- to 60-minute sessions intense enough to raise your pulse to between 60 and 90 percent of your maximal heart rate. Such exercise should be aerobic, meaning something like running, brisk walking, biking, swimming, or cross-country skiing, as opposed to, say, weight training. Is running better than walking for this purpose? Depends. Running is certainly more of a workout per unit of time, but if your idea is that 15 minutes of running equals 30 minutes of walking, forget it — sustained exertion is the key. Then again, while any exercise is better than none, if your wife thinks a no-sweat half-hour amble around the neighborhood is the equivalent of your three-mile run, she’s kidding herself too.
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