Vigorous walking is an often-underestimated exercise. It doesn't build your cardiovascular system to the degree jogging does, and it's not as time-efficient, but it's easier on your legs and is therefore less likely to land you on the injured list -- and it's still a great fat-burner. You may prefer walking outdoors for the scenery and fresh air or prefer a treadmill for the convenience; perhaps you indulge in both. You may have heard that one or the other is better for weight control. What's the real story here?
When the Treadmill Is Better
If your aim is to get in as intense a workout as you can in the interest of making unwanted pounds disappear, the treadmill -- while boring almost to the point of tears for some -- may be a clearly superior choice. For example, if you live in a flat area, the only "hills" you can walk up will be ones you create using the 'mill. If your home is on a major street with few or no sidewalks, or if your work hours prevent you from walking outdoors in daylight during some or all times of the year, you're better off on a treadmill for simple safety reasons that trump any discussion about calorie expenditure.
Walking on the Flat
A study of nine men published in "Journal of Sports Science" in 1996 demonstrated that running above a given speed on a level road surface involves a slightly higher use of oxygen than running at that speed on a treadmill set to a zero or 1 percent grade. This is because on a treadmill, you don't have to overcome the effect of air resistance that's present when you are actually moving forward. This means you burn marginally more calories once you start moving at a certain speed outdoors than you do on a treadmill. But below about 7.5 mph, or an eight-minute mile, this effect all but disappears. Walking on a treadmill with a zero or 1 percent incline and walking on the level outdoors do not burn significantly different numbers of calories or amounts of fat. If you weigh 155 pounds, you'll burn about 335 calories an hour walking at 4 mph.
Depending on where you live, it's generally possible to seek out hilly terrain and therefore do a good deal of walking on uphills. At any given walking pace, when you are climbing, you are obviously working harder than you are when ambling along at the same pace on the level, so you are burning more calories. On a treadmill, you usually have the option of inclining the belt to as steep a grade as 10 percent and sometimes more, a pitch few U.S. roads can match. Furthermore, on a treadmill, you can do your entire walk "uphill" without having to return to the bottom of the "hill." Outside, this is clearly not an option, so if you walk uphill rather than on the level, you will have an easier time burning more calories on a treadmill than on roads or trails.
On a treadmill, you can exert a far greater degree of control over your walking environment than you can outside. You precisely monitor and manipulate the speed and the percent grade, and not only that, most motorized treadmills now allow you to input your weight and can tell you in real time how many calories you have burned based on the speed and pitch of the belt. Therefore, if your chief aim in walking for exercise is to burn fat and calories, using a treadmill is probably your best choice.
L.T. Davidson has been a professional writer and editor since 1994. He has been published in "Triathlete," "Men's Fitness" and "Competitor." A former elite cyclist with a Master of Science in exercise physiology from the University of Miami, Davidson is now in the broadcast news business.