Get off the never ending path to nowhere
Everytime I’m at the gym, I notice the banks of “cardio” machines. In fact, for many consumers, the number and availability of the card machines is the single most important factor in decision making when buying a guy membership (which is why gyms can get away with less-than-optimal weight areas but they can’t get away from sub-par cardio equipment.) In the winter time (in Canada) I can understand how using a cardio-machine can be useful. Personally, I abhor most forms of cardio for the sake of doing cardiovascular work. I’ve been a competitive swimmer, and a competitive rower and we never did cardio just to do cardio. We worked out because we wanted to get faster. That being said, if it’s nice outside and you can run (or bike, or climb stairs, or row), there are a number of very good reasons why you should leave the gym and the endless run/bike/stairclimb to nowhere, but I’m going to focus on the biomechanics and energetics of running/walking.
Treadmill running is very different than overground running. Treadmill _walking_ is very different than overground walking. This has been shown time and time again in numerous biomechanics studies. But when it comes to fat loss and treadmill running or even treadmill intervals (if you’re of the school that cardio, in any of its many forms, including HIIT is important for fat loss), these differences are quite important. To understand some of these differences though, we need to have a language for gait.
It doesn’t matter where you start in the gait cycle, but there are essentially four phases to any given step that you take: Heel strike, stance phase, push off, and swing through. Stance phase is sometimes divided into double stance and single stance phase, but for the purposes of our discussion, the distinction is likely not very important. Heel strike occurs from the moment your foot touches the ground to the time you start to put weight on your foot. Stance phase starts as soon as you start to put weight on your foot to just before your foot leaves the ground. Push-off, or toe-off happens in the time that you push off the ground with your foot behind you; and swing through is the period of time your foot is in the air.
On top of the four gait phases, there are two more terms to understand: Stride length, which is the length (in distance) between one heel strike and the next heel strike of the same foot (or any identical points of the gait cycle really); and cadence, which is the number of steps you take in a minute (counting for both feet).
When you walk overground, your heel strikes the ground and you move your weight over your foot. Your body passes over your foot and then as you strike with your other foot in front of you, the foot behind you pushes off. The biggest difference between treadmill running/walking and overground running/walking is the relative position of your body to your foot while it’s on the ground. On a treadmill, your heel strikes the belt, and your foot moves under your body behind you. As your foot is assisted off the ground behind you, you strike the treadmill belt with your other foot.
Sounds almost identical, doesn’t it? However, in overground walking, you are moving the weight of your body over your foot. In treadmill walking, the belt does all the moving for you. You’re not propelling your weight; the belt is propelling your foot. Your foot, in fact, leaves the belt because a) it feels comfortable for you to do so, and b) because it’s not ALLOWED to stay on the belt past a certain angle of your hip due to simple laws of physics and the length of your leg. Your stride length is therefore dictated by how fast the treadmill is running and how fast you can react to your leg flying by beneath you. Your cadence is also dictated by the speed of the belt (you can run faster by increasing your stride length or cadence, or both; but generally, stride length is a relative constant and most people adapt to faster speeds by increasing their cadence).
The relative up-down motion you perform while running is generally pretty small in comparison to the forward-backward motion. Since the forward motion is basically taken away from treadmill running, the energetics of treadmill running are vastly altered. You therefore, require less energy to run on a treadmill than you do when you’re running overground. Even at much higher speeds and inclines, the only mass you’re moving on a treadmill is the mass of your leg as you carry it through space on the swing-through. If you’re on a non-powered treadmill, you’re also moving the weight of the belt (although the force required to move it once it’s going is somewhat reduced because you’re not moving it from rest with each step).
From a training perspective, treadmill running might be good for generally conditioning in the same way a lat pulldown is somewhat useful for working your way up to chin-ups/pull-ups. Obviously, the treadmill is better than nothing if nothing is truly your alternative. Eating less to lose fat is a great alternative, but it won’t really make you run faster if that’s your goal. But now that its almost summer, there’s no reason from a weather, biomechanics or energetics standpoint to stay indoors.
EDIT (Feb 2, 2012): There was a small debate in comments before the Facebook change, however, there are a number of studies to support the metabolic difference as well as the biomechanical differences between overground and treadmill running:
1) Differences in gait while walking were observed in mid- to late-stance phase (mid-stance is the phase of walking where your foot is directly under you, and late-stance being just before you push off with your toes) between treadmill and overground conditions, particularly at higher speeds
White, SC et al. Comparison of vertical ground reaction forces during overground and treadmill walking. Med Sci Sports Exerc 30(10): 1537-42, 1998
2) Differences in gait while running were also observed in mid-stance phase were reported between overground and treadmill running, with differences in oxygen debt (overground incurring 36% more oxygen debt than treadmill running).
Frishberg BA. An analysis of overground and treadmill sprinting. Med Sci Sports Exerc 15(6)478-85, 1983.
This is contrasted by another more recent study that concluded that there was no metabolic difference between the two conditions though the data were only collected at the last 150m of a 950m run.
Basset DR et al. Aerobic requirements of overground versus treadmill running. Med Sci Sports Exerc, 17(4):477-81, 1985.
3) Differences in muscular activity were noted between overground and treadmill running with greater activation of the soleus muscle during push-off in the overground condition.
Baur H et al. Muscular activity in treadmill and overground running. Isokinetics and Exercise Science 17: 165-171, 2007.