Treadmill versus road: to lean or not to lean?

One thing that all the new schools of running technique agree on is that you should lean forward while running. A definition of "leaning" could simply be that your center of gravity is out of line with your point of contact with the ground. If you take this as your definition then it is impossible to run without leaning because you need some horizontal force to overcome air resistance. We tend to think of pushing off when we run and that it is friction that drives us forward, but friction is just a reaction force that is always less than or equal to the force opposing it. Think about it: you can't push off without moving your center of gravity out of line with your point of contact. Instead, the horizontal force comes from the residual horizontal component of force that results when you cancel the force of gravity with the ground reaction force, the force that prevents you from falling through the floor. These two forces act through different points and cause a rotational torque, rather like gripping a pencil at each end with both hands and then moving your hands vertically closer together. Seen another way, your center of gravity moves forward as you "fall" by leaning forward, while you move your legs forward to regain balance momentarily (i.e., point of contact with the ground under your center of gravity). So not only should you lean forwards in order to run, but it is actually impossible to run unless you are leaning forwards. When the running schools talk of leaning, they mean leaning properly, that is to say, leaning from the ankles. The reason for this is that this ensures that all the relevant muscles in the body from the feet to the core are all engaged. The antithesis of this is the classic K-bend where the runner bends from the waist; in this case the core is not engaged and the other muscles are forced to make up for this causing overuse injuries in the hip flexors as well as potential back problems.

Very bad running posture

What about when you run on a treadmill? One would think that relativistic principles would mean that there is no difference between running on the road or running on the treadmill: in one case you are moving past the road; in the other the "road" is moving past you. The key difference is that there is air resistance when you are running on the road so you must produce a horizontal force to counter this in order to advance; in a vacuum, after an initial push, you could "run" on ice by hopping up and down vertically. The difference (as I understand it and think of it) is that, on the road you must lean forward to produce this force while on the treadmill the moving belt pulls you out of balance and you must try to keep upright. So you see, there is a very subtle difference based on cause and effect but the upshot is that you must actively lean while running on the road and must try to stay upright while running on the treadmill.

This post arose after I saw a video on youtube on so-called "Good Form Running". I agree with the principles but, unfortunately, even though it is most practical to demonstrate them on a treadmill, it is also not the place to be leaning forward.