It seems to me that nobody can quite agree on exactly what is happening during the running gait.
The running gait is characterized by an alternation of support: at one point, your body is supported on the ground by your left leg, then you’re suspended in the air, and then it’s supported by your right leg (and then subsequently back to your left leg). It’s how you get from these support phases—also called “stance phases”—to being suspended (and back again) that people vehemently disagree on.
Many in the running community say that the motive force of running is produced by a strong push of the leg muscles against the ground. But Dr. Nicholas Romanov of The Pose Method suggests a different—and in my opinion, far more parsimonious—interpretation of what happens: instead of “pushing,” the body accelerates its center of gravity by repositioning itself relative to the point of support (the foot on the ground).
UPDATE # 1: All repositioning occurs due to muscle activity, and the speed and effectiveness with which the body (or a specific body part) can reposition is commensurate to the power of the relevant muscles.
We typically think of “acceleration” as “the thing that makes cars go from 0 to 60.” But even a slight weight shift is an acceleration. When the slowest snail takes one tiny step, it’s accelerating it’s body (and then promptly decelerating it). Similarly, a slight weight shift constitutes an acceleration of the part of the body that moved. A greater weight shift is an even bigger acceleration. If you string together enough tiny weight shifts (or big ones) in a close enough sequence, you get a really big acceleration!
In this post, I’ll argue that the most logical way of producing a human movement (and that of any segmented organism) is by shifting the most easily-movable part first.
If you look at the body from a design perspective, you’ll see that it’s a stack of different parts (feet, calves, thighs, hips, etc.), all separated by joints. In the standing posture, each of these parts provides support for the part above it, much like a stack of bricks. But the difference is that the body’s joints let each brick move semi-independently of all the other bricks. The question, then, isn’t “how do we run?” It’s waaay more basic than that. The question is: to get from A to B (over and over again), how does a stack of things have to move?
You could simply lift the bottom brick—along with all the bricks on top of it—and move it that way. That’s not particularly convenient, though: it requires a lot of energy in very little time. But there’s another way: start from the top brick. That way you only have to move one brick at a time, shifting bricks in quick succession.
This is the logic that your body (and the body of any segmented organism) uses to move. If you’re standing on two feet and want to lift your left foot, you don’t start by lifting your foot. You start by shifting your weight—starting by your shoulders, and moving down the body—onto your right leg, effectively removing all the weight off your left foot.
(This takes all the top “bricks” off the foot first.)
I’ve just described to you a process intrinsic to any human movement, which Dr. Romanov calls unweighing. This is the simplest process: if you want to move a limb, you first shift all the weight you can off it first, and then you move that limb. What makes Dr. Romanov’s theory parsimonious is that you need very few ideas to successfully describe human movement as a whole. Case in point: the movement of the entire body is simply a large-scale version of unweighing.
If you want to move, you create a forward weight shift in the direction you want to go. This effectively takes your weight off your feet and puts it in the space ahead of you.
Let’s talk running. During stance, one leg has the entire “stack of bricks” on top of it, and the other one is suspended in air (and already traveling forward), with nothing pulling it to the ground but its own weight. (UPDATE #2: In terminal swing, that leg actively reaches for the ground in order to provide new support). But when one leg is in early stance and midstance, which do you move? Do you push with the leg that has all the bricks on top of it, or do you move the foot with nothing holding it in place—the “topmost” brick?
That’s the question Dr. Romanov answers with the Pull. The Pull describes the process of getting the back leg off the ground, and recycling it forward to produce the next step. But part of the hidden importance of the Pull is that it is also a weight shift: whereas in the previous weight shift you drifted your shoulders a few inches to one side, in the Pull, you aid the elastic recoil of your tendons in pulling your foot from the ground. This brings the mass of your entire leg ahead of the foot currently supporting you on the ground.
In a proper landing, your foot will touch the ground just ahead of your hips, torso, and head. There’s a slight deceleration due to the foot’s contact with the ground, but the body as a whole continues to travel forward, vaulting over the support leg. If the leg that just came off the ground—the “Pull” leg—moves forward fast enough, the body can add more of its mass ahead of the point of support.
We already know that a small weight shift—drifting the shoulder to one side—causes you to move (read: accelerate) slightly to that side. Now imagine how much more acceleration you can create by pulling the leg and moving its mass ahead of the body.
Mainstream thought questions whether this kind of weight shift can create enough momentum to offset wind resistance, plus the braking effect of landing, plus any power leaks that the person might have. The argument goes that if it can’t, the “pushing” argument is more likely the correct one.
But I hope I’ve convinced you that the best way to move a stack of things is by moving one part at a time in order to tip the stack in the direction you want (and then continue to move the parts in order to create more acceleration). Supposing that this—the best way to move a stack of things—somehow wasn’t enough to overcome wind resistance and the braking effect of landing, there’s no way that you could do it with pushing (a.k.a. moving from the bottom brick) because, well, it isn’t as effective.
So if the question of running is “what is the best way to offset wind resistance and braking?” the answer would still be to reposition the most easily movable limb in order to create a weight shift to move the body in the desired direction.