Gravity and Running: A High Schooler's Perspective

[Woodpecker]

Hey. I'm a serious runner, and lately I've become very interested in biomechanics and running form. One thing I've found very interesting is a subset of running systems based around the concept of using gravity for forward motion. The general idea is that by keeping a straight body, but bending forwards at the ankles, one goes into something of a freefall forwards. The legs are then swung under the body to catch it in its fall. The benefit, obviously, is that the job of providing forwards momentum shifts from the muscles of the legs to gravity. Does that actually make sense from a physics perspective? Something doesn't seem right about it to me, but I'm a high-school student just taking Honors Physics this year.

 

[Doc Al]

I'm no runner, but I've read similar "explanations"--for example, Danny Dreyer's "Chi Running"--that say gravity pulls you forward. Doesn't make sense to me, unless you're going downhill.

Such a running style might be more efficient and healthy than other styles, but not because you are mysteriously propeled by gravity. Perhaps other styles, due to an unbalanced posture, require more muscular effort than necessary, making a more balanced style seem effortless in comparison.

 

[Astronuc]

The benefit, obviously, is that the job of providing forwards momentum shifts from the muscles of the legs to gravity.

Basically one is describing the process of moving the body's CM forward of the pivot (hips).

But then the legs have to provide a force to counter balance the force of gravity. There may be a slight benefit if the CM is moved slightly forward, but out much further, I don't think so, based on personal experience. Where that benefit is realized certainly varies according to physiology.

There is also the matter of 'swing' or rotating the leg about the pivot as opposed to pushing. I think pushing with the leg (thigh (quadriceps) muscle) is more effective than swinging the leg (using hamstrings), and certainly puts less stress on the hamstrings. Ideally there is a balance on work performed by quadriceps (thighs) and hamstrings, and perhaps the angle of the body may depend on which set of muscles is better developed (or possibly least damaged).

Notice that sprinters start off inclined and gradually become more erect as speed increases (perhaps this reflects transfer of work from quadriceps to hamstrings as the thighs tire sooner if they do more work in the beginning). Then at the end, some lean forward, certainly to get head across the finish line, but also to 'fall forward'.

 

[cesiumfrog]

Gravity won't ever provide a forward force per se (assuming you are running on flat ground), but in some ways that's a moot point (like noting the magnetic field does no work, as you watch a magnet lift some filings).

As you attempt to accelerate yourself forward, pushing your feet across the ground, the ground not only conducts a linear forward force onto your centre of mass, but also (since your feet are located well below your centre of mass) the ground applies a torque to your body (tending to make you fall backward).

If you lean forward first, gravity will supply an opposing torque, and you will accelerate forward. If you did not lean forward at all then your feet would move forward out from under you, and you would fall on your .. And this is also why you lean backwards when you want to stop running (ie. deaccelerate). It's also critical to the control of segway-style balancing platforms, incidentally.

Once you reach maximum velocity this reason no longer applies (marathon runners do not stay doubled forward like sprinters). It may still be advantageous for other reasons: a forward posture may allow longer stride, since the thighs can move forward more than back. Don't stress too much over the explanation someone gives: it can be very impractical to try to manipulate your body optimally while trying to analyse the mechanics, whereas sometimes a completely non-physical instruction is far more helpful (eg. in aikido, sometimes visualising "projecting your energy forward" will suffice to move your body into an optimal posture, despite not being a physical explanation).. it'd be a contrast between low-level-programming a robot and coaching a human.

 

[ganstaman]

There is also the matter of 'swing' or rotating the leg about the pivot as opposed to pushing. I think pushing with the leg (thigh muscle) is more effective than swinging the leg (using quads), and certainly puts less stress on the quads. Ideally there is a balance on work performed by thighs and quads, and perhaps the angle of the body may depend on which set of muscles is better developed (or possibly least damaged).

Notice that sprinters start off inclined and gradually become more erect as speed increases (perhaps this reflects transfer of work from thighs to quads as the thighs tire sooner if they do more work in the beginning). Then at the end, some lean forward, certainly to get head across the finish line, but also to 'fall forward'.

I'm a bit confused by this. The quads are the (first or second) biggest/most powerful muscles in the thighs. So when you differentiate above between "quads" and "thigh muscles" I'm not sure exactly what you mean.

I have noticed this effect when I was in high school, but it also could easily have been psychological.

I think the physics in the first post is sound -- you let gravity pull you forward in relation to your legs instead of making your legs force you forwards. If this is right, then the forward lean has your legs absorbing your downward motion (as they normally do anyway), but they also swing freely forward instead of propelling forward -- seems like less work to me.

Leaning forward also affects other muscles and joints. As you move faster, you must swing your leg through faster (have to beat the speed of the rest of your body or else you fall), so your hip muscles must work harder. I also imagine that you are landing slightly off balance still (center of gravity still in front of you). This may cause some slight ankle instability as well as shifting some of the load of absorbing the shock of hitting the ground to possibly a different muscle. So I think that while this may work in the short term, it's not sustainable (otherwise I'd imagine that that's how we'd run naturally).

I also considered the effect of air resistance when running head high/chest out vs head down and into the wind. Not sure if that's a significant effect at all, especially at the low speeds we'd be running.

 

[Astronuc]

I'm a bit confused by this. The quads are the (first or second) biggest/most powerful muscles in the thighs. So when you differentiate above between "quads" and "thigh muscles" I'm not sure exactly what you mean.

I was using the term quads incorrectly when I was thinking of hamstrings (particularly biceps femoris). Correction applied.