Let’s start with the big force, the vertical one. This is a little complicated because the magnitude of this force changes over time as the foot lands and takes off. It starts off at 0 as the foot lands, then increases to more than your weight as you decelerate your body from its bobbing down (because you bob up and down as you run). This force then starts to make you bob up and as you take off the forces decrease back to zero at the point your foot leaves the ground. If you average all of those forces over time you can think of them as a single average vertical force applied somewhere between where your foot landed and where it took off relative to your center of mass.

What should be obvious from that is that if your foot lands directly under your center of mass then the average force is behind your center of mass and there is a torque that will cause you to fall forward. So your intuition and that of other commenters is confirmed by physics. The reality is that, more or less, where your foot lands and where it takes off have to be roughly the same distance in front of and in back off your center of mass. So, the longer your stride, the further forward your foot will have to land. Of course, no one really thinks about this because your body automatically does what it needs to do to keep you from falling over.

The horizontal force will be small and will tend to make you fall over backward. It can easily be counteracted by a small forward lean. It could also be counteracted by moving the point where your foot lands slightly more forward of your center of mass than what it would otherwise have to be.

I think the net of this is that the runners with an upright body position will need for their foot to land slightly further forward than those with a slight forward lean. Based on that, I’d think that a slight forward lean should be more efficient that an upright body position because it will allow the foot to land slightly further back which should be a slightly softer landing. However, I doubt that the distance is significant compared to other aspects of the gait.

What must be happening when people try to bring their landing back is that they have to compensate by taking off sooner which shortens their stride. This fits with the recommendation I’ve read to favor a short, compact stride and a fast foot turnover. However, the foot must land in front of your center of mass unless you want to fall over.

To add to your point:
“I just think we need to be a bit clearer and more precise when describing running mechanics, because to not do so leads to confusion and a lack of clear understanding.”

I think … we need to be a bit clearer on the running objective (context) when we talk about running technique. Is the objective in this moment outright speed or is it top aerobic speed or is it the highest level of efficiency (irrespective of speed)? And how does that objective in this moment relate to the individual and their current fitness level?

In nature, it is very simple. “Form follows function [or objective]”. It is not the other way around or “Form ignores objective”. And it is also not “Form follows somebody’s objective (and form).”

The objectives of ChiRunning and the objectives of elites are not likely the same. Similar maybe, but not the same. One is more about efficiency and preventing injury with speed as a (optional) by-product; the other is more about speed while being efficient and reducing the risk of injury.

For example in nature, take your dog Jack. Watch Jack chase a squirrel vs. running along side you. Different technique? I think so. When Jack wants to run for speed, there is more reaching and more muscle. When Jack wants to run at less than HIS outright speed, there is less and less reaching and muscle. Does Jack run with his speed form often? Not likely. He knows better.

Thanks again for the great information here. My point is that information without context also creates confusion.
David.

One factor worth noting that hasn’t been mentioned extensively is how ‘optimal’ changes as weight rises among runners. The elites shown in most of the footage here have very low BMIs (common in high-level endurance runners), so, even if they are ‘breaking’ a bit when they overstride and/or heel-strike, the forces at work on their vulnerable areas are simply not as great as they are for heavier runners. In fact, most of these fellows likely check in somewhere in the 140-160 lb range, so they, in effect, can ‘get away’ with lots more heel striking, overstriding, etc than a runner who is 25-30 pound heavier without suffering undue injury.

In my experience, the 185-200+ lb runner (which is the range I fall in), is the one who, usually, must be VERY conscious about overstriding, heel-striking, and using poor form over long distances. When I first started running, injury/discomfort was a very common occurance for me. I tried every recommended shoe, stretch, and excercise with zero success at eliminating injury… until I learned to land forefoot and under or close to my center of gravity. Once I reduced the forces at work on my vulnerable areas (knee, shin, hip, etc), the inuries subsided and have never returned. Perhaps my case is merely anecdotal, but I have talked to a lot of other heavier runners who notice the same when they rigidly monitor the factors surrounding their landing.

Great site!!

Reagrds,

Blake

In my previous comment I’m assuming that your foot’s forward velocity has stopped before hitting the ground. If it hasn’t then it is still moving forward and there will be a sudden horizontal braking force as your foot impacts the ground and stops. If your knee is flexed at all then your quads have to exert a counteracting force. The braking force will slow you down and it will also cause a torque rotating you forward. I think that this braking torque will have to be counteracted by your hip flexors which will have to work harder to accelerate and counteract the braking force. Note, this really has nothing to do with where your foot strikes. That is determined by what I previously described. I don’t think that this will necessarily make your foot strike have to be further forward because your hip flexors will wind up creating an equal and opposite torque to keep you from slowing down.

Your slow motion videos show this very clearly with the elite runners. I think the common feature they all exhibit is that their foot has stopped moving forward as it lands. When that is the case, it doesn’t matter as much if they heel strike slightly. A heel strike doesn’t necessarily imply a braking force. The braking force is caused by the fact that the foot hasn’t stopped yet.

The other factor is stride length. If you are standing straight up with your foot directly under your hip joint and your hip directly under your center of mass, then your hip flexors don’t have to exert any forces to balance you. However as your foot moves forward or back from your hip your hip flexor has to do work to counteract the resulting torque. The magnitude of this force relative to hip angle is very non linear and is quite small for small angles but it increases rapidly past a certain point. What does this mean? It means that the longer your stride is the more work your hip flexors are having to do just to counteract gravity which is a quite strong force. So, this is yet another argument for a compact, short stride.

Out of all of this I think that the focus on where the foot lands relative to the center of mass is a distraction. What is more important is to land lightly with the foot already stopped and to keep the leg turnover high and the stride as short as possible. This has the possibility to dramatically decrease the forces exerted by the hip flexors which are relatively small, weak muscles and thereby increase running efficiency and endurance.

So, don’t run like the guy at the gym who sounds like he is trying to pound the treadmill through the ground and first increase your speed by increasing your leg turnover and then by increasing your stride length.

I know that this is a simplification of the forces and torques involved. But I think it illustrates the key points.

Steve Magness stated: “Looking at elite athletes, when racing and training, they generally have higher turnover, minimal ground contact time, and a foot strike that is under their center of gravity.”

So Steve used to think elites had a foot strike under their center of gravity but has obviously shifted his thinking.

Romanov doesn’t mess it up, nor does he preach physical impossibilities. He allows for loading the biomechanical spring by touching the foot to the ground within a certain range of the mid stance, or Pose— within a about a frame of video taping at 24fps. He says to release the ground in about the same amount of time.

Dryer is way off the mark, plus he heel strikes when he runs.

I’ve seen video of Michael Johnson and Haile Gebrselassie getting in and out of the running Pose all within a single frame. That’s pretty close to landing directly beneath the GCM.

I do understand why many people gets confused by Chi though, since there’s no differing between cues and how actual running should look.

Overall a great physics lesson/analysis. Thanks, was very helpful.

One minor issue is that while the distance of landing in front of the CoG and take off behind the CoG is coordinated, almost every runner will have significantly greater distance between takeoff and their CoG then between landing and their CoG.

I just ran some quick calculations and using the toe as the constant measuring point, at landing my toe was 51cm in front of my hips/roughly my CoG. At take off my toe was 64cm behind my hips/CoG.

Here is part of what I wrote on a running thread a while back, please correct me if I am wrong:

“Gravity will not push you horizontally (forward nor backwards), only
vertically (downwards). We cannot convert a vertical force into a
horizontal force by “leaning forward”. Gravitational force between us and the earth either accelerates us toward the center of the earth or pins us down against its surface if the ground is level. If we are pinned down the result is a force equal to your weight.

Now, if the ground is sloped, the vertical force will have a
horizontal acceleration component. We will then either accelerate
towards the bottom of the slope or brake against the ground by
friction or by being pinned horizontally by another element such as
wind, water, a tree, a wall, whatever….

The most significant way we accelerate ourselves horizontally while
running is by applying a horizontal force between our feet and the
ground by means of friction, that is the push off. We can also
accelerate if we are being pushed by an external force such as wind.”

Best regards,
Luis

Looks to me like that image was taken well after initial foot contact – it’s
very hard to observe the exact moment landings in still images due to the
precision of timing needed. Slow motion video is the way to go.

Pete

My point in this post was not to advocate that people emulate elites when
they run. It is for that reason that I also included video of an unshod
Kenyan boy, and my 6yo son (I could have included myself too). I simply used
these images to demonstrate my point about landing location.

My point in this post is that advice to land “directly under the center of
gravity” is a cue and does not happen in reality. I fully realize that exact
location of footstrike will vary slightly, but I could include images of
hundreds of non-elite runners here as well that I have looked at in my
videos to show the same thing. I have no problem with this cue as it helps
to get people to shorten stride, I just think it’s important to recognize it
for what it is.

With regard to my dog, when he chases a squirrel at a sprint, his gait
changes from a trot to a gallop, which is a wholesale change in gait, not
simply more reaching and muscle. Details of human gait surely change as we
speed up, but the dog transition is more like us changing from a walk to a
run than from running slow to running fast.

Pete