The question becomes, why do we have to land in front of the COG and not directly underneath it like Pose or Chi running advice?
Many well intentioned ideas or concepts come about because we have no patience. In regards to running form, that means minimizing the braking forces on the ground. In theory this is a great idea, minimize braking so we can spent more time in the propulsion phase. However, in practice it doesn’t turn out so well.
The idea of the pawback developed based on this theory. Coaches thought that if we could start the backwards movement of the leg before we even hit the ground then braking forces would be minimized as the foot would already be moving backwards. Similarly, the land right underneath your COG idea probably developed because people noted that you couldn’t have propulsion until the foot was past the COG and on its way backwards. They reasoned that if we eliminated the time between when the foot touched the ground and when it was under the COG, we’d minimize braking and useless time spent on the ground waiting to be able to push off. In essence, both ideas came about because of wanting to get quick.
Theory and practice don’t always work out so nicely unfortunately.
Loading up-Why patience is a virtue
Many of the proponents of the landing directly underneath your COG point to the fact that propulsion can’t occur until the body is over the COG. Only then can the body begin to push off and propel it self forward. They claim that time spent from landing in front of the COG to when this propulsion can actually occur is just time spent waiting. Their principle idea is to eliminate this needless waiting and get on with the propulsion phase, or in the case of POSE, the pull/fall phase.
The problem with this line of thinking is that it ignores the impact forces and elastic energy usage. If you recall that the bodies tendons, ligaments, and muscles act in a similar way to a spring, it only makes sense that there has to be a compression in that spring system before it can rebound and release energy. Put another way, there has to be energy storage before there can be energy release. Along these same lines, the initial impact of the collision of the foot to the ground has to be absorbed.
When you land slightly in front of the COG it allows for these process to take place. The body has time to absorb the impacts and move into a propulsion phase. If somehow you were able to land directly under your COG, you’d waist part of the time in which you could be applying propulsive forces to the ground because part of that time would be spent absorbing the passive forces.
To help clear this up, let’s look at some data.
The above chart shows a typical ground reaction force graph. The top graph is vertical GRF, while the bottom one is anterior-posterior (forward-backward) GRF. I like this illustration as it shows exactly where passive and active forces take place and is easy to understand.
The anterior-posterior (bottom) graph shows us the crossover point where we go from deceleration to propulsion. The first half of the graph there is a frictional force backwards against our foot, preventing it from continuing to slide forward as we land. On the latter half of the graph, the frictional force is forward to prevent us from slipping backwards and we push behind us. We can use this transition point as a guideline to show what’s going on in terms of propulsion.
If we combine this graph with the vertical GRF, things get very interesting. Note that the transition point corresponds with peak GRF, and as pointing out in the picture, this is due to active muscle forces and the rebound of that stored elastic energy.
Finally, take a look at this screenshot from the Lieberman study video on youtube.
While it’s not exact since it’s an incomplete picture, note that the peak vertical GRF corresponds nicely for when the center of pressure on the foot is underneath the approximate CoG. The red line in the picture marks the runner’s approximate CoG. By approximation the Center of pressure in the foot is on the ball of the foot at this point, so it falls pretty close to directly underneath the CoG. While it's hard to get an exact mark, it's obvious that the foot is under the CoG sometime around or just before peak vertical GRF.
This fits in nicely with the data from the anterior/posterior transition point and shows that the stride can likely be seperated into an absorption and a propulsion phase. Although hard to tell in the graphs, the absorption phase takes less time than the propulsion phase, as the foot extends backwards from the CoG more than it extends in front of the CoG upon landing. This can be seen in the Lieberman videos.
For some rough info, I ran some data on myself.
As you can see the time and distance from initial contact to when the approximate center of pressure (approx average of where on the foot the load is) is under the CoG is less than the time and distance from when the CoG is crossed to when toe off occurs. Initial foot strike occurs ~27cm in front of the CoG and it takes .086sec for the foot to get under the CoG. On the other hand it takes .133sec to go from the CoG to toe off. Toe off occurs ~50cm behind the CoG.
What about footstrike?
If you are a keen observer, you’d notice that the first graph is of a heel striker. Let’s take a look at a midfoot striker and see if the premise holds true.
The above data is from a separate study by Peter Cavanagh (1995) (link here). The left column is heel striking and the right column is midfoot striking. The top row is mediolateral GRF (side to side), the middle row is anterior-posterior GRF (front to back) and the bottom is vertical GRF.
As you should be able to see, even in the midfoot strike, the anterior/posterior transition point corresponds to the peak vertical GRF.
So what’s the takeaway message?
Have patience. We need to land just in front of the COG to allow for absorption of the impact. This doesn’t mean reach out with your leg and land way out in front of your COG. If that was done then the delay between when impact occurred and when propulsion occurred would be longer, thus more energy dissipation and less use of elastic energy.
In an earlier post I mentioned using cue’s to improve form. Landing under your CoG still may be a reliable cue at first as you try and overcorrect the problem of landing way out in front of you. The problem arises when we mix cues and what’s actually going on.
So be aware of what’s a cue and what actually happens.