What do we “know” about running injuries?: Forces

What do we “know” about running injuries? If we reduce the question down to its core elements, it is conversation about managing forces. It’s a conversation about tissues being able to dampen external forces and use them to produce internal forces to act back upon the ground. It’s about them being able to handle these demands with forces at given magnitudes over a certain accumulated volumes both acute and chronic.

So what are these forces at play? Vertical ground reaction forces (vGRF) are the highest magnitude forces at play during running. These forces can be broken down into 2 smaller elements. There is the impact peak, which is the initial “shock” of braking. The body taking the punch from the earth as we return from our brief moment of floating. Many studies show that the rate at which this initial loading occurs may be as important or more important to certain common injuries. ( see work by Willie, Dicharry etc..)

Vertical Ground Reaction Forces (vGRF) are the highest magnitude (and most studied!) forces acting on the body during running. These forces can be broken into several components and qualities as seen in the figure above

The rate of loading is broken down into instantaneous and average. A lower loading rate is akin to “riding with the punch”. The high forces can be dispersed over a longer time reducing impact peak. When we are watching runners and see a smooth glide versus choppy stomper this is the quantification of that aesthetic visual. The stomper takes the punch by ramming his head into it, while the glider patiently rides the wave. 

Characteristic profile of the “stomper”. The impact peak designates a braking shock to the system.

Second, there is the active peak. This is moment the rubber band is released. Potential energy being converted into kinetic. Occurring right around mid stance, this is the moment of maximum propulsion. The forces are internal tissue compressing and acting on each other to allow a punch to be delivered to to the ground. 

Characteristic profile of the “glider”. A more gradual rate of loading smooths out the forces and removes the harsh impact peak.

With force, it’s all in how we manage it. How efficient we can be. The difference between graph one with the choppy peak and graph two with the smooth peak are all in the movement profile of the individual. Central to this is the long debate over striking patterns. Conventional wisdom may tell you that the high impact peak directly relates to the heel striker while the smooth slope is related to the forefoot striker. While these are correlated they are not directly linked.  What factors are more directly linked?

Stay tuned.

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