Here is what we know, when we put our foot on the ground, we, as humans who sit too much and tend to get into sagittal plane activities too often, things like swimming, biking, walking, running -- and do not challenge the frontal/lateral plane enough earn our way into functional problems: "Walking appears to be passively unstable in the lateral direction, requiring active feedback control for stability. The central nervous system may control stability by adjusting medio-lateral foot placement, but potentially with a metabolic cost. This cost increases with narrow steps and may affect the preferred step width." -Donelan study
For well over 6 years now I have been working on solidifying my thoughts and theories on the cross over gait. I did our 3 part video series back in 2011 and Ivo and I have built our theories to deepen the roots on this concept since then. Since then, the more research I come across continues to serve these initial theories well and help me to hone them for my clients and runners. Some still dismiss the concept because "many professional runners have a very narrow step width and they are fine" -- that is not the point, it is deeper than that. More recently I have found it more helpful to explain it as, "a narrow step width, like all things off of the mechanical norm, have a place and some value when the environment requires it. However, it comes down to a challenge between the two issues of Economy and Liability, perhaps better put, Economy vs Stability. A narrow step width may be more economical for moving through the sagittal plane in many ways, if they have sufficient lateral (frontal plane) endurance, but if one goes too far or for too long, that economy can become a liability and injury risk can build as one begins to tease that lateral plane." I will ask my athletes, "how long can you be in this running economical place before you run out of gas and liabilities start to mount into the more metabolically demanding frontal plane?". Endurance and strength are the major factors, built on skillful movement. The question remains for many athletes, "how long can you run with a narrower step width, with your present lateral hip-pelvis-core endurance and stability, before you exhaust the endurance of your protective mechanisms and expose the liabilities of those more risky frontal plane mechanics ?"
Again, from the Donelan study:
"Walking appears to be passively unstable in the lateral direction, requiring active feedback control for stability. The central nervous system may control stability by adjusting medio-lateral foot placement, but potentially with a metabolic cost. This cost increases with narrow steps and may affect the preferred step width.
These results suggest that (a). human walking requires active lateral stabilization, (b). body lateral motion is partially stabilized via medio-lateral foot placement, (c). active stabilization exacts a modest metabolic cost, and (d). humans avoid narrow step widths because they are less stable."
- Dr. Shawn Allen, one of the gait guys
J Biomech. 2004 Jun;37(6):827-35. Mechanical and metabolic requirements for active lateral stabilization in human walking. Donelan JM1, Shipman DW, Kram R, Kuo AD.