As we dig deeper into the layers of foot biomechanics, we need to discuss the very important topic of subtalar joint MPP or maximally pronated position. Typically, we have a certain measureable range of motion from our subtalar joint neutral position to both invert and evert. Let us take a NCSP of the subtalar joint that is standing with heel vertical (0 degrees) and has a range of motion into eversion of a typical 10 degrees. When you measure the RCSP (relaxed calcaneal stance postion) the common range of this foot is 5 degrees inverted to 13 degrees everted. If neutral is zero or heel vertical, then a inverted heel means that the subtalar joint has supinated for some reason. This is a bad problem and I will talk alot next week about supinators. What about if the heel stood in RCSP in an everted position? This means that the subtalar joint is pronating away from its neutral. Four of the most common measurements of heel eversion would be 2-3 degrees, 5-7 degrees, 10 degrees, and 13-15 degrees. The ranges are only for measurement differences. So, if you measured 2-3 degrees you are probably dealing with a slight case of pronation, but a standard orthotic device should be helpful. If you measured 5-7 degrees, the pronation is more severe or just compensation of the same degrees of forefoot varus. More severe implies more correction with orthotic devices, stable shoes, exercises, taping, etc. If you measured 10 degrees, by our measurements the subtalar joint is maximally pronated with the lateral walls not be able to more any more. The adaptability of the subtalar joint to pronate any more to adapt to the ground is gone. If a surface or activity or shoe requires more pronatory (eversion) motion, that motion probably will be coming from the foot joint now stacked in a loose bag of bones, or the knee joint (here comes either foot or knee pain for sure). If you measured, 13-15 degrees (yet only measured 10 degrees), the subtalar joint is either subluxed or the knee is torqued excessively. Neither good!
What are the three methods of measuring MPP? MPP is a version of both RCSP, NCSP, and OCSP. And, as you are reading, these are some of my most important measurements. You stand both feet in RCSP looking from behind. You then measure RCSP for each foot.
From RCSP, you can see if the patient can pronate farther in two ways. The first is to internally rotate the entire limb, one foot at a time, without lifting the lateral side of the foot off the ground. Once this is done, re-measure the RCSP. This is the MPP. If it is within 2 degrees of the RCSP, assume that the original RCSP was the MPP (or that the patient was functioning in their maximally pronated position).
Dr Kevin Kirby, biomechanics expert, proposed another way of obtaining MPP. With the patient in RCSP, have the patient attempt to pronate their feet more (again one at a time after measuring RCSP) by firing their lateral extensors (lifting up the 4th and 5th toes). Quickly remeasure RCSP for the MPP value.
Back in the 1950s, when the biomechanics world was dark, and various arch supports and varus wedges and stiff soled dress shoes and lowdye taping was all that was available for pronators, along came "light" called Root Biomechanics. Terms like RCSP and NCSP were introduced and so much more. Later, the genius of Dr Kevin Kirby told us to pay attention to MPP and OCSP and so much more. Who will be coming next?
