This new thread is split off from the thread Are Root Biomechanics Dying?
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Jeff, Eric and Colleagues:
This is one of the best discussions we have had on the midtarsal joint for some time, therefore, I have taken the liberty of creating a new thread that hopefully will be a valuable learning resource for many of those that are following along.
Jeff, I am totally in agreement with you that there is great inter-individual variability in the magnitude and quality of midtarsal joint (MTJ) motion. However, I am in disagreement with you that the examination technique you have described, the same one that John Weed, DPM, taught personally to me and I taught to other podiatrists and podiatry students for 10-15 years, is actually measuring "the axis of motion of the MTJ", or as you stated: "I find it far more important to assess the spatial location of the MTJ axis."
Instead, the examination technique you describe is a measure of MTJ stiffness, not of "the spatial location of the MTJ axis". In other words, when you perform this examination technique and find it more difficult to move the MTJ in one plane along one axis of motion rather than in another plane along another axis of motion, you are not determining "the spatial location of the MTJ axis" you are measuring the stiffness of the MTJ along each of it's many possible joint axes. You might claim that the MTJ has a vertical axis if the abduction-adduction direction of the navicular and cuboid (NC) relative to the rearfoot has the least joint stiffness. [Joint stiffness being defined as the amount of external force or external moment applied across the joint divided by the amount of joint rotation that occurs as a result of the applied external force or moment.] You might also claim that the MTJ definitely does not have a vertical axis if your applied manual force on the NC in the adduction-abduction direction produces less joint MTJ motion than when an adduction-plantarflexion and abduction-dorsiflexion external force is applied. Your examination technique rather than finding "the spatial location of the MTJ axis" is, I believe, determining the axis of motion of the MTJ that has the most compliance (i.e. has the least stiffness). Is this important functionally? I would bet it has some importance, but we simply don't know yet since no research, to my knowledge, has been done using this test which measures MTJ stiffness in multiple planes of motion.
Therefore, I believe that this difference in MTJ joint stiffness at multiple planes of externally applied manual force during the examination technique you describe may give us some valuable information, especially when some quality research is done, that may allow us to determine whether MTJ joint stiffness has anything to do with preferred motion patterns of the NC relative to the rearfoot. In my recent private discussions with podiatric researchers (I won't mention names, but you all know them), the concept of MTJ stiffness is a "hot topic" now and certainly your excellent description of the technique may eventually give impetus to researchers to see if, what I will call Dr. Root and Weed's MTJ Maximum Compliance Axis Test, has any correlation to the kinematic and kinetic function of the foot and lower extremity during weightbearing activities.
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