- Image courtesy of http://moon.ouhsc.edu
We actually discussed four axes yesterday, but for the sake of keeping this post at a reasonable length I'll just be highlighting two right now with more to come later. Feel free to update or add to any of the posted info by commenting below!
Talocrural Joint
The first axis we discussed was found within the talocrural joint (TC). The TC is commonly referred to as the ankle joint as is made up of the articulations of the tibia and fibula on the talus. Typically, the motions that are thought of when discussing ankle motion are plantarflexion (PF) and dorsiflexion (DF). While it is true that the structure of this joint allows for predominantly PF and DF, it is important to note that other motions are allowed to happen here, as well.
Because the axis is not purely a frontal plane axis (allowing for purely sagital plane motion--PF and DF), there is motion that occurs through the other cardinal planes as PF or DF is happening. Specifically, as PF happens around the TC, adduction (AD) of the foot occurs, as well. Likewise, as DF happens, abduction (AB) of the foot occurs. Additionally, there is a slight bit of rotational motion that is allowed for by the bones, but according to Greg Roskopf this motion is not clinically significant due to various soft tissues and other structures restricting it.*
How do you find the angle and location of the TC axis? Roughly speaking, it is going to be a line the runs directly through your lateral malleolus up and in through your medial malleolus. So, we are looking at an axis that runs inferior/posterior/lateral to superior/anterior/medial.
In terms of degrees, the axis is going to sit about 82 degrees from the sagital plane (8 degrees from the transverse plane) and 20-30 degrees from the frontal plane pointed medially.* So, as you can see, because the axis sits almost 90 degrees from the sagital plane, there will be a lot of motion in that plane allowed by the structure. However, you can also see why saying that only DF and PF occur at the TC would not be an accurate statement as there will be other motions allowed here.
Subtalar Joint
The second axis we discussed was found within the subtalar joint (ST). The ST is made up of the articulations of the calcaneus and the talus and motions here are commonly discussed as pronation (PR) and supination (SP). Much like the TC, we'll see that the direction this axis runs is from plantar/posterior/lateral to dorsal/anterior/medial. The angle at which it runs is completely different than the TC, though.
Based on the numbers we discussed yesterday, we are looking at an axis that sits roughly 42 degrees from the transverse plane (48 degrees from the frontal plane) and roughly 16-23 degrees medially from the sagital plane.* We also discussed that there can be significant variance in the exact placement of the axis, for example having higher arches may make indicate a more vertical ST axis.
Once again, because the axis doesn't sit purely in one plane, motion around this axis is going to be occurring throughout all three cardinal planes. Because of where the axis sits, you can see the structure potentially allows for a lot of inversion (INV) and eversion (EV) combined with AD and AB. Some PF and DF appears to be allowed, as well, but not nearly to the extent that is allowed by the structure of the TC.
Going back to PR and SP (mentioned above), PR seems to be the combination of motions that occurs when the calcaneus EV (AB and DF), and likewise SP seems to be the combination of motions that occurs when the calcaneus INV (AD and PF).
It is important to note that the motions that occur around these joints are not independent motions, meaning you do not move through the sagital plane and then the front plane and then the transverse plane, for example. More accurately, the motion that is allowed by each of these joints is a combination of motions through all three cardinal planes, but it is only one motion. In other words, all three named motions are happening at one time and are one motion, but because we do not have a name for that one specific motion, we have to name it using the system we already have--a naming system based on which of the three cardinal planes the motion occurs in.
So that's a brief recap of what we discussed yesterday during the Precision Human Performance RTS/MAT study group. We went into greater detail with some stuff but those are pretty much the high points of those two axes from what I recall.
Your body. Your training.
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*Roskopf, Greg. Advanced Foot Function. p 47, 50. February, 2013.
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