Calcaneus' instantaneous axis of rotation ?

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Hello,

As "the plantar shape of the calcaneus is inherently unstable, not stable" [ Kirby KA - "The effect of plantar calcaneal shape on foot biomechanics" in Foot and Lower Extremity Biomechanics, vol. 1], can we discuss about an instantaneous axis of rotation of calcaneus relative to supporting surface ?

How this instantaneous axis of rotation of calcaneus relative to supporting surface influence the movement of subtalar joint?



Daniel

 

The movement of the calcaneus relative to the supporting surface will change the location of ground reaction force. It's the location of ground reaction force that will effect the moment from ground reaction force about the STJ axis. So, the instantaneus axis of rotation of the calcaneus relative to the floor is not the variable that one should use to assess STJ moment or movement.

Another way of looking at it: Lower your heel to the ground with the forefoot in the air. It is possible to either supinate or pronate your STJ while lowering your forefoot to the floor. The movement of the calcaneus relative to the floor is somewhat independent of the movement of the STJ.

Eric

 

Is something which I don't understand !
So if
- A = the movement of the calcaneus relative to the supporting surface
- B = location of ground reaction force
- C = the moment from ground reaction force about the STJ axis
If A influence B and B influence C doesn't mean that A influence C [without considering brain influence !] ?

I've lowered my heels to a plan which has three positions:

1- paralel to the ground,
2- inclined in the frontal plane,
3-inclined in the sagital plane

The effort exerted by the body to maintain balance in all three situation was different, I think 3 > 2 > 1 . Doesn't mean that the movement of the calcaneus relative to the floor will influence the movement of the STJ ?
In my mind the "pure" movement of the talus relative to calcaneus around the STJ could be assessed when one of these two bones are somehow fixed. When the calcaneus is not fixed on the ground, the movement of the STJ [and of the talus relative to the calcaneus] will be influenced by the movement of the calcaneus relative to the supporting surface. But is not clear for me how could be described this influence ! Of course, the problem is more complicated when we'll consider the influence of the rest of the body !

Daniel

 

Daniel:

Sorry I missed your question earlier.

The instantaneous axis of rotation of the calcaneus relative to the ground is determined by the rotational motions of the calcaneus relative to the ground. The instantaneous axis of rotation of the subtalar joint (STJ) is, instead, determined by the rotational motions of the calcaneus relative to the talus. Therefore, if the talus is rotating or translating relative to the ground, the spatial locations of the axis of rotation of the calcaneus and the spatial location of the STJ axis will not be the same. In other words, the calcaneal axis and the STJ axis are two very separate axes of rotation and may or may not be related to each other depending on what the translations and rotations of the talus are above the calcaneus.

Of course, if the calcaneus rotates or translates during weightbearing activities, this motion of the calcaneus may also influence both the spatial location of the STJ axis and the location of ground reaction force acting on the plantar calcaneus. In turn, calcaneal rotation and/or translation may affect the moments acting across the STJ axis.

Hope this helps.

 

Actually the movement of the calcaneus relative to the talus is not influenced by the motion of either bone relative to the ground. The STJ axis describes the motion of the talus relative to the calcaneus. Both bones can be moving relative to the ground when they are moving relative to each other.

The Van Langallan paper that someone posted a copy of in a recent thread is a wonderful reference that can help you understand the difference between the rotation of the bone relative to the ground and the rotation of one bone to another.

Eric