In this second in a series of podiatric biomechanics thought experiments, I have created a model of the foot including the tibia and talus as one rigid body, the talo-tibial unit, which is attached to a rigid foot model by way of the subtalar joint axis, that, in this model is a hinge perpendicular to the frontal plane. The foot has only two weightbearing surfaces at the approximate locations of the 1st and 5th metatarsal heads. F1 is the ground reaction force (GRF) acting plantar to the medial weightbearing surface of the foot and F2 is the GRF acting plantar to the lateral weightbearing surface of the foot. It may be assumed that both the total GRF acting on the plantar foot and the compression force acting through the talo-tibial unit to the STJ axis is equal to 400 Newtons.
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In the three examples shown, the STJ axis is positioned in a medial location (left), a central location (center) and a lateral location (right). The distance from F1 to F2 in each foot is 8 cm. In the foot with the medially deviated STJ axis, the STJ axis is 2 cm from F1 and 6 cm from F2. In the foot with the central STJ axis location, the STJ axis is 4 cm from F1 and 4 cm from F2. In the foot with the laterally deviated STJ axis, the STJ axis is 6 cm from F1 and 2 cm from F2.
Here are my questions:
1. Determine how the magnitudes of GRF change at the medial and lateral weightbearing surfaces of the foot (i.e. F1 and F2) as the axis moves from a medial to central and lateral location.
2. Why does this change in weightbearing pattern occur on the plantar foot in all three examples if the tibia is still at the same angle to the foot in all the examples?
3. If the weightbearing surfaces of the foot were movable dorsally, which surface in which foot would tend to have more dorsiflexion force on it?
4. Which foot would have more equal tendency for dorsiflexion of the weightbearing surfaces?
5. In the foot with the medially deviated STJ axis, how could we better design the leg-foot model to have reduced GRF plantar to the medial weightbearing surface by adding tensile load-bearing structures to it?
6. In the foot with the medially deviated STJ axis,how could we better design the leg-foot model to have reduced GRF plantar to the medial weightbearing surface by adding compression load-bearing structures to it?
7. In the foot with the laterally deviated STJ axis, how could we better design the leg-foot model to have reduced GRF plantar to the lateral weightbearing surface by adding tensile load bearing structures to it?
Interested in anyone's thoughts on this little experiment.
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