Five toed windlass

I'm not quite sure what you mean by lateral transmission. When an object applies force to another object you can describe that force with its direction, magnitude and point of application. So, if a muscle were to "help" the plantar fascia it would have to apply forces parallel to the plantar fascia.

It's been a few years since I did any anatomical dissection. My recollection is that the plantar fascia moved relative to the muscles deep to it. There were some fibrous connections, but they were oriented in such a way that the muscles could not add horizontal force to the fascia. On the other hand the fascia enveloping the abductor hallucis muscle did have some muscular attachments, however this fascia was on the medial side of the foot, relatively separate from the plantar aponeurosis bands.

Eric

 

Luke:

Like Eric, I don't know what you mean by "lateral force transmission". However, the flexor digitorum brevis muscle belly is attached along its length to the central component of the plantar aponeurosis (the FDB literally must be dissected away from the plantar fascia when a plantar fasciectomy is performed). The abductor hallucis is covered by the medial component of the plantar aponeurosis and the lateral component of the plantar aponeurosis is variable, generally attaching to the styloid process of the 5th metatarsal.

The bottom line is that all the plantar intrinsic muscles that attach to the calcaneus have the mechanical ability to reduce the tensile force on the central component of the plantar aponeurosis since these muscles also cause a rearfoot dorsiflexion moment and a forefoot plantarflexion moment with their contractile activity. Without these important muscles, the plantar fascia would likely develop either partial or complete ruptures with running or other sports activities at least by the time the individual was a young adult. I think most podiatrists underestimate the strength potential of the plantar intrinsic muscles. Their proper function is vital to normal gait function and very important for the protection of the plantar fascia and plantar ligaments from pathological tensile stresses during weightbearing activities.

 

Eric and Kevin

Sorry of my term "lateral force transmission"was a little ambiguous. I am referring to the ability of one muscle, through it's it's own contraction to directly transmit force to adjacent structures. In this case it is the intrinsics directly transmitting force to the Plantar fascia via fascial connections.

I have attached an article by Bojsen-Moller and colleagues describing "intermuscular force transmission" between the posterior compartment leg muscles. They also speculate that this mechanism minimises the contractile load spacially, reducing the the potential stress concentrations in muscles and adjacent aponeuroses.

If this was possible in the foot, this could also have a significant impact on the distribution of strain in the fascia?

I hope this is a little more clear?

 

Luke:

I can't get too excited about the idea of "intermuscular force transmission" being anything too signficant with the plantar intrsinsics, or for any other muscles within the body for that matter. Of course there may be some frictional effects between the muscle sheaths of one muscle and another but I believe these can certainly be ignored in most calculations of the mechanical effects of muscles. I like to devote my energies toward the ideas that intuitively seem to be the most mechanically significant, and not toward the ideas that likely have little importance in the overall function of the foot and lower extremity.

What is interesting about the flexor digitoum brevis (FDB) and the plantar fascia (i.e. the central component of the plantar aponeurosis) is that the muscle fibers of the plantar aspect of the FDB muscle belly are directly attached to the dorsal aspect of the plantar fascia. In addition, in plantar calcaneal spur resection surgery, the spur is never attached to the plantar fascia but rather seems to be deeper than the plantar fascia, at the origin of the FDB and possibly the abductor hallucis and quadratus plantae. Possibly this plantar calcaneal spur formation at the origin of these plantar intrsincis muscles is an indicator of just how hard these muscles work (i.e. duration and magnitude of their tensile forces) for the foot during a patient's lifetime?

Have we been selling the plantar intrinsics short all these years?

 

I now understand what you meant by lateral force transmission. When you have a muscle belly that takes some of its origin or insertion from an intramuscular septa, or fascia, or plantar fascia, that muscle will increase tension and possibly move the septa or fascia. The direction of the force will be essentially unchanged. If the muscle pulls on the fascia the fascia pulls on the muscle from Newton's third law. This also applies to the proximal attachments of the fascia. The muscle will be pulling on one end of the fascia and the bony attachement of the fascia will be pulling in the opposite direction at the other end of the fascia.

Specifically, thinking about the plantar intrinsics and their attachment to the plantar fascia. (I was wrong in the earlier post about how they attach.) The intrisics attach proximally along the undersurface of the calcaneus. and some of them will attach distally along the course of the fascia. When they contract the muscles will create an anterior pull on the calcaneus and posterior pull on the distal half of the fascia. Effectively this will increase the surface area of the proximal attachment of the plantar fascia. Distally, the plantar fascia splits and I am assuming that the mechanically important attachment of the fascia is the base of the proximal phalanges. So, distally, the proximal pull from the fascia acting on the base of the phalanges, would have the passive tension in the fascia and the additional tension from the pull of the muscle. So, as I said above, this would effectively increase the surface area of the proximal attachment of the fascia. Otherwise, I don't think that it really changes the mechanics that much. The above theorizing is done with the assumption that muscle pulls the fascia proximally.

The article is interesting, but I don't think it radically changes how we think about the mechanics of the foot. I have often said that when I talk about the plantar fascia, I'm actually talking about everything that has a distal attachment at the base of the proximal phalanges. That is why I don't think these lateral forces change the overall picture much.

Eric

 

I know that the concept is open for discussion, but when discussing the mechanics of the foot and looking at timing of the DF of digits during propulsion and the effect it has on the PF function, the high gear/low gear concept comes in to mind.
I am preparing a presentation for manual therapists where i'm discussing the sagital plane fascilitation of motion model and the link with the windlass mechanism, the manipulation of the calcaneocuboïd joint etc... anyway... going through some gait analysis I came up with this patiënt who had a difference in propulsion style. The screenshots are of maximal DF MTPJ with met heads making still contact with the ground, and then about 2 frames later.

There is going to be a difference in mechanics between the left and the right foot for sure.
So another parameter to take into account, no?