The myth of toes curling to grip the ground

Not sure what you mean here Eric . Are you saying there is no lever arm ?

I have been showing the figure in question purely because you refuse to post the radiograph from which you say you measurements have been taken .

Looking at your model ( the unused digit which you won't show ) you have no lever arm and no moment arm in either of your digits. How are you measuring lever arm ? From where to where ?

You said so earlier in the thread Eric -

"Toe curling will occur if there is not the perfect amount of contraction of the lumbricles, short flexors and long flexors. Contraction of the long flexors can produce hammertoes/claw toes."

What is most interesting about your model to me is that both of the digits lack any moment arm for the FDB at the MTPJ .

I have been trying to provide evidence of what causes very weak toes to collapse under load throughout this thread .Your counter arguments have been unpersuasive and, on a number of occasions, evasive.

 

What evidence do you have that the moment arm of my model is inaccurate? An accurate model reproduces reality. The model reproduces the reality of collapsed toes.

 

The fact that the model has almost no lever arm at the MPJ is exactly the point. That is the reality of the situation. An FDB with a small lever arm at the MPJ will create collapsed toes.

One way to measure lever arm is to measure the distance from the line of action of force to the axis of rotation. I described this already in post #78.

Another way to measure lever arm of a tendon is to move the joint through a certain number of degrees of motion and then measure the distance the tendon travels. This is similar to a bicycle wheel with multiple sprockets. The bigger the sprocket the farther the chain will move with a given amount of rotation of the wheel.

 

Below is your video.

The two digits are materially different with regard to the extra ring on the digit not being activated . Which of the two are you proposing as an accurate model ? The version being illustrated in motion or the version not in use ? If you can answer this I can more accurately reply to your question. If you see the digit with the extra proximal ring close to the hinge as being the more accurate, could you show a video of this?

I am happier relying to more a precise question.

 

Eric, with regard to the figure below where would you estimate the axis of rotation to be?

 

Eric, as previously stated, I can't find an MRI scan of the lesser toe MTPJs that is open access so will just go with the 1st MTPJ seen in the scan below .

I have marked in an approximation of the axis of rotation in yellow and the moment arm in green . As you can see the moment arm is far from negligible as it is in your model. Also the angle of pull of the short toe flexors is greater than 30 degrees to the long axis of the proximal phalanx .

So my model is functionally more correct than yours but I still would not rely on it. Since your model is so inaccurate and your free body analysis is likely also way off , it can be concluded that you have yet to offer any evidence that the toe collapse seen in the picture is the result of increased FDB activity.

This unsupported notion, that isolated contraction of the FDB will produce dorsiflexion at the MTPJ , is held by others in the podiatry profession, but where did it first come from? Has the idea simply sprung from very poor models?

 

Gerry, look at the model again. The lever arm of the tendon relative to the joint axis in the model is actually bigger than the one in your picture. A rough estimate of 2cm thick for a metatarsal head the center of rotation is roughly in the center of the metatarsal head so that would make the lever arm 1cm. My model has a lever arm of roughly 1.5 cm. Why do you think the lever arm in the model is negligible?

As to your criticism of my model not going through the eye screw on the base of the proximal phalanx: If tendon did go through that eye screw that would decrease the lever arm of the tendon at the MPJ. Gerry, you can't have it both ways. You can't criticize the model for having to small a lever arm at the MPJ and also criticize it for not using an even smaller lever arm.

Come on Gerry, at least put in the time to make an argument. Say why my analysis is way off.

Gerry, would evidence that FDB activation causes the hammertoe configuration (toe collapse) reduce the value of your intrinsic foot muscle strengthening machine because you could not claim that it prevents hammertoes?

The model is evidence that FDB tendon force produces toe collapse. Your criticism of the model (negligible lever arm) is wrong. The model produces the exact same motions seen in toe collapse. You have not provided an alternate explanation of toe collapse that makes any sense (muscle weakness cannot cause the motion of toe collapse).

 

Eric ,
Where I have talked about "moment arm" you seem to have read "lever arm" . They are not the same thing !

 

They can be the same. Go ahead and explain why you think the difference matters.

 

But they are not the same here and should not be used interchangeably. Please explain why you think the difference does not matter. After all, it is your mistake .

 

Lever arm and moment arm can be considered to be the same when there is a 90 degree angle between the force and the lever. They can be interchangeable be in this case.

This moment arm/ lever arm criticism is irrelevant to the mistake you made was saying that the moment arm in my model was negligible. The moment arm of the tendon in the model can be seen in the video in the non weight bearing portion of the video. After the PIPJ has reached its end of range of motion, the moment arm at the MPJ becomes the distance from the string to the wire that is the joint axis. This moment arm is able to plantar flex the phalanges of the model when non weight bearing. However weight bearing, tension in the FDB tendon will cause the collapsed hammertoe position.

 

Nowhere in the video you produced is there a 90 degree angle between the force and the lever. You used the wrong terminology, we all make mistakes, let's move on.

In an earlier post, and in complete contradiction to the quote above, you said;

"The fact that the model has almost no lever arm at the MPJ is exactly the point. That is the reality of the situation. An FDB with a small lever arm at the MPJ will create collapsed toes."

Eric, you are tying yourself in knots because you are trying to defend a poor model.

 

Gerry, you are playing both sides again. You complained that I didn't use a smaller lever arm in the video and you are also claiming that it should be a larger lever arm. The FDB has a small lever arm (as stated in the quote) at the MPJ. With no resistance to plantar flexion FDB can create a small moment to plantar flex the MPJ. When the foot is on the ground, the ground provides resistance to plantar flexion of the MPJ. When the FDB contracts it creates a plantar flexion moment on the intermediate phalanx. The head of the phalanx cannot go into the ground so the back end of the phalanx goes up and this dorsiflexes the MPJ.

In our discussion so far the best explanation of the cause of toe collapse is tension in the FDB tendon.

 

Nope ,that would be you . In fact you are even playing with 2 different models and refuse to pick one as your "best representation".

Now we are getting somewhere . The FDB has a larger moment arm in real life than in your models where it is near
negligible ( or as you put it " almost no lever arm" ) at the ranges of motion under consideration . Remember, any distance between the wire axis and the line of pull on the ligament (string ) measured on your models must be scale down before comparisons can be made with in vivo measurements . Nobody has toes 12-13 inches long .

You said "In our discussion so far the best explanation of the cause of toe collapse is tension in the FDB tendon."

No Eric, that is only your opinion backed by an inaccurate model and a fee body diagram that you refuse to show anyone .
Do you have any better evidence ?​

 

You have not presented a better explanation. The model exactly creates the motion seen in vivo.

 

You presented a better explanation in post #23 .
You said "Contraction of the long flexors can produce hammertoes/claw toes"

Your model can be manipulated to produce a number of outcomes including the one seen in my video .

 

What I believe we have established then Eric, is that based on a proportionally representative model, isolated contraction of the FDB will plantarflex the proximal phalanx of the lesser toe and not cause it to dorsiflex.

Further evidence is required and would probably best be provide by using exactly the same experimental set up as was used by Luke Kelly in one of his PhD papers where the foot was placed under load and then the FDB was activated via direct electrical stimulation. From memory he found that navicular height increased but made no mention of collapsing toes .

If surgical techniques are linked to the belief that FDB contraction can cause hammer toes then might it be prudent to ask Kelly what he found with regard to proximal phalanx movement and then, if necessary, investigate further ?