Thought Experiment 8
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In the continuing series of Thought Experiments (Gedankenexperiment), we will be considering the mechanical effects that a single isolated force acting on the plantar foot may have on the multiple joints of the foot and ankle.
In the illustration below, a single isolated force, Force A, is shown to be acting on the plantar aspect of the second metatarsal head. As a result of Force A, there will multiple moments acting across the joint axes of the foot and lower extremity. In this case, how does the foot respond to Force A? Assume the Achilles tendon is exerting a passive tensile force on the posterior calcaneus in response to Force A.
1) What is the direction of external moment at the subtalar joint (STJ) from Force A?
2) What is the net moment acting across the STJ? (Consider the effects from Achilles tendon also.)
3) What is the direction of external moment at the 2nd metatarsal-2nd cuneiform joint from Force A?
4) What is the direction of external moment at the 2nd cuneiform-navicular joint from Force A?
5) What is the direction of external moment at the talo-navicular joint from Force A?
6) What is the direction of external moment at the ankle joint from Force A?
7) Do these moments occur simultaneously or does the force "flow" from one joint to the other (i.e. one joint first experiencing its moment, then the next joint, then the next, in response to Force A)?
8) If these multiple moments are occurring, and the foot is in relaxed bipedal stance, what do we know regarding the direction and magnitude of the moments occurring within the foot (i.e. internal moments) that keep the joints of the foot stable, and at rest in the standing position? In other words, what are the direction and magnitude of the internal moments at these joints during relaxed bipedal stance?
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Related threads:
Thought Experiment #1: Tie Tensile Force in Loaded Arch
Thought Experiment #2: Effect of STJ Axis Location on Met Head GRF
Thought Experiment #3: Effect of STJ Axis Location on PT Muscle Tension
Thought Experiment #4: Effect of STJ Axis Location on Sinus Tarsi Compression Forces
T.E. #5: Effect of Foot Orthoses on Sinus Tarsi Compression Force
T.E. #6: Effect of Foot Orthoses on PT Tendon Force Required to Cause STJ Supination
T.E. #7: External Forces on Objects: Problems with Only Motion Observation -
Q1) Force A would result in a pronation moment at the STJ
Q2) The net moment occurring would depend on the antagonistic pull here of the achilles tendon. If this was occurring in late midstance when the tendo-achilles (TA) is contracting I would suspect the net moment would be a supination moment as the TA would be likely to be overriding the force through the forefoot. If force A is greater then the pull of the TA, then a pronation moment would occur. If both forces were equal, equilibrium would result.
Q3) A dorsiflexion moment would occur at the 2nd met-2nd cuneiform joint from force A
For Q4 & Q5 I am not completely sure. I will have a go and say it results in a plantarflexion moment at the 2nd cuneiform-navicular joint. Dorsiflexion and possible adduction moment of the talonavicular joint.
Q6) Force A will cause a dorsiflexion (with some abduction) moment at the ankle joint.
Q7) I would believe these moments to occur one after another (but very closely followed) as the distal force would act like a chain reaction to the more proximal joints as the force is applied.
Q8) The internal moments of the foot will antagonise the external moments of the foot to obtain equilibrium and this results in static bipedal standing.
Again not completely sure, but those are some of my thoughts on paper.
Regards,
Dean -
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By the way, Dean, instead of using the word "antagonize", I would use the word "counterbalance" to describe the actions of opposing moments to attain rotational equilibrium.
Good job.......so far. -
For those who want a little extra reading on Thought Experiments, Albert Einstein used Thought Experiments effectively in formulating his theory of Special Relativity.
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Kevin,
I hope I am on the right track. I didn't do physics in high school, although did a higher level of maths, so I hope some of my mathematical reasoning is correct.
Dean -
All your responses are correct. There needs to be an abuction force on the foot in order to have an abduction moment at the ankle joint. Excellent work!! -
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A little bit out of my area of expertise, but it is my understanding that when one object contacts another object, the atoms of each respective object never touch the other, but rather are repelled from each other by molecular forces and by their "electron clouds". -
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What do you think?
BTW I seem to remember reading that a Gecko was able to hang on to vertical sheets of glass, because the atoms of it's feet merged with those of the glass??????? Not quite but Van da Vaals forces attracting the atoms together.Last edited: Dec 5, 2007 -
You aren't off the hook quite yet. Now, if the force plantar to the second metatarsal head in the illustration from my initial posting was moved to being directly plantar to the subtalar joint (STJ) axis, considering also the effects from Achilles tendon tension, would the STJ supinate, pronate or be stable in rotational equilbrium? -
However if the Achilles tendon was simply creating a passive tensile force to the calcaneus originally to obtain rotational equilibrium, then this external force of the Achilles tendon may also be reduced along with force A to enable equilibrium at the STJ.
I hope you could understand that Kevin.
Regards,
DeanLast edited: Dec 6, 2007 -
Your answer above is perfect. Therefore, if Force A was directly plantar to the STJ axis, and the Achilles tendon was placed under tensile force because of Force A in my illustration, then a net STJ supination moment would occur, tending to cause STJ supination. This occurs because tensile force within the Achilles tendon causes a STJ supination moment in this foot. Lesson? You cannot just look at the center of pressure relative to the STJ axis to determine net STJ moments.
Next question:
If the STJ axis in my illustration was laterally deviated, so that the STJ axis passed between the 2nd and 3rd metatarsal heads, would the foot tend to pronate, supinate or be stable? Why? -
Regards,
Dean -
Excellent! Another perfect answer. I am impressed.
Here's another question:
If Force A in the illustration is non-vertical, and directed superior-medially, will this change in angle of the vector of Force A cause increased STJ supination moment, increased STJ pronation moment or cause STJ rotational stability? Also, will the inclination angle of the STJ axis have any affect on this? -
If Force A was directed superior-medially I would believe this would result in less pronation moment at the STJ axis, leading to an overall increase in STJ supination moment occurring. I say this because isn't force the greatest when it is applied perpendicular to the point of rotation. The angle of the force vector as you now suggested would decrease the ability of this external moment to create a pronation moment at the STJ axis, and the amount of force would need to be increased to obtain rotational equilibrium here.
The inclination angle of the STJ axis would play an affect. If the angle is increased a greater moment arm would be present for Force A, increasing the amount of pronation moment. And vice versa if the inclination angle was decreased. Furthermore I believe if the inclination angle changed it would also change the direction of motion occuring at the STJ. For example if the inclination angle increased, more transverse plane motion would occur with a decrease in the frontal plane motion.
?
Regards,
Dean -
Attached is a paper that I did with Bart Van Gheluwe and Friso Hagman (two biomechanics researchers from Belgium) that uses this concept to determine the mechanical effects of simulated genu valgum on the foot (Van Gheluwe B, Kirby KA, Hagman F: Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait. JAPMA, 95:531-541, 2005).
Next questions for you:
How would one orient the line of action and vector of Force A on the second metatarsal head so that it would have the maximum potential to cause a STJ pronation moment?
How would one orient the line of action and vector of Force A on the second metatarsal head so that it would have the maximum potential to cause a pure compression force at the subtalar joint?Last edited: Dec 7, 2007 -
However if you are talking about the force vector being superior-medially directed, then to increase the pronation moment, this vector would need to manipulated so it would be applied closer to perpendicular to the axis of rotation. In this instance conservatively you could use a forefoot valgus pad or full length valgus pad (such as those used for lateral knee OA). This type of padding would not only aid in orienting the force vector more perpendicular to the axis of rotation but also increase its moment arm, resulting in an increase net pronation moment. (A diagram would help here to show you what exactly I mean, although Iam not great at putting my mind on paper with a drawing).
My understanding of the pure compression force of the subtalar joint axis you mean a force which is neither pronating or supinating the STJ. Therefore to obtain this in this situation increased supination moment would need to occur. This could be done by something similar as previously stated but the opposite of; a forefoot varus pad (which I wouldn't think would be ideal in aiding propulsion). This would reduce the affects of the force vector from Force A at causing a pronation moment, resulting in increased supination moment (from the Achilles tendon and shifting the GRF more medially), ideally to the state of compression on the STJ axis, so there is no over-riding supination or pronation moment occurring.
I hope you can decipher that. A diagram would of really helped, but I'm not computer friendly when it comes to them.
Regards,
DeanLast edited: Dec 7, 2007 -
Final questions:
When an amputation of the distal half of the 5th metatarsal occurs, would one expect there to be an increase in STJ pronation moments, an increase in STJ supination moments or no change in STJ moments during weightbearing activities?
Why?
What might happen to a foot clinically over time that has such an operation? -
The amputation of the distal half of the 5th MPJ would reduce STJ pronation moments during weightbearing activities. This is because ground reaction force lateral to the STJ axis causing a pronation moment would be decreased significantly. Leading to STJ supination moments being unopposed.
I would believe several things could happen to this foot over time. As a result of the overwhelming STJ supination moments, chronic peroneal overuse would occur. Lateral ankle sprains would be a problem. This could also lead to a plantarflexed 1st ray due to the increased tension of the peroneus longus and to promote ground contact of the medial forefoot. The peri-amputation area would be a high pressure area as the ground reaction force would be applied to a lesser area, possible resulting in hyperkeratotic disorders etc. The centre of pressure line would also be disrupted, I'm gathering it would be delayed in going medially in the forefoot due to loss of the effect of ground reaction force on the lateral forefoot (???).
I guess the list could be endless, depending on foot type and other biomechanical factors??.
Regards,
Dean -
Dean:
Again, your are right on. The removal of the distal half of the second metatarsal would lessen the contribution of this segment of bone to the STJ pronation moments acting on the foot so that, all other factors being equal, reduced magnitudes of STJ pronation moment and a more supinated foot would tend to result. The likely scenario of this net gain in STJ supination moment from 5th metatarsal partial amputation would be the following:
1. A shift in the center of pressure to a more lateral position on the plantar foot.
2. A significant increase in ground reaction force on the 4th metatarsal head (may lead to hyperkeratotic lesion/ulcer at 4th metatarsal head).
3. An increase in supination instability of the foot (may lead to increased tendency toward inversion ankle sprains).
4. Increased contractile force in the peroneal muscles during weightbearing activities (may lead to peroneal tendinopathy).
5. Increased external dorsiflexion moments at the calcaneo-cuboid and 4th metatarsal-cuboid joints (may lead to lateral dorsal midfoot interosseous compression syndrome).
You only finished your degree a few weeks ago, Dean?? I just wish I had a classroom full of students like you over the past 22+ years that I have been teaching these same subjects both here in the States and internationally. I guess I'm going to have to come up with some harder questions for you next time! -
Again thankyou for the time spent. Look forward to further discussions.
Regards,
Dean -
When you get around to wanting to do something useful with your superior knowledge in biomechanics, you should consider writing a paper for publication in one of the podiatry journals. I am serious....no joke.
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