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All, just been going back over the gastrocnemius: supinator or pronator thread. Kevin's post there got me thinking:
Thinking out loud: The direct effect of the peroneus brevis (PB) on the subtalar joint (STJ) is to cause a STJ pronation moment (due to the lateral location of the peroneus brevis insertion relative to the STJ axis), the indirect effect of PB contractile activity is to increase the ground reaction force (GRF) plantar to the medial forefoot. This increase in GRF on the plantar medial forefoot may either cause a STJ supination moment or a STJ pronation moment, depending on the spatial location of the STJ axis and the location of the GRF vectors on the structures of the plantar forefoot. etc. Right?

So for a given contractile force of either GS or PB there must also be an axial position where we get neither an increase in pronation nor supination moment- right?

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Simon

Not sure if I quite get the gist of your question.

My answer at present is - If the peroneous brevis can apply a force then there must be an equal and opposite force(s) from somewhere and if that PB force creates a moments about a certain point of interest than that reaction force(s) must also produce an equal and opposite moment. Equilibrium exists at all times whether the system is static or dynamic.

Dave

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Descartes seems to consider here that beliefs formed by pure reasoning are less doubtful than those formed through perception.

How do joint segments accelerate if equilibrium exists at all times?

__________________
Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Dave:
I think what Simon is saying (and I'm not trying to speak for him, especially when it comes to biomechanics........but since Kevin must be busy seeing patients SOMEBODY has to chime in!!!!!!!) is that due to the change in spatial location of the STJ axis a constant force vector will change in relation to it. Thus, depending on the axis location (which is dynamic) the force (in this case the PB) can go from being an everter to an inverter THUS as some point in time the force applied will be neutral.

Steve

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DrSArbes
Fellow American College of Foot & Ankle Surgeons
Board Certified Foot & Ankle Surgery, ABPS
Adjunct Professor OCPM
Green Bay, Wisconsin, USA

I've thought a lot about this for a long time. In my residency I took an amputated foot and dissected it and pulled on the tendons. When held in such a position that the first ray was not on the ground, the lateral column was on the ground, and the tendon was pulled, the first ray plantar flexed. When the first ray was touching the ground, eversion of the rearfoot was more obvious.

Also, I made a presentation on this at a PFOLA conference where I made a model of the STJ axis and the first ray and peroneus longus muscle. The axis was variable. When there was a laterally positioned axis plantar flexion of the first ray caused supination of the STJ. When the axis was centrally positioned, the first ray remained stationary, as the STJ everted. With a medially positioned STJ axis the first ray dorsiflexed as the STJ everted. So, the function of peroneus brevis is very dependent upon STJ axis position.

The peroneus brevis muscle simultaneously causes a plantar flexion moment of the first ray and a pronation moment at the STJ. The joint that moves is the one with the least resistance. With a laterally positioned axis, plantar flexion of the first ray (or more precisely a plantar flexion moment on the first ray that is resisted by ground reaction force.) will shift the center of pressure more medially, which will cause an increased supination moment from the ground. If the center of pressure is far enough medial to the STJ axis then the supination moment from the ground will overcome the pronation moment from the muscle.

Yes. See explanation above. There has to be a crossover point.

Cheers,

Eric

Eric,
Just to confirm, you are talking about peroneus brevis and not peroneus longus above. Right?

__________________
Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Simon:

This is an excellent question and analysis. The lesson here is that we must consider both the internal moments from muscle contractile activity and the external moments that come from the interaction of the plantar foot with the ground, or ground reaction force (GRF), if we are to determine the true function of one of the extrinsic muscles of the foot.

In the example of the peroneus brevis (PB), the internal moment will always be a STJ pronation moment since there is always a pronation moment arm for the PB tendon due to its insertion on the 5th metatarsal base. However, if PB contractile activity increases the medially located GRF, then it is possible that the external moment could be a STJ supination moment if the center of pressure (CoP) becomes medial to the STJ axis as a result of the internal STJ pronation moment from PB contractile activity. Of course, as Simon mentioned, the STJ spatial location will largely detemine what magnitude of PB contractile force is required to cause the foot to reach that STJ equilibrium condition, before the maximally pronated STJ position is reached (and the STJ no longer can rotate).

In regards to Dave' comments, if there is no rotational acceleration of the STJ, or in other words, if the STJ is static or rotating at constant velocity, then the conditions of rotational equilibrium could apply. However, if the STJ is accelerating or decelerating under the influence of PB contraction, then the conditions of equilibrium could not strictly apply, but would need to be estimated under the constraints of quasi-static biomechanical analysis.

Hope this helps.

__________________
Sincerely,

Kevin

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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

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Simon

you wrote
[quote]How do joint segments accelerate if equilibrium exists at all times?[quote]

Kevin replied
Kevin's comment is contentious but essentially correct. The inertial force is considered in all calculations then consideration of all forces in the system will return a sum of zero or equilibrium, which is a proof that the calculations are correct. But let's not focus on that point since I'm keen to explore the OP.

Dave

__________________
Descartes seems to consider here that beliefs formed by pure reasoning are less doubtful than those formed through perception.

Steve, very nearly. What I was saying was that PB can simultaneously exert both pronatory (internal) moments and supinatory (external) moments about the STJ. That the net effect of these will be dependent upon STJ axial position.

Now let me talk you through that osteotomy... ;b)

__________________
Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Oops,

Sorry, that should be longus. I'll edit and fix it. Most of the discussion applies to both except for the direct plantar flexion moment from the muscle applied to the 1st metatarsal.

Eric

So, to restate that which has already been stated: in a foot with a laterally deviated STJ axis, gastroc-soleus (GS) external moment is unlikely to pass onto the lateral side of the axis and the peroneus brevis (PB) external moment will be relatively higher due to the greater lever arm it can exert on the medial side of the axis. Therefore, the STJ supination moments should be greater in this foot when compared to a foot with medially deviated STJ axis. By contrast, a foot with a medially deviated STJ axis should have the GS external moment lateral to the STJ axis (= pronation moment) and a greater lever arm for the PB internal moment versus it's external lever arm (= pronation moment > supination moment). So in the foot with medially deviated axis, the phasic activity of the PB may well be one of the causes of late stance phase pronation.

How elegant your system of biomechanics appears Prof. Kirby. To which you answer:
"We call that physics"

Nice to just check your own understanding every once in a while.

Now lets start to bring in the other muscles.. you were talking about peroneus longus, Eric....

__________________
Who? What? When? Why? Yeah? And? So? What?

"My mission drive is to open up my eyes, 'cause the wicked lies and all the sh!te you say..." http://www.youtube.com/watch?v=V4NW5S1UTPQ

"Science is the antidote to the poison of enthusiasm and superstition."

Simon:

This is good stuff. Never really considered the peroneus brevis specifically, until you asked the question, but when answering your question it seems quite clear to me that this separation of the STJ and ankle joint moments into both internal and external moments is not only a good way to analyze the mechanical effects of contractile activity of the extrinsic muscles of the foot but is probably the best way to analyze their mechanical actions.

Now, do the same analysis for the gastrocnemis and soleus relative to STJ axis location using internal and external moments for clarification and you have come up with an answer to the age-old question of why an "equinus deformity" causes excessive STJ pronation in one foot but may also cause increased STJ supination and early heel-off in another foot. Eric Fuller and I discussed this a little in our chapter on Tissue Stress (still unpublished) that we wrote about four years ago and I have also discussed these topics a little in my newsletters.

A complete discussion of these ideas would be a great article for JAPMA since this method of analysis is very powerful and may be the next big step forward for us in understanding the function of the foot and lower extremity during weightbearing activities.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

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Hi Simon,

One difficulty of using the terms external or internal moment is that you have to have defined the system that you are talking about. For example, tension in the peroneus brevis tendon will be an internal moment if you define the system as the foot and leg wheras if you define it as just the bones of the foot it would be an external moment.

I think that the best solution to this is describe the force in terms of the structure that applies the force and then describe the structure the force is applied to. (This is what is supposed to be done in free body diagram analysis.) For example, Tension in the peroneus brevis tendon applies a force to the base of the fifth metatarsal. This force, whose direction is in the direction of the tendon, will create a pronation moment about the STJ axis, by inspection of the location of the force relative to the STJ axis. This force, by inspection, will cause a very small moment about the ankle joint axis, because the line of action points very close to the ankle joint axis.

I agree that there should be a term for what a muscle does to the position of ground reaction force. For example, tension in the Achilles tendon will cause a plantar flexion moment at the ankle joint. A plantar flexion moment at the ankle joint will shift the center of pressure anteriorly. This anterior shift in the center of pressure will change the moment from ground reaction force around various joint axes. The change can be calculated by examining the location of the line of action of force relative to the location of a particular joint axis. Here's a suggestion for that term. The change in location of the center of pressure of ground reaction force caused by muscle activition. (Suggestions for other terms welcome.)

The Hicks experiment was a guite elegant demonstration of this and it is still as good today as it was in the 1950's. I learned more about muscle function from that article than from any other source. I was amazed at how much I could still get from it on the third read. Sorry, don't have the exact site for the article but it was something like the function of the foot IV: the action of the muscles. I believe it was published in 1954. JH Hicks was the author.

Regards,

Eric Fuller

Eric and Colleagues:

The terms "internal moment" and "external moment" are commonly defined in most biomechanics labs as being as I suggested: internal moments being those moments due to forces acting from within the body and external moments being those moments due to ground reaction force or other forces acting externally on the body.

In fact, one of the main current confusions in foot and lower extremity biomechanics research in studies that use inverse dynamics is that joint moments are most commonly reported as internal moments, not as external moments.

For example, if, in a force plate study, the center of pressure (CoP) was lateral to the bisection of the foot and no rearfoot movement occurred, then this would be reported as an "inversion moment of the rearfoot" since internal moments are standardly reported by the software packages that perform inverse dynamics analysis (so I have been told). [Internal rearfoot inversion moment in this case may have been due to posterior tibial muscle contraction, deltoid ligament tensile force, etc.] However, before a few years ago, I would have said that this lateral position of the CoP relative to the foot would have caused a subtalar joint pronation moment since I have, for many years, used the convention of describing moments about joint axes of the body as external moments from the effects of ground reaction force, for example. Now, I try to make the point of describing moments as either external or internal moments to avoid confusion with either the researchers or clinicians, since often times we are speaking very different languages.

At the PFOLA meetings for the last few years, I have had a few conversations with both Chris MacLean and Irene Davis about this potentially very confusing difference in terminology between the clinical podiatry world and the foot and lower extremity research community. As a result, they have tried to make a point of being sure the audience knows whether their results are in terms of internal or external moments, when they present their research data.

Chris Kirtley also has a nice way of describing these concepts in this excerpt from his book, Clinical Gait Analysis.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
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Kevin, I'm not sure everyone else will be clear on where I thought the difficulty was. In performing free body diagram analysis you have to define the body. It is quite easy to tell what are internal forces and what are external forces once you have defined what the body is. There is a tendency for medical professionals to think of the body being the whole human body. In engineering it is common to examine just part of a structure and define that as the "body". An example of that would be to examine just the foot "cut off" at the ankle. Or sometimes two objects can be defined as "the body." An example of that would be a bicycle and rider. (Thought experiment: what is the external force that accelerates the system of bicycle and rider.)

The concept of internal and external are somewhat helpful, but not really necessary and as you pointed out, sometimes confusing. Just define your "body" and then add in the forces that are applied to it. If we examine the foot "cut off" from the body. There is ground reaction force. This force is actually two equal and opposite forces. (Newton's third law). There is a fore applied to the foot from the ground and there is a force from the ground applied to the foot. So, if we are examining the foot we need to take the force from the ground applied to the foot and we ignore the force from the foot applied to the ground.

When the center of pressure of ground reaction force is found, we can then calculate moments from ground reaction force. When the CoP is anterior to the ankle joint the force from the ground acting on the foot will cause a dorsiflexion moment at the ankle. If the foot is not moving, it is in equilibrium and there should be no net moment acting on the foot. If we define the body as the whole body, there is an internal plantar flexion moment at the ankle. If we define the body as "just the foot" then there is an external plantar flexion moment, at the ankle, acting on the foot, from the structures touching the foot. (e.g. tibia and Achilles tendon) The same moment is internal with one definition of body and external with another definition of body.

So, using the term internal moment, can be a convenient short hand, but the best way to accurately describe what is happening is to describe the force or moment from one object applied to the other object. Example: When the center of pressure of force, from the ground is applied to the foot, is lateral to the projection of the STJ axis, that force will cause a pronation moment applied to the foot about the STJ axis. Yes, it is long, but sometimes you have to be wordy to be precise.

Regards,

Eric

Eric:

For the two or three others who are still following along.............

I can't disagree with you regarding your analysis. I understand what you are saying, but it is really a matter of establishing proper definitions for our terminology so that researchers and clinicians can all understand each other. So far, over the past decade, the convention used in the majority of foot and lower extremity biomechanics/foot orthosis research literature is to report internal moments, not external moments, when inverse dynamics analysis is performed on the joints of the foot and lower extremity.

I just wanted to point this fact out to the others following along so that confusion of terminology is minimized. I know, I am often confused by the results of this type of research unless the researchers have made it very clear as to whether they are reporting internal or external moments in their research. .

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************