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The role of tibialis posterior fatigue on foot kinematics during walking
Michael B Pohl , Melissa Rabbito and Reed Ferber
Journal of Foot and Ankle Research 2010, 3:6doi:10.1186/1757-1146-3-6

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anyone have this paper? I'd like to read it all.
Sounds like they used "normal" feet to fatique and that in my mind is why the experiment failed.
thanks
Bruce

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Bruce E. Williams, D.P.M.
Breakthrough Podiatry

Bruce,

JFAR is open access - full paper here: http://www.jfootankleres.com/content...7-1146-3-6.pdf

Ian

Bruce

I read this paper yesterday and they do point out the shortcomings of their research, which includes the fact that they studied feet of young people with what might be termed normal feet. They also considered the redundancy factor of the whole set of inversion or supinating muscles. I thought it was a good paper and did do what it set out to do within the limitations of the research question.

What I feel they did not consider was the external moments about each STJ during stance and the relative internal moments required to balance them.

So if the external moments could be balanced by only using 10% of the initial max force of MVC of the post tib, then fatiguing the muscle to allow only 70% of the MVC will still give it plenty of force available to keep the STJ with the same or similar position and angular velocity.

I think a table showing the deflection excursion of all and each subject would have been useful so that we could see more precisely how individual results varied.


Cheers Dave

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

I think the cocept that not all feet are equal is a concept that Root Orien, Weed, et al got right. Their foot clasification system was a good first attempt at categorizing feet. I think STJ axis position is something that would make a huge difference in STJ moment from the ground and that would have a large effect in a study looking at posterior tibial muscle fatigue as it relates to gait and stance.


Regards,

Eric

Dave:

I agree with Eric. Your mechanical analysis is "spot-on", as you British lads like to say.

A foot with a normal subtalar jiont (STJ) axis location, let's say with a 3.0 cm supination moment arm for the posterior tibial tendon, would not need much contractile force to generate the internal STJ supination moments to allow normal walking. And the patient with a medially deviated STJ axis, let's say with a 0.5 cm supination moment arm for the posterior tibial tendon, would likely be maximally pronated at the STJ throughout midstance and would therefore be deriving most of their internal STJ supination moment from the floor of the sinus tarsi of the calcaneus, rather than from their mechanically-disadvantaged posterior tibial muscle.

This brings me to an important point that I have mentioned before here on Podiatry Arena, but I think is worth bringing up again, especially considering that this mechanical fact is not widely understood by most clinicians. The point is that when we are doing muscle testing on patients, we are not actually measuring the contractile strength of the muscle in question, but we are rather measuring the contractile strength of the muscle and the length of the muscle moment arm to the joint axis.

In nearly all lower extremity joints, the length of these muscle moment arms are fairly constant from one size of foot/leg to another of equal size. However, for the peculiar case of the subtalar joint, the moment arms for the posterior tibial tendon, for example, may vary by at least a factor of 10 when comparing a foot with a medially deviated STJ axis to a laterally deviated STJ axis. This has a huge effect on the apparent muscle strength of the posterior tibial and peroneal muscles, for example. There is simply no other joint within the human body like the subtalar joint that demonstrates these wide variations in axis spatial location relative to muscle tendon moment arms of its prime movers when comparing one foot to another. In addition, there is simply no other joint within the human body like the subtalar joint that demonstrates these wide variations in axis spatial location relative to muscle tendon moment arms of its prime movers within the normal the range of motion of that joint.

Until biomechanics researchers start realizing that the STJ axis is constantly moving and has wide interindividual variation in spatial location and stop assuming that the primary inversion-eversion axis of the foot is an unmoving axis that always exists along the midsagittal bisection of the foot in their kinetics studies, then they will continue to have difficulty in understanding the questionable results of their kinetic studies of the human rearfoot.

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Sincerely,

Kevin

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

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Why do we test tib post dysfunction by asking our patient to do a single heel lift?

Because we do tests we were taught without thinking about it. I've seen people with essentially no posterior tibial strength do a single leg raise on the affected side. It's rare, but you'll see it. Now, if you ask that person to stand on one leg and supinate their foot, they won't be able to do it, if they keep their forefoot on the ground.

With PT dysfunction you get rapid STJ pronation. When you have rapid STJ pronation it will often be stopped by the medial forefoot hitting the ground. This will put high forces on the first met head and stress the structures of the first ray. You often see medial column faulting with long term PT dysfunction. A medial column that hurts when you try to lift your heel off the ground is probably what limits the single leg raise.

The PT muscle is at best a very weak plantar flexor of the ankle. PT dysfunction does not directly effect the normal ankle plantar flexors. There is no reason for the development of weak ankle plantar flexors other than disuse because it hurts to walk when you lift the heel off of the ground.

Regards,

Eric

Thanks Eric,

Given the size of the tib post muscle I wonder is it really capable of controlling pronation when walking and/or running?

Depends on the length of the Lever arm to the STJ axis, strength of muscle etc. The more reduced the lever arm length the less mechancial advantage the more load on the muscle during STJ pronation the greater the work required to cause an internal STJ supination moment.

there will lots of discussion in these threads for you to read threads on PTTD

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Mike Weber.



Crazy Mary

Also, remember it is a multi-redundant system: it is not working alone.

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

In addition to the other comments, remember that the posterior tibial muscles is a pennate muscle. This increases its physiological cross sectional area. What pennation does is reduce the distance the muscle can shorten, but increase the force of the contraction. I wouldn't look at the size of the muscle, but the results of contraction of the muscle. Can people choose to slow their pronation more? If so the muscle is strong enough. (You could try the thumb tack orthotic. Put a tack pointy side up under the medial arch. Can you prevent the arch fom contacting the point for an extended period of time?)

Regards,

Eric

Good point about it being pennate..I'll have another look next week when I'm dissecting again.
Do you think an FHL pathology would give the same 'heel stuck to the floor' result when testing a single heel lift as per my question re tib post?

FHL = Functional Hallux Limitus or = Flexor Hallucis Longus.

The FHL muscle is a plantar flexor of the Ankle, but creates a much smaller plantar flexion moment than Gastroc and Soleus muscles because of much smaller muscle mass (and physiological cross sectional area) and much smaller lever arm at the ankle. I just saw a patient who was post Achilles tendon rupture, poorly treated, who over a year has developed a hallux hammertoe. This is what you would expect if you don't get any plantar flexion power out of the Achilles and you try to get plantar flexion power out of the FHL muscle. She, couldn't do a single leg raise on the ruptured side and had no plantar flexion strength from the Achilles.

Regards,

Eric

Sorry, I meant Flexor Hallucis Longus:
So would you conclude a FHL tear/tendinopathy could prevent single heel lift due to pain inhibition rather than impaired muscle function?

Interesting question. In theory you could not contract the muscle as the ankle moves, so there would be very little stress on the tendon. On the other hand I ruptured a plantaris tendon and could not dorsiflex my ankle past perpendicular without severe pain. When looking at cadaver feet, when you move the ankle joint the FHL tendon does not move that much (but more than the peroneals or posterior tib and FDL). Those other muscles do very little at the ankle.

Regards,

Eric