Foot orthoses reduce internal supination moment- how?

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All,

One of the consistent findings that appears to be emerging from studies of the kinetic effects of foot orthoses seems to be that foot orthoses reduce the internal supination moment- right?

So, it may be argued that by the addition of increased external supination moment from foot orthoses, the body can "rest" and "reduce stress" on the tissues that generate internal supination moment such as the tibilias posterior. Indeed, it may be argued that this mechanism is responsible for the success of foot orthoses in the treatment of tibialis posterior dysfunction. So how do we explain results from EMG research that suggest:

"Tibialis Posterior (TP) activity was recorded from five participants (age range: 25–69 years) with flat-arched foot posture using three different styles of foot orthoses. This study found no systematic changes in TP EMG with the three types of foot orthoses. A similar result was reported for another study investigating three styles of athletic footwear, each with a custom-made midsole aimed at inducing foot pronation and supination during running. This study investigated TP EMG amplitude and temporal characteristics in 10 males (average age: 27 years), however no significant changes were reported for TP EMG among the three shoe styles." http://www.jfootankleres.com/content/2/1/24 ???????

Here is the abstract to the foot orthoses study quoted above:
http://www.sciencedirect.com/scienc...serid=10&md5=9a34a5f1009546fb6a6d953144085302

And here be the running shoe study:
http://www.jelectromyographykinesiology.com/article/S1050-6411(05)00116-1/abstract

These studies appear to question the tissue stress approach to podiatric biomechanics (at least in the treatment of TP dysfunction).

Have a nice day ;-)

 

Two possible reasons:
1. We do not know if the foot orthoses used had design features that actually were aimed at reducing the supination or inversion moments (and it the moments were actually reduced; and if they were or were not did that correlate to the post tib EMG)
2. None of them took into account the subtalar joint axis position. We have some pretty sound theoretical rationale about STJ axis position and the posterior tibial tendon lever arms.

 

Interesting point! If post tib EMG research does not take into account the lever arms, how relevant is that research going to be?

 

An anecdotal experience that shaped my thinking on how orthotics work. Patient with PT dysfunction. Dispensed devices with med heel skive and high medial arches and pain in post tib tendon increased. Remade devices, kept skive but lowewred arch height and PT pain went away. Came back six months later and wondered if it would be ok to wear the high arched devices. I was about to say no, but then she added she had been wearing them for a month and liked them better because she felt they gave her better support.

From this experience, and a few others, I concluded that there is a pain/discomfort avoidance (CNS mediated) response to standing on a device that adds pressure under the medial arch that causes the posterior tibial muscle to increase its activity. This is often a good thing in most cases. However, this can be a bad thing in some cases, especially PT dysfunction.

In relation to this thread, there will probably be some individuals who may have an increase in posterior tibial activity with an orthosis, especially those with high arches and without a medial heel skive. When experimenting with orthoses it is critical to describe how the orthotic was made (as Craig pointed out.) There is more than one way to supinate a STJ.

So, it is still concievable that a medial heel skive could shift the center of pressure more medially on the heel and this would reduce external pronation moment on the STJ even though the EMG results from the above study does not agree with this.
Cheers,

Eric

 

Good discussion topic, Simon.:drinks

One other possible factor is that, compared to inverse dynamics research with force plates and 3D motion anaylsis systems which indirectly give us information on internal rearfoot inversion moments (these studies don't really give us true internal STJ supination moments since STJ axis spatial location is unknown), performing an EMG on a posterior tibial muscle doesn't necessarily correlate directly with the tensile force within the posterior tibial tendon at any instant in time. The PT tendon may have a lot of tensile force within it due to passive eccentric stretching of the tendon with only a relative small amplitude of EMG activity of the PT muscle.

In other words, it would very interesting to see how closely quantitative EMG assessment of the posterior tibial muscle correlated to posterior tibial tendon tensile forces....I'll bet the correlation is not that great. Plus the electromechanical delay (i.e. time from EMG signal initiation to tensile force development in muscle) of approximately 50 msecs may also have an effect in assessing PT muscle function relative to EMG activity.

 

My understanding is that EMG- force response is curvelinear- EMG tracks force output at lower levels (dont know where that cut off is, and would presume big inter-individual variation), but gets very fuzzy as MVC progresses. Also, concentric actions produce relatively more EMG per unit F compared to eccentric ones. Good for sequencing and timing, not so good for F! As KK also notes, net tensile stress is a function of passive and active contributions. If pathology were in the passive structures- eg esp the bone, does the lack of EMG change in the presence of reduced internal inversion moments support the TSM? (Can you actually reduce stress in say bone, and not the muscle attaching to it??)

Given the flat footed study population, you might presume a medially deviated STJA. With a smaller lever arm, does Tib Post work harder or just give up?? Further, the presence or absence of pain must be a massive variable in the CNS (read motor) response to changing inputs.


Scattered thoughts from a virgin poster.

Jason R

 

1. We do know that: "The results show that the combined molding and posting foot orthosis significantly reduced eversion and eversion moments during walking compared to a posting type and a proprioceptive orthosis in several test parameters." so we can probably take the leap of faith to say that they were trying to reduce external pronation moment.
2. Indeed, and if eversion was reduced then the medial excursion of the STJ axis should also have been reduced resulting in an increased lever arm for the TP.

 

So what we are saying then is that an inverse dynamics approach is fundamentally flawed because we are using the long axis of the foot rather than the STJ axis and EMG is a pretty blunt instrument too. So how do we quantify the direct and indirect effects of foot orthoses? Moreover, how do foot orthoses reduce internal supination moment? It seems that the effect of foot orthoses on the EMG activity of tibialis posterior is variable and other EMG studies suggest an increase in tibialis anterior activity in association with foot orthoses, so how is that reduction in internal moment achieved?

 

I think we can't rely necessarily too heavily on either inverse dynamics or EMG if our goal is to know whether the posterior tibial tendon has increased or decreased tensile force with or without different types of foot orthoses. In inverse dynamics, it could be any structure causing the rearfoot inversion moment: the floor of the sinus tarsi of the calcaneus, the PT, FDL, FHL, and/or the gastrocnemius-soleus muscles or deltoid ligament. Also, since in inverse dynamics the inversion-eversion axis chosen to be at the intersection of the sagittal and transverse plane passing through the middle of the foot, the true inversion-eversion axis of the rearfoot, the subtalar joint axis, could be off as much as 35 degrees in some feet within the transverse plane and could be displaced a couple of centimeters medial or lateral to the position of the axis chosen by the software of the computer. However, at this time, inverse dynamics is the best we have to go with other than EMG which has it's own problems or other than putting strain gauges within the posterior tibial tendon which you could probably only do in live subjects in Sweden.

 

OK, so given the degree of potential error that exists in the inverse dynamics research, how much certainty can we give to the observation of reduced internal supination moment in association with foot orthoses?

The more I analyse foot orthoses research, the more I think it is all so flawed that it is virtually worthless

Carl Rogers suggested that the only thing of value in ones education is personal experience......

 

Simon,

Good discusion point...

Although I really know this has been said in other threads, let me clarify that inverse dynamics calculates the "net internal moment" from external ground reaction forces and aceleration of the center of mass of the desired segment. The point here is that "net internal forces" could be derived knowing the external forces acting on the segment, the mass of the segment and the aceleration of the desired segment. The term "net" implies the sum of all the structures producig an internal forces/moments acting on the segment. Some of the biomechanics laboratories has used the term "muscle" moments (refering to net internal moments) which implies that the muscles are the main responsible of that phenomenon which is true for some joints such us the hip in the sagital and frontal plane but it could be some misleading for other joints and planes.

This could be a problem in inverse dynamics interpretation as it does not specify the structure producing the internal moment on the joint (muscle, capsule-ligaments or bone to bone force). At the same time, co-contraction can be involved in the interpretation of the net internal moments, i.e. an increase in ankle plantarflexion moments could be derived from an increase in the activity of ankle plantarflexion muscles or a decrease in the activity of ankle dorsiflexion muscles. Research in the kinetics effects of foot orthoses showed that they decrease the "net internal inversion moment", which could be a decrease in the activity of the inversion muscles, a decrease in the tensional force of the capsular-ligament structures, a decrease in the bone to bone pronation forces or an increase of the activity of pronation muscles.

In this context the results of the studies you mentioned are not a problem to the interpretation of the effects of foot orthoses effect in the treatment of posterior tibial tendon if we agree that the main problem in PTTD is the pronation forces the foot is suffering during daily activities instaed of a problem in the posterior tibial tendon itself. Lots of my patients with PTTD do not have pain in the tendon itsef and they do have pain in the medial plantar area of the arch as the foot is deforming, probably derived from soft tissues damage in the plantar medial area of the foot. Those patients can have a benifit reducing the net internal moments without reducing the activity of the PT muscle.

On my opnion, the problem of the axis location is not really relevant to this point if we assume the axis has not have great displacements during the stance phase. I mean, taking the subtalar joint axis as the being fixed located in the axis of the foot (from 2º-3º metatarsal to the center of the heel), foot orthoses by means of inverse dynamics reduced the net internal moment. This is because they increase the external supination moment (the center of pressure is more medial to the axis)and/or reduce the pronation aceleration of the foot. If we take the real position of the axis in each subjetc instaed of the axis of the foot, the results would probably have been the same as foot orthoses would increase the external supination moment (the center of pressure is more medial to the axis) and/or reduce the pronation aceleration of the segment (foot) unless the axis experiences great displacements over the plantar foot during the gait cycle.

Really, really nice discusion point. Sorry for the very long post...

 

I did a mistake here...

foot orthoses by means of inverse dynamics reduced the net internal moment. This is because they increase the external supination moment (the center of pressure is more medial to the axis)and/or INCREASE the pronation aceleration of the foot.

Sorry...

 

Like Javier said, I think we can assume that the posterior tibial muscle tendon has less tensile force in it with an anti-pronation orthosis because patients generally report a decrease in PT tendon pain within a few seconds of wearing an anti-pronation foot orthosis. Certainly, with this clinical correlation to the inverse dynamics research, I am confident that the PT tendon has less tension with an anti-pronation orthosis and that the decrease in internal rearfoot inversion moment seen in many orthosis research studies is at least partially caused by a decrease in tensile force within the posterior tibial tendon when an anti-pronation orthosis is worn. I think that also, if EMG studies could be done on the posterior tibial muscle with proper anti-pronation orthoses and without pain from the deeply implanted EMG lead necessary for studying this particular muscle, that these studies will also eventually show a decrease in EMG activity with anti-pronation orthoses.

Great discussion!:drinks

 

Javier:

I think you were right with your original statement. An increase in external STJ supination moment would tend to decrease the pronation acceleration of the STJ.

 

Kevin,

In inverse dynamics internal and external moments are in the same equation (Internal moments + External moments = mass moment of inertia*angular acceleraton of the segment).

A decrease in the net internal supination moments on the foot segment calculated by means of inverse dynamics could be derived from an increase in the external supination moment with no change in pronation aceleration, or could be derived from an increase in pronation aceleration with no change in the external moments, or some type of combination of both effects.

An increase in external STJ supination moment would tend to decrease the pronation acceleration of the STJ, ONLY IF internal moments are the same. Because if the increase in the external moments is acompanied by an decrease in the internal moments in the same magnitud or higher, the segment would be stable or accelerate into pronation. Is important to take this into consideration when interpreting some weird kinematic effects seen in foot orthoses research.

Cheers

 

Javier:

I think we are on the same page. I thought you were saying that an increase in external STJ supination moment would cause a increase in pronation acceleration of the foot. I think I misunderstood you from the way it was written.