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Kevin and all

This has been put to me by a colleague i.e . that it is possible for the STJ to be pronated by the action of the peroneals when the STJ axis is laterally deviated.

The assumption is that excessive relative supination moments in early stance phase due to STJ lateral position causes early and over activation of the peroneals thru the stance phase and so the foot becomes pronated in the mid to late stance phase.

This was described to them by a college friend who apparently had seen the explanation of this action by Kevin Kirby at a Biomechanicsl summer school.

The prescribed way to reduce the pronation is to use a lateral rearfoot post to equalise the moments about the STJ by GRF instead of peroneal action and thereby restoring the normal phasic firing pattern of the peroneals.

Can someone explain this for me as I cannot imagine how this works. I could imaginge this if the STJ axis were laterally translated but medially rotated so the GRF caused a net supination moment at early stance and a net pronation moment at mid - late stance but would you say this is due to excessive peroneal activity or moments due GRF CoP position relative to the STJ axis.

all the best Dave Smith

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

Dave:

We have been discussing a similar mechanism with Bruce Williams in another thread.

We must remember that STJ axis spatial location that we determine in a non-weightbearing setting may not be the same that we see in a static weightbearing setting either due to differences in forefoot inversion/eversion relative to the rearfoot, differences in STJ rotational positions or due to inverter/everter muscle activity during standing. For example, many feet with laterally deviated STJ axes in non-weightbearing exam will appear to have normal STJ axis spatial location during standing due to tonic peroneal muscle activity.

If a patient has a laterally deviated STJ axis at the rearfoot and forefoot, one may see these feet in early stance in a more supinated position at the STJ but then undergo STJ pronation in late midstance. John Weed, DPM, often lectured to us as podiatry students about this clinical observation seen in "rigid forefoot valgus" feet and he thought it was due to increased peroneal activation in late midstance. I believe this phenomenon occurs since the central nervous system (CNS) detects that the lateral STJ axis combined with the relatively medial center of pressure (CoP) is causing so much external STJ supination moment in late midstance that unless it activates the peroneals with more efferent output in late midstance, then the foot may develop supination instability, and cause an inversion sprain/injury.

However, if the patient has a lateral axis at the rearfoot and a medial axis at the forefoot (i.e. abnormally adducted and laterally translated STJ axis), or has a reduced medial forefoot dorsiflexion stiffness so that during late midstance the medial forefoot is dorsiflexing excessively under increasing load from ground reaction force (GRF), then these feet may develop late midstance pronation simply due to the external STJ pronation moments being created by GRF, and not necessarily due to increased efferent output to the peroneals from the CNS.

The above theories fit best with my clinical observations thus far. Hope this makes sense.

__________________
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
**************************************************

Dave, we also need to be mindful of both the internal and external moments produced by peroneal activity as we discussed here:

http://www.podiatry-arena.com/podiat...eroneus+brevis

__________________
Science is the antidote to the poison of enthusiasm and superstition

My location

Foot A: medially deviated STJ axis. In gait at heel off the Achilles tendon shifts the center of pressure forward. This forward shift of the center of pressure increases the pronation moment from the ground. This increase in pronation moment from the ground is greater than the supination moment from the Triceps surae muscles. So no additional pronation moment from the muscles is needed.

Foot B: laterally deviated STJ axis. In gait at heel off the Achilles tendon tension shifts the center of pressure anteriorly. If the center of pressure is close to the axis there will be a very small pronation moment from the ground. The tension in the Achilles tendon will cause a larger supination moment than there is pronation moment from ground reaction force and if no other muscles were acting then there would be supination of the STJ because there is a net supination moment.


So, to prevent uncontrolled supination the patient could increase their peroneal activity to one of two points. It could increase the activity to the point where the supination moment from the Achilles is just matched by the pronation moment from the peroneal muscles. Or, the peroneal muscles could be activated to the point where the STJ becomes maximally pronated and there is some supination moment from the end of range of motion of the STJ in the floor of the sinus tarsi.

It could be very difficult to contract the peroneal muscles to create an net zero moment about the joint, because the moment from ground reaction force could be variable. So the peroneal tension would have to vary each step with the variation in ground reaction force. This variation would have to be anticipated in advance because of the time needed for a reflex arc may not be short enough to react to changes in moment from the ground. This choice for peroneal activity may feel "too unstable" for the patient to prefer this option. (Unexpected changes in moment from ground reaction force will cause dramatic changes in the movement pathway.)

Constant peroneal tension that would keep the STJ in the maximally pronated position would create a more consistent gait path. If the peroneal muscles kept the STJ in a maximally pronated position, there could be variation in the moments without variation in position. To explain that further: On a particular step, there is a 50 Nm supination moment from the Achilles tendon a 10 Nm pronation moment from the ground and 65 Nm pronation moment from the peroneal muscles and then there would need to be a 15Nm supination moment from the floor of the sinus tarsi. (Kevin has called this last moment residual pronation moment) Now, if there was a change in surface that was stepped on, there may now only be a 5Nm pronation moment from the ground. So, with constant peroneal activity there would only be a 10 Nm supination moment from the floor of the sinus tarsi to maintain equilibrium at the STJ. (Numbers pulled out of thin air for this example.) So with a change in moment from the ground, there is no change in the movement, but there is a change in internal moments. This is how I explain late stance phase pronation caused by muscles.

It would be easy to evaluate this effect. Take some folks with Late stance phase pronation and measure their peroneal activity with and without a valgus wedge and compare that to the motion seen. There has been some work done on variation in post tib and peroneal activity in gait. Matsusaka N. Control of the medial﷓lateral balance in walking. Acta Orthopaedica Scandinavica. 57(6):555﷓9, 1986 Dec.

Hope this helps,

Eric

Eric

Yes thanks for that excellent and comprehensive explanation: I see the logic of your reasoning here, I am a little sceptical about that statement tho.

Matsusaka N. Control of the medial﷓lateral balance in walking. Acta Orthopaedica Scandinavica. 57(6):555﷓9, 1986 Dec.

Only the papers up to 1997 are available on line so I can't read the whole paper but from the abstract it appears they only made conclusions about the general difference between variation in frontal plane active muscular control of joint stability when compared to saggital plane muscular control. They found a significant 79% increase in the variation of muscle activity comparing saggital to frontal plane. However the abstract does not say what the base line activity variation was for the control. So if the variation was very small then 79% increase although statistically significant may be be clinically insignificant. Do you have a copy of the full paper so we can get a clearer picture?

I have read my reference books and searched for papers on peroneal activity but what I have found only show normal range, or reduced range due to pathology, of phasic firing and possible contraction magnitudes of peroneals based on EMG data.

I'll have to go away for a think and come back later

Cheers Dave

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

Hi Dave,

Unfortunately, I only have a hard copy burried somewhere after a move. It has been probably 10 years since I read the paper. What I do remember was that it was not entirely clear as to exactly was compared to muscle activity. But the point that I got from the paper was that muscle activity varied with some external factor. The EMG pattern was not the same for every step. It makes perfect sense that some steps need more supination moment and others need more pronation moment and that would depend on external variables.

In decerbrate cats they found a reflex walking pattern where there was a consistent EMG pattern. Then someone tried to get the de cerbrarl cats to walk uphill or down hill and there was a change in EMG pattern. Again EMG changes in response to external stimuli.

There is often a reliance on EMG patterns for level walking as a "normal" firing pattern. Is walking on a level surface normal? We have to be able to adapt to different needs for locomotion and to do that we need to use muscles differently each step.

Cheers,

Eric

Eric Here's a pattern that I see, what do you think of this explanation? Subject has Lateral STJ axis, Equinus ankle, foot cannot clear the ground in normal swing thru. Subject pronates the foot during swing thru (everted abbducted and dorsiflexed) The action of peroneus longus also plantarflexes the 1st ray so that at foot strike the into eraly stance the fore foot is extremely valgus and low 1st MPJ. As the 1st MPJ become weight bearing the PL is further tensions as the 1st ray is dorsiflexed. (maybe the PL is actively relaxing but GRF acting on the 1st MPJ is passively tensioning PL) therefore there are continuous pronation moments by the PL force. This scenario leads to FncHL which forces the subject to make a compensation. Some choose to adduct the hip ie translate the pelvis sideways over the stance leg. This moves the CoP laterally and reduces GRF on the 1st MPJ to allow some windlass action (the PL can actively plantarflex the 1st ray) but at the same time increases pronation moments from GRF so the foot is still relatively pronated and has stayed that way right thru stance even tho the STJ axis might be lateral and it might appear that the peroneals are actively firing thru most of the stance phase.

Dave

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

Dave:

The peroneus longus is not a swing-phase muscle and is also a plantarflexor of the ankle joint so I doubt it has any contribution to swing phase kinetics and kinematics. Also STJ pronation normally occurs during early swing phase to help the individual better clear the digits away from the ground without additional hip or knee flexion. STJ pronation motin will occur during swing unless the foot is already maximally pronated at the end of propulsion. It is not until the end of swing phase that STJ supination occurs normally, under the control of the anterior tibial muscle.

__________________
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
**************************************************

Hi Dave,
I agree the pattern is rare, but exists. However, I'm not sure about the explanation. Foot position during swing is a choice. My working theory is that the person chooses the position that is the most comfortable at heel contact.

Another explanation for the extreme valgus forefoot position is that in stance the foot tends to supinate because of the lateral position of the STJ axis. In stance, to get equilibrium about the STJ axis there has to be high forces on the lateral forefoot and low forces on the medial forefoot. With the heel on the ground, in this lateral axis foot, there is supination moment from forces on the heel and medial forefoot. Therefore there must be high forces on the lateral forefoot to get the same moment from both sides of the STJ axis. (the axis could be so far lateral that the peroneals need to be active in stance as well to maintain STJ equilibrium.) When there are chronically low forces on the first ray it will tend to achieve a plantar flexed position.

There is often a misunderstanding of the concept of the Windlass being active. Many think the windlass becomes active when the toe dorsiflexes. The implied part of the statement is that it is not active when the toe is not dorsiflexed. I disagree with that notion. The windlass can create a plantarflexion moment on the 1st ray and a STJ supination moment when the windlass unwinds and toe plantar flexion is stopped by forces from the floor. It is the tension in the fascia that creates those moments and that tension can be present regardless of toe position. This is a true passive system.

This passive tension in the windlass is what causes functional hallux limitus. The motion is limited by a plantar flexion moment acting on the hallux from tension in the plantar fascia. Touch a few arches in people standing. In those that have difficult to dorsiflex halluxes there will be high tension in the fascia before the toe dorsiflexion is attempted.

The peroneus longus, on the other hand, is not a passive system. If there is a force dorsiflexing the first ray it will make the tendon slide and also the muscle belly elongate unless there activation of the muscle belly to produce force. Take one of your arms and move the other arm. You can elongate the muscles unless you choose to resist.

I don't think hip position and center of pressure under the foot are necessarily linked. Yes, if there is increased varus angulation of the leg there will be a tendency to increase force on the lateral foot, but only if you use up all of the foot eversion available. I think it would be easier to increase pronation moments by using the peroneal muscles (longus and brevis).

Cheers,
Eric

The lever arm of peroneus longus to plantar flex the ankle joint is very small. In cadaver dissections that I've done it barely moves with ankle plantar flexion dorsiflexion. I know that peroneus longus ankle plantar flexion function is out there in the literature, but it just can't plantar flex the ankle because it has no leverage.

Regards,

Eric

Kevin and Eric

Sorry to take so long to get back, very busy with other stuff lately.

Kevin you wrote
I know that normally the peroneals only fire from early mid stance to propulsive phase. Are you saying the peroneals never fire in swing phase in any circumstances?
As you say STJ pronation often occurs in swing phase so if peroneals are not firing one must assume that the eversion moment content of pronation are produced by the long digital and hallux flexor. Is this possible if the STJ axis is very lateral? Could the subject, in this case, choose to use the peroneal group to evert the STJ?

Eric
Just to clarify and perhaps expand your explanation, what do you think of the following - The diagrams show a pronated, vertical and supinated stj. The moment arm d of the achilles is long on the short on the supinated case and short in the pronated case, the opposite is true in the case of the peroneal moment arm d1 i.e. the relative length of the moment arms change with joint position.. PS assume the dot on the talus is the STJ lateral axis position.



Obviously the cross section area of the Soleus is much bigger then the peroneal group and so relative max force is potentially much greater in the soleus and this muscle is working hard creating high Achilles tension in the propulsive phase. If, in the laterally deviated STJ axis case, the soleus overpowers the peroneal group then GRF is unlikely to be able to resist supination since its moment arm is small. Therefore it would be advantageous to the peroneal group to maintain the longest moment arm possible through out the propulsive phase. Therefore it would be feasible to keep the STJ pronated to maintain this scenario and avoid eversion sprains.

Cheers Dave

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

I'm not sure that you can make the assumption that peroneal leverage changes with STJ position. For the lever arm to change the point of application of force would have to change its distance from the axis or the direction of pull relative to the axis would have to change. Thinking about the position of the styloid process (insertion of peroneus brevis) relative to the axis with STJ motion, I don't think the distance changes appreciably. The direction of pull changes some, but I don't feel that it is that much.

Another way of assessing leverage is to look at tendon excursion with joint motion. (excursion = distance a point on the tendon travels.) When I was looking at tendon excursion in cadavers there was not a noticible rate of change of movement of the tendon.

This is much different than the anterior tibial tendon that can change from creating a supination moment to creating a pronation moment with motion of the STJ.

Although, I think the lever arm of the peroneals at the STJ doesn't change much it may be enough that your explanation is plausible. It is probably equally plausible as my "perceived stability" explanation of STJ pronation with a laterally positioned STJ axis. They both may be true.

Regards,
Eric

Dave:

I would imagine that in a patient with peroneal spasm, the foot would have peroneal activity during swing phase, but other than that, I would not expect either the peroneus brevis or peroneus longus to be active during swing phase since it is a plantarflexor of the ankle, not a dorsiflexor of the ankle. However, I could be wrong in this conjecture and would be happy to be corrected if someone could provide me some evidence to the contrary. It would be the peroneus tertius, extensor digitorum longus (EDL)and extensor hallucis longus (EHL) which could produce STJ pronation moment and ankle joint dorsiflexion moment, not the FDL or FHL.

If the STJ axis is quite lateral, I would imagine that only the EDL and the peroneus tertius, of the ankle joint dorsiflexors, could produce STJ pronation moment. Certainly I could see the possiblility of an individual especially using the peroneus brevis to aid in foot clearance in early swing but don't know if this actually happens or not. Sounds like this will just need to be your next paper for JAPMA.

__________________
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
**************************************************

Dave:

Your diagrams are excellent. They look very similar to a few of my drawings from many years ago when I was wrestling with the ideas of STJ axis location and muscle moment arms.

I believe that what happens during late midstance and propulsion when the STJ axis is too lateral, is that the central nervous system shuts down the efferent activity to the gastrocnemius-soleus muscles early in late midstance/early propulsion so that excessive STJ supination moment will not occur during propulsion which would tend to cause inversion ankle sprains. In these patients, it is typical to see that propulsive phase is shortened and the stride length is also shortened. It is also common to see that addition of valgus forefoot wedges to the shoe or foot (which increase the external STJ pronation moment) will increase the length of propulsive phase and lengthen the stride of the individual, probably since the central nervous system now "recognizes" that the gastrocnemius-soleus can be activated more forcefully and for a longer duration with the forefoot valgus wedge during the latter half of stance phase without increasing the tendency toward supination instability of the STJ and increasing the tendency toward inversion ankle injuries.

__________________
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
**************************************************

"During the swing phase of the gait cycle, no peroneal activity was detected in the injured and uninjured legs."

From:

Valter Santilli, Massimo A. Frascarelli, Marco Paoloni, Flaminia Frascarelli, Filippo Camerota, Luisa De Natale and Fabio De Santis: Peroneus Longus Muscle Activation Pattern During Gait Cycle in Athletes Affected by Functional Ankle Instability, Am J Sports Med, 2005, 33: 1183.

Attached Files

Peroneus Longus Activation Pattern During Gait in Athletes with Ankle Instability.pdf (277.1 KB, 1 views)

__________________
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
**************************************************