Windlass mechanism not main stiffener of the foot at push off !

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So another very interesting paper from Farris and Kelly (see link below ) .

This paper lays out a case that the windlass mechanism is not the primary stiffener of the foot during push off , but rather that this stiffening is provided by muscular forces including a major contribution from the intrinsic foot muscles . The authors argue that this contribution helps limit strain in the plantar fascia and that strengthening of the intrinsics may help treat, or prevent ,plantar fasciitis .

Foot stiffening during the push-off phase of human walking is ...

royalsocietypublishing.org › doi › rsif.2020.0208
4 days ago - Several plantar intrinsic foot muscles course a parallel path to the plantar aponeurosis from the heel to the toes (figure 1). These muscles are active during the late stance phase of gait [14,15] and appear to be important for modulating foot mechanics [16–19].

 

Here are some quotes from the Farris /Birch/Kelly paper .

"Also, that the plantar intrinsic muscles can offload plantar aponeurosis strain might be of interest to clinicians and athletic trainers who seek to rehabilitate or prevent plantar fasciitis. This is a common syndrome resulting from repeated loading of the plantar structures in the foot [23]. Our findings suggest that strengthening of the plantar intrinsic muscles has the potential to shield passive structures from larger strains that could be contributing to this condition."

and

"In summary, our results show that maintaining tension across the plantar aspect of the foot involves active muscular contraction of ankle plantar flexors and plantar intrinsic muscles. Therefore, the passive windlass mechanism is not the foot's source of rigidity for push-off against the ground during gait. In fact, within the foot, increasing push-off for changes in gait mechanics relies primarily on foot muscles and not winding of the plantar aponeurosis via the windlass mechanism."

 

In general the work of Kelly ,Farris and their numerous coworkers is to be applauded since they are improving understanding of the physiology and mechanics of the foot which may lead to better treatments and/ or performance .

However , in this latest paper Farris et al have not touched on the transverse arch of the foot and the changes that take place in this during transition from mid to late stance . I feel pretty sure that the metatarsal parabola will give rise to very significant changes in foot stiffness around push off time .

You could do worse than have a look at the video linked to below . In this simple 10 second clip the hand is used as an analogy for the foot with the knuckles representing the tarsal ( formed into a bit of an arch shape by the elastic band) ,
and the fingers the metatarsals .
See how the relationship of the finger tips ( met heads )changes as the hand lifted .
Note that this effect is quite different from Vankandesen's fish fin mechanism

 

Good luck to them, I hope people listen. What he is describing is biotensegrity, a view that some of us have been talking about for years, only to be shot down at every turn. That said, there is more to this than intrinsic musculature. The extrinsic muscles. 1) p tib, p longus, p brevis, abductor hallucis, and the great toe flexors provide transverse arch control and a direct transfer of force proximal in the kinetic chain on loading. 2) The peripheral nervous system.....those mechanoreceptors that constantly monitor and manipulate tension synergies.....are vastly underappreciated. 3) Related to muscle synergies is conversion of concentric action to eccentric for repetitive motion such as gait, pitching a baseball, and typing to name a few. The world of biomechanics is a bit resistant to change........
Cheers, K

 

Some aspects of biomechanics are indeed resistant to change no matter what the evidence says .
My own pet peeve is the use of the towel curl exercise to strengthen the intrinsics .

This exercise or associated exercises which employ apparatus that provide resistance to toe curling via toe bars etc , just don't target the intrinsics sufficiently for strength gain no matter the level of effort .Dr Sarah Ridge and her colleagues recently demonstrated this using a custom built rig and indwelling electrodes .And yet some researches, particularly it the Far East , persist in measuring toe flexor strength with toe grip dynamometers which can't meaningfully pick up the majority of intrinsic flexor strength .
Often wasted research time if you ask me and frustrating to see .

 

On the subject of progressive resistance training and the muscles of the foot , a paper was published a few years ago which looked at the effects of this type of exercise as a treatment for Charcot-Marie -Tooth disease . The study , led by Professor Joshua Burns concluded as follows ;


"Interpretation
6 months of targeted progressive resistance exercise attenuated long-term progression of dorsiflexion weakness without detrimental effect on muscle morphology or other signs of overwork weakness in paediatric patients with Charcot-Marie-Tooth disease."

Here is a link to an article on the subject ;


First effective treatment of children with Charcot-Marie-Tooth ...

www.sydney.edu.au › news › 2017/07/17 › first-effecti...
17 Jul 2017 - Led by Professor Joshua Burns from University of Sydney and The Children's Hospital at Westmead, the study found that progressive resistance exercise could help to significantly reduce the muscle weakness experienced by patients with the genetic disease.


---------------------------------------------------------------------------------------------------------------------------------------------------------------
Forward to the present day and Professor Burns is moving to improve methods of strengthening the intrinsic muscles of the foot . He is basing his method around the short foot exercise but has correctly identified that,during exercise , the toes should remain straight but flex around the MTFJs causing pressure to be applied to the ground through the tips of the toes and the ball of the foot to leave the ground ( see link to photographs from the paper and to the paper itself ) . He and his team are calling their system the "Archercise biofeedback device ".

https://images.app.goo.gl/dTEDiE1swjaX3c6P7


Feasibility of the Archercise biofeedback device to strengthen ...

jfootankleres.biomedcentral.com › articles
13 Jul 2020 - Toe flexor and foot arch exercises focused on intrinsic foot muscle strength and ... We investigated the feasibility of using a novel medical device, known as ... J. McKay, Claire E. Hiller, Elizabeth J. Nightingale & Joshua Burns.
by PJ Latey - ‎2020

 

LOL... Agree with you RE the towel exercises. Not all that functional. My favorite ther-ex for cases that require significant rehab is walking backwards in sand. It is cheaper for the pt to build a shallow frame and fill it with sand that go to weeks of PT.

 

I have come across a few instances of foot health care professionals doing rehab with professional athletes in sandpits and the like .

Re the towel exercise I feel that the evidence teaches that this is not only not a functional exercise but also that it only exercises the intrinsics ( QP apart ) as ancillary muscles .

So, for example, if an individual places their toes over a bar of a custom made dynamometer and pulls back on the bar with all the might their toes can muster,the quadratus plantae and the long toe flexors will contract to near the max force they can apply but the intrinsic muscles, except QP , will become activated to nowhere near their maximum levels .

So toe curls will strengthen the long toe flexors but will never strengthen the intrinsics no matter how hard a subject tries .

It might help to think of the bicep curl .This exercise is good for building bicep strength but during its execution some muscles in the back activate and act as ancillaries to stabilise the shoulder . So the bicep curl activates some muscles in the back but not sufficiently to effectively strengthen them . And so it is with the intrinsics and toe toe curls



This is probably not a big problem if simple towel curls are being used as an exercise but could well introduce muscle imbalances if toe curls are done with a progressively increasing resistance level. The extrinsic may end up overpowering weak intrinsics detrimentally affecting function .

 

Here is something for you to noodle on.....
When I was working on my masters and doctorate, I worked in the gross lab and for three years was able to dissect anything I wanted...and do experiments. I think that the extrinsic moves are more important than given credit. Primarily, post tib, ant tib, peroneus long, pro brev, flex hallux abductor hallux. Their combined action, just before the foot begins to load, is to shorten and narrow the foot. For instance post tib and P longus make a sling under the navicular and cuboid. Post tib doesnt stop at the navucular, it continues through the foot and sends tendons to the lateral tarsal and sometimes to the base of the 4th and 5th mets. P longus inserts on the base of the first. So when they shorten, they tilt the rearfoot up......at the same time, abductor H and P brevis shorten the foot.....in otherwosrds, the make the midfoot joints congruent. When the foot loads, those muscles act eccentrically to maintain foot position, but more importantly to store and transfer energy from loading up the leg and to trigger muscle synergies needed for gait via mechanorecptors. BUT! What keeps the mets from simply flattening out? The attachments of the muscles mentioned are all proximal to the mets? Enter the intrinsics......

All the towel exercises in the world won't produce that action.....but walking backward is the same functional position as sprinting......an action where all the foot muscles have to fire together to maintain structural stability.


Sorry about the brevity, errors and lack of detail....typing this on my phone.
Cheers,K

 

I largely agree with what you are saying about the extrinsic foot muscles and the support they provide for the structure of the foot and indeed some manifestations of the short foot exercise ,where the toes don't push against the ground , are almost entirely about these muscles and not the intrinsics at all .

Going back to that pet peeve of mine ,the toe curl exercise and its variants , a couple of papers have measured the activity levels of the abductor hallucis during this exercise and the results point towards this muscle being ancillary to the flexor digitorum longus which contributes by far the most force during the exercise .

For anyone who might not be familiar with the term , "EMG max "refers to the electrical activity in a muscle during its maximum voluntary contraction . Researchers sometimes record this figure then express subsequent effort levels in the muscle as a percentage of this figure . So if the studied muscle is worked at half the level of its voluntary maximum you might expect to see an EMG figure of about half the EMG max figure .

So back to those papers .
The first is Jung et all 2011 (see link below ). These researchers found that the EMG level in the abductor hallucis ,during a standing toe curl exercise ,was 17.5 % max. That is a low figure and demonstrates that the abd hallucis is not a major player in this movement but is ancillary . At 17.5 % max you will be there for a very long time doing a huge number of repetitions to get a strengthening effect , if you ever get one at all .

A more recent paper is that of Bruening et all 2019 ( Link also below ) . For this research the investigators built just about the best possible toe curl measurement device possible ,indeed its doubtful if the abductor hallucis could have been involved anymore than it was in the curling action . However, even with the test subjects pulling on the ring ( curling their big toe back towards the heel ) with all their might ,the EMG figure did not get much above 30 % EMG max for the abd hallucis . So with a following wind and the best possible set up ,toe curls will strengthen the flexor hallucis longus but not the abd hallucis ,or at least not effectively .
Actually that EMG max figure used by Bruening et al was arrived at by recording the EMG in abd hallucis during a single leg calf raise with support and as such is liable to be much lower than the true EMG max . The 30 % figure for the toe curl exercise is probably a good bit higher than it should be .

As far as I am aware these two papers are the only ones to measure the activity in the abd hallucis during toe curl activies and as you can see it is a very poor choice for strengthening this muscle ( and probably ,QP aside and in my opinion , a poor choice for strengthening the other intrinsics as well )

Jung DY, Kim MH, Koh EK, Kwon OY, Cynn HS, Lee WH. (2011) A comparison in the muscle activity of the abductor hallucis and the medial longitudinal arch angle during toe curl and short foot exercises. Physical Therapy in Sport 12;30-35

Bruening et all
https://link.springer.com/article/10.1186/s12891-019-2981-6

 

Going back to the original post in this thread and the paper by Farris et al ,I have to say that this paper appears not to have gone down well among some well regarded and influential individuals .

In discussing this very interesting piece of search further it may be a good idea to look first at Welte et al 2018 . They used the rig shown below to study the windlass mechanism where the plantar fascia wraps around the MTPJs which in turn raises the medial arch of the foot and more closely approximate the met heads with the heel (Hick ).
The authors thought this would stiffen the foot but the opposite turn out to be the case .

So looking at the diagram , Welte and colleagues increased the MTPJ angle to engage the windlass . They found that for the same applied force down through the tib/fib the bit on wheels ,well ball bearings , moved more when the windlass was engaged (MTPJ ANGLE ) . Or in other words ,engagement of the windlass produced increased tension in the plantar fascia but reduced foot stiffness ; so increased PF tension does not necessarily equate to increased foot stiffness in vivo .

Here is how it was explained in the paper .
Quote
"The windlass did, however, modulate the energetics of the arch during dynamic loading. When the windlass mechanism was engaged, the arch was shortened significantly, which probably placed other tissues crossing the arch closer to their resting length. The arch could therefore go through a greater excursion due to the nonlinear elastic properties of the arch-spanning tissues. This would functionally reduce the bulk stiffness of the arch and lead to greater elongation, and consequently facilitate greater energy absorption and dissipation during cyclic loading of the foot. While the effect of perturbing the MTPJ angle on the specific internal structures of the foot cannot be determined with these methods, it is possible that modification to the shape and length of the arch changed the proportion of the load delivered to the arch's soft tissue structures. Differences in viscosity between these structures could also influence the dissipated energy in the arch."

 

With regard to the very recent Farris ,Birch ,Kelly paper ( link below ) I would have to say that I have issues with some of the interpretation , most particularly with the assertion that the windlass is not engaged until the tension in the PF and achilles tendon is decreasing .

However ,this is the first paper to factor in the intrinsic foot muscles when considering tension within the PF during gait and as such is a very important step forwards. It highlights the importance of the intrinsic foot muscles both with regard to the forces around the MTFJs and draws attention to the protective function that the intrinsics seem to have over the plantar fascia .

How accurate can previous models be ( finite element / cadaver based experiments ) if they have left the intrinsics out of the equation ?

If the intrinsics have a protective role to play when it comes to strain within the plantar fascia then it is very likely the extrinsic toe flexors are also involved .Strengthening of these muscles may be key to treating/ preventing plantar fasciitis .It is known that the plantar flexors respond quickly and to a high degree to approprate exercise regimes ,and using the right apparatus 3x 10 mins a week is enough to promote strength gains even in intrinsic minus feet with neuropathy .

Ps I have no idea if Luke Kelly reads the material I post here but with regard the the "initial windlass phase " as I am calling it , friends and family involved in research tell me they would never reference a blog site in papers for publication ,and so ,if that is the way things are done , no harm no foul .

 

Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism

Dominic James Farris,
Jonathon Birch
and
Luke Kelly
Published:15 July 2020https://doi.org/10.1098/rsif.2020.0208

So above is a link to the Farris paper and below is a quote from that paper .

"In summary, our results show that maintaining tension across the plantar aspect of the foot involves active muscular contraction of ankle plantar flexors and plantar intrinsic muscles. Therefore, the passive windlass mechanism is not the foot's source of rigidity for push-off against the ground during gait. In fact, within the foot, increasing push-off for changes in gait mechanics relies primarily on foot muscles and not winding of the plantar aponeurosis via the windlass mechanism."

But could the windlass mechanism reduce transverse compression of the intrinsics at push of and so facilitate IFM linear force production leading to improved foot stiffness ? I think it probably does .

 

The Farris paper allows that the plantar fascia contributes to foot stiffness at push off but not that the windlass contributes to this .

Think of standing on a book with your toes free in space over the end of the book . If the calf muscles contract to lift the heels off the book then only the balls of the feet will be in contact with the book . The plantar fascia will be under tension through muscular contraction giving stiffness but the windlass is not engaged as the toes are free in space .Farris and colleagues have calculated that adding the windlass in contributes little to foot stiffness over and above the muscular contraction of the calf muscles .

However ,according to the work of Farris ,the intrinsic foot muscles do make a significant contribution to foot stiffness during push off .

BUT ,are not the intrinsics being compressed between the bony arch of the foot and the plantar fascia early in push off ? I believe so but the action of the windlass mechanism ,raising the arch ,will reduce passive transverse compression of the intrinsics ,allowing them to produce more force linearly .

The existence of passive transverse compression of the intrinsics between the bony arch of the foot and the plantar fascia is conclusively proven by the existence of the plantar venous pump . It is a major player in foot biomechanics esp during weight acceptance .

Plantar venous pump or plexus is not powered by muscle contraction or necking down . Read text via link and please think for yourself .With respect ,Fox and Gardner got this mechanism wrong .

Link https://biomch-l.isbweb.org/forum/biomch-l-forums/general-discussion/32337-venous-foot-pump

 

The following may be novel !

So a couple of papers now (Welte , Farris ) have demonstrated the the windlass mechanism does not stiffen the foot at push off but actually has the opposite effect . This is explained as follows ;

From Welte 2018
"The windlass did, however, modulate the energetics of the arch during dynamic loading. When the windlass mechanism was engaged, the arch was shortened significantly, which probably placed other tissues crossing the arch closer to their resting length. The arch could therefore go through a greater excursion due to the nonlinear elastic properties of the arch-spanning tissues. This would functionally reduce the bulk stiffness of the arch and lead to greater elongation, and consequently facilitate greater energy absorption and dissipation during cyclic loading of the foot. While the effect of perturbing the MTPJ angle on the specific internal structures of the foot cannot be determined with these methods, it is possible that modification to the shape and length of the arch changed the proportion of the load delivered to the arch's soft tissue structures. Differences in viscosity between these structures could also influence the dissipated energy in the arch."

Now in Farris et al 2020 it was found that muscular contraction ( including the IFM ), and not the windlass stiffened the foot at push off . But if the windlass mechanism does not stiffen the foot because the windlass action more closely approximates the heel with the base of the toes , then surely the same holds true for intrinsic muscles which span the whole of the longitudinal arch .

Contraction of the abductor hallucis will shorten the arch ( indeed this is the basis of the short foot exercise ) causing the plantar ligaments to come closer to their resting lengths and so reducing foot stiffness in much the same fashion as the windlass .

And yet we know ,again through Farris , that the intrinsics are required for foot stiffness at push off .

It would appear that the intrinsics that do not span the arch are the ones that are stiffening the foot at push off .

This is likely through the effects of muscles like the adductor hallucis and interosseous muscles controlling the transverse arch of the foot .

Any thoughts ?

 

IMO and with regard to push off during gait , toe flexion exercises might be better for foot mechanics than exercises which shorten the foot .
Recent evidence suggests that during push off ,the windlass mechanism shortens the foot and offloads the plantar ligaments giving reduced foot stiffness .
During this phase of gait ,much of the stiffness of the foot probably comes from the accentuation of the transverse arch which is a function of the metatarsal parabola #parabolaeffect
Looking at the work of #FarrisDominic and #Weltelauren ,it would appear that the windlass mechanism does not stiffen the foot at push off . By the same logic they use to explain this lack of stiffening, other structures which span the longitudinal arch and which might serve to more closely approximate the heel and the metatarsal heads during push off ,offloading the plantar ligaments , might also fail to contribute to foot stiffness .
However ,also from the work of Farris ,we know that anaesthetizing the plantar intrinsic foot muscles leads to a poorer foot lever and gait changes during push off .
IMO this highlights the importance of the intrinsics which do not span the arch but which help with the stabilization of the transverse arch .
These muscles are unlikely to be effectively targeted by arch shortening exercises but would be better strengthened by toe flexion exercises .

Image from Venkadesan et al Yale

 

Below is an image showing the plantar ligaments of the foot .
In mid stance these take much of the load on the foot and give the foot its stiffness . According to the work of Farris et al and Welte et al ,as gait progresses into push off the toes dorsiflex and the windlass mechanism is engaged ,causing the met heads to become more closely approximated to the heel , with a net decrease in foot stiffness due to reduced tension in the plantar ligaments (shown in the drawing )




At push off much foot stiffness probably comes from the accentuation of the transverse arch as outlined in previous posts #parabolaeffect .
The muscles giving core support to the distal aspect of the transverse arch will likely not be those that span the longitudinal arch ,since these will also offload the plantar ligaments ,but rather those that do not span the longitudinal arch like adductor hallucis ,interosseus and flexor hallucis brevis .

IMO it is likely that in the experiment where Farris et al anaesthetized the intrinsics and found push off was impaired during walking and running , it was the loss of function of the muscles highlighted in 2 shades of pink in drawing below, which caused impairment rather than the muscles which span the full longitudinal arch like abductor hallucis .
So different intrinsic foot muscles have very distinct roles at various stages of gait .

 

It is thought by some that strengthening the muscles which span the longitudinal arch of the foot can help "share the load " , taking strain off of the plantar fascia and helping with heel pain .

More likely in my opinion , is that strengthening the intrinsics with more distal origins and which help support the transverse arch , can have a bigger impact on tension within the fascia .

The #parabolaeffect ,acting in concert with transverse arch supporting muscles like the adductor hallucis and interossei ,will produce a stiff " dome " like structure which means less strain on the fascia than would
otherwise be the case .

The interosseous muscles act to flex some of the toes at the MTP joints while extending them at the interphalangeal joints and so would be best strengthening by toe flexion exercises around the MTP joints and not toe curls .

Actually , strengthening of the transverse arch maintaining muscles in general seems far more likely to be achieved by flexing around the MTP against resistance , than by toe curls ( which seems to be a poor exercise for the intrinsics ) or the short foot exercise .

 

So how can the dorsal interossei act to help maintain the more distal aspect of the transverse arch ?

Well , looking at the hand it is understood that the 1st dorsal interosseous muscle can adduct the thumb , that is draw the 1st metacarpal towards the 2nd . I have not seen it written , but likely the other dorsal interossei act in the same way , more closely approximating the metacarpals when they contract .

In the foot ,the dorsal interossei are not big muscles and it is likely that the adductor hallucis plays a much more significant role in transverse arch control ,however they could easily contribute far more than has been previously realised ,to prevent the metatarsals being separated at their more distal aspects .

The #parabolaeffect and the transverse ligament are the key aspect to distal foot stiffness at push off , but Farris et al have demonstrated that the intrinsic foot muscles play a crucial role which other supporting structures cannot substitute for .

 

There is evidence that atrophy of the forefoot intrinsic musculature is specifically associated with degradation of foot function .

Much of the stiffness of the foot at push off comes from tension in the plantar fascia . In 2012 Hamill et al looked at relationship between the foot intrinsics and unilateral plantar fasciitis .

They found that the volume of intrinsics was broadly similar between healthy feet and those with plantar fasciitis but that when the forefoot muscles were considered separately , a significant reduction was present in feet with PF .

Here is the interpretation from the Hamill paper -

"Interpretations: While the total volume of plantar intrinsic foot muscles was similar in healthy and plantar fasciitis feet, atrophy of the forefoot plantar intrinsic foot muscles may contribute to plantar fasciitis by destabilizing the medial longitudinal arch. These results suggest that magnetic resonance imaging measures may be useful in understanding the etiology and rehabilitation of chronic plantar fasciitis."

So at the time Hamill and colleagues believed the atrophy of the intrinsics caused a destabilization of the medial longitudinal arch . I disagree with them and believe the atrophy causes a destabilization of the distal aspect of the transverse arch which in turn leads to weakness in the foot "dome " ( McKenzie ) ,which increases work required of the plantar fascia leading to plantar fasciitis .

Likely this sort of weakness will not be reversed by toe curls or the short foot exercise but requires toe flexion exercises ,around the MTFJ , against increasing resistance .

Rehabilitation protocols for treating plantar fasciitis via intrinsic strengthening have never been particularly successful but I am becoming more and more convinced that this is because the wrong muscles are being targeted with the wrong exercises .It seems to have been that way for a very long time .








Use of MRI for volume estimation of tibialis posterior and plantar intrinsic foot muscles in healthy and chronic plantar fasciitis limbs
Ryan Chang 1, Jane A Kent-Braun, Joseph Hamill
Affiliations expand

• PMID: 22166747
  
• DOI: 10.1016/j.clinbiomech.2011.11.007

 

Interesting . Re the above here is a paper which appears to contradict the idea that it is atrophy of the intrinsic musculature of the forefoot that contributes to plantar heel pain . This leads me to think that the lack of proper functioning of the #parabolaeffect ,first introduced by me on this forum, may be key in some instances of heel pain .
The intrinsics cannot support and control a distal transverse arch which does not exist . The distal aspect of the transverse arch becomes significant at push off only if the met heads are able to move relative to each other .


Intrinsic foot muscle atrophy in individuals with chronic plantar ...

www.jsams.org › article › pdf
Introduction: Plantar heel pain (PHP) is a prevalent muscu- loskeletal condition and common cause of foot pain presenting to sports medicine practitioners.
by MF Smith · ‎2019 · ‎Cited by 1 · ‎Related articles

 

On the subject of plantar heel pain and the intrinsic foot muscles ,a protocol was published by a group from UQ a few months ago .
I only read the publication recently but was greatly heartened to find that they have not included any form of toe curl exercises in their trial , never mind marble pick ups .
I am not sure that hopping on a domed foot is a good idea for people with plantar heel pain but that aside this looks set to give an initial answer to the question of whether foot strengthening exercises should be used routinely in the treatment of heel pain .
Not big numbers as yet but, and in my opinion , at least they are going about things in the right way . First researchers to do so !
Wonder what they will find /have found .


J Foot Ankle Res. 2020; 13: 20.
Published online 2020 May 8. doi: 10.1186/s13047-020-00384-1
PMCID: PMC7206811
PMID: 32384905
Foot exercise plus education versus wait and see for the treatment of plantar heel pain (FEET trial): a protocol for a feasibility study

Melinda M. Franettovich Smith,1 Natalie J. Collins,1,2 Rebecca Mellor,1 Alison Grimaldi,1,3 James Elliott,4,5 Mark Hoggarth,5 Kenneth A. Weber II,6 and Bill Vicenzino1
Author information Article notes Copyright and License information Disclaimer

 

Below is a link to another study that looked at intrinsic muscle volumes in the feet of people with chronic bilateral heel pain .

As with the MF Smith et al 2019 paper , Cheung et al found significant intrinsic atrophy in the distal part of the foot only .

So what would cause atrophy of only the muscles that span the length of the foot and not of those that don't, like the adductor hallucis ,and would this lead to plantar heel pain .

Here is the discussion of the Smith conference paper -

"Discussion: Atrophy of the abductor hallucis was evident in individuals with longer duration of PHP symptoms only. This suggests that changes are a consequence of PHP symptoms and/or reduced function, rather than a contributor to the development of PHP, but this requires further investigation in prospective studies. These findings suggest that interventions that have been shown to induce abductor hallucis hypertrophy, such as exercise and footwear, may produce beneficial outcomes in the management of individuals with PHP.

Plantar heel pain can have a number of causes but it begins to look to me , that in cases of bilateral chronic heel pain , the problem is a with the insertion of the abductor hallucis . A tendonitis if you like .
This fits with pain first ,then muscle atrophy .

I just really wish that " jump on one leg with the foot in a domed position " had been left out of the FEET trial . The natural mechanical pattern is toes dorsiflexed to absorb force on landing . #initialwindlassphase

Note : If plantar heel pain were being caused by something other than abd hal tendonitis /insertion problems ,then we could reasonably expect to see later atrophy not just of the abd hal but of the more distal intrinsic musculature also .