Functional lesser toe deformity.

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Functional lesser toe deformity, is there such a thing and can it contribute to falls in the older population?

What I mean by "functional lesser toe deformity" is probably best understood by looking at the foot on the left of the included image. When the intrinsic foot musculature of a foot is weak and demands are placed on the foot, the foot will collapse in such a way that the lesser toes adopt a hammer toe position . This effectively shortens the foot and reduces the base of support, likely compromising balance.

In my experience ( I am not qualified in foot and ankle biomechanics but I have been involved in university studies around the subject) where the lesser toes of a foot collapse under load they can easily be strengthened so that they do not.

 

Does it make a difference if toes adopt a hammer toe configuration only under load, for example when balancing or walking, or if the configuration is present even at rest and the toes cannot straighten at the MTPJs ? I suspect not. That is to say, toes which collapse under load will have the same overall effect on gait and balance as toes which are in a permanent hammer toe configuration.

We know that lesser toe deformities are associated with an increase in falls risk , but are clinicians dealing with falls in the elderly failing to do anything about toe weakness? Are lesser toe intrinsic muscle deficits being ignored and toes being left to collapse under load giving functional toe deformity?

Some really promising falls prevention programmes exist but as far as I can see none directly address intrinsic foot muscle weakness.

 

Which muscles prevent the toes from adopting a hammer toe/ claw toe configuration when proximal part of the foot is under the most load. Some feel that the flexor digitorum brevis has a role to play in this but others think that activation of the FDB encourages hammer toe configuration. The same certainly cannot be said for the lumbricals or the interossei. The lumbrical for example, both flex the distal phalanx of the lesser toes and extend the interphalangeal joints. If the lumbricals are strong enough and " switched on " as they are during balance and walking on a flat surface the toes will not collapse . Collapsing lesser toe structure may be indicative of weak intrinsics generally, but certainly means an effective shortening of the foot and presumably impaired balance .

How would you strengthen the lumbricals ? By moving the toes around the MTPJs against resistance whilst keeping the interphalangeal joints straight .

Calf raises will not strengthen the lesser toes and toe curls certainly wont strengthen muscles like the lumbricals or interossei since this action requires bending at the toe joints.

 

An error in the post above "The lumbrical for example, both flex the distal phalanx of the lesser toes and extend the interphalangeal joints." should read "The lumbricals, for example, both plantar flex the lesser toes at the MTPJs and extend the interphalangeal joints "

How might we tell if a persons intrinsic foot musculature is weak, for example an older person or someone with diabetes? One test that might be done under supervision is as follows -

An early marker for intrinsic atrophy could be what the toes do during Vele's forward lean . Often with neuropathy, the more distal musculature is affected soonest . If an individual with straight, relaxed toes, but weak lumbricals and interossei, loads up the fore foot then the toes will collapse into a hammer toe or curled configuration.

Toes that remain straight under load are better at assisting in balance than toes that curl or hammer ( more force applied to ground further from the COP, ie longer lever arm. Also, research has shown that the FDB generates more muscular force when the toes remain straight at the IPJs rather than curling or going into a hammer toe configuration. )

From experience, if you strengthen the IFM of a foot that collapses when the forefoot is loaded, then the collapsing stops.

I have included an image that shows a hammer toe configuration. The PIP will not "pop up" if intrinsics attached to the base of the 1st phalanyx are strong enough, regardless of the action of the FDB.

 

For a dentist who has spent a huge amount of time studying feet, you should know anatomy and terminology better. The number refers to which toe. The phalanx is delineated by proximal, intermediate, or distal. What intrinsic muscles do think insert on the base of the proximal phalanx of the the lesser toes? I don't know of any.

I would agree that the lumbricals can plantar flex the proximal phalanx at the metatarsal phalangeal joint.

Your term collapse rankles my sensibilities. Collapse implies that gravity is causing the motion that you see in the picture you posted. You would get the same motion (dorsiflexion of proximal phalanx, plantar flexion of intermediate phalanx and hyperextension of the distal phalanx) if you put your feet on a wall and gripped with your toes like the person did in the picture in the first post in this thread. That motion is caused by active contraction of a muscle.

I would agree that lumbrical contraction could counter act the moments created by EDB and contracting both EDB and lumbricals simultaneously might be able to get you straight toes.

We should also look at the amount that straight versus hammered toes could change COP. Take a 250 mm foot and look at the difference in length of the foot with hammered versus straight toes. The difference is what 10mm? That's a 4% difference. Ankle plantar flexors are much more important for AP balance.

 

Dorsal Interossei Eric ?
Insertion

The two heads of each muscle form a central tendon which passes forwards deep to the deep transverse metatarsal ligament.[1] The tendons are inserted on the bases of the second, third, and fourth proximal phalanges[2] and into the aponeurosis of the tendons of the extensor digitorum longus[3] without attaching to the extensor hoods of the toes.[1]

 

Hi Eric ,
I ran into a firewall problem with the last post so couldn't complete it.

Muscles which insert on the base of the proximal phalanxes of the lesser toes include the lumbricals, the plantar interossei, the dorsal interossei, flexor digiti minimi brevis and abductor digiti minimi ( a large muscle as intrinsics go). To use your own words "you should know anatomy .. better."

The lumbricals do plantar flex the proximal phalanx at the metatarsal phalangeal joint.

I like the term since it conveys the idea of structural failure under load due to weakness. "Buckle" would also be a good term.

Eric , would you expect strong, healthy toes to go into a hammer toe configuration during Veles forward lean or remain straight at the PIP joint ?

 

Eric, you are not alone in thinking that the lumbricals of the foot do not attach at the bases of the proximal phalanges of the lesser toes, with a number of anatomy sites making this mistake, but in fact they do. This allows these muscles to extend the IPJs, plantarflex the toes at the MTPJs and adduct toes 2 through to 5.

In view of the above comment, I think it would be a good idea to revise the intrinsic muscles of the foot with a particular emphasis on those that insert onto the bases of the proximal phalanxes of each of the lower toes, before expanding on how this anatomy impacts on toe function and deformity.

Each lesser toe has at least 3 such attachments, not none, and they are likely crucial when it comes to lesser toe deformity.
Muscles inserting onto the base of the proximal phalanx of each lesser toe-

2nd toe
1 lumbrical on medial aspect of base as well as extensor hood
2 dorsal interosseous inserted into medial aspect of base
3 dorsal interosseous on lateral aspect of base

3rd toe
1 dorsal interosseous – inserted on lateral aspect of base of proximal phalanx (pp)
2 plantar interosseous – inserted on medial aspect of base pp
3 lumbrical - medial aspect of base pp

4th toe
1 dorsal interosseous -lateral aspect base pp
2 plantar interosseous – medial aspect base pp
3 lumbricals - medial aspect base pp

5th toe
1 lumbrical -medial aspect of base pp
2 plantar interosseous- medial aspect of base pp
3 flexor digitorum minimi- base pp
4 abductor digiti minimi -base pp

In short , most of the intrinsic muscles of the foot insert onto the bases of the proximal phalanxes of the lesser toes .



Note on the short and long toe extensors

Eric,you said "I would agree that lumbrical contraction could counter act the moments created by EDB and contracting both EDB and lumbricals simultaneously might be able to get you straight toes."

There seems to be a persistent misunderstanding abroad concerning the way the toe extensor muscles contribute to foot biomechanics during gait, with some believing co-contraction of the toe flexors and extensors is a thing. In fact, they are largely separable as ably demonstrated in Co-ordination of intrinsic and extrinsic foot muscles during gait Zelik et al 2014-

“Conclusions: The sequential peak activity of MTP flexors followed by MTP extensors suggests that their biomechanical contributions may be largely separable from each other and from other extrinsic foot muscles during walking.”

During Veles forward lean I would be very surprised if a healthy foot shows much toe extensor activity at all.

When I say “straight toes” some dorsiflexion at the distal PIP, to give a toe wink effect would, be found in a healthy strong foot. Thank you for allowing me to clarify this.

When Vele’s forward lean is done by a person with weak intrinsic foot muscles, and weak lumbricals and interossei in particular, hammer or claw toes will form transiently with the extension of the proximal phalanges at the MTPJs.

In my experience, this tendency to collapse can be reversed with foot strengthening exercises where the toes remain “straight” at the proximal IP joint, extended at the distal IP joint, and flex around the MTPJs against resistance
Could flexible hammer and claw toes be treated using this exercises method? I have no evidence but believe it may well be possible.


Eric ,you said " Ankle plantar fkexors are much more important for AP balance "
Perhaps you would benefit from reading this paper from 2029

Toe weakness and deformity increase the risk of falls in older people Karen J Mickle et al 2009
Interpretation
Reduced toe flexor strength and the presence of toe deformities increase the risk of falling in older people. To reduce this risk, interventions designed to increase strength of the toe flexor muscles combined with treatment of those older individuals with toe deformities may be beneficial

 

Can functional lesser toe deformity contribute to the risk of plantar plate tears?

Well, flexible lesser toe deformities, like hammer and claw toes, are thought to contribute to the risk of tears and these can be caused by weak intrinsic foot musculature.

If flexible toe deformities can increase the risk of plantar plate tears during activities like sports, then it seems logical to me to conclude that "functional lesser toe deformities" will increase the risk by the same mechanisms.

It is important to remember that the majority of the intrinsic muscles of the foot insert onto the bases of the proximal phalanxes of the lesser toes. These muscles seem to be largely ignored by some clinicians ,which is extraordinary give their importance to foot mechanics.

If functional lesser toe deformities increase the risk of plantar plate tears, wouldn't it be wise to reverse these deformities, where present, by strengthening the lumbricals and interossei ?

 

Other common foot problems that are associated with lesser toe deformities include -
Morton's Neuroma and Metatarsalgia

. If, for example, flexible lesser toe deformities contribute to these conditions, then won't functional lesser toe deformities do the same ? (" Functional lesser toe deformities "is an expression I have coined to describe toes which adopt a hammer toe/claw toe configuration when under load. Or at least I think it was me that first coined it!)

It would appear that many conditions may be traced back to weakness in the lumbricals and interosseus muscles since when healthy these act to help keep the toes straight under load at the interproximal joints and flex the toes around the MTPJs.

Worth noting also, if the toes collapse under load then the flexor digitorum longus, which functions isometrically, will not be able to function as well as it does if the IP joints don't "fail" . Functional hammer toes cannot apply the same level of force to the substrate as healthy toes which do not collapse/buckle, although developed hammer toes ,where the tendon of the flexor digitorum longus becomes shortened ,may be better at applying force. This may in part explain the possible progression from "functional lesser toe deformity" to flexible hammer toe deformity.

 

Some might say, "if lesser toe deformity can be reversed or prevented by strengthening exercises then where is the evidence?"

That would be a fair enough question. In my view, which I believe is supported by the evidence, you need to strengthen the lumbricals ,interossei and FDB if you want to prevent functional lesser toe deformity or prevent it getting worse. Toe curling exercises will get you nowhere since these involve turning the lumbricals and interossei "off" to allow flexion the IP joints (these muscles straighten the IP joints) .

Functional foot strengthening exercises like hopping or calf raises may also fail if loading the toes during such exercises causes a collapse of the toes into a hammer toe configuration from the start . The very muscles you are trying to target are not in use during the exercises if the toes display functional lesser toe deformity. You might even be strengthening the flexor digitorum longus which might cause weakening of the lesser toe intrinsics.

Previous research has shown extrinsic foot muscle strengthening done before intrinsic strengthening can cause significant intrinsic foot muscle weakening.
Ketachi et al Differential effects of intrinsic- versus extrinsic-first corrective exercise programs on morphometric outcomes and navicular drop in pediatric flatfoot

If you come across a subject who's toes collapse under load, I believe it may be wise to address this at once or certainly before embarking on functional exercises based around collapsing toes. A foot may need to function properly before it can be strengthen uniformly.

If anyone is working on research concerning functional exercises and the intrinsic foot muscles they could do worse than take cognisance of this thread.

Get the foot working properly, with toes staying straight and flexing around the MTPJs ,then strengthen the whole.

 

So if you read one site that says one thing and another site that says another how do you choose which one is mistaken?

Sarafian's text the Anatomy of the foot is one place to look. The book has the pictures of the dissected feet to prove that the guy did look closely at the anatomy.

On the lumbricles: Sarafian p.247 "The lumbicle tendons are located on the plantar and tibial aspects of the corresponding deep transverse intermetatarsal ligament. ... At the distal end of the deep transverse intermetatarsal ligament the lumbrical tendon is directed anteriorly and dorsally. It joins the extensor hood and most of the fibers remain concentrated on the tibial border distally, reaching the extensor medial and lateral slips. Few fibers insert on the base of the proximal phalanx. "

The lumbricals are able to plantar flex the proximal phalanx not because they attach to the base of the phalanx, but because of the downward pull, from the tendon, on the hood. It has this downward pull because the tendon goes inferior to the transverse metatarsal ligament. (The interosseous muscles are dorsal to the transverse intermetatarsal ligament)

It is not just that the muscles insert on the base, it is very important where on the base of the phalanx that they insert.


What I needed to have said was that I don't know of any muscles that insert on the plantar base of the proximal phalanx. Muscles that insert on the medial or lateral sides (Plantar and Dorsal interossei) are not able to create a plantar flexion moment when the toes are straight. As the proximal phalanx moves relative to the metatarsal the direction of pull changes. When the toe is dorsiflexed the attachment of the tendon moves dorsally and this gives the tendon a dorsiflexion lever arm. In my dissections of feet with hammertoes I've seen interosseous muscles that only have a dorsiflexion action because the toe sits dorsiflexed.

In short, strengthing the lumbricals would potentially increase a source of plantar flexion moment at the MPJ to counteract the weight bearing action of FDB which is to cause dorsiflexion of the MPJ. The interosseous muscles would not be helpful for this.

 

I can see why someone wanting to sell something would like to call normal function of a muscle structural failure. The "load" that causes the motion is tension in the FDB tendon.

I would expect a foot with a strong healthy FDB to go into hammertoe configuration. You might be able to train someone to simultaneously contract their lumbricals at the same time as they contract the FDB so that their toes might stay straight.

I can put my hand on a table and create the hammertoe shape, or not, depending on which muscles I contract, or relax. It is a voluntary muscle.

 

Good to see that you have gone away and read up on the anatomy of the foot although your comments about the contribution the interosseus muscles make to plantar flexion at the MTPJs seem to be merely your opinion and not supported by the literature.

Depending on circumstances ( contraction state of associated muscles), the flexor digitorum brevis flexes the lesser toes at the PIP joint and also at the MTPJs .

You contend the contraction of the FDB would give a hammer toe configuration. However, the lumbricals and interossei are active at the same time as the FDB during gait on a flat surface, and these ( lumbricals and interossei) act to extend the interproximal joints of the toes. If the interproximal joints are being extended then, without question ,the FDB will flex the lesser toes at the MTPJs. That is what makes these muscles so important .

You think we have to train this ? Why are you using your hand and not your foot ? A hand is not a foot.

 

Lesser toe deformities like hammer or claw toes involve flexion at one or both of the joints between the 3 bones that make up each of the toes 2-5 ,the IP joints .Hammer toes also involve extension at the toe knuckle joint or MTPJ.

The lumbricals and interossei act to extend the IP joints and flex the toes around the MTPJ's and so strong healthy lumbricals and interossei will tend to allow the foot to function with straight toes that flex/extend around the MTPJ's during gait or manoeuvres such as Vele's forward lean.

The actions of the flexor digitorum longus and flexor digitorum brevis (FDB) will change along with lumbrical /interossei activity. For example, lack of activity in the lumbricals/interossei may see contraction of the FDB produce a hammer toe configuration ( flexion at the PIP and extension at the MTPJ's) but simultaneous activity in these 3 muscles will result in straight toes flexing around the MTPJ's .

Weakness in the muscles which hold the toes straight ( extended )at the IP joints will, IMO, lead first to "functional lesser toe deformity "and then, depending on the demands placed on the foot, perhaps to flexible deformity and then fixed deformity as the foot adjust to strength deficits.

Weak feet may not exhibit flexible lesser toe deformity if gait is adjusted by the CNS to avoid the usual later stages of toe off, however, "functional lesser toe deformity" will probably be present.

Back to the 1st sentence in this thread " Functional lesser toe deformity, is there such a thing and can it contribute to falls in the older population? "

I think there is, but then I am not a foot health care professional, as Eric has pointed out.

With regard to the simultaneous contraction of the toe extensors and flexors, Zelik 2015 et al found that-

"peak MTP flexor activity occurred significantly before peak MTP extensor activity during walking (P < 0.001)." And that" The sequential peak activity of MTP flexors followed by MTP extensors suggests that their biomechanical contributions may be largely separable from each other and from other extrinsic foot muscles during walking.
Paper https://doi.org/10.1007/s00421-014-3056-x

So, at least in the initial stages of hammer toe claw toe formation, an imbalance between extensors and flexors is unlikely to be the cause since these muscles are active at different phases of gait. The timing of activity in the extensors changes as deformity worsens .

Anyway, one of the major causes of lesser toe deformity ,lumbrical /interossei weakness . How can you tell if this is present ? Place the toes under load and look for IP flexion ,or buckling under load .

 

There is literature that supports the idea that the interossei will dorsiflex the toes when the toes are in a dorsiflexed position. I have seen this when pulling on the tendons in a cadaver. What literature do you have that says that the interossei plantar flex the MPJ?

Would you agree that the interosseous muscles inert on the side, and not the bottom of the proximal phalanx?

Sometimes the lumbricals and interossei are not active at the same time as the FDB. For evidence look at the picture you posted in post #9

 

Could you supply a reference for literature that supports the idea of the interossei acting to dorsiflex the toes at the MTPJ's ? I would be very interested to read it.

For example Dorsal Interossei of Feet : Wheeless' Textbook of Orthopaedics https://share.google/e9yuqZnHzG4Y4OZ8Z
On the dorsal interossei
" Action:
- assists in flexing the proximal phalanx and extending the middle and distal phalanges.
- abducts the toes from the longitudinal axis of the 2nd toe."

In the picture, the hammer toe configuration is caused by FDB contraction and the lack of force production from the lumbricals and interossei. I have taken to calling this type of toe buckling/collapse as "functional lesser toe deformity" .
People with healthy feet do not walk in such a way that the toes "claw up"/"hammer"at toe off.

I have read texts that advocate lumbrical/ interossei strengthening through toe curls/towel curls . Since this involves flexion at the IP joints, and the lumbricals, dorsal interossei and plantar interossei all extend these joints, I fail to see how such exercises could work. Trying to correct toe deformities caused by deficiency in plantar IP extensors, using toe curls, may actually make things worse . IMO such exercises are at best, useless for the purpose.

 

Since I appear to be introducing a new concept in this thread ,it is worth repeating that I am not a qualified foot health care professional and nothing I say is meant to be medical advice. Instead, the thread is aimed at researchers. The opinions I have expressed on the short foot exercise are by no means accepted by all, they are just my opinions.

That said, “Functional lesser toe deformity” may be a wide spread and largely undiagnosed problem with many believing the foot adopting, for example, a hammer toe configuration under load, is healthy and normal. I believe it is a clear sign of intrinsic foot muscle weakness and, if it is very common, then so is toe flexor weakness. A recent study of younger people found that their toe flexor strength increased by 57% after 6 months walking around in minimal footwear.

The lumbricals and interossei are small muscles but their function is vital to overall foot mechanics.

How would these muscles be strengthening ? Well ,the doming exercise looks like a good idea to me.

 

In a previous thread it was suggested that although the toes adopt a hammer position in Vele's forward lean they might not do the same during walking and running. In actual fact, the load placed on the toes is roughly the same during a single footed forward lean as it is during walking ,or at least it is with a strong healthy foot. However, research shows that the body compensates for lack of intrinsic muscle strength during toe off by altering gait strategies and thus reducing the load placed on the toes. Toes with atrophied lumbricals and interossei, therefore, need not "hammer" or "curl" during walking.(This might ,in part, be why the intrinsic minus feet of some diabetics do not display lesser toe deformity)

Farris et al 2019, used a tibial block to prevent input from the intrinsics and found "the impaired push-off in the nerve block condition led to a compensatory increase in hip joint positive work in late stance and early swing phases of walking and running (Fig. 5 and SI Appendix,Fig.S1), along with an elevated stride frequency" .

Permanent toe deformity may require weak intrinsics and loading that challenges the weakened toes ,to become established.

Weak lumbricals and interossei are clearly going to be a problem for the human foot. To strengthen these muscles, all of which attach to the bases of the proximal phalanges and act to plantar flex the toes at the metatarsophalangeal joints and extend the toes at the IP joints , surely you need to be working the toes in this self same pattern? Straight toes around the MTPJ's.

How can the short foot exercise, which involves pulling/pushing the met heads back towards the heel using muscles that span the arch, target the interossei?

 

Further to toe deformity being, in part, due to a combination of weakness in the intrinsic toe flexors coupled with loading that places a demand on the toes that they can't meet without buckling, it is interesting to look at 2 groups that load the forefoot and toes differently, men and women.

Healthy women load up the toes during standing and gait more than men do. Women also have a greater incidence of flexible and fixed toe deformities than men . Footwear factors aside , could the incidence of weak toes be the same between males and females but differences in loading patterns lead to differences in fixed deformity incidence?

Very little attention is given to the lumbricals and interossei of the foot and yet these are the muscles that keep the toes straight at the IP joints.

Yomamoto et al 2020
"Discussion
The most important finding of this study was that women applied a significantly higher peak pressure on
the hallux, toes, forefoot, and medial aspect of the foot while both standing and walking than men."

 

It is true that the lumbricals and interossei can be volitionally switched off to achieve motor tasks. For example, if you pick up marbles using your toes/feet you will likely need to switch these muscles off to grab the object in question since grabbing marbles with your toes involves flexing them at the interproximal joints . This demonstrates why exercises like marble pick ups or towel curls won't be effective when it comes to activating the lumbricals and interossei.

However, IMO, it would generally be incorrect to claim that toes under load during Vele's forward lean will usually assume a hammer toe configuration unless a person undergoes a period of lumbrical/ interossei training to keep the toes straight during loading. That would imply that the lumbricals and interossei are not generally used for postural tasks .

My personal observation, base around watching a number of people progress through a toe strengthening process, is that toes which buckle under load no longer do so when the intrinsic foot muscles become stronger.

With regard to applying force to the ground, the lesser toes can do so more effectively if the toes remain straight and torque around the MTPJs . Research shows that the FDB is more active if the toes are not being curled, and the FDL ,which acts isometrically, can apply force through a greater range of ankle plantarflexion if its insertion is at the end of a straight toe (curling toes see the insertion moving back towards the origin in the lower leg ).

IMO, the idea that healthy toes can apply more force to the ground when curled or in a hammer configuration than when straight, is wrong .

 

I think you are wrong here. You agree that lumbricals are voluntarily controlled. That is they work some of the time and not others. There is a simple explanation of sometimes seeing the hammertoe configuration. When FDB fires without lumbricals you get the hammertoe configuration. The contraction of FDB plantar flexes the intermediate phalanx and this causes increased ground reaction force on the distal aspect of the toe. This force on the distal aspect of the toe creates a dorsiflexion moment acting on the proximal phalanx at the MPJ. Now, if you contract the lumbrical at the same time as the FDB the lumbrical can create a great enough plantar flexion moment at the MPJ, and dorsiflexion moment at the PIPJ to prevent the toe from achieving the hammertoe position. This would imply that the lumbricals are used some of the time and not others.

So in the first quote above you claimed "it would generally be incorrect to claim that toes under load during Vele's forward lean will usually assume a hammer toe configuration unless a person undergoes a period of lumbrical/ interossei training to keep the toes straight during loading." How do you get someone to strengthen their lumbricals without training them to use the lumbricals?

 

I said they can be . For example, getting dressed this morning I dropped a sock on the floor then picked it up using the toes of my foot. The lumbricals/interossei would not have been involved in the act of curling the toes around the sock since they straighten them. Later, walking around the house, the toes would have remained straight during gait and the lumbricals interossei would have been active . If they were not active, the toes would have flexed at the IP joints as muscles like the flexor digitorum brevis, quadratus plantae and flexor digitorum longus became active.

I didn't say this, I said "it would generally be incorrect to claim that toes under load during Vele's forward lean will usually assume a hammer toe configuration unless a person undergoes a period of lumbrical/ interossei training to keep the toes straight during loading."

What this means is that if you take a person with strong healthy feet and ask them to posture forwards into Vele's forward lean the toes will remain straight at the IP joints without the subject ever having to practice keeping the toes straight. Straight toes are the default position of a healthy foot . Hammer toes are the default of a weak foot. With regard to the intrinsics, most habitually shod people have weak feet , even some world class runners.

In the nomenclature I am proposing "functional toe deformity" is different than "flexible toe deformity" but probably precedes it.

 

When considering lesser toe deformity it is important to consider the forces generated in the plantar fascia and plantar plate during weight bearing. These forces will be transmitted to the proximal phalanx of each lesser toe giving a plantar flexing moment at the MTPJ. This force helps resist the dorsiflexing forces exerted on the proximal phalanx of the lesser toes by the flexor digitorum longus and flexor digitorum brevis and so improves digital purchase.

What seems very clear to me, is that proximal phalanx plantar flexion forces at the MTPJ, derived from tension in the plantar fascia and plate, are not enough to resist the dorsiflexing , hammer toe creating, forces generated by contraction of the FDB and FDL. This is evidenced by the simple observation that for many people Vele's forward lean will produce contraction of the FDB and so functional hammer toes .The proximal phalanx plantarflexing force generated by the fascia and plate is not enough to counter the dorsiflexing force generated by the FDB and FDL during this simple manoeuvre.

The lumbricals and interossei are clearly vital for plantarflexing the proximal phalanx at the MTPJ and maintaining digital purchase during gait . Tension in the fascia will not do the job by itself . Muscular input is required.

 

Aha, functional lesser toe deformity is not a new concept at all but called Dynamic Toe Deformity at La Trobe.

Dr Ana Morais Azevedo was conducting a PhD study into the concept. Here is a link CLINICAL TIPS … ANA AZEVEDO ON SINGLE-LEG HEEL RAISES – DOWN UNDER THE FOOT – The BMJ Open Sport & Exercise Medicine blog

Physios at Australian ballet try to avoid the toes adopting a hammer position when under load . See A above .
I wonder how the PhD is progressing and if lumbrical/interosseus weakness has been identified as the culprit of " dynamic toe deformity" . After all, these muscle keep the toes straight.

Quote from article
"Why does dynamic deformity (toe curling) occur while performing SLHR( single leg heel raise)? Toe curling (i.e., claw, hammer, mallet toes) presents as flexion and/or extension of the interphalangeal and/or metatarsophalangeal joints (Figure 1). If the person cannot keep the neutral position of the interphalangeal and metatarsophalangeal joints, by maintaining contact of the toes and metatarsal heads against the floor, this may be a clinical sign of a deficit in the strength of the intrinsic foot muscles and overactivity of the extrinsic muscles such as flexor digitorum longus and flexor hallucis longus. "

 

A thread on podiatry arena called " The myth of toes curling to grip the ground" was started by me in April 2022, which predates the BMJ linked to blog from Latrobe, so I will go back to calling buckling under load "functional lesser toe deformity" since it does appear I may have been the first to go in print saying that hammer toes under load do not happen if all the intrinsic foot muscles are strong .

In the "myth of toes curling to grip the ground" thread I wrongly identified the FDB as the muscle which was failing to do its job under load when hammer toes transiently appear. It's pretty clear to me now that weak lumbricals and interossei are the culprit.

In the literature, one of the actions of the FDB is given as plantar flexion at the metatarsophalangeal joints in toes 2-5 . IMO ,this is true only if the lumbricals and interossei, which keep the toes straight ( extended at the IP joints) are strong and active. If the lumbricals and interossei are weak then the FDB acts to pull the middle and distal phalanges ( toe bones) in underneath the proximal phalanx wedging it up in a dorsiflexed position and this creates a downward force under the distal phalanx( ground reaction force) .

So ,with a loaded foot ,there is tension in the plantar fascia and this acts to plantarflex the toes at the MTPJs . The proximal phalanx is pulled to the ground and applies force ( or put another way, the toe bone nearest to the foot knuckle joint is pulled down to the ground as the fascia is put under tension by forces acting on the foot ). In closed chain the FDB can act to apply pressure to the ground not by flexing the toe at the MTPJ but by wedging the remaining 2 bones in under the already plantarflexing proximal phalanx ( the proximal phalanx plantar flexing due to the force applied by the plantar fascia through the plantar plate)

It is a complete mystery to me that the lumbricals and interossei seem to be almost completely ignored when it comes to lesser toe deformity . These muscles keep the toes straight for goodness sake.

Eric, a muscle can attach on the side of a proximal phalanx and still plantarflex the bone in question. For example, the adductor hallucis. You still have not provided a reference for literature "that supports the idea that the interossei will dorsiflex the toes when the toes are in a dorsiflexed position. " Could you provide one?

Figure ;This is an example of the middle and distal bones of a toe being wedged in underneath the proximal phalanx by FDB /FDL contraction to create downward pressure on the end of the toe and so generate ground reaction forces . The proximal phalanx ( the longest of the depicted bones) is being flexed downwards ( plantarflexed )by tension in the plantar fascia.

 

The linked to video, below ,from Eric Fuller does a good job of illustrating the action of the flexor hallucis brevis (FDB)) when this is the only structure applying a force to the bones of a lesser toe other than ground reaction forces, that is to say when plantarflexing forces from the plantar fascia are removed from the equation along with the straightening effect of the interossei and the lumbricals. All that happens in the model is the middle phalanx is plantarflexed at the proximal IP joint and this causes the proximal phalanx to dorsiflex at the MTPJ. No significant ground reaction forces would be produced beneath the "bones "of a subject toe at all .

So you could say that lack of tension in the plantar fascia produces a floating toe, and no amount of FDB strengthening will change that since , without the lumbricals or interossei, the FDB does not produce a plantarflexing moment around the MTPJs ( see video)

If we added a plantar fascia to Erics model then the situation would change . The plantar fascia would apply a plantarflexing moment ( force) to the proximal phalanx . The FDB, when active, would plantarflex the middle phalanx at the proximal IP joint and this would cause the proximal phalanx to tend to dorsiflex at the MTPJ. The proximal phalanx is being "held down to the ground "( plantarflexed) by tension in the plantar fascia so when the middle phalanx is plantarflexed by the action of the FDB , ground reaction forces will now be developed ( I should probably provide a drawing here ).

In a healthy foot with strong active lumbricals and interossei, things change yet again .Toes which remain straight (extended) at the proximal IP joint when the FDB is activated will not causes dorsiflexion at the MTPJ but plantarflexion . The forces being generated by the FDB will compliment and add to those being generated by tension in the plantar fascia . They will not be opposing forces.

Much the same sort of mechanics applies to the forces generated by the flexor digitorum longus . If we have a plantarflexing moment of the proximal phalanx due to tension in the PF, then flexion at the IP joints caused by FDL activity will see a net dorsiflexing moment at the MTPJs with forces from the fascia being overpowered by the dorsiflexing action at the MTPJs of FDL activity . Keep the IP joints extended and the FDL will act to plantarflex the entire toe at the MTPJ.

With straight IP joints all of ,(1) tension in the plantar fascia ,(2) force from the FDB , (3) force from FDL
(4 )Forces from the lumbricals and interossei , will act together to plantarflex the toe at the MTPJ.

Likely the four named forces acting together would make injury to the forefoot area much less likely than would be found in "functional lesser toe deformity" where some forces are opposing and not acting together.

Healthy lumbricals and interossei are absolutely vital to foot health. Key!