The abductor hallucis muscle ;what does it do ?

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As people will know ,the abductor hallucis muscle is one of the larger intrinsic muscles of the foot ,but what does it actually do ?

If you look at an anatomy textbook you will probably find its actions described as "toe flexion and abduction" . However ,if you look at some modern research texts , you might find its actions described thus -
"The action of the abductor hallucis is to flex and supinate the first metatarsal, invert the calcaneus, externally rotate the tibia, and elevate the medial longitudinal arch. (Nicholas Campitelli -Podiatry Today 2014 " with no mention of toe flexion/abduction at all .

In my opinion ,if you are going to discuss the role of the abductor hallucis muscle in supporting /raising the medial longitudinal arch ,you must acknowledge the levering effect that a plantarflexing great toe has on the mechanics of the foot as a whole . Generally ,this does not happen .

 

Following on from the previous post , in 2014 Kelly et al produced a paper which looked at the activity of 3 intrinsic toe flexor muscles with the foot under load . The muscles were the abductor hallucis ,the flexor digitorum brevis and the quadratus plantae .

The authors were looking at how the muscles behaved as the load on the foot was increased and also at the effects of electrical stimulation of the muscles in a preloaded foot . What I don't understand is how you can look at foot mechanics from the point of view of toe plantar flexor activity without factoring in toe plantar flexion .

Particularly perplexing is figure 6 drawing (a) ,(see below ) . In this drawing the bones of the foot in a loaded condition are represented by the grey shaded image and the position of the bones after additional stimulation of the abductor hallucis is represented by the red outline . Looking at the position of the proximal end of the first phalanx relative to the 1st metatarsal head ( MTPJ) the authors seem to be suggesting that additional stimulation of the abductor hallucis ,a toe plantar flexor ,actually produces toe dorsiflexion . Is this actually the case or is this schematic drawing simply inaccurate ?

Certainly, if I stand on one foot and press my toes down my COG shifts towards the heel and the MTFJs lift .

I just don't get how you can talk about toe plantar flexors supporting the arch but omit the levering effect produced by the toes as they plantarflex .

Any thoughts ?

Paper link
https://doi.org/10.1098/rsif.2013.1188

 

Here is a drawing of the rig used by Kelly and his colleagues in their 2014 paper . It is designed to simulate loading of the foot in a quite standing position .

In their analysis of the experimental results , the researchers seem to have assumed that changes in foot shape , brought about by electrical stimulation of the 3 toe flexor muscles being studied ,were due to forces being applied between the metatarsal heads and the calcaneus and there is no mention of toe plantar flexion at all . I feel that this is a mistake and here's why .

In a nut shell , if an individual stands on one leg then pushes their toes down into the ground the centre of mass is moved towards the heel offloading the longitudinal arch and giving a reduction in loaded arch length and an increase in arch height .

Think of a person crossing a chasm on a long plank of wood . The closer the person gets to the centre of the plank the more it bends or distorts . Walking across an arched plank will see it bend and flatten much like the weight bearing arch of the foot but, if you move the person closer to the end point of the arched plank you will see the plank under less stress and hence less strain / deformation .

It is entirely possible that the changes in foot shape observed by Kelly and his colleagues occurred not because the intrinsic foot muscles are supporting the arch but because causing the toes to plantarflex produced a shift in the location of the COG towards the heel.

I just don't understand how you could conceive of an experiment in which you stimulate the toe plantar flexors to study subsequent changes in foot shape and then not factor in the consequences of toe plantar flexion on foot mechanics in your analysis .

The authors looked at , among others , the quadratus plantae muscle . This attaches to the most distal phalangeal bones of the lesser toes via the tendons of the flexor digitorum longus muscle and so produces a very similar plantarflexion action . Can it be seriously suggested that contraction of the QP will shorten and raise the longitudinal arch but that no significant toe flexion will be produced ?

In my opinion ,for this experiment and for a number of other experiments subsequently carried out by this group into the intrinsic foot muscles ,the foot should be viewed as a whole ,from heel to toe tip and not merely from heel to met head .

 

I off course mean no offense to Luke Kelly and his colleagues by these posts, but their paper came into my head again today when I was doing squats in the gym .
How can the muscles of the medial arch of the foot, such as flexor hallucis brevis ,abductor hallucis and flexor hallucis longus ,support the medial longitudinal arch during the squat action if there is no pressure being generated under the big toe ? These muscles are all hallux plantar flexors ( including the abdh ).