The EHB....In all its glory...

The extensor hallucis brevis : An overlooked "miracle worker"

The Extensor Hallicus Brevis, or EHB as we fondly call it is an important muscle for descending the distal aspect of the 1st ray complex (1st metatarsal and medial cunieform) as well as extending the 1st metatarsophalangeal joint. It is in part responsible for affixing the medial tripod of the foot to the ground.  Its motion is generally triplanar, with the position being 45 degrees from the saggital (midline) plane and 45 degrees from the frontal (coronal) plane, angled medially, which places it almost parallel with the transverse plane. With pronation, it is believed to favor adduction (1).

It arises from the anterior calcaneus and inserts on the dorsal aspect of the proximal phalynx. It is that quarter dollar sized fleshy protruding, mass on the lateral aspect of the dorsal foot.  The EHB is the upper part of that mass. It is innervated by the lateral portion of one of the terminal branches of the deep peronel nerve (S1, S2), which happens to be the same as the extensor digitorum brevis (EDB), which is why some sources believe it is actually the medial part of that muscle. It appears to fire from loading response to nearly toe off, just like the EDB; another reason it may phylogenetically represent an extension of the same muscle (2-4).

Because the tendon travels behind the axis of rotation of the 1st metatarsal phalangeal joint, in addition to providing extension of the proximal phalynx of the hallux (as seen in the child above), it can also provide a downward moment on the distal 1st metatarsal (when properly coupled to and temporally sequenced with the flexor hallicus brevis and longus), assisting in formation of the foot tripod we have all come to love (the head of the 1st met, the head of the 5th met and the calcaneus).

Why is this so important?

The central axis of a joint (sometimes called the instantaneous axis of motion) is the center of movement of that articulation. It is the location where the motion will occur around, much like the center of a wheel, where the axle attaches. In an articulation, it usually involves one bone moving around another. Lets look at an example with a door hinge.

A hinge is similar to a joint, in that it has parts with is joining together (the door and the jamb), with a “joint” in between, The axis of rotation of the hinge is at the pivot rod. When the door, hinge and jamb are all aligned, it functions smoothly. Now imagine that the hinge was attached to the jamb 1/4” off center. What would happen? The hinge would bind and the door would not operate smoothly.

Now let’s think about the 1st metatarsal phalangeal joint. It exists between the head of the 1st metatarsal and the proximal part of the proximal part of the proximal phalanyx. Normally, because the head of the 1st metatarsal is larger than the heads of the lesser ones, the center of the joint is higher (actually,almost 2X as high; 8mm as opposed to 15mm). We also remember that the 1st metatarsal is usually shorter then the 2nd, meaning during a gait cycle, it bears the brunt of the weight and hits the ground earlier than the head of the 2nd.

The head of the 1st metatarsal should slide (or should we say glide?) posteriorly on the sesamoids during dorsiflexion of the hallux at pre swing (toe off). It is able to do this because of the descent of the head of the 1st metatarsal, which causes a dorsal posterior shift of the axis of rotation of the joint. We remember that the head of the 1st descends through the conjoined efforts of supination and the coordinated efforts of the peroneus longus, extensor hallucis brevis, extensor hallucis longus, dorsal and plantar interossei and flexor hallucis brevis (which nicely moves the sesamoids and keeps the process going smoothly)(1, 5).

Suffice it to say, if things go awry, the axis does not shift, the sesamoids do not move, and the phalanyx crashes into the 1st metatarsal, causing pain and if it continues, a nice spur you can write home about!

Treating and needling this muscle is easy, as it is very accessible on the dorsum of the foot and due to the decreased receptor density, is not too uncomfortable. We like to needle the peroneus longus and short flexors as well, as they all have the function of lowering the head of the 1st ray. Check it out in this quick how to video.

1. Michaud T: Human Locomotion: The Conservative Management of Gait Related DisordersNewton Biomechanics; First Edition 2011

2. https://www.physio-pedia.com/Extensor_Hallucis_brevis

3. http://www.wheelessonline.com/ortho/extensor_hallucis_brevis

4. Becerro de Bengoa Vallejo R., Losa Iglesias M.E., Jules K.T.  Tendon Insertion at the Base of the Proximal Phalanx of the Hallux: Surgical Implications (2012)  Journal of Foot and Ankle Surgery,  51  (6) , pp. 729-733.

5. Zelik, K.E., La Scaleia, V., Ivanenko, Y.P. et al. Eur J Appl Physiol (2015) 115: 691. https://doi.org/10.1007/s00421-014-3056-x

Proprioception trumps Biomechanics

As I sit here on a rare Friday afternoon, not working (OK, I am writing this, so sort of working) and looking out at the lake (picture above), while on a family camping trip, I think about a walk on the rocks this morning with my kids. I was watching my very skilled 7 year old jump from rock to rock while my 3 ½ year old, that thinks he is seven, tried to follow his older brother.

I had my foot on a rock which lowered the front of my foot in plantar flexion and stood on that leg. I noticed that my balance was not as great as it was when my foot was in dorsiflexion. This made me think about pronation and supination. Yes, it is not uncommon for me to think about such things, especially when I have some spare time. That is one of the things about being a foot and gait nerd; these sorts of things are always on our minds.

So, why was my balance off? Did I need more proprioceptive work? Were my foot intrinsics having issues? No, it was something much more mundane.

Pronation consists of dorsiflexion, eversion and abduction. This places the foot in a  “mobile adaptor” posture, reminiscent of our hunter/gatherer ancestors, who needed to adapt to uneven surfaces while walking over terra firma barefoot. Supination, on the other hand (which is the position my foot was in), consists of plantarflexion, inversion and adduction. It places the foot (particularly the midtarsals) in a locked position for propulsion (think of the foot position during toe off).

So why when my foot was plantar flexed and adducted while standing on this rock so much more unstable in this supposedly more stable, supinated position? I would encourage you, at this point, to try this so you can see what I mean. When I placed my foot in dorsiflexion on the rock, I was much more stable. A most interesting conundrum for a biomechanist.

Experimenting for a few minutes, alternating plantar flexion and dorsi flexion, gave me the answer. When we are walking on the flats, our foot is (usually) not pushed to the extremes of dorsiflexion; with the front of the foot up on a rock, it is much more so. This “extra” upward force on the front of the foot, provides much more sensory input (and thus proprioception) from the ball of the feet. Take a look at the sensory homunculus and you can see how much brain real estate is dedicated to your foot, especially the front portion. With this information, we are able to apply more  force through the posterior compartment of the leg,which is stronger than my anterior compartment (as it is with most folks).

When the front of your foot is in plantar flexion (ie, your heel is on the rock), we have less sensory input to the balls of the feet, and rely more on the anterior compartment (weaker in many folks, including myself) to counterbalance the weight of our body.

Mystery solved: proprioception trumps biomechanics; more proof that the brain is smarter than we are.

The Gait Guys. Solving the worlds great gait questions, one at a time.