Flexor Hallucis Brevis

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Description: 

Flexor hallucis brevis (FHB) is one of the third layers (of four layers) of plantar muscles. It is located adjacent to the plantar surface of the 1st metatarsal and contains 2 sesamoid bones.  

Origin:

The flexor hallucis brevis originates along the plantar aspect of the cuboid and cuneiforms.

Insertion:

The medial head of the FHB is the larger head of the two. It inserts medially on the plantar plate. Also, it forms a common tendon with the abductor hallucis muscle before the insertion point at the base of the proximal phalanx. Within this tendon sits the tibial sesamoid bone.

The lateral head of the FHB inserts laterally on the plantar plate. It forms a common tendon with the adductor hallucis muscle before inserting on the lateral aspect of the base of the proximal phalanx. Within this tendon sits the fibular sesamoid bone.

Nerve:

The medial and lateral head of the flexor hallucis brevis is innervated by the medial plantar nerve. Both heads are represented by the spinal segments S1 and S2.

Artery:

Supplied by branches of the posterior tibial artery.

Function

Flexion of the great toe at the metatarsophalangeal joint with the assistance of the flexor hallucis longus.

Clinical Significance

Dysfunction of the flexor hallucis brevis will commonly present as pain in the ball of the foot when extending the big toe, difficulty and pain during gait, and toe deformities. This may be due to muscle injury of the FHB or sesamoiditis.

Aggravation of the muscle may occur when the second toe is longer than the other toes, walking or running on uneven ground, wearing high heels or shoes that are too small.

Assessment

Palpation: 

It is nearly impossible to palpate the FHB muscle as it is located deep in the foot. (The third layer from 4 layers of the foot muscles).

Muscle strength: 

FHB strength can be measured manually by MMT grading. Start the test by having the patient in supine/long sitting position with the foot hanging over the table. With your hand, hold the foot just below the ankle for stabilization and ask the patient to flex the big toe while you resist the movement by your fingers of the other hand.

Treatment

Stretching:

Stretch the muscle by moving the big toe into hyper extension as much as possible, hold then relax.

Strengthening

Pulling the towel would always be the good option for strengthening the big toe and other four toe flexors, but make sure that the patient is using the FHB muscle so she/he should flex the great toe by bending it at the MP joint. You can progress the exercise by putting a weighted object on the towel or use a resistance band.

References