Flexor hallucis longus !
The flexor hallucis longus is a deep calf muscle that curls the big toe and helps push off when walking.
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What it is
A deep calf muscle.
What it does
Curls the big toe and helps push off.
Key facts
- Origin
- Lower two-thirds of posterior fibula; interosseous membrane
- Insertion
- Base of distal phalanx of big toe
- Action
- Flexes big toe; weak plantar flexion; supports medial longitudinal arch
- Nerve
- Tibial nerve (S2–S3) (some texts: L5–S2)
- Blood supply
- Fibular artery; posterior tibial artery
Relations
- Course
- From the lower fibula it runs down and medially across the back of the lower tibia, through the groove on the posterior talus, under the sustentaculum tali, deep to the flexor digitorum longus tendon in the sole and between the two sesamoids of the big toe
- Medial
- Posterior tibial vessels and tibial nerve, then flexor digitorum longus
- Deep
- Fibula and interosseous membrane; the fibular artery runs close to the fibula under or within it
- Superficial
- Soleus and, above the heel, the fat pad in front of the Achilles tendon (Kager's fat pad)
- Lateral
- Fibularis muscles, beyond the posterior intermuscular septum
How to find it
It is deep and hard to feel at the ankle. With the big toe flexed against resistance, the tendon can be felt in the sole along the medial forefoot and between the sesamoids under the first metatarsal head.
Examination
- Resisted flexion of the big-toe tip while the proximal phalanx is held: weakness suggests a flexor hallucis longus or tibial nerve lesion
- Big-toe extension compared with the ankle plantarflexed and then dorsiflexed: less extension with the ankle dorsiflexed points to a tight or tethered flexor hallucis longus (functional hallux limitus)
On imaging
On MRI, some fluid in its sheath at the ankle can be normal because the sheath often communicates with the ankle joint. A lateral ankle radiograph shows an os trigonum behind the talus.
Clinical notes
- Posterior ankle impingement from an os trigonum often coexists with tenosynovitis of this tendon
- Its tendon is transferred to the calcaneus for chronic Achilles rupture or severe insertional Achilles disease
- Cuts of the medial sole can divide the tendon, leaving the big-toe tip unable to flex
Anatomical variants
- A tendinous slip linking it with flexor digitorum longus in the sole is common
- Rare accessory muscles, such as the peroneocalcaneus internus, can lie beside it in the tarsal tunnel
Development
A derivative of the ventral (flexor) muscle mass, hence its tibial nerve supply. After the medial rotation of the lower limb in week 7 it lies on the fibular side of the deep posterior compartment, so its tendon has to cross the back of the ankle to reach the big toe.
Related conditions
Test yourself
Think of the answer, then tap the question to see it.
Where does the flexor hallucis longus tendon lie in the tarsal tunnel?
Most posterior, in a groove between the two tubercles of the posterior talus, then under the sustentaculum tali.
What is «dancer's tendinitis»?
Catching of the flexor hallucis longus tendon in its fibro-osseous tunnel behind the talus in ballet dancers on pointe, sometimes with triggering of the big toe.
Why is the flexor hallucis longus tendon the key landmark in posterior ankle arthroscopy?
The tibial nerve and posterior tibial vessels lie just medial to it, so work stays lateral to the tendon.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- flexor longus hallucis TA2 2668
- Official Spanish (DeCS)
- Músculo flexor largo del dedo gordo
- English (TA2)
- Flexor hallucis longus
- Also called
- flexor hallucis longus muscle
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Bony Pelvis and Lower Limb: Calf Flexor Hallucis Longus Muscle; 8.5 Development of the Appendicular Skeleton (OpenStax (Rice University), 2022) · adapted, CC BY 4.0.
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Representative model, for educational purposes only.
