The flexor hallucis longus muscle (french : muscle long fléchisseur propre du gros orteil) - named from its action upon the great toe (hallux) and its length - is a strong fusiform sheet.
- The lower two-thirds of the postero-internal (or flexor) surface of the fibula external to the oblique line ;
- the intermuscular septa between it and the tibialis posticus in front, and the peronei outside ;
- the deep fascia covering its posterior surface ;
- the lowest portion of the interosseous membrane.
- The under surface of the base of the last phalanx of the great toe ;
- by a small slip into that part of the flexor longus digitorum tendon which is distributed to the second and third toes.
The muscle arises by fleshy fibres which pass with a bipenniform arrangement into the tendon. This tendon appears first just below the middle of the leg at the back of the muscle near its inner border. The fleshy fibres are inserted into it as far as the ankle joint, and just above this point the tendon passes through the groove at the outer part of the back of the lower end of the tibia. The tendon then grooves the back of the astragalus, and afterwards the under surface of the sustentaculum tali, where it lies external to the tendon of the flexor longus digitorum. From this point it passes forwards in the second layer of the muscles of the sole, lying above and crossing the tendon of the flexor longus digitorum, to which it gives a small slip. It then crosses beneath the inner head of the flexor brevis hallucis, lies in the groove between the sesamoid bones of that muscle, and is finally inserted into the base of the last phalanx.
From the posterior tibial nerve by branches which enter the muscle in the upper part of its posterior surface near its inner border.
This muscle, which is much more powerful than the flexor longus digitorum, is a strong flexor of the last phalanx of the great toe, and is of great importance in walking, as it presses the great toe firmly against the ground. The ungual phalanx of the great toe is the last part of the foot to leave the ground when : the step is completed ; and until this is the case the flexor longus hallucis is strongly contracted. It will also help to flex the first phalanx of the great toe upon its metatarsal bone and it will act upon the joints which intervene between the first metatarsal bone and the astragalus ; and, finally, it will assist in the extension of the ankle joint.
Superficially, in the leg, it is covered by the soleus, and in the foot by the abductor hallucis, the flexor longus digitorum, the external plantar vessels and nerve; on its outer side are the peronei ; on its deep aspect in the leg lie the tibialis posticus and the peroneal vessels ; and, after passing over the back of the ankle and other joints, it lies upon the inner head of the flexor brevis hallucis.
An accessory portion of the muscle may be inserted into the sustentaculum tali or the inner surface of the calcaneum. The slip to the flexor longus digitorum tendon may vary in the number of toes to which it is distributed.