The Flexor Hallucis Longus is a crucial muscle in the posterior compartment of the leg. As its name implies, this muscle is largely responsible for the flexion of the big toe (hallux), but its role extends beyond this singular movement, contributing significantly to foot stability and propulsion.
Location
The Flexor Hallucis Longus is located in the deep posterior compartment of the leg, lying laterally to its counterpart, the Flexor Digitorum Longus, and medial to the tibia.
Origin and Insertion
Origin: It originates from the middle two-thirds of the posterior surface of the fibula and the interosseous membrane.
Insertion: The muscle descends down the leg and posterior aspect of the ankle, passing along the medial side of the calcaneus (heel bone). It proceeds into the sole of the foot and attaches to the plantar surface of the distal phalanx of the big toe.
Structure
The muscle is long and slender at its origin, becoming more tendinous as it nears the ankle. As it approaches its insertion, the tendon of the Flexor Hallucis Longus runs in a groove behind the distal end of the tibia and talus and then moves along the plantar aspect of the foot to reach the big toe.
Function
- Toe Flexion: Its primary function is to flex the big toe, especially during the terminal stance and pre-swing phase of walking, providing propulsion.
- Foot Arch Support: The FHL plays a supporting role in maintaining the arch of the foot, especially during push-off in walking or running.
- Ankle Stabilization: The Flexor Hallucis Longus also provides stability to the ankle joint by resisting excessive supination, ensuring balanced weight distribution across the foot.
Innervation: The Flexor Hallucis Longus is innervated by the tibial nerve, which originates from the sciatic nerve.
Blood Supply: The blood supply to the FHL is from the posterior tibial artery, a major artery of the leg that ensures an adequate supply of nutrients and oxygen to the muscle.
Clinical significance
- FHL Tendinopathy: Repetitive strain or overuse can lead to inflammation of the FHL tendon, especially where it runs in the groove behind the talus and tibia. This is commonly seen in ballet dancers, often referred to as “dancer’s tendinitis.” It presents as pain at the posterior aspect of the ankle, particularly during toe-pointing movements.
- Hallux Limitus/Rigidus: Dysfunction of the FHL can contribute to stiffness (limitus) or rigidity (rigidus) in the big toe joint. It can cause pain and limited motion in the toe, affecting walking and other activities.
- Stenosing Tenosynovitis or “Trigger Toe”: This condition involves the locking or snapping of the big toe due to thickening of the FHL tendon or its sheath. It’s another pathology observed in ballet dancers, especially those frequently performing demi-pointe and pointe work.
- Ankle Impingement: An enlarged or inflamed FHL tendon can contribute to posterior ankle impingement, especially during plantar flexion of the foot.
- Hallux Valgus (Bunions): While primarily a structural deformity, a weakened or dysfunctional FHL can exacerbate the symptoms or progression of bunions.
- Surgical Harvesting: The FHL tendon is sometimes harvested for use in other tendon transfer or reconstructive surgeries, especially in the foot and ankle. For instance, in cases where the Achilles tendon is damaged beyond repair, a segment of the FHL can be used as a graft.
- Diagnostic Importance: An MRI or ultrasound of the ankle might be recommended if there’s suspicion of FHL pathology. These imaging modalities can detect changes in the tendon’s thickness, inflammation, or other structural abnormalities.
