The Flexor Hallucis Longus (FHL) is a powerful muscle located in the deep compartment of the posterior lower leg. This muscle plays a significant role in controlling the mechanics of the foot, specifically the large first digit, or big toe. The FHL is one of the three deep muscles in this region, contributing to the strength and stability required for activities like walking, running, and jumping. Its long tendon allows it to exert considerable force, impacting the foot’s function beyond simple toe movement.
Anatomy and Path of the FHL
The Flexor Hallucis Longus muscle belly originates primarily from the posterior surface of the fibula over its lower two-thirds. A portion of the muscle also arises from the interosseous membrane connecting the fibula and tibia. This long muscle descends down the leg, forming a tendon that is nearly the length of the entire muscle.
The tendon’s path is notable as it travels behind the ankle, passing through the tarsal tunnel on the inner side. It navigates a groove on the posterior surface of the talus bone and runs under the sustentaculum tali (a bony shelf on the heel bone). This complex route changes the tendon’s direction of pull before it continues forward along the sole of the foot. The tendon finally inserts onto the plantar surface of the base of the distal phalanx of the big toe.
Primary Function
The primary action of the Flexor Hallucis Longus is the powerful plantar flexion of the hallux, or big toe. The FHL flexes all joints of the great toe, making it a strong and efficient flexor. While this powerful flexion allows for grasping objects, its more common function is providing a rigid lever for pushing off the ground.
A secondary function of the FHL is its contribution to plantarflexion of the ankle joint (pointing the foot downward). It assists the larger calf muscles in this motion. The FHL also plays a role in mildly inverting the foot and supporting the arch structure.
Stabilizing the Foot and Propulsion
The practical application of the FHL is most evident during dynamic movements, especially the gait cycle of walking and running. The muscle is highly active during the “toe-off” or propulsive phase. As the heel lifts and body weight shifts forward, the FHL provides a final, strong push-off force by flexing the big toe against the ground. This action accelerates the body forward.
Throughout the stance phase, the FHL functions to maintain the integrity of the foot’s structure. It acts as a dynamic support for the medial longitudinal arch, preventing the arch from collapsing under the body’s weight. The muscle’s stabilizing role is also important for balance, especially when traversing uneven surfaces.
Common FHL-Related Conditions
When the Flexor Hallucis Longus is subjected to excessive or repetitive strain, it can develop pathological issues, most commonly FHL tendinopathy. This condition involves irritation, inflammation, or micro-tearing within the tendon, often presenting as pain deep in the ankle or under the big toe. The pain is aggravated by movements that require rising onto the toes or forceful push-off.
A more specific issue is flexor hallucis longus syndrome, sometimes called “dancer’s tendonitis.” This syndrome involves the tendon becoming impinged or irritated as it travels through the narrow fibro-osseous tunnel behind the medial malleolus. This is common in populations requiring repetitive, forceful plantarflexion, such as ballet dancers, runners, and other athletes. The muscle belly itself can sometimes be too long, contributing to friction and subsequent pain where the tendon passes through the ankle joint.