Flexor hallucis longus (FHL) pain is discomfort localized to the inner ankle and foot, commonly reported by active individuals. This condition is frequent among athletes, such as ballet dancers and runners, whose activities require extensive push-off or repetitive ankle pointing. The pain typically stems from an overuse or strain injury to the tendon that controls the big toe. Understanding the mechanics of this injury is the first step toward recovery.
Understanding the Flexor Hallucis Longus Muscle
The Flexor Hallucis Longus (FHL) is a deep muscle located in the posterior compartment of the lower leg. It originates from the back of the fibula bone, forming a long tendon that travels a complicated path to the foot. This tendon passes through a narrow fibro-osseous tunnel behind the ankle bone, specifically underneath the bony prominence known as the sustentaculum tali.
The tendon continues along the sole of the foot before inserting onto the base of the big toe’s end bone, the distal phalanx. The primary action of the FHL is to flex the great toe, but it also assists with pointing the ankle downward (plantarflexion) and stabilizing the arch during movement. Because of its winding course around the ankle, the FHL tendon is highly susceptible to friction and compression injuries in this tight space.
Primary Causes of FHL Pain
FHL pain is most frequently attributed to overuse injuries, primarily FHL Tenosynovitis. This condition involves inflammation of the synovial sheath, the protective membrane that surrounds the tendon as it glides through the ankle tunnel. Repetitive, forceful movements, such as rising onto the toes, cause the tendon to rub excessively against the bony canal, leading to irritation and swelling.
This inflammation restricts the tendon’s smooth movement, sometimes referred to as “Dancer’s Tendinitis” or “Trigger Toe.” In chronic cases, the condition progresses to FHL Tendinopathy, a degenerative change within the tendon structure itself. Tendinopathy is characterized by a breakdown of the tendon’s collagen fibers due to repeated micro-trauma without sufficient time for repair.
Acute causes of pain, while less common, include a sudden strain or a partial tear of the FHL muscle or tendon fibers. These injuries happen during a sudden, forceful push-off or a traumatic ankle sprain that abruptly overstretches the tendon. An extra bone fragment behind the ankle, called an os trigonum, can also cause impingement and friction on the FHL tendon.
Diagnosing and Initial Management
A healthcare provider diagnoses FHL pain using a patient’s history of activity combined with a detailed physical examination. The clinician will palpate the area behind the inner ankle bone to locate tenderness, a common sign of FHL involvement. Specific clinical tests are performed, such as resisting the patient’s attempt to flex their big toe, which reproduces the pain and confirms FHL irritation.
Imaging studies are used to rule out other conditions or determine the extent of the injury. X-rays can identify bone spurs or the presence of an os trigonum that may be impinging on the tendon. Magnetic Resonance Imaging (MRI) is the preferred method for visualizing soft tissues, revealing inflammation in the tendon sheath, chronic degeneration, or any partial tears.
Initial management focuses on reducing inflammation and preventing further tendon irritation. The R.I.C.E. principles—Rest, Ice, Compression, and Elevation—are recommended immediately following symptom onset. Rest involves temporarily avoiding activities that require forceful big toe push-off, sometimes necessitating a walking boot for severe cases. Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) can be used briefly to manage pain and local swelling.
Long-Term Treatment and Rehabilitation
Conservative treatment, primarily a structured physical therapy program, is the standard long-term approach for FHL issues. Rehabilitation focuses on restoring the tendon’s capacity by introducing controlled, progressive loading. This process begins with gentle range-of-motion exercises for the ankle and big toe to maintain flexibility.
Strengthening exercises are then introduced, emphasizing eccentric loading, which involves strengthening the FHL while the muscle is lengthening. This type of exercise encourages healing and improves the tendon’s tolerance to stress. Footwear modifications, including custom orthotics, may be recommended to correct excessive pronation or stabilize the foot, reducing strain on the FHL tendon.
If conservative treatments fail to provide relief after several months, other interventions may be considered. Corticosteroid injections can be administered under ultrasound guidance to reduce inflammation around the tendon sheath. For chronic cases resistant to non-surgical methods, or when a triggering sensation is caused by an anatomical obstruction, surgery may be necessary. Surgical options, such as tenolysis, involve releasing the tight sheath to allow for smooth gliding.