FDL stands for flexor digitorum longus, a muscle in the back of your lower leg that controls the curling motion of your four smaller toes. It runs from the shinbone (tibia) down through the ankle and splits into four separate tendons, one for each of the second through fifth toes. The FDL plays a key role in walking, balance, and pushing off the ground with each step, and it’s a muscle that orthopedic surgeons and physical therapists frequently reference when treating foot and ankle problems.
Where the FDL Is Located
The flexor digitorum longus originates on the back surface of the tibia, just below a bony ridge called the soleal line. From there, its muscle fibers travel downward behind the inner ankle bone and into the sole of the foot, where the single tendon fans out into four separate tendons. Each of those four tendons attaches to the base of the last bone (the distal phalanx) of the second, third, fourth, and little toes.
The FDL sits in a deep compartment of the calf, tucked behind more superficial muscles like the calf muscles you can see and feel. It’s controlled by the tibial nerve, which carries signals from the S2 and S3 spinal nerve roots in your lower back. Because of its deep position and long path through the ankle, the tendon passes through a tight tunnel behind the inner ankle alongside other tendons, nerves, and blood vessels.
What the FDL Does
The primary job of the FDL is bending your four smaller toes downward, a motion called flexion. You use this every time you grip the ground while standing, push off during walking or running, or curl your toes inside your shoes. The muscle also contributes to pointing the foot downward (plantar flexion) and helps support the arch of the foot.
During walking, the FDL is most active in the push-off phase of each stride. As your heel lifts and your weight shifts forward onto the ball of your foot, the FDL contracts to stabilize your toes against the ground and generate forward propulsion. This makes it essential for balance, particularly on uneven surfaces. Its line of pull also closely mirrors that of the posterior tibial tendon, another important muscle for arch support, which is why surgeons sometimes use the FDL as a replacement when that tendon fails.
How the FDL Connects to the Big Toe Tendon
One anatomically interesting feature of the FDL is that it crosses and interconnects with the tendon of the flexor hallucis longus (FHL), the muscle that curls the big toe. These two tendons intersect on the sole of the foot, and in about 85% of people, a fibrous slip connects the FHL to the FDL. Less commonly (about 12% of people), the connection runs in the opposite direction, from the FDL to the FHL. A small percentage, roughly 3%, have a crossed connection between the two.
These interconnections matter clinically. When the FDL tendon is harvested for reconstructive surgery, the natural connection from the FHL can help preserve some residual curling ability in the smaller toes. But the interconnections also complicate the surgical harvesting process, since surgeons need to account for variable tendon lengths and branching points that differ from person to person.
FDL Tendon Transfer for Flat Foot
The most common surgical use of the FDL involves transferring it to replace a damaged posterior tibial tendon, the primary tendon responsible for maintaining the foot’s arch. When the posterior tibial tendon degenerates (a condition called posterior tibial tendon dysfunction, or PTTD), the arch gradually collapses and the foot rolls inward. This is one of the leading causes of adult-acquired flatfoot.
In stage II PTTD, where the foot is still flexible, surgeons can reroute the FDL tendon to take over the posterior tibial tendon’s job. The FDL is the most widely studied and commonly used tendon for this procedure. Its path through the ankle runs in a similar direction to the posterior tibial tendon, making it a natural substitute. The trade-off is strength: the FDL generates only about 28% of the force that the posterior tibial tendon produces. The FHL is stronger at roughly 56%, but the FDL’s alignment and accessibility make it the preferred choice in most cases. The goals of the transfer are pain relief, restoring the arch, rebalancing the foot’s mechanics, and preventing further deformity.
One practical consequence of the transfer is reduced toe-gripping ability. Since the FDL tendon is detached from the toes and rerouted, patients lose some active flexion in their smaller toes. For most people, this is a minor functional loss compared to the benefit of restoring arch support and eliminating pain.
Common FDL Injuries and Symptoms
The FDL tendon can develop tenosynovitis, an inflammation of the sheath surrounding the tendon. This typically causes pain and swelling along the inner ankle or the sole of the foot, often worsening with activity. Swelling is usually the earliest sign, followed by a noticeable gap between how far you can curl your toes on your own versus how far someone else can move them for you. As the inflammation progresses, pain and restricted motion become more prominent.
Tenosynovitis of the FDL can result from overuse (common in runners and dancers), inflammatory conditions like rheumatoid arthritis or gout, or less commonly from infection. Non-infectious cases tend to develop gradually and worsen over weeks to months if untreated. In rare cases of infectious tenosynovitis affecting the toe tendons, symptoms include a swollen finger or toe held in a slightly bent position, tenderness along the entire tendon sheath, and significant pain when the digit is straightened passively.
Anatomical Variations
Some people have an extra muscle called the flexor digitorum accessorius longus (FDAL) that runs alongside the FDL. This variant is present in roughly 4% to 12% of the population, with one study finding it in 12% of individuals and in 20% of men specifically. The FDAL is usually discovered incidentally on MRI or during surgery. In most cases it causes no symptoms, but it can occasionally contribute to crowding in the tarsal tunnel behind the ankle, leading to nerve compression and pain.
Exercises That Target the FDL
Strengthening the FDL is straightforward and requires no special equipment. The classic exercise is the towel scrunch: sit with your foot flat on a towel, then grip and pull the towel toward you using only your toes. This isolates the toe flexors and can be done daily.
Resisted toe flexion is another direct approach. With your foot in a neutral position or pulled slightly upward, press your four smaller toes down against resistance, whether that’s a therapist’s hand, a resistance band, or simply the floor. The key is keeping your ankle still so the work stays in the toe flexors rather than the larger calf muscles.
Balance training is a more functional way to engage the FDL. Standing on one leg, using a wobble board, or balancing on a foam pad all force the FDL to fire repeatedly as your toes grip and adjust to keep you upright. These exercises build both strength and the neuromuscular coordination the FDL needs for its stabilizing role during walking and running.