FHL Transfer: Procedure, Recovery, and Muscle Adaptations

Flexor hallucis longus (FHL) tendon transfer is a surgical procedure that repurposes the tendon responsible for curling the big toe downward to reconstruct or reinforce a damaged Achilles tendon. The operation has become a workhorse solution for chronic Achilles tendon ruptures and severe tendinopathy that have not responded to conservative treatment, and the transferred muscle has a striking ability to grow in response to its new mechanical role. The procedure raises practical questions about what happens to the big toe afterward, how the muscle adapts, and what recovery actually looks like, all of which the research addresses in useful detail.

When FHL Transfer Is Used

The FHL tendon sits deep in the calf, running behind the ankle and under the foot to the big toe. Its path places it conveniently close to the Achilles tendon, and the muscle itself contributes to pushing off the ground during walking and running. These anatomical advantages make it a logical donor when the Achilles is beyond direct repair. The technique was originally developed for neglected Achilles ruptures where the tendon ends have retracted and scarred too far apart to be stitched back together, but the indications have expanded over time to include chronic tendinosis and insertional tendinopathy.

Before surgery, imaging with MRI is standard to measure the size of the Achilles defect and to evaluate the condition of the FHL muscle itself.² This step matters because the anatomy of the FHL muscle varies between people. A cadaver study found three typical patterns of the muscle belly, with some specimens having only a single lateral belly rather than the usual two-part structure. The researchers recommended preoperative MRI specifically to make sure the harvested tendon will be long enough to bridge whatever gap exists in the Achilles.

Surgical Approaches and How They Compare

Surgeons can harvest the FHL tendon through several routes, and the choice of approach affects how much tendon length is available and how many complications tend to follow. The three main options are the single-incision technique (one cut behind the ankle), the double-incision technique (a second cut in the arch of the foot to release the tendon further along its path), and an arthroscopic approach using small portal incisions and a camera.

The single-incision method yields roughly 5 centimeters of usable graft. Adding the second incision in the foot arch extends the harvest to about 8 centimeters on average, a significant increase.³ An anatomical study confirmed similar numbers and also tested a minimally invasive technique that pulled the tendon even further, averaging over 17 centimeters.⁴ More tendon length gives the surgeon more material to work with when bridging large gaps, but the trade-off is greater surgical complexity and a second wound site.

When researchers compared complication rates across the three approaches, arthroscopic transfer had the lowest rate of postoperative problems, followed by the single-incision and then the double-incision technique. The overall difference was statistically significant, and the authors suggested that arthroscopy and single-incision methods should be preferred when feasible.⁵ That said, when the Achilles defect is very large, the extra length available from a double-incision harvest may outweigh the marginally higher complication risk.

How the Tendon Is Anchored

Once harvested, the FHL tendon needs to be securely fixed to the calcaneus (heel bone). Two main methods dominate: drilling a bone tunnel and pulling the tendon through it, or anchoring the tendon with an interference screw pressed into a bone socket. The question of which holds better has been tested in biomechanical studies with slightly mixed but ultimately reassuring results.

One cadaver study found that interference screw fixation produced similar ultimate strength and peak stress compared to bone tunnel fixation. The screw group did show a modest decrease in tissue stiffness and absorbed more energy before failure, but those differences did not translate into a weaker overall construct.⁶ A separate biomechanical study tested screw fixation against suture fixation through bone and found that the screw group showed less loosening during repeated loading cycles and trended toward higher ultimate failure loads, though the difference was not statistically significant.⁷ In practice, both fixation methods work well, and the choice often comes down to surgeon preference and the specific defect being repaired.

The Muscle Actually Grows to Meet Its New Job

One of the more remarkable aspects of FHL transfer is what happens to the muscle after surgery. When the FHL takes on the workload of the damaged Achilles, it does not simply maintain its original size. It hypertrophies, sometimes dramatically.

A study comparing MRI scans of the operated leg to the non-operated leg found an average increase of 52% in FHL muscle cross-sectional area after transfer for chronic Achilles rupture. Individual patients ranged from a 9% to a 104% increase. The researchers interpreted this as evidence of strong adaptive capacity, particularly in patients whose calf muscle complex had been severely weakened before surgery.⁸ Another study looking at MRI before and after surgery (rather than comparing legs) found a median increase of about 20% in FHL cross-sectional area across all patients, with the growth reaching about 30% in a subgroup where the native Achilles insertion was also reattached.⁹

Earlier observations had already noted this pattern. In one series of patients treated for chronic Achilles problems, hypertrophy of the FHL muscle exceeding 15% was seen in multiple patients, and the researchers concluded that muscle growth strongly suggested the transferred tendon was functionally contributing to push-off power rather than just acting as a passive scaffold.¹⁰

The degree of hypertrophy varies between patients, and the reasons for that variability are not fully settled. It likely depends on how severely the original calf muscles were weakened, how much load the FHL is required to take on, the patient’s activity level during rehabilitation, and individual biological differences in the muscle’s response to training stimuli. But the basic finding is consistent: the FHL muscle does not just passively survive the transfer. It actively remodels to handle its increased mechanical demand.

What Happens to the Big Toe

Harvesting the FHL tendon means sacrificing the main muscle that flexes the big toe. This naturally raises concerns about losing push-off power and changing how the foot functions during walking. The reality is more nuanced than you might expect.

Biomechanical testing has quantified the immediate effect. After simulated FHL harvest, great toe flexion pressure dropped by roughly 30% on average, and total forefoot push-off pressure decreased by about 20 to 25%, depending on the foot’s specific anatomical crossover pattern at the knot of Henry (the point where the FHL and the other toe-flexor tendon cross in the arch).¹¹ A related study confirmed these ranges, showing decreases in great toe flexion pressure from 21% to 34% depending on the anatomical variant.¹²

Those lab numbers sound alarming, but the clinical picture is more forgiving. In a study of patients followed long-term after FHL transfer for large Achilles defects, all had measurable weakness in big toe flexion, yet none complained of any noticeable functional problem in daily life.¹³ Pedobarography (pressure mapping during walking) has shown measurable unloading of the first toe and a shift of force to the metatarsal heads after FHL harvest, but after about two years, patients’ gait appeared normal to clinical observation, with the loading asymmetry well compensated.¹⁴

The likely explanation is that the other toe flexor, the flexor digitorum longus, partially takes over big-toe flexion through that knot of Henry crossover, and the intrinsic muscles of the foot also adapt. The body compensates surprisingly well for the loss of one tendon’s contribution, at least for everyday activities. Whether elite athletes would notice the deficit during high-level performance is a different question, and one the current literature cannot fully answer.

Recovery and Rehabilitation

Postoperative protocols after FHL transfer have traditionally been conservative: a period of non-weight-bearing in a cast or boot, followed by gradual progression to full weight-bearing over several weeks. A typical standard protocol keeps the patient immobilized for around six weeks before beginning range-of-motion exercises, with full return to activity taking several months.

There has been growing interest in whether patients can safely begin weight-bearing and movement earlier. A systematic review of tendon transfers in the foot and ankle using interference screw fixation found that early rehabilitation protocols (starting weight-bearing or motion sooner than the traditional six-week mark) resulted in zero early tendon failures across all the studies that reported them. Function improved significantly under both early and standard protocols, and patient satisfaction was high either way. The authors noted, however, that evidence specifically supporting early loaded activities remains limited, so the trend toward accelerated rehab is promising but not yet definitive.¹⁵

Rehabilitation typically progresses through phases: initial protection (boot or cast, crutches), early mobilization (gentle ankle range of motion, toe movement), strengthening (calf raises, resistance band exercises), and sport-specific training for athletes. The timeline from surgery to return to sport varies widely and depends on the severity of the original injury, the patient’s age, and how well the transferred muscle adapts.

Return to Activity and Functional Outcomes

Most studies report strong improvements in pain and function scores after FHL transfer. A study of patients over 50 with chronic insertional Achilles tendinopathy found significant improvements across all validated ankle scoring systems, and every patient returned to daily activities.¹⁶ For the typical patient whose goal is walking, climbing stairs, and getting through a normal day without pain, the operation delivers well.

Sports participation is a trickier outcome. A study tracking return to sport after FHL transfer for neglected Achilles ruptures found that the median activity level score was maintained at preinjury levels. That is the good news. The more honest picture: roughly a third of patients returned to sport at a level one point lower than before the injury, with less favorable clinical outcome scores.¹⁷ The surgery restores substantial function, but expecting a complete return to pre-injury athletic performance is unrealistic for a meaningful minority of patients.

How the surgery is performed may also influence outcomes. A comparison of endoscopic FHL transfers versus open procedures found that the endoscopy group improved their ankle function score by about 53% from baseline, while the open group improved by roughly 41%. Both approaches produced good results, but the endoscopic technique appeared to offer a slight edge in functional improvement.

Complication Profile

The complication rates for FHL transfer are generally low, and the procedure compares favorably to alternatives. A meta-analysis of isolated FHL transfer for chronic Achilles rupture reported no cases of nerve injury or associated numbness, a notable finding given that nerve damage is a recognized risk with other Achilles reconstruction techniques. The analysis also flagged smoking as a risk factor for postoperative complications.¹⁸

One retrospective study compared endoscopic FHL transfer to open primary Achilles repair for acute ruptures and found that the overall complication rate was significantly lower in the FHL group (about 5% versus 28% in the open repair group), with the difference driven mainly by wound-related problems.¹⁹ Hallux-related complaints in the FHL group did not affect daily activities, consistent with the pattern seen in other long-term follow-up studies.

The most commonly reported donor-site morbidity is weakness of big toe flexion, which as discussed above tends to be measurable on exam but functionally invisible to the patient. Wound healing issues, infection, and sural nerve damage are all possible but uncommon, and minimally invasive techniques appear to reduce these risks further.

FHL Versus Peroneus Brevis Transfer

The other tendon commonly used for Achilles reconstruction is the peroneus brevis, which runs along the outer side of the ankle and helps with foot eversion (turning the sole outward). The choice between FHL and peroneus brevis has been debated for years, and the evidence suggests both work well without a clear winner.

A biomechanical cadaver study found that the peroneus brevis produced higher failure loads than the FHL when tested to destruction. However, the researchers noted that these peak loads were far above what the tendon would experience in real life, making the difference unlikely to matter clinically. Stiffness and energy absorption were similar between the two.²⁰

Clinical comparisons tell a similar story. A head-to-head study found no significant difference in final ankle scores between FHL and peroneus brevis groups. The complication profiles differed in character rather than severity: 20% of FHL patients developed big toe flexion weakness, while the peroneus brevis group saw superficial infections and sensory changes on the outer sole.²¹ Another comparative study concluded bluntly that both tendons were equally effective at restoring plantar flexion power.²²

The practical considerations that tip the decision one way or the other tend to be patient-specific. The FHL is generally preferred when the surgeon wants a tendon that runs in a similar line to the Achilles and whose muscle belly contributes to ankle push-off. The peroneus brevis may be favored when preserving big toe flexion is a priority, but at the cost of sacrificing some ankle stability on the outer side. For patients who need lateral ankle stability (those with chronic ankle instability, for instance), taking the peroneus brevis could worsen their existing problem. And for patients who depend on strong big toe grip, such as rock climbers or ballet dancers, the FHL may not be the first choice either. In most cases, though, both options produce good functional results and the decision depends on the surgeon’s assessment of the individual patient’s anatomy and activity demands.

FHL Transfer for Non-Achilles Problems

While Achilles reconstruction is by far the most common indication, the FHL tendon is also transferred for other conditions. One example is hallux claw toe deformity, where the big toe curls rigidly downward due to muscle imbalance. In a small series of patients treated with FHL transfer for this problem, all deformities were corrected and alignment was maintained at follow-up, with all patients reporting satisfaction.²³ The FHL can also be used to address peroneal tendon problems on the outer ankle, though this is a less common application and the evidence base is thinner.

The versatility of the FHL as a donor tendon comes from its favorable anatomy: it is relatively strong, its muscle belly is positioned well for various redirections, and the functional cost of harvesting it is low enough that most patients adapt without meaningful complaints. As surgical techniques continue to evolve, particularly with endoscopic and minimally invasive methods, the procedure is likely to become less invasive while maintaining the strong outcome profile that has made it a reliable option for reconstructive foot and ankle surgery.