The Role of the Flexor Hallucis Longus in Acute Ankle Injury
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The Role of the Flexor Hallucis Longus in Acute Ankle Injury
The Flexor Hallucis Longus (FHL) has long been considered a site of potential overuse injury, especially in dancing cohorts (De-la-Cruz-Torres et al., 2020; Newman et al., 2021; Wentzell, 2018). What appears to be less widely discussed is the implications of the FHL in the management of acute ankle inversion injuries, which are far more common in most sporting populations (Van den Bekerom et al., 2013). This blog aims to highlight the importance of the FHL in such injuries, especially in cohorts where ankle inversion injuries are common, but overuse FHL injuries are rare, such as court sports and field sports.
Anatomy of the FHL
The FHL originates on the posterior aspect of the distal two thirds of the fibula and interosseous membrane. The muscle courses inferomedially and its tendon runs through the posteromedial compartment of the ankle with the tibialis posterior (TP) and flexor digitorum longus (FDL), colloquially known together as Tom, Dick and Harry. It’s important to note that the FHL tendon sits more laterally and deep to its “Tom and Dick” counterparts, coursing directly between the medial and lateral processes of the posterior talus (Figure 1 & 2). The tendon sheath of the FHL creates a fibro-osseous tunnel securing it in its groove.
The tendon continues beneath the sustentaculum tali of the calcaneus to the plantar aspect of the foot. Beneath the 1st cuneiform, the FHL crosses dorsally and medially to the FDL at a fibrous intersection known as the “Knot of Henry”. The FHL tendon continues along the medial, plantar foot, between the sesamoid bones of the 1st toe and inserts at the distal phalanx of the 1st toe.
(Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018).
Figure 1 - Reproduced from Sharp et al., 2020. The FHL sits between the posteromedial and posterolateral process of the talus.
Figure 2 - Reproduced from Major et al., 2020. Axial MRI slice showing the FHL in its fibro-osseous tunnel (green arrow) communicating much more closely with the ankle joint than the FDL (blue arrow) and tibialis posterior (red arrow).
Function of the FHL and Role in Athletic Performance
The FHL is a multi-joint muscle, spanning and acting upon the joints of the ankle and medial column of the foot all the way to the 1st interphalangeal (IP) joint. It serves as the primary flexor of the IP and metatarsophalangeal (MTP) joints of the 1st toe and has secondary roles as an ankle plantar flexor, rear foot invertor and medial longitudinal arch stabiliser (Murdoch et al., 2021; Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018).
In athletic movements, such as high-speed running, changing direction and jumping, the forefoot is often the only ground contact point for the body. It is imperative that the 1st toe can resist extension and provide a rigid base to transfer the forces produced by the powerful muscles of the hip, knee and ankle into the ground (Goldmann et al., 2013; Yuasa et al., 2018). It is also understood that, to transfer this force effectively, the foot must be held in supination and the medial longitudinal arch actively stiffened to act as a stable base (McKeon et al., 2015). The importance of the FHL to athletic performance appears to be supported by small cohort studies correlating its function with change of direction speed (Yuasa et al., 2018), horizontal jump distance (Goldmann et al., 2013), and ground reaction force absorption in landing (Oku et al., 2021).
Due to its position, wrapped behind the posteromedial ankle, the FHL requires adequate mobility to avoid acting as a tether restricting ankle dorsiflexion (Michelson et al., 2021). Restricted ankle dorsiflexion is a commonly reported, ongoing, impairment following ankle inversion injury (Van den Bekerom et al., 2013), and it appears that the FHL has the potential to be a contributor.
Aetiology of FHL Injuries Secondary to Ankle Inversion Injuries
The most common mechanism of ankle injury is forced ankle plantar flexion and inversion, which distracts the anterolateral aspect of the joint, commonly injuring anterolateral structures like the anterior talofibular ligament (ATFL) (Van den Bekerom et al., 2013). On the other side of the joint, the structures in the posteromedial aspect of the joint are compressed (Van den Bekerom et al., 2013), commonly including the FHL tendon within its fibro-osseous tunnel (Sharpe et al., 2020). The FHL tendon can also be implicated secondary to ankle injuries as the proximity of its synovial sheath to the ankle joint (directly communicating in approximately 20% of individuals) can allow ankle effusion to leak into it, acting as a pseudo-tenosynovitis (Major et al., 2020).
Assessment of the FHL should be a staple component of the assessment of the injured ankle. FHL pain may be suspected if posteromedial ankle pain is reported, especially with activities involving ankle plantar flexion such as toe-off in gait or the top of a calf raise (Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018). The FHL can be directly palpated in its talar groove (Sharpe et al., 2020), instructing the patient to wiggle the 1st toe whilst palpating can help confirm the accuracy of palpation. The FHL is also stressed with the posteromedial impingement stress test (Figure 3), resisted 1st toe flexion, resisted ankle inversion and a full range calf raise (Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018). Pain in the posteromedial ankle with these tests should increase suspicion that the FHL is a source of symptoms.
Figure 3 - The posteromedial stress test involves passively moving the ankle in to end range plantar flexion and inversion then applying an overpressure at the posteromedial heel.
Along with assessing whether the FHL may be a source of symptoms, it is important to ascertain whether the function of the FHL has been impaired, which may have implications for returning to performance (Goldmann et al., 2013; Oku et al., 2021; Yuasa et al., 2018). The most commonly described test for assessing FHL flexibility involves assessing maximal 1st MTP extension range with the ankle held in dorsiflexion and the 1st metatarsal head stabilised (Michelson et al., 2021;Sharpe et al., 2020). This can be compared to 1st MTP extension range in ankle plantar flexion (FHL off stretch). Another, potentially more functional, test that could flag whether FHL flexibility is limiting ankle dorsiflexion is to compare the ankle dorsiflexion lunge test with and without a small wedge under the first toe. If the range decreases with the wedge, it could be hypothesised that FHL flexibility is the limiting factor and worth addressing.
As a secondary ankle plantar flexor and inverter, impaired FHL function may contribute to deficits in strength testing of these movements. To get a clearer understanding of FHL muscle function, 1st toe flexion strength must be assessed, as the FHL is the primary driver of this movement (Murdoch et al., 2021; Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018). In athletic tasks, active 1st toe flexion is most often required in 1st MTP extension (Goldmann et al., 2013; Yuasa et al., 2018), so testing in this position may be advised. This theory appears to be supported by a recent, small, cohort study that showed 1st toe flexion strength in 45 degrees of extension was correlated with change of direction performance but flexion strength in flexion was not (Yuasa et al., 2018).
Figure 4 demonstrates a possible set-up to take this measurement (depending on equipment available). The hip, knee and ankle are standardised to 90 degrees, the pad of the 1st toe is centred on the dynamometer which is fixed by the wall and the floor. Although the degree of 1st toe extension would vary from patient to patient in this position, the goal is to standardise the position for reliable re-testing of the patient. Small cohorts of healthy participants in their 20’s have shown an average 1st toe flexion strength of ~20% of body weight (Mickle et al., 2016; Spink et al., 2010). No evidence exists for an “ideal” strength level for athletes, but a common target is >30% of bodyweight and >90% limb symmetry.
Figure 4 - Position for dynamometry assessment of 1st toe flexion strength in extension. The toe is centred on the pad of the dynamometer which is fixed in place by the wall and floor. The hip, knee, and ankle are in 90 degrees of flexion.
The implications of the FHL for ankle inversion injury rehabilitation will depend on the results of the objective and subjective examination. Factors include; the severity of the ankle injury, whether or not the FHL is a likely source of symptoms, FHL flexibility restriction, strength deficits of the FHL and strength deficits of other muscles of the foot and ankle. Figure 5 is a simplified model of the decision-making process regarding the FHL after ankle inversion injury.
Figure 5 - Flowchart to guide decision making around the FHL following ankle inversion injury based on assessment findings.
The proposed management of the symptomatic FHL tendon has been outlined in several papers, with limited evidence, primarily considering dancing cohorts. The conservative management includes non-steroidal anti-inflammatory medications (NSAIDs), ice, non-provocative loading of the FHL (such as mid-range isometric toe flexion in ankle neutral), strengthening of the kinetic chain (especially the hip), and external support for protection (may include weight bearing aids, arch support, toe spacers, limiting end range 1st toe extension, and/or limiting end range ankle plantar flexion) (Newman et al., 2021; Sharpe et al., 2020; Wentzell, 2018). If conservative management fails to get symptoms under control, medical intervention may be necessary, anecdotally, this is not commonly required following ankle inversion injury.
Beyond managing the painful FHL, rehabilitation must consider any functional impairments of the FHL (evidence is lacking for the rehabilitation of FHL function so the following recommendations are based on clinical opinion). If the FHL appears to be limiting dorsiflexion range of motion, then targeted FHL mobility exercises may be of benefit (examples in the video below). Passive treatment modalities targeting the FHL muscle such as massage or dry needling may also be considered.
If the FHL is found to be weak, that weakness must be considered within the context of the whole assessment. For example, if 1st toe flexion shows a 50% deficit from expected values, but so too does ankle plantar flexion, ankle inversion and ankle eversion, exercising each muscle group in isolation would be inefficient. In this case, more benefit would be gained by focussing on functional plantar flexion exercises that load the triceps surae, foot intrinsic muscles, flexor compartment muscles and peroneals to a high degree simultaneously. These exercises may include calf raise variations and toe walking variations with cueing encouraging:
- Pressure through the 1st and 5th toe - driving through the forefoot.
- Driving the heel up in a straight line.
- A stable, upright stance.
- Slow, controlled, deliberate, movement quality.
Conversely, if 1st toe flexion shows a 50% deficit from expected values but the other muscle groups do not, more targeted FHL loading may be advisable. Examples of these exercises are provided in the video below, they include similar functional exercises with greater constraints to encourage active 1st toe flexion. Isolation 1st toe flexion exercises (video) are another option in this scenario but may not be necessary with appropriate loading of more functional variations. Isolation 1st toe flexion exercises may be more appropriate in severe ankle injuries when the early stages of rehabilitation involve a period of modified weight bearing status.
To summarise, the FHL is not to be forgotten in the management of ankle inversion injuries in any sport due to its anatomical proximity to the ankle joint and function in athletic movements. A structured and thorough physical examination will help determine whether the FHL is a source of symptoms following ankle inversion injury and whether the function of the FHL has been impaired. The results of the examination can be used to guide rehabilitation and ensure that a forgotten FHL is not the source of ongoing functional impairments and, consequently, performance deficits.
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Major, N. M., Anderson, M. W., Helms, C. A., Kaplan, P. A., & Dussault, R. (2020). Musculoskeletal MRI (Third edition.. ed.): Philadelphia, Pennsylvania : Elsevier.
McKeon, P. O., Hertel, J., Bramble, D., & Davis, I. (2015). The foot core system: a new paradigm for understanding intrinsic foot muscle function. British journal of sports medicine, 49(5), 290. https://doi.org/10.1136/bjsports-2013-092690
Michelson, J., O’Keefe, J., & Bougioukas, L. (2021). Increased flexor hallucis longus tension decreases ankle dorsiflexion. Foot and ankle surgery, 27(5), 550-554. doi:10.1016/j.fas.2020.07.007
Mickle, K., Angin, S., Crofts, G., & Nester, C. (2016). Effects of Age on Strength and Morphology of Toe Flexor Muscles. The Journal of Orthopaedic and Sports Physical Therapy, 46(12), 1065-1070.
Murdock, C. J., Munjal, A., & Agyeman, K. (2021). Anatomy, Bony Pelvis and Lower Limb, Calf Flexor Hallucis Longus Muscle. In StatPearls. StatPearls Publishing.
Newman, D. P., Holkup, K. C., Jacobs, A. N., & Gallo, A. C. (2021). Recalcitrant Flexor Hallucis Longus Dysfunction: A Case Study Demonstrating the Successful Application of an Adaptable Rehabilitation Program With a Two-Year Follow-Up. Cureus, 13(4), e14326. https://doi.org/10.7759/cureus.14326
Oku, K., Kimura, D., Ito, T., Matsugi, A., Sugioka, T., Kobayashi, Y., Satake, H., & Kumai, T. (2021). Effect of Increased Flexor Hallucis Longus Muscle Activity on Ground Reaction Force during Landing. Life (Basel, Switzerland), 11(7), 630. https://doi.org/10.3390/life11070630
Sharpe, B. D., Steginsky, B. D., Suhling, M., & Vora, A. (2020). Posterior Ankle Impingement and Flexor Hallucis Longus Pathology. Clin Sports Med, 39(4), 911-930. doi:10.1016/j.csm.2020.06.001
Spink, M., Fotoohabadi, M., & Menz, H. (2010). Foot and Ankle Strength Assessment Using Hand-Held Dynamometry: Reliability and Age-Related Differences. Gerontology (Basel), 56(6), 525-532.
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Wentzell M. (2018). Conservative management of a chronic recurrent flexor hallucis longus stenosing tenosynovitis in a pre-professional ballet dancer: a case report. The Journal of the Canadian Chiropractic Association, 62(2), 111–116.
Yuasa, Y., Kurihara, T., & Isaka, T. (2018). Relationship Between Toe Muscular Strength and the Ability to Change Direction in Athletes. Journal of human kinetics, 64, 47–55. https://doi.org/10.1515/hukin-2017-0183
About the Author
Simon graduated with Honours from the University of Sydney with a Bachelor of Applied Science (Physiotherapy) in 2014, and has since completed a Masters of Sports Physiotherapy in 2021. He is also an ASCA accrediated Level 1 Strength and Conditioning coach.
Simon grew up playing representative level AFL and basketball, his passion for these sports from a young age lead him into the physiotherapy world with a keen interest in musculoskeletal and sporting injuries and rehabilitation.
Simon is currently a physiotherapist for the Sydney Kings in the Australian NBL.
He has previous experience as head physiotherapist with Sydney University NEAFL AFL teams, Sydney Comets Waratah League basketball teams, NPL soccer teams and a number of other elite sporting athletes in the private setting.
Through his experience, Simon has developed a keen interest in musculoskeletal injuries including, but not limited to: hip, knee and shoulder injuries and injuries of the muscles and tendons.
Simon believes in a holistic, patient centred approach to therapy. He believes in empowering patients with the tools to manage their injuries and look after their bodies to ensure long term success.