Flexor Hallucis Longus (FHL):

Origin: Posterior surface of the distal 2/3rds of the fibula, interosseous membrane, and adjacent intermuscular septa and fascia.

Insertion: Base of the distal phalanx of the great toe, plantar surface .

Nerve: Tibial nerve arising from the sciatic nerve via the sacral plexus, originating from nerve roots L5, S1, and S2.

Action:

Phalanges: flexion of the interphalangeal joint of the great toe and assists in flexion of the metatarsophalangeal joint

Ankle: Plantar flexion of the foot, as well as, aaccesory  invertor of the foot and ankle.

Relative Location:

The flexor hallucis longus (FHL) is located lateral to the tibialis posterior. The anterolateral border combined with the periosteum of the fibula and posterior intermuscular septum.

Integrated Function:

Stabilization:

Stabilizes the tibiotalor & subtalor joints,transverse tarsal, tarsometatasal, metatarsophalangeal, interphalengeal joints.

Eccentric action :

The FHL eccentrically decelerate extension of the metatarsophalangeal and interphalangeal joints, as well as dorsiflexion and eversion of the ankle.

Synergists:

The FHL work synergistically with the tibialis posterior to eccentrically decelerate eversion during the mid-stance of gait cycle. In addition, it also helps medial gastronemus and plantaris . It also assist in push off and landing mechanics during gait cycle.

A functional relationship may exist between the FHL, the tibialis posterior and tibialis anterior in control of the talus. Some evidence state that the FHL may control talar inversion via the groove in the talus and the sustentaculum tali of the calcaneus.

Arthrokinematics:

The FHL cross several joints, but likely have their largest impact on the interphalangeal joints – capable of producing plantar glide. In claw toe deformity the extensor hallucis longus and extensor digitorum longus may also contribute to dysfunction by producing excessive dorsal glide of the metatarsophalangeal joints.

Course of FHL:

The FHL has an interesting course, running through a groove on the posterior medial talus and continuing through a groove inferior to the sustentaculum tali.

During contraction of the FHL this creates an anterior and superior force on the medial side of the talus. This may have functional implications on talar and calcaneal mechanics, specifically an ability to contribute to inversion and a varus tilt of the talus and calcaneus . It potentially anterior glide of the talus on the tibia.

Fascial Integration of the FHL

The most obvious relationship exists between the posterior tibial fascia and the combined FHL and FDL. Which links the deep posterior compartment muscles by function and fascia.

Additionally, the  tendinous slips of the FHL may distribute the load in the forefoot, especially during toe-off phase of gait cycle. This increases the weight-bearing on the forefoot and eventually helps the FHL support the medial longitudinal arch of the foot .

Lower Leg Dysfunction (LLD):

These muscles are long due to excessive eversion at the subtalar joint and tilt of the talus, but rather than resulting in a decrease in tone, the inhibition of prime movers results in over-use and hyper-facilitation of these synergistic muscles.

When tibialis posterior is inhibited , the FHL become synergistically dominant. That is to state that the overactive fibularis muscles result in altered reciprocal inhibition of the tibialis posterior and a relative increase in FHL activity to compensate for a lack of force production in inversion. Furthermore , there is evidence suggest that the FHL and FDL may play a similar role at the metatarsalphalangeal joints, compensating for inhibited short toe flexors.

Excessive pronation:

The change in arthrokinematics and activity may limit extension of the toes  a compensation pattern that generally leads to excessive pronation (eversion) from heel-off to toe-off during the stance phase of gait cycle.

To sum up, the FHL is long and over-active, acting as overactive synergists. This clearly indicate that this muscles should be released, but do not stretched or activated. This trigger points in these muscles are often mistaken for gastrocnemius and soleus trigger points during self-administered release technique.

Anantomy

image coutsey : wikipedia.com

FHL Trigger Points :

Palpation results in tenderness (trigger points or tender points) and may result in radiating symptoms along the muscle and its tendons. Based on the theoretical model of trigger point development it would seem likely that “trigger points” are dysfunction at the “motor point” of a muscle, and release will decrease tonicity via reflexive inhibition or ischemic pressure .

 

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