The Ankle and Foot 315Flexor Hallucis Longus
The flexor hallucis longus (FHL; flexor, flex + hallux, great toe + longus,
long) originates from the posterior aspect of the fibula, runs medi-
ally across the back of the leg to cross behind the medial malleolus,
and then forward along the medial portion of the plantar aspect of
the foot to attach to the undersurface of the great toe (figure 6.18).
As its name implies, the primary action of this muscle is to flex the
great toe at the MTP and IP joints. However, in movement, this action
appears to be more important in establishing firm contact of the great
toe on the ground in closed kinematic chain movements and produc-
ing a powerful push-off such as during walking, running, and jumping
(Femino et al., 2000; Moore and Agur, 1995) rather than actually pro-
ducing flexion that bends the big toe under. The flexor hallucis longus
also assists with ankle-foot plantar flexion and inversion of the foot.
Similar to what occurs with the tibialis posterior, this inversion can
serve to raise the medial longitudinal arch, lock the transverse tarsal
joint, and thereby stabilize the foot. The course of the flexor hallucis
longus under the sustentaculum tali is particularly advantageous
for supporting the talus and preventing excessive pronation. During
standing on the toes, this muscle is also important for preventing
excessive “rolling in,” supporting the medial longitudinal arch, and
helping to stabilize the hallux.
Palpation: You can palpate the distal portion of the flexor hallucis
longus by placing two fingers under the proximal phalanx of the great
toe while actively flexing the great toe.Flexor Digitorum Longus
The flexor digitorum longus (FDL; flexor, flex + digit, finger or toe + longus,
long) originates from the posterior aspect of the upper tibia and from
the fascia covering the tibialis posterior, and runs down the posterior
medial tibia (figure 6.19). As its name implies, the primary function of the flexor digitorum longus is to
flex the digits of the toes. The distal tendon of the flexor digitorum longus divides into four separate
tendons, each inserting into the base of the distal phalanges of toes 2 through 5, producing flexion of
both the distal and proximal IP joints. This motion produces a pressing of the toes against the ground
that is key to application of force to the ground during push-off in closed kinematic chain movements
such as running or jumping, and can be used more subtly to help maintain stability. The flexor digitorum
longus also assists with ankle-foot plantar flexion, foot inversion, and maintaining the longitudinal arch
during dynamic weight bearing.Attachments and Primary Actions of Flexor Digitorum and Hallucis Longus
Muscle Proximal attachment(s) Distal attachment(s) Primary action(s)
Flexor hallucis longus
(FLEK-sor HAL-u-sis LON-gus)Posterior aspect of lower
two-thirds of fibula
Lower interosseous
membranePlantar surface of base of
distal phalanx of great toeGreat toe flexion
A-F plantar flexion
Foot inversionFlexor digitorum longus
(FLEK-sor di-ji-TOR-um LON-gus)Posterior surface of tibia
and tibialis posterior fasciaBases of distal phalanges
of toes 2-4; each tendon
passes through opening in
corresponding tendon of
flexor digitorum brevisFlexion of lesser toes
A-F plantar flexion
Foot inversionFIGURE 6.18 The flexor hallucis
longus (right foot, posterior view).