Tibialis Posterior
| Tibialis Posterior | |
|---|---|
| Muscle Type | |
| Origin | Posterior tibia (below soleal line), posterior fibula, and interosseous membrane |
| Insertion | Navicular tuberosity, cuneiforms, cuboid, bases of 2nd–4th metatarsals |
| Action | Inverts foot, assists plantarflexion, supports medial arch |
| Synergists | |
| Antagonists | Peroneus Longus, Peroneus Brevis, Peroneus Tertius, Tibialis Anterior |
| Spinal innervation | L4, L5, S1 |
| Peripheral Innervation | Tibial nerve |
| Vasculature | Posterior tibial artery and fibular artery |
The tibialis posterior is a deep posterior leg muscle that inverts the foot and assists in plantarflexion, and it is crucial for maintaining the medial arch of the foot, often called the “key stabilizer” of the arch.
Muscle Type
Tibialis posterior is a thick, central muscle in the deep posterior compartment of the leg. It lies between FDL and FHL in the leg (deep to soleus). Its tendon passes behind the medial malleolus through the tarsal tunnel and fans out under the foot to multiple attachments. It is classified as a pennate muscle, built for stability and postural control rather than large range movements.
Origin
It originates from the posterior surfaces of both the tibia and fibula, and the interosseous membrane between them. Specifically, the origin spans the lateral part of the posterior tibia (below the soleal line), the medial part of the posterior fibula, and the intervening interosseous membrane. This broad origin places tibialis posterior in the center of the deep compartment.
Insertion
The tibialis posterior tendon runs behind the medial malleolus (in the tarsal tunnel) and inserts primarily on the navicular tuberosity on the medial side of the foot. It also sends slips to the cuneiform bones, the cuboid, and the bases of the 2nd, 3rd, and 4th metatarsals (and often to the sustentaculum tali of the calcaneus). In effect, tibialis posterior has a broad insertion on the tarsal and metatarsal bones of the midfoot and hindfoot (excluding the talus). The most clinically significant attachment is the navicular, as detachment or dysfunction there leads to arch collapse.
Action
Tibialis posterior’s main action is foot inversion – turning the sole of the foot inward (medially). It is the deepest invertor, working with tibialis anterior (which is an invertor in the anterior compartment) to invert the subtalar and transverse tarsal joints. Tibialis posterior also plantarflexes the ankle (assists in pointing the foot down) since it passes behind the ankle joint. Importantly, it acts as a dynamic supporter of the medial longitudinal arch of the foot: during stance, it contracts to prevent the arch from collapsing (i.e., prevents over-pronation). This role is fundamental in walking and running, giving tibialis posterior a significant postural function.
Synergists
Tibialis posterior works synergistically with tibialis anterior to invert the foot (TA inverts while dorsiflexing, TP inverts while plantarflexing). Together, these two form a force couple that supports the arch (TA from above, TP from below). In plantarflexion, tibialis posterior contracts along with gastrocnemius, soleus, FDL, FHL, and fibularis longus/brevis. Notably, fibularis longus, though an evertor, also helps support the transverse arch by its attachment across the plantar foot to the first metatarsal; it works in a complementary fashion with tibialis posterior (which supports the medial arch) – this pair is sometimes called a “stirrup” for the foot. During supination (which includes inversion and plantarflexion), tibialis posterior and the triceps surae act together to lock the foot for propulsion.
Antagonists
The primary antagonist of tibialis posterior is peroneus (fibularis) brevis and peroneus tertius for foot inversion – these muscles evert the foot, opposing inversion. Fibularis longus also everts and thus antagonizes tibialis posterior’s inversion (though they synergize in arch support as noted). Tibialis anterior can act as an antagonist in terms of ankle motion (TA dorsiflexes while TP plantarflexes), but they synergize in inversion. In a collapsed arch (pes planus), often tibialis posterior is weak or dysfunctional, and the antagonistic evertors (fibularis muscles) may dominate, causing an everted (pronated) foot posture.
Spinal Innervation
Tibialis posterior is innervated by the L4 and L5 spinal nerve roots (and some S1) via the tibial nerve. In fact, the L4–L5 contribution is relatively significant (tibialis posterior is one of the few posterior compartment muscles with a strong L4 component), which is why tibialis posterior dysfunction can be seen in L4 radiculopathy.
Peripheral Innervation
The tibial nerve innervates tibialis posterior. The nerve branch to tibialis posterior comes off in the upper leg and enters the muscle’s deep surface. Clinically, this corresponds to the same tibial nerve that innervates the other deep flexors; thus, isolated tibialis posterior palsy is rare unless the branch is selectively injured.
Vasculature
Tibialis posterior is mainly supplied by the posterior tibial artery and the fibular (peroneal) artery. The muscle’s central location allows it to receive perforating branches from both arteries. The fibular artery runs lateral to it, sending branches medially, while the posterior tibial artery runs along its medial side. In the foot, branches of the medial plantar artery supply the insertion area around the navicular. Venous drainage is via the posterior tibial vein and fibular vein.
Clinical Application
Tibialis posterior dysfunction is a common cause of acquired flatfoot deformity in adults. In posterior tibial tendon dysfunction (PTTD), the tendon becomes inflamed or torn, leading to collapse of the medial arch, hindfoot valgus (heel tilting outward), and forefoot abduction (“too many toes” sign when viewed from behind). Patients present with medial ankle pain and difficulty with single-leg heel raise (they cannot invert the heel on tip-toe).
Early treatment is conservative (orthotics, strengthening exercises for tibialis posterior), but advanced cases may require surgical tendon transfer or osteotomies. The strength of tibialis posterior can be examined by asking the patient to invert and plantarflex the foot against resistance. Injury to the tibial nerve (e.g., tarsal tunnel syndrome) can weaken inversion and contribute to an everted foot posture. Runners with excessive pronation might strain the tibialis posterior, leading to shin pain.
Strengthening tibialis posterior (e.g., exercises with resistance bands in inversion, or eccentric heel raises with inversion bias) is often included in rehab for flatfoot or shin splints. In severe flatfoot, the FDL tendon is sometimes transferred to the navicular to substitute for tibialis posterior function (because FDL is nearby and somewhat redundant with FHL).
A tight tibialis posterior can contribute to an overly supinated foot. Overall, tibialis posterior is pivotal for arch integrity; thus it’s sometimes termed the “spring” of the foot’s arch. Recognition and treatment of tibialis posterior issues are critical in preventing progressive pes planus deformity.
