The ankle joint complex connects the lower leg to the foot, enabling movement and stability during walking and standing. It primarily consists of the talocrural joint, which allows up-and-down movement, and the subtalar joint, which permits side-to-side rotation. Movement is powered by extrinsic muscles originating in the calf and shin whose long tendons cross the ankle joint line. These muscles are categorized by the specific action they produce, allowing for precise control during locomotion.
Muscles of Dorsiflexion
Dorsiflexion is the action of lifting the forefoot toward the shin, essential for clearing the toes off the ground during walking. The muscles responsible for this motion are located in the anterior compartment of the lower leg. The most powerful is the Tibialis Anterior, which originates on the outer surface of the tibia and inserts on the medial side of the foot. Its primary role is to pull the foot upward, but it also contributes to turning the sole inward (inversion).
Two other muscles assist in dorsiflexion while controlling the toes. The Extensor Hallucis Longus acts mainly to lift the big toe, secondarily aiding in general dorsiflexion. The Extensor Digitorum Longus extends the four lesser toes and helps lift the entire foot. Weakness in these anterior muscles, supplied by the deep fibular nerve, can cause “foot drop,” where the foot cannot be lifted properly. The Tibialis Anterior also controls the rate at which the foot lowers to the ground after the heel strike, preventing the foot from slapping down.
Muscles of Plantarflexion
Plantarflexion, the movement of pointing the foot downward, provides the propulsive force for walking, running, and jumping. The largest and strongest muscles responsible for this motion are the Gastrocnemius and Soleus, collectively known as the triceps surae. These muscles converge to form the Achilles tendon, which inserts onto the heel bone.
The Gastrocnemius is superficial and originates above the knee joint, making it a biarticular muscle acting on both the knee and the ankle. The Soleus lies deep beneath the Gastrocnemius, originating below the knee, making it a monoarticular muscle acting only on the ankle. This difference means the Gastrocnemius is most effective when the knee is straight, while the Soleus provides powerful plantarflexion regardless of knee position.
The deep posterior compartment contains three additional muscles that assist in plantarflexion: the Tibialis Posterior, the Flexor Digitorum Longus, and the Flexor Hallucis Longus. The latter two primarily flex the toes, but their pull across the ankle joint contributes secondary force to pointing the foot downward. These deep muscles are active in stabilizing the foot’s arch and controlling the foot during ground contact.
Muscles Controlling Side-to-Side Motion
Side-to-side movement, tilting the sole inward (inversion) or outward (eversion), occurs primarily at the subtalar joint. These movements are driven by tendons that cross the ankle and are essential for navigating uneven terrain and maintaining balance. The muscles responsible for eversion are found in the lateral compartment of the leg: the Fibularis Longus and Fibularis Brevis.
The Fibularis Longus and Brevis tendons pass behind the lateral malleolus (outer ankle bone) and exert an outward pull on the foot. The longus tendon also wraps under the foot to provide dynamic support for the arch. These lateral muscles protect the ankle from inversion sprains by rapidly pulling the foot into eversion.
Inversion is controlled by two muscles: the Tibialis Posterior and the Tibialis Anterior. Although the Tibialis Anterior is known for dorsiflexion, its insertion point makes it a strong invertor. The Tibialis Posterior, located in the deep calf, is the main invertor and supports the medial arch of the foot.
Clinical Significance of Ankle Muscle Function
Understanding the function of these muscle groups is important for diagnosing and treating common lower leg and foot conditions. Overuse or strain on the triceps surae and its Achilles tendon can lead to Achilles tendinitis, characterized by pain and stiffness at the back of the heel. This condition often results from tight calf muscles or a sudden increase in activity.
Pain along the shinbone, known as shin splints, frequently involves irritation of the Tibialis Anterior or Tibialis Posterior muscles. Strengthening the dorsiflexors is a focus in rehabilitation, as anterior compartment weakness contributes to this issue. When the ankle is forced into excessive inversion, the lateral ligaments are often damaged in an ankle sprain. This highlights the protective role of the fibularis muscles, which must be strengthened to restore dynamic stability.