The Complex System of the Ankle and Foot



The extrinsic muscles of the ankle and foot primarily originate in the leg, with some attaching proximally to the femur. In contrast, the intrinsic muscles are entirely contained within the foot, having both their proximal and distal attachments located within the foot itself. This structural difference allows for finer, more precise movements of the foot.

The extrinsic muscles of the leg are systematically arranged into three distinct compartments:

  1. Anterior compartment
  2. Lateral compartment
  3. Posterior compartment

Each compartment is innervated by a distinct motor nerve, all of which originate from the sacral plexus (L4-S3) and travel through the sciatic nerve.


Nerve Innervation of the Leg and Foot

At the lateral side of the fibular head, the common fibular (peroneal) nerve (L4-S2) divides into two major branches:

  • Deep fibular nerve
  • Superficial fibular nerve

Deep Fibular Nerve

The deep fibular nerve innervates the muscles of the anterior compartment, including:

  • Tibialis anterior
  • Extensor digitorum longus
  • Extensor hallucis longus
  • Fibularis (peroneus) tertius

It also extends its motor control to the only intrinsic muscle on the dorsum of the foot, the extensor digitorum brevis. Additionally, it provides sensory innervation to a small triangular area of skin between the first and second toes.

Superficial Fibular Nerve

The superficial fibular nerve innervates the lateral compartment muscles, including:

  • Fibularis (peroneus) longus
  • Fibularis (peroneus) brevis

It also sends sensory branches distally, supplying the skin over the lateral and dorsolateral aspects of the leg and foot.

Tibial Nerve (L4-S3) and Its Branches

The tibial nerve provides motor innervation to the remaining extrinsic and intrinsic muscles of the foot and ankle.

The posterior compartment muscles are divided into two layers:

  1. Superficial layer:

    • Gastrocnemius
    • Soleus (together with the gastrocnemius, forming the triceps surae)
    • Plantaris
  2. Deep layer:

    • Tibialis posterior
    • Flexor hallucis longus
    • Flexor digitorum longus

As the tibial nerve approaches the medial side of the ankle, it gives off sensory branches to the heel and then divides into the:

  • Medial plantar nerve (L4-S2)
  • Lateral plantar nerve (L4-S3)

These plantar nerves are responsible for sensory innervation across the plantar surface and motor control of all intrinsic foot muscles, except for the extensor digitorum brevis (which is innervated by the deep fibular nerve).


Comparison with Upper Limb Nerve Patterns

The medial plantar nerve is functionally analogous to the median nerve of the upper limb, while the lateral plantar nerve corresponds to the ulnar nerve. This similarity reflects a parallel organization of nerve distribution between the hands and feet, optimizing fine motor control in the hands and weight-bearing stability in the feet.