Foot Eversion: Muscles, Anatomy, and Clinical Significance

Which muscles evert the foot?

The primary muscles responsible for everting the foot are the Fibularis Longus (Peroneus Longus) and Fibularis Brevis (Peroneus Brevis), with the Fibularis Tertius (Peroneus Tertius) providing synergistic assistance.

Understanding Foot Eversion

Foot eversion is a complex movement involving the rotation of the sole of the foot laterally, away from the body's midline. This action combines abduction (moving the foot away from the body), dorsiflexion (lifting the foot upwards), and pronation (flattening of the arch). Eversion is a critical component of dynamic balance, gait, and adapting the foot to uneven surfaces. Understanding the muscles that execute this movement is fundamental to comprehending ankle stability, injury mechanisms, and effective rehabilitation.

Primary Evertors of the Foot

The primary muscles responsible for initiating and powerfully executing foot eversion are located in the lateral compartment of the lower leg. They are collectively known as the fibularis (formerly peroneal) muscles.

• Fibularis Longus (Peroneus Longus)

  • Origin: This long, superficial muscle originates from the lateral condyle of the tibia, and the head and upper two-thirds of the lateral surface of the fibula.
  • Insertion: Its tendon runs behind the lateral malleolus, crosses the sole of the foot obliquely, and inserts onto the base of the first metatarsal and the medial cuneiform bone.
  • Action: The Fibularis Longus is a strong evertor and also acts as a powerful plantarflexor of the ankle. Its unique insertion across the sole of the foot also provides crucial support for both the transverse and longitudinal arches of the foot.
  • Innervation: Superficial Fibular Nerve.

• Fibularis Brevis (Peroneus Brevis)

  • Origin: Lying deep to the Fibularis Longus, this muscle originates from the lower two-thirds of the lateral surface of the fibula.
  • Insertion: Its tendon also passes behind the lateral malleolus but inserts directly onto the tuberosity on the lateral side of the fifth metatarsal bone.
  • Action: The Fibularis Brevis is a primary evertor of the foot and a weaker plantarflexor compared to the Fibularis Longus. Its direct attachment to the fifth metatarsal makes it particularly important in stabilizing the lateral aspect of the foot.
  • Innervation: Superficial Fibular Nerve.

Secondary Evertors and Synergists

While the fibularis longus and brevis are the main drivers of eversion, other muscles contribute synergistically or have a minor eversion component.

• Fibularis Tertius (Peroneus Tertius)

  • Origin: This small muscle is often considered a part of the Extensor Digitorum Longus, originating from the lower anterior surface of the fibula and the interosseous membrane.
  • Insertion: It inserts onto the dorsal surface of the base of the fifth metatarsal bone.
  • Action: Unlike the other fibularis muscles, the Fibularis Tertius primarily acts as a dorsiflexor of the ankle, but it also provides synergistic assistance in foot eversion due to its lateral pull.
  • Innervation: Deep Fibular Nerve.

• Extensor Digitorum Longus

  • Origin: Originates from the lateral condyle of the tibia, the anterior surface of the fibula, and the interosseous membrane.
  • Insertion: Its four tendons insert into the middle and distal phalanges of the lateral four toes.
  • Action: While its primary role is to extend the lateral four toes and dorsiflex the ankle, its overall pull can contribute a minor amount to foot eversion, particularly when working with the Fibularis Tertius component.
  • Innervation: Deep Fibular Nerve.

Anatomical Context and Biomechanics

Eversion is one part of the complex triplanar motion of the foot and ankle, which includes:

• Inversion: The opposite of eversion, where the sole of the foot turns medially. Primarily performed by the Tibialis Anterior and Tibialis Posterior.
• Dorsiflexion: Lifting the foot upwards towards the shin.
• Plantarflexion: Pointing the foot downwards.

The fibularis muscles play a crucial role in dynamic ankle stability, particularly in preventing excessive inversion, which is the most common mechanism for ankle sprains. During walking and running, they help to control the pronation phase of gait and prepare the foot for push-off. Their ability to adapt the foot to uneven ground is essential for balance and injury prevention during activities like trail running or hiking.

Clinical and Functional Significance

Understanding the evertor muscles has significant implications for both clinical practice and fitness training:

• Ankle Sprains: Weakness or imbalance in the evertor muscles can increase the risk of lateral ankle sprains (inversion sprains), as they are the primary antagonists to the stronger invertor muscles. Strengthening these muscles is a cornerstone of ankle injury prevention and rehabilitation.
• Shin Splints: While often associated with the anterior tibialis, imbalances or overuse of the fibularis muscles can contribute to lateral shin pain.
• Proprioception and Balance: The fibularis muscles are rich in proprioceptors, contributing to the body's awareness of foot position in space. Training these muscles enhances balance and coordination.
• Rehabilitation: Following ankle injuries, targeted exercises for the evertors (e.g., resistance band eversion, balance board exercises) are critical for restoring function and preventing re-injury.
• Athletic Performance: Athletes in sports requiring agility, quick changes of direction, and powerful push-offs (e.g., basketball, soccer, running) rely heavily on well-conditioned evertor muscles for optimal performance and injury resilience.

Conclusion

The Fibularis Longus and Fibularis Brevis are the undisputed primary muscles responsible for foot eversion, supported by the synergistic action of the Fibularis Tertius and, to a lesser extent, the Extensor Digitorum Longus. These muscles are vital for dynamic ankle stability, adapting the foot to uneven terrains, and preventing common ankle injuries. For anyone involved in physical activity, from daily walking to high-performance sports, understanding and adequately training these critical muscles is paramount for maintaining healthy, functional ankles and feet.