The question of whether the peroneal tendons function as flexors or extensors requires an understanding of their true mechanical actions on the foot and ankle joint. These tendons are structures of the lower leg that provide dynamic support, but they do not fit neatly into the simplistic flexor or extensor categories typically applied to the ankle. Their primary and most defining actions are eversion and plantarflexion, which are complex movements that stabilize the foot in multiple planes of motion. The muscles that become these tendons are therefore best classified by their specific functions rather than by the traditional, binary labels.
Anatomical Location and Nomenclature
The peroneal tendons belong to muscles originating from the fibula, the smaller bone on the outside of the lower leg. The two main tendons are the peroneus longus and the peroneus brevis (also known as fibularis longus and fibularis brevis). These muscle-tendon units run down the lateral side of the leg and wrap behind the bony prominence of the outer ankle, which is called the lateral malleolus. The tendons are held securely in this groove by a sheath of tissue known as the superior peroneal retinaculum. This anatomical positioning provides the leverage necessary to pull the foot outward. Although “peroneal” is commonly used, modern terminology often favors “fibular” because it directly references the fibula bone.
Primary Actions: Eversion and Plantarflexion
The most important function of the peroneal tendons is eversion, which involves turning the sole of the foot outward, away from the body’s midline. This lateral movement is distinct from the simple up-and-down motion of the ankle joint. The peroneus brevis contributes significantly to this action, providing a large portion of the total eversion power. The tendons also contribute to plantarflexion, the action of pointing the foot downward. The peroneus longus plays a role in this downward motion, particularly by helping to control the position of the first ray. The combined action of eversion and plantarflexion helps the foot adapt to uneven surfaces during walking and running.
Clarifying the Flexor and Extensor Terminology
The confusion about the peroneal tendons stems from the traditional use of “flexor” and “extensor” to describe ankle movements. In the ankle, pulling the foot upward toward the shin (dorsiflexion) is sometimes referred to as extension. Conversely, pointing the foot downward (plantarflexion) is sometimes referred to as flexion. While the peroneal tendons assist in plantarflexion, classifying them simply as flexors is misleading because their most specialized function is eversion. Because the peroneal tendons are located in the lateral compartment and their primary role is side-to-side stabilization, a functional description like “everter and plantarflexor” is much more accurate than a simple flexor or extensor label.
Role in Lower Leg and Ankle Stability
The functional importance of the peroneal tendons lies in their ability to stabilize the ankle joint, particularly against sudden, forceful movements. The most common type of ankle injury is an inversion sprain, where the foot rolls inward. The peroneal tendons act as dynamic protectors, contracting rapidly to counteract this inversion and prevent the foot from rolling too far. This counteracting force helps maintain balance, especially when walking or running on uneven ground. These tendons also contribute to static stability by resisting the forward displacement of the talus bone within the ankle joint, and the peroneus longus tendon affects passive stability when the lateral ankle ligaments are compromised.