The peroneal muscles of the lower leg
Anatomy and supply
The peroneal muscles are a group of two muscles at the lower leg. They lie within the peroneal compartment located at the lateral fibular region. When lowering the foot they can be easily seen forming the surface of the lateral lower leg. Both peroneal muscles are supplied by the superficial fibular nerve (L5-S1). In detail, they are:
- Fibularis longus muscle/ Peroneus longus muscle (left): originates at the head and the upper body of the fibula and the intermuscular septa. It inserts at the plantar side of the medial cuneiform and first metatarsal bone.
- Fibularis brevis muscle/ Peroneus brevis muscle: has its origin more distal at the fibula and is covered by the fibularis longus muscle to its most part. Its tendon inserts at the tuberosity of the fifth metatarsal bone.
The tendons of both muscles run caudally towards the foot behind the lateral malleolus and then ventrally along the lateral foot edge. There they are led by two canal-like peroneal retinacula. The superior fibular retinaculum stretches between the lateral malleolus and calcaneus. The inferior fibular retinaculum stretches between the inferior extensor retinaculum of the anterior muscles of the lower leg and calcaneus.
The fibularis longus and brevis muscles move both the upper and lower ankle joints. In the upper ankle joint they force a depression of the foot (plantar flexion) as their tendons run behind the flexion-extension axis. In the lower ankle joint their contraction leads to an eversion (pronation) which means that the medial foot edge is lowered whereas the lateral foot edge rises. Additionally the tendon of the fibularis longus muscle supports the transverse arch of the foot.
When the superficial fibular nerve is paralysed the pronation of the foot is severely restricted. Thus the supination predominates so that during the lifting of the foot it simultaneously drifts medially (equinovarus position). When the affected patients try to walk it strikes how they place the lateral foot edge first in each step of the affected leg. Furthermore one may observe a shrinking of the lateral calf due to an atrophy of the peroneal muscles. Common causes for a lesion of the superficial fibular nerve include injuries to the fibular head, a too tight cast or inappropriate splint positioning of the leg and even polio (infantile paralysis).