By Elizabeth A. Murray, Mount St. Joseph University
The leg can be referred to as the crural or sural region, and the leg muscles are surrounded by a dense covering of fascia called the crural fascia. Two deep extensions—called the anterior crural intermuscular septum and posterior crural intermuscular septum—connect the outer sleeve of fascia down to the fibula. So, along with the interosseous membrane, these help divide the leg into its three subdivisions: the anterior, lateral, and posterior compartments.
The anterior compartment of the leg consists of four muscles that work together to facilitate movement. (Image: Medicalstocks/Shutterstock)
Muscles in the Anterior Leg
There are four muscles in the anterior leg—tibialis anterior, extensor hallucis longus, extensor digitorum longus, and a small muscle—almost like a bonus tendon—called fibularis tertius, also known as peroneus tertius.
In anatomy, the word peroneus is gradually being replaced by fibularis. This is partly because peroneus is Latin for the Greek term for the fibula, so this will mix the Greek for the fibula with the Latin word tibia. The other issue relates to chronic mispronunciation; some people call these “perineal” muscles, and we know the perineum is the diamond-shaped region between the thighs, which is nowhere near the leg or the fibula.
Tibialis Anterior
When one feels along their shin, the muscle mass immediately lateral to the sharp crest of the tibia is tibialis anterior. It takes origin from the proximal half of the tibia’s lateral surface, as well as the interosseous membrane. The large tendon of tibialis anterior can be easily seen passing across the anterior and medial aspect of the ankle, to insert on the medial cuneiform and the base of the first metatarsal.
Because it crosses the anterior ankle, does tibialis anterior dorsiflex or plantar flex? The right answer is dorsiflex. And because it crosses the medial aspect of the ankle to insert on the foot, does it invert or evert the foot? Yes, tibialis anterior causes inversion.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Extensor Hallucis Longus
The extensor hallucis longus muscle originates off the interosseous membrane, but also off the anterior fibula.
As its name implies, extensor hallucis longus inserts on and extends the great toe, and as a longus muscle, it goes the long way and inserts on the distal phalanx. But its tendon crosses the anteromedial aspect of the ankle, so it also dorsiflexes and inverts the foot.
Extensor Digitorum Longus
The extensor digitorum longus muscle takes origin off the interosseous membrane, too, but additionally off the lateral condyle of the tibia and the anterior fibula. It inserts on the middle and distal phalanges of the four lateral toes.
Extensor digitorum longus’s name reveals its function in toe extension, but because its tendon crosses the anterior ankle, and then fans out laterally towards the smaller toes, extensor digitorum longus can also assist in ankle dorsiflexion and eversion of the foot.
Fibularis or Peroneus Tertius
In most people, there’s an additional tendon that runs off the distal and lateral portion of extensor digitorum longus but doesn’t attach to a toe. When present, this tendon is called fibularis or peroneus tertius, though its muscle belly is indistinguishable from extensor digitorum longus.
Fibularis tertius inserts on the tuberosity or tubercle at the base of the fifth metatarsal. Because this tendon crosses the anterior and lateral aspect of the ankle, it dorsiflexes and everts the foot.
Fibularis/Peroneus Longus and Fibularis/Peroneus brevis
Tertius means “third”, and there are two other fibularis buddies in the lateral leg. But before leaving this compartment, we need to remember that all four muscles of the anterior leg are supplied by the deep fibular nerve, off the common fibular branch of the sciatic nerve.
The two muscles in the slender lateral leg compartment are fibularis longus and fibularis brevis—also known as peroneus longus and brevis. The fibularis longus originates on the proximal lateral aspect of the fibula, and has a much longer tendon, as its name implies. That tendon sits superficial to the belly of fibularis brevis, which originates more distally on the lateral fibula.
Both muscles send their tendons across the lateral ankle and function to evert the foot. But fibularis brevis inserts on the tuberosity of the fifth metatarsal, with its tertius companion—while the fibularis longus really goes the long way—it passes under the sole of the foot from lateral to medial and inserts on the inferior aspect of the base of the first metatarsal—that’s the one on the big toe side.
Not only does this facilitate eversion of the foot, the tendon of fibularis longus helps support the sole of the foot as well. Both muscles of the lateral compartment are supplied by the superficial fibular nerve, a branch of the common fibular off the sciatic nerve.
Common Questions about Muscles in the Anterior Compartment of the Leg
When one feels along their shin, the muscle mass immediately lateral to the sharp crest of the tibia is tibialis anterior. It takes origin from the proximal half of the tibia’s lateral surface, as well as the interosseous membrane. The large tendon of tibialis anterior can be easily seen passing across the anterior and medial aspect of the ankle, to insert on the medial cuneiform and the base of the first metatarsal.
In most people, there’s an additional tendon that runs off the distal and lateral portion of extensor digitorum longus but doesn’t attach to a toe. When present, this tendon is called fibularis or peroneus tertius, though its muscle belly is indistinguishable from extensor digitorum longus.
The leg muscles are surrounded by a dense covering of fascia called the crural fascia. Two deep extensions—called the anterior crural intermuscular septum and posterior crural intermuscular septum—connect the outer sleeve of fascia down to the fibula. So, along with the interosseous membrane, these help divide the leg into its three subdivisions: the anterior, lateral, and posterior compartments.
