The sciatic nerve innervates nearly all of the muscles in the back of your thigh, the entire lower leg, and the foot. It also provides sensation to most of the skin below your knee. Formed from spinal nerve roots L4 through S3, it is the thickest nerve in the human body, and its reach explains why sciatic nerve problems can cause such widespread symptoms in the leg.
Where the Sciatic Nerve Begins
The sciatic nerve forms from five spinal nerve roots (L4, L5, S1, S2, and S3) that merge in the pelvis as part of the lumbosacral plexus. It carries fibers from both the front and back divisions of this plexus, which is why it controls such a wide range of movements and sensations. The nerve exits through the buttock, passes deep beneath the gluteal muscles, and travels down the back of the thigh.
At or just above the back of the knee, the sciatic nerve splits into its two terminal branches: the tibial nerve and the common peroneal (fibular) nerve. These two branches were bundled together the entire time, wrapped in a shared sheath, but they supply completely different parts of the lower leg and foot. Understanding this split is key to understanding what the sciatic nerve innervates, because each branch has its own territory.
Muscles in the Back of the Thigh
Before it divides, the sciatic nerve directly supplies the hamstring muscles: the biceps femoris (both heads), semitendinosus, and semimembranosus. These are the muscles that bend your knee and help extend your hip, powering movements like walking, running, and climbing stairs. The nerve also supplies part of the adductor magnus, a large inner-thigh muscle involved in pulling the leg inward.
What the Tibial Branch Controls
The tibial nerve is the larger of the two terminal branches and takes over the back and bottom of the lower leg and foot. Its motor territory is extensive.
In the calf, the tibial nerve powers the muscles responsible for pointing your foot downward (plantarflexion) and curling your toes. The superficial layer includes the gastrocnemius, soleus, and plantaris, the muscles that form the visible bulk of your calf and generate the push-off force when you walk or stand on your toes. The deeper layer includes the tibialis posterior (which supports the arch and turns the foot inward), the flexor digitorum longus (which curls the four smaller toes), and the flexor hallucis longus (which curls the big toe). The popliteus, a small muscle behind the knee that “unlocks” the joint from a fully straight position, is also tibial nerve territory.
Once the tibial nerve reaches the foot, it splits into medial and lateral plantar branches. Together, these supply the small intrinsic muscles of the sole: the abductor hallucis, flexor digitorum brevis, the lumbricals, the interossei, and several others that fine-tune toe position and help maintain balance. In total, the tibial nerve controls the muscles that let you push off the ground, grip with your toes, and stabilize your foot during movement.
Tibial Nerve Sensory Areas
The tibial branch also carries sensation from a large swath of skin. Through its cutaneous branches, it covers the back and outer side of the lower leg (via the sural nerve), the outer edge of the foot, the heel (both the medial and lateral sides), and the entire sole of the foot. If you’ve ever experienced numbness on the bottom of your foot from sitting in an awkward position, that’s the tibial nerve’s sensory territory being temporarily compressed.
What the Common Peroneal Branch Controls
The common peroneal nerve wraps around the outside of the knee and then divides into superficial and deep branches. Together, these control the front and outer compartments of the lower leg, handling a completely different set of movements than the tibial nerve.
The superficial peroneal nerve powers the peroneus longus and peroneus brevis, two muscles on the outer side of the shin that evert the foot (turn the sole outward). These muscles are critical for ankle stability, especially on uneven ground.
The deep peroneal nerve supplies the muscles that lift your foot and toes upward (dorsiflexion): the tibialis anterior, extensor hallucis longus, extensor digitorum longus, and sometimes the peroneus tertius. It also controls the extensor digitorum brevis on top of the foot. These are the muscles that keep your foot from dragging when you take a step forward. When the common peroneal nerve is damaged, the result is foot drop, a noticeable difficulty lifting the front of the foot during walking.
Common Peroneal Sensory Areas
Sensory coverage from the peroneal branch includes the outer side of the lower leg, most of the top of the foot, and the skin between the first and second toes (a classic test point for deep peroneal nerve function). Between the tibial and peroneal branches, the sciatic nerve accounts for nearly all sensation below the knee, with the exception of a strip along the inner calf and foot supplied by a separate nerve (the saphenous nerve, which comes from higher up).
Joint Actions the Sciatic Nerve Makes Possible
Putting this all together, the sciatic nerve and its branches enable several essential joint movements:
- Knee flexion: bending the knee, powered by the hamstrings
- Plantarflexion: pointing the foot downward or pushing off the ground, powered by the calf muscles (tibial nerve)
- Dorsiflexion: lifting the foot upward to clear the ground while walking (deep peroneal nerve)
- Foot inversion: turning the sole inward (tibial nerve, via tibialis posterior)
- Foot eversion: turning the sole outward for ankle stability (superficial peroneal nerve)
- Toe flexion and extension: curling and lifting the toes, controlled by both branches
Without a functioning sciatic nerve, you would lose the ability to bend your knee, move your ankle in any direction, or control your toes. Walking would be essentially impossible.
What Happens When Innervation Is Disrupted
Because the sciatic nerve supplies so much of the lower limb, damage or compression produces symptoms that follow predictable patterns. Sciatica, the most common sciatic nerve problem, typically causes pain that radiates from the buttock down the back of the leg, following the nerve’s path. Tingling and numbness tend to appear in the specific skin areas (dermatomes) that correspond to the affected nerve roots, often the outer calf, top of the foot (L5), or the area around the outer ankle bone (S1).
Muscle weakness from sciatic nerve compression depends on which fibers are affected. If the tibial component is involved, you may notice difficulty pushing off when walking or weakness when curling your toes. If the peroneal component is affected, foot drop is the hallmark sign, where the foot slaps the ground because you can’t lift it properly. In severe or prolonged cases, the muscles supplied by the affected branch can visibly shrink from disuse.
The pattern of weakness and numbness often tells clinicians exactly where along the nerve the problem is occurring, whether at the spine, in the pelvis, or further down the leg. This is possible precisely because the sciatic nerve’s innervation map is so well defined.