Four muscles work together to invert your foot, turning the sole inward. The two primary drivers are the tibialis posterior and the tibialis anterior, with two smaller muscles in the back of the lower leg playing a supporting role. Inversion happens mainly at the subtalar joint, just below the ankle, and the normal range of motion is about 20 degrees.
The Two Primary Inverters
The tibialis posterior is the strongest inverter of the foot. It runs deep along the back of your lower leg, originating from the upper two-thirds of the tibia and fibula. Its tendon wraps around the inside of your ankle and fans out to attach to several bones on the underside of your midfoot, most notably the navicular bone and the medial cuneiform. When it contracts, it pulls the sole of the foot inward and slightly downward. Beyond inversion, this muscle is the key stabilizer of your medial arch, the curve along the inside of your foot. When the tibialis posterior weakens or its tendon degenerates, adults often develop flat feet and poor arch control.
The tibialis anterior sits in the front compartment of your lower leg, running along the outer edge of your shinbone. Its tendon crosses the top of your ankle and inserts on the inner side of the foot at the base of the first metatarsal and medial cuneiform. It performs two actions simultaneously: it pulls the foot upward (dorsiflexion) and turns the sole inward (inversion). You use this muscle every time you lift your foot to clear the ground while walking. Because it dorsiflexes and inverts at the same time, it’s the reason your foot naturally tilts slightly inward as you swing it forward during a step.
Supporting Muscles
Two deep muscles in the back of the lower leg assist with inversion, though it isn’t their primary job. The flexor hallucis longus, which curls your big toe, and the flexor digitorum longus, which curls the smaller toes, both run tendons along the inner side of the ankle. Their line of pull gives them a mechanical advantage for pulling the sole inward, so they contribute to inversion whenever they’re active. They matter most during dynamic movements like pushing off the ground in walking or running, when the foot is loaded and multiple muscles fire together.
Where Inversion Happens
Inversion is primarily a subtalar joint movement. The subtalar joint sits directly beneath the true ankle joint, between the talus (the bone that connects your foot to your leg) and the calcaneus (your heel bone). Its axis of rotation is tilted, averaging about 42 degrees in the side-to-side plane and angled 23 degrees inward relative to the long axis of your foot. This angled axis is what allows the rocking, inward-turning motion of inversion rather than a simple hinge movement. The transverse tarsal joint, a set of joints in the midfoot, contributes additional range as well.
This subtalar motion also serves a compensatory role. If your lower leg has a slight angular alignment issue, your subtalar joint can adjust through inversion and eversion to keep your foot flat on the ground.
Nerve Supply to the Inverters
The inverter muscles are controlled by two different nerves branching from nerve roots in the lower spine (L4 through S3). The tibialis anterior receives its signal from the deep peroneal nerve, which runs through the front of the leg. The tibialis posterior, flexor hallucis longus, and flexor digitorum longus are all supplied by the tibial nerve, which runs through the back of the leg.
This split in nerve supply is clinically useful. If someone develops foot drop, where the foot hangs limp and can’t be lifted, testing inversion strength helps pinpoint where the problem is. A person with foot drop from a pinched nerve root in the spine (L4-L5) will typically have weak inversion because the tibialis posterior is affected. Someone with foot drop from damage to the peroneal nerve at the knee will still have strong inversion, because the tibial nerve supplying the tibialis posterior is intact. To test this, a clinician asks you to turn the sole of your foot inward against resistance with the foot slightly pointed downward.
The Muscles That Oppose Inversion
Inversion and eversion are opposite movements, so the muscles that evert the foot (turning the sole outward) act as antagonists to the inverters. The primary evertors are the peroneal muscles, also called the fibularis longus, brevis, and tertius, which run along the outer side of the lower leg. The extensor digitorum longus, which lifts the smaller toes, also contributes to eversion. These muscles don’t just oppose inversion; they actively stabilize the ankle during standing and movement. A healthy balance between the inverter and evertor groups keeps the ankle from rolling too far in either direction.
Why Inversion Matters for Ankle Sprains
Forced inversion, where the foot rolls inward beyond its normal 20-degree range, is the most common mechanism behind ankle sprains. When the foot rolls in too far, the ligaments on the outer side of the ankle get stretched or torn. Three ligaments make up the lateral complex on the outside of the ankle, and they tend to injure in a predictable sequence. The anterior talofibular ligament is the weakest of the three and tears first. Roughly 70% of lateral ankle sprains involve this ligament alone. More severe inversion injuries progress to damage the calcaneofibular ligament and, in the worst cases, the posterior talofibular ligament.
Strong inverter muscles don’t just produce movement. They also act as a braking system, eccentrically controlling how fast and how far the foot rolls. Well-conditioned tibialis posterior and tibialis anterior muscles help absorb sudden inversion forces before the ligaments reach their limit, which is one reason ankle strengthening exercises are a staple of sprain prevention and rehabilitation programs.