The tibialis posterior is a deep muscle of the lower leg, recognized as a primary stabilizer for the foot and ankle. It is one of the deepest muscles in the calf, nestled between the larger, more superficial muscles. This muscle’s long tendon extends down behind the ankle, acting like a flexible cable that helps maintain the structural integrity of the foot. Understanding its location and function is fundamental to grasping why it is frequently associated with lower leg pain.
Exact Anatomical Placement
The tibialis posterior muscle is situated within the deep posterior compartment of the lower leg, lying behind the shin bone, or tibia. Its origin is extensive, beginning on the upper two-thirds of the inner surface of the tibia and the upper portion of the fibula, the two long bones of the lower leg. This muscle belly is also attached to the interosseous membrane, a sheet of tissue that connects the tibia and fibula along their length.
The muscle fibers merge into a powerful tendon that travels down the leg, passing through the ankle area. The tendon wraps directly behind the medial malleolus, the prominent bony bump on the inside of the ankle. This course allows the tendon to function as a pulley, redirecting the muscle’s force toward the underside of the foot.
The tendon inserts primarily onto the tuberosity of the navicular bone, a tarsal bone on the inner side of the foot. It also sends extensions to the cuneiform bones, the cuboid bone, and the bases of the second, third, and fourth metatarsals, creating a broad attachment that supports the entire midfoot structure.
Role in Foot and Ankle Stability
The anatomical positioning of the tibialis posterior allows it to perform two primary mechanical actions for foot stability and movement. Its most direct action is the inversion of the foot, which turns the sole inward toward the midline of the body. This movement is important for navigating uneven surfaces and maintaining balance during dynamic activities.
The secondary role of the muscle is providing dynamic support to the medial longitudinal arch of the foot. As the foot contacts the ground during walking or running, the tibialis posterior contracts to resist the natural tendency of the arch to collapse (pronation). By acting like a flexible stirrup, the muscle helps maintain the arch’s height and shape, ensuring proper weight distribution and shock absorption.
Common Causes of Pain and Injury
The repetitive and high-tension demands placed on this muscle make its tendon vulnerable to overuse injuries.
Posterior Tibial Tendon Dysfunction (PTTD)
PTTD is the most notable overuse injury, involving chronic inflammation and eventual degeneration of the posterior tibial tendon. This condition impairs the tendon’s ability to support the foot’s arch and is the most frequent cause of adult-acquired flatfoot, where the arch gradually collapses and the heel and toes begin to turn outward.
Initial symptoms often include pain and tenderness along the inside of the foot and ankle, directly over the tendon’s course. As the condition progresses, the pain may shift to the outside of the ankle due to new stress placed on surrounding ligaments and joints. Risk factors that increase the strain on the tendon include:
- Age
- Obesity
- An existing flat foot structure
- Engaging in high-impact sports like running
Medial Tibial Stress Syndrome (MTSS)
The tibialis posterior is also implicated in Medial Tibial Stress Syndrome (MTSS), commonly referred to as shin splints. This condition causes pain along the inner edge of the tibia, typically in the lower two-thirds of the shin bone.
The inflammation associated with MTSS is thought to involve the repetitive pulling of the muscle’s fascial covering and connective tissue where it attaches to the bone. Overuse, poor biomechanics, and a sudden increase in activity intensity can contribute to the strain on the tibialis posterior, leading to the pain characteristic of shin splints.