The outside, or lateral side, of the knee is a complex intersection of bone, muscle, and connective tissue that manages the immense forces placed on the leg during movement. These structures are designed for both mobility and stability, allowing the leg to bend and straighten while preventing unwanted side-to-side motion. The structure often felt running along the outer thigh toward the knee is not a single large muscle at the joint line. Instead, it is a thick, rope-like structure that continues from musculature higher up in the hip, providing lateral support, especially when bearing weight.
The Iliotibial Band and Tensor Fasciae Latae
The structure extending along the outside of the thigh is the Iliotibial Band (IT Band), which is technically not a muscle. The IT Band is a long, thick sheath of fascia, a dense connective tissue that provides support and encases muscles. This fibrous band originates high up at the hip and stretches down the entire length of the thigh, attaching to the upper part of the shin bone (tibia) at Gerdy’s tubercle.
The muscle responsible for generating tension in this connective tissue is the Tensor Fasciae Latae (TFL). The TFL is a small, spindle-shaped muscle located near the front of the hip and pelvis. Along with a portion of the Gluteus Maximus muscle, the TFL inserts directly into the Iliotibial Band at the hip. When the TFL contracts, it pulls on the IT Band, transmitting force down the thigh and influencing the mechanics of the knee joint.
This relationship means the TFL acts as the motor, and the IT Band acts as the long extension that delivers force to the lower leg. The TFL is a hip flexor and internal rotator. Its tensioning of the IT Band allows it to exert influence on the knee, even though the muscle belly is nowhere near the joint. The IT Band is a crucial anatomical link, bridging the power generated by hip muscles to the lateral stability required at the knee.
How This Structure Stabilizes the Leg
The Iliotibial Band complex is a major contributor to the lateral stability of the leg, particularly during activities involving single-leg support, such as walking and running. Its primary function is to act as a dynamic strut, assisting in maintaining the alignment between the hip and the knee. This stabilization is important in the frontal plane, preventing the knee from moving excessively side-to-side.
During the stance phase of gait, the IT Band resists the tendency of the torso’s weight to cause the hip and knee to collapse inward. The TFL and the upper fibers of the gluteus maximus actively regulate the tension in the band to counteract these forces, providing continuous support. This regulation ensures the knee joint remains aligned over the foot, which is fundamental for efficient movement.
The band also works synergistically with knee ligaments, particularly the anterior cruciate ligament (ACL) and the fibular collateral ligament, to provide robust lateral support. When the knee is fully extended, the IT Band is under tension, which helps to lock the joint and contributes to static posture. Its attachment to the tibia ensures it maintains a mechanical advantage for managing rotational forces during dynamic movement.
Common Sources of Lateral Knee Discomfort
The most frequent source of discomfort related to this structure is Iliotibial Band Syndrome (ITBS), an overuse injury commonly seen in runners and cyclists. ITBS results from irritation and inflammation of the tissues on the outside of the knee. This occurs where the IT Band passes over the lateral femoral epicondyle, the bony prominence on the outer side of the thigh bone.
The mechanism of injury is often described as repetitive friction as the band slides back and forth over the bony landmark with each bend and straighten of the knee. This rubbing causes localized inflammation in the underlying bursa or in the band itself, leading to a stinging or burning sensation on the outer knee. Pain associated with ITBS often begins after a specific duration or distance of activity. It may intensify when the knee is bent at roughly a 30-degree angle, a common position during the mid-stance phase of running.
Several factors contribute to the development of ITBS, with biomechanical issues and training errors being the most prevalent. Weakness in the hip abductor muscles (gluteus medius and gluteus minimus) can lead to poor hip control, increasing strain on the TFL and the IT Band. Training mistakes, such as a sudden increase in mileage, running on uneven or sloped surfaces, or inadequate recovery time, also raise the risk. Other factors include excessive foot pronation, slight leg length discrepancies, and pre-existing tightness in the IT Band.