The inside of your knee, known anatomically as the medial side, is the area closest to your opposite leg. This region contains a complex arrangement of muscles, tendons, and protective sacs called bursae. The muscles inserting here originate high up near the hip, providing stability and movement to the entire lower limb.
Identifying the Medial Knee Muscle Groups
The medial side of the knee is primarily defined by two distinct muscle systems that provide support and movement. The first is the Pes Anserine group, a convergence of three separate tendons named for its “goose’s foot” appearance. These tendons belong to the sartorius, gracilis, and semitendinosus muscles, each originating from a different compartment of the hip and thigh. The conjoined tendons insert onto the anteromedial surface of the tibia, approximately five centimeters below the joint line. The sartorius is the longest muscle in the body, the gracilis is an adductor muscle, and the semitendinosus is one of the hamstring muscles.
The other major muscle system is the Vastus Medialis Obliquus (VMO), the teardrop-shaped muscle located on the lower, inner portion of the quadriceps group. The VMO is the most distal part of the vastus medialis muscle and is positioned immediately adjacent to the kneecap. Its oblique fibers angle downward toward the patella and connect directly into it. This specific fiber orientation gives the VMO its specialized function within the larger quadriceps. Unlike the Pes Anserine group, the VMO is an integrated part of the large muscle mass on the front of the thigh.
Role in Knee Stability and Movement
The two medial muscle systems work together to ensure the knee joint operates smoothly and remains stable during dynamic activities. The VMO has a specialized role focused on the mechanics of the patella. Its oblique pull counteracts the kneecap’s natural tendency to shift outward during extension. This muscle provides the final, precise adjustment needed to keep the patella centered within the trochlear groove of the femur as the knee straightens.
The Pes Anserine group acts as a dynamic stabilizer against external forces. These three muscles help resist valgus stress, which is a force that pushes the knee inward. They also contribute to the internal rotation of the tibia, a movement important for locking the knee during the stance phase of walking and running. Additionally, the muscles of the Pes Anserine group assist in knee flexion, or bending the knee.
Common Sources of Inner Knee Pain
One frequent cause of discomfort on the medial side is Pes Anserine Bursitis or tendinitis. This involves inflammation of the bursa, a small fluid-filled sac beneath the Pes Anserine tendons that reduces friction against the bone. Pain is localized below the joint line on the inner side of the tibia and often worsens with activities like climbing stairs or rising from a chair.
Common causes include:
- Overuse from repetitive activities like running.
- Tight hamstring muscles.
- Obesity.
- Valgus knee alignment, which places greater mechanical stress on the area.
Another source of medial knee pain stems from VMO muscle issues, specifically weakness or imbalance. If the VMO is not strong enough, it fails to correctly guide the patella as the knee bends and straightens. This poor patellar tracking can lead to Patellofemoral Pain Syndrome, where pain is felt around or under the kneecap, sometimes referring to the inner side. Repetitive stress, such as from running or jumping, can worsen this pain.
Strain in the adductor muscles, particularly the gracilis component, can also refer pain down to the knee region. Adductor tightness transmits tension down to its insertion point on the tibia. This can mimic the localized pain of bursitis, making the inner knee a common area for referred pain from the hip and upper thigh.