Knee Anatomy, Muscles & Ligaments

 

The knee is the largest joint in the human body — and one of the most complex. It endures forces of up to 5–6× your body weight during running and squatting. A thorough understanding of its architecture is the foundation of effective self-care.

Figure 1: Cross-section of the right knee joint — key bony and soft-tissue structures

 

1.1  Key Bony Structures

 

Structure	Description & Clinical Significance
Femur (Thigh bone)	The largest bone in the body. The distal end forms two rounded condyles that articulate with the tibia. The femoral groove (trochlea) guides the patella during knee flexion.
Patella (Kneecap)	A sesamoid bone embedded in the quadriceps tendon. Acts as a mechanical pulley, increasing the quadriceps' leverage by up to 30%. Prone to maltracking when hip/quad strength is imbalanced.
Tibia (Shin bone)	The main weight-bearing bone of the lower leg. Its flat upper surface (tibial plateau) receives load from the femoral condyles, cushioned by the menisci.
Fibula	The slender bone on the outer lower leg. Provides LCL and biceps femoris attachment. Bears ~15% of lower leg load.

 

 

1.2  Ligaments — The Stabilisers

Ligaments are tough fibrous bands that connect bone to bone, providing passive stability to the knee. There are four primary ligaments:

Ligament	Location	Function	Common Injury
ACL (Anterior Cruciate)	Deep — centre	Prevents the tibia from sliding forward; controls rotational stability. Most critical for sport.	Pivoting, landing injuries
PCL (Posterior Cruciate)	Deep — centre	Prevents the tibia from sliding backward. Stronger than ACL — less commonly torn.	Dashboard injury, falls on flexed knee
MCL (Medial Collateral)	Inner (medial) side	Resists valgus (knock-knee) forces. Supports medial joint stability.	Contact sport tackle from outside
LCL (Lateral Collateral)	Outer (lateral) side	Resists varus (bow-leg) forces. Works with popliteus to control tibial rotation.	Rarer; high-energy trauma

 

1.3  Muscles — The Active Controllers

Muscles are the dynamic stabilisers of the knee. Weakness or imbalance in these groups is the root cause of most non-traumatic knee pain. Understanding your muscle roles helps you prioritise which exercises matter most for your recovery.

 

 

 

 

 

Muscle / Group	Type	Role at the Knee	Rehab Priority
Quadriceps (Rectus femoris, Vastus group)	Extensor	Extend the knee; absorb landing forces. The VMO (inner quad) specifically controls patellar tracking.	HIGH — target first in PFPS & post-surgery
Hamstrings (Biceps femoris, Semimembranosus)	Flexor / Stabiliser	Flex the knee; co-contract with quads to protect the ACL during dynamic movements.	HIGH — imbalance increases ACL risk
Gluteus Medius & Maximus	Hip controller	Controls femoral rotation and valgus alignment. Weakness leads to knee cave — the primary driver of PFPS.	CRITICAL — hip before knee in most protocols
Calf (Gastrocnemius)	Plantarflexor / Knee flexor	Assists knee flexion; contributes to knee stability during push-off phase of gait.	Moderate — especially in runners
Popliteus	Rotator / Unlocking	Unlocks the knee from full extension; controls tibial rotation. Often overlooked but critical for PFP.	Moderate — target in lateral pain
IT Band / TFL	Stabiliser	The iliotibial band runs along the outer thigh. Excessive tightness or weakness causes friction at the lateral femoral condyle — IT Band Syndrome.	Moderate in runners & cyclists

 

🔑 Key Insight:  The hip controls the knee. Research consistently shows that gluteus medius weakness is the single greatest predictor of patellofemoral pain, IT band syndrome, and ACL injury risk. Before targeting the knee directly, always assess and strengthen the hip.

1.4  Cartilage & Soft Tissues

Two additional structures play a vital role in knee health:

•    Menisci — Two C-shaped fibrocartilage discs (medial & lateral) act as shock absorbers, distributing load evenly across the joint. They also deepen the joint socket, adding stability. Meniscal tears are among the most common knee injuries, especially with rotational mechanisms.

•    Articular cartilage — A smooth, glassy layer covering bone ends in the joint. Unlike muscle, cartilage has no direct blood supply and heals very slowly. Damage leads to osteoarthritis over time. Protecting and loading cartilage appropriately is central to long-term knee health.

•    Bursae — Small fluid-filled sacs reducing friction between tissues. Inflammation of the prepatellar or infrapatellar bursa (bursitis) can cause localised swelling and tenderness.

•    Synovial membrane & fluid — Lines the joint capsule and produces synovial fluid, which lubricates and nourishes the cartilage. Excess fluid (effusion) is a sign of active inflammation.