The muscles acting on the knee joint can be categorized by their primary function: flexion (bending the knee) or extension (straightening the knee). While some muscles primarily act on the hip, they can also assist in knee movement.
Knee Extensors (Straightening the Knee)
-
Quadriceps Femoris: This is the main group of muscles responsible for knee extension and consists of four muscles:
-
- Rectus Femoris: Also flexes the hip.
-
- Vastus Lateralis:
-
- Vastus Medialis:
-
- Vastus Intermedius:
Knee Flexors (Bending the Knee)
- Vastus Intermedius:
-
-
Hamstrings: Located on the posterior side of the thigh, these muscles primarily flex the knee and extend the hip. The hamstrings group includes:
-
- Biceps Femoris:
-
- Semimembranosus:
-
- Semitendinosus:
-
-
Gastrocnemius: This calf muscle also assists in knee flexion. It plantarflexes the ankle as well.
-
Popliteus: Located at the back of the knee, the popliteus muscle helps to "unlock" the knee by laterally rotating the femur on the tibia (or medially rotating the tibia if the femur is fixed). It also weakly flexes the knee.
-
Sartorius: This long, strap-like muscle also assists in knee flexion, hip flexion, and hip abduction and external rotation.
-
Gracilis: While primarily a thigh adductor, the gracilis muscle also flexes and medially rotates the knee.
Here's a table summarizing the key muscles involved:
| Muscle Group | Muscles | Primary Function(s) |
|---|---|---|
| Knee Extensors | Rectus Femoris, Vastus Lateralis, Vastus Medialis, Vastus Intermedius (Quadriceps Femoris) | Knee Extension |
| Knee Flexors | Biceps Femoris, Semimembranosus, Semitendinosus (Hamstrings), Gastrocnemius, Popliteus, Sartorius, Gracilis | Knee Flexion |
It is important to note that muscle actions can vary depending on the position of the body and the presence of other forces. This list represents the primary muscles contributing to knee movement.
