Knee flexors are the muscles responsible for bending the knee joint, bringing the heel towards the buttocks.
These muscles, working together, enable activities like walking, running, jumping, and kicking. While the hamstrings are the primary muscles involved in knee flexion, several other muscles contribute to this movement and also assist in rotation of the knee.
Muscles Involved in Knee Flexion
The following muscles are the key players in flexing the knee:
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Hamstrings: This group consists of three muscles located on the back of the thigh:
- Biceps femoris
- Semitendinosus
- Semimembranosus
- Hamstrings are the primary knee flexors.
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Gracilis: Located on the inner thigh, it also assists in hip adduction and internal rotation of the tibia when the knee is flexed.
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Sartorius: The longest muscle in the human body, it crosses both the hip and knee joints, contributing to flexion, abduction, and external rotation of the hip, as well as flexion and internal rotation of the knee.
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Gastrocnemius: A major calf muscle that also crosses the knee joint and helps with plantarflexion of the ankle.
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Plantaris: A small muscle in the posterior compartment of the leg that assists with knee flexion and ankle plantarflexion.
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Popliteus: Located at the back of the knee, it unlocks the knee joint by laterally rotating the femur on the tibia (or medially rotating the tibia if the femur is fixed), allowing knee flexion to occur.
Function and Importance
Knee flexors play a vital role in various movements:
- Locomotion: Essential for walking, running, and other forms of movement.
- Posture: They contribute to maintaining proper posture and balance.
- Athletic Performance: Crucial for athletes in sports requiring running, jumping, and kicking.
- Everyday Activities: Used in activities like sitting, squatting, and climbing stairs.
Knee Flexor Actions
| Muscle | Primary Action | Secondary Actions |
|---|---|---|
| Hamstrings | Knee Flexion | Hip Extension, Tibial Rotation (when knee is flexed) |
| Gracilis | Knee Flexion | Hip Adduction, Tibial Internal Rotation |
| Sartorius | Knee Flexion | Hip Flexion, Hip Abduction, Hip External Rotation, Tibial Internal Rotation |
| Gastrocnemius | Knee Flexion | Ankle Plantarflexion |
| Plantaris | Knee Flexion | Ankle Plantarflexion |
| Popliteus | Knee Flexion (initiates) | Knee Joint Unlocking, Tibial Internal Rotation |
Proper function and strength of the knee flexors are essential for optimal lower limb biomechanics and injury prevention.
