Knee Flexion: Anatomy, Muscles, and Biomechanics

How does knee flexion occur?

Knee flexion, the bending of the knee joint, is a complex physiological movement primarily driven by the contraction of the hamstring muscles, facilitated by the intricate biomechanics of the tibiofemoral and patellofemoral joints.

Understanding Knee Flexion: A Definition

Knee flexion refers to the decrease in the angle between the thigh and the lower leg. It is the action of bending the knee, bringing the heel closer to the buttocks. This movement occurs in the sagittal plane around a frontal (or mediolateral) axis. The knee joint, a modified hinge joint, allows for a significant range of flexion, typically from 0 degrees (full extension) to approximately 140-155 degrees, depending on individual anatomy and soft tissue restrictions.

The Anatomy of the Knee Joint

To understand knee flexion, it's crucial to appreciate the key structures of the knee:

• Bones:
  • Femur: The thigh bone, its distal end forms the superior part of the knee joint.
  • Tibia: The larger of the two lower leg bones, its proximal end forms the inferior part of the knee joint.
  • Patella: The kneecap, a sesamoid bone embedded within the quadriceps tendon, which articulates with the femur.
• Articular Cartilage: Covers the ends of the femur and tibia, and the posterior surface of the patella, providing a smooth, low-friction surface for movement.
• Menisci: Two C-shaped cartilaginous pads (medial and lateral meniscus) located between the femur and tibia, acting as shock absorbers and improving joint congruity.
• Ligaments: Provide stability to the joint, preventing excessive movement:
  • Cruciate Ligaments (ACL & PCL): Anterior and Posterior Cruciate Ligaments, located within the joint, prevent anterior and posterior displacement of the tibia relative to the femur.
  • Collateral Ligaments (MCL & LCL): Medial and Lateral Collateral Ligaments, located on the sides of the joint, prevent excessive side-to-side movement.
• Joint Capsule: Encloses the joint, containing synovial fluid for lubrication.

Key Muscles Responsible for Knee Flexion (Primary Movers)

The primary muscles responsible for initiating and executing knee flexion are collectively known as the hamstrings. These muscles originate on the ischial tuberosity of the pelvis and cross both the hip and knee joints, inserting on the tibia and fibula.

• Biceps Femoris:
  • Long Head: Originates from the ischial tuberosity.
  • Short Head: Originates from the linea aspera of the femur.
  • Insertion: Head of the fibula and lateral tibial condyle.
  • Action: Powerful knee flexor, also externally rotates the tibia when the knee is flexed.
• Semitendinosus:
  • Origin: Ischial tuberosity.
  • Insertion: Pes Anserinus (medial surface of the proximal tibia).
  • Action: Knee flexor and internal rotator of the tibia when the knee is flexed.
• Semimembranosus:
  • Origin: Ischial tuberosity.
  • Insertion: Medial tibial condyle (posterior aspect).
  • Action: Knee flexor and internal rotator of the tibia when the knee is flexed.

Synergistic Muscles and Stabilizers

While the hamstrings are the primary drivers, several other muscles assist in knee flexion, acting as synergists or contributing to stability:

• Gastrocnemius:
  • Origin: Medial and lateral condyles of the femur (above the knee joint).
  • Insertion: Calcaneus (heel bone) via the Achilles tendon.
  • Action: Primarily a plantar flexor of the ankle, but also assists in knee flexion, particularly when the ankle is dorsiflexed or during powerful movements like jumping.
• Popliteus:
  • Origin: Lateral femoral condyle.
  • Insertion: Posterior surface of the proximal tibia.
  • Action: Initiates knee flexion by "unlocking" the knee from full extension (internally rotating the tibia on the femur or externally rotating the femur on the tibia), a crucial part of the "screw-home mechanism."
• Sartorius:
  • Origin: Anterior superior iliac spine (ASIS).
  • Insertion: Pes Anserinus (medial surface of the proximal tibia).
  • Action: Flexes, abducts, and externally rotates the hip; also assists in knee flexion.
• Gracilis:
  • Origin: Pubic symphysis.
  • Insertion: Pes Anserinus (medial surface of the proximal tibia).
  • Action: Adducts the hip; also assists in knee flexion.

The Biomechanics of Knee Flexion

Knee flexion involves a complex interplay of movements at the tibiofemoral and patellofemoral joints:

• Rolling and Gliding: As the knee flexes, the femoral condyles roll posteriorly on the tibial plateau while simultaneously gliding anteriorly. This combination ensures that the femur does not roll off the back of the tibia, maintaining joint congruity.
• Screw-Home Mechanism: When the knee moves from full extension into flexion, the popliteus muscle plays a critical role. At full extension, the tibia externally rotates slightly on the femur (or the femur internally rotates on the tibia) to "lock" the knee, providing stability for standing. To initiate flexion, the popliteus internally rotates the tibia (or externally rotates the femur), "unlocking" the joint.
• Patellar Tracking: During flexion, the patella glides inferiorly within the trochlear groove of the femur. Proper tracking is essential to prevent excessive friction and pain. Muscle imbalances (e.g., weak vastus medialis obliquus) or structural issues can disrupt this tracking.
• Ligamentous Control: The cruciate and collateral ligaments guide and limit the range of motion during flexion, preventing unwanted anterior/posterior or medial/lateral displacement. The Posterior Cruciate Ligament (PCL) becomes taut during flexion, helping to prevent excessive posterior translation of the tibia.
• Open vs. Closed Kinetic Chain:
  • Open Kinetic Chain (OKC): When the foot is free to move (e.g., seated hamstring curl). The tibia moves on a relatively fixed femur.
  • Closed Kinetic Chain (CKC): When the foot is fixed to the ground (e.g., squat, lunge). The femur moves on a relatively fixed tibia. Both types of movements involve knee flexion, but the muscle activation patterns and joint forces differ.

Factors Influencing Knee Flexion

Several factors can influence the range and efficiency of knee flexion:

• Muscle Strength: Strong hamstrings and synergistic muscles are essential for powerful and controlled flexion.
• Flexibility: Adequate flexibility of the quadriceps (antagonists to the hamstrings) and surrounding soft tissues is crucial; tight quadriceps can restrict full flexion.
• Joint Health: Conditions like osteoarthritis, meniscal tears, or ligamentous injuries can significantly limit the range of motion and cause pain during flexion.
• Swelling: Fluid accumulation within the joint capsule can restrict movement.
• Neural Control: The nervous system coordinates muscle activation and relaxation, ensuring smooth and controlled movement.

Practical Implications for Movement and Training

Knee flexion is fundamental to nearly every human movement, from simple daily activities to complex athletic maneuvers:

• Daily Activities: Walking, running, climbing stairs, sitting down, and standing up all rely on efficient knee flexion.
• Sports Performance: Essential for jumping, landing, sprinting, kicking, and changes of direction in sports.
• Exercise: Hamstring curls, squats, lunges, deadlifts, and many other exercises involve and strengthen the muscles responsible for knee flexion. Understanding the biomechanics helps in designing effective and safe training programs.
• Injury Prevention and Rehabilitation: Identifying limitations in knee flexion or imbalances in the muscles that control it is critical for preventing injuries and guiding rehabilitation strategies after knee trauma or surgery.

Conclusion

Knee flexion is a sophisticated biomechanical action driven primarily by the hamstrings, supported by synergists, and guided by the intricate anatomical structures of the knee joint. Its proper execution is vital for functional movement, athletic performance, and overall musculoskeletal health. A comprehensive understanding of "how" this movement occurs provides a foundation for effective exercise programming, injury prevention, and rehabilitation strategies.