Knee Flexion and Extension

Brian Lee

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Quick‑Reference Table

Knee Flexion

• Definition: Bending the knee so the heel moves toward the buttock, decreasing the angle between the femur and tibia.
  
• Range of Motion (ROM): ~0° (full extension) → 135° (deep flexion)
  
• Prime Movers:
  • Hamstrings:
    • Biceps femoris (long & short heads)
      
    • Semitendinosus
      
    • Semimembranosus
      
  • Assistants:
    • Gastrocnemius (crosses knee)
      
    • Gracilis
      
    • Popliteus
      
• Functional Role:
  • Swing phase of gait (foot clearance)
    
  • Sitting, squatting, climbing

Knee Extension

• Definition: Straightening the knee, increasing the angle between femur and tibia toward 180°.
  
• Range of Motion (ROM): ~135° (full flexion) → 0° (full extension); some individuals permit 5–10° hyperextension.
  
• Prime Movers:
  • Quadriceps group:
    • Rectus femoris
      
    • Vastus lateralis
      
    • Vastus medialis
      
    • Vastus intermedius
      
• Functional Role:
  • Stance phase of gait (support)
    
  • Rising from a chair, jumping, kicking