Knee Joint Biomechanics: Flexion and Extension

Introduction

• Focus on knee joint kinematics: flexion & extension
• Other possible motions: medial/lateral rotation, abduction/adduction
• Discuss the principal motions: flexion & extension
• Role of ACL, PCL, and menisci in these movements

Knee Complex Biomechanics

• Degrees of Freedom: 3 rotational (flexion/extension, abduction/adduction, medial/lateral rotation)
• Translatory Motions: anterior/posterior, medial/lateral translation, pure spin
• Tibiofemoral Joint: Focus on this joint for flexion/extension
• Axis of Motion: Instantaneous, not fixed; shifts with each movement

Principal Motion: Flexion and Extension

• Axis for Motion: Trans-epicondylar axis through femoral condyles
• Flexion: Tibia fixed, femur moves backward (posterior roll + anterior glide) during squatting
• Extension: Femur rolls anteriorly, slides posteriorly during rising from squat

Arthrokinematics vs. Osteokinematics

• Arthrokinematics: Gliding, sliding, spinning within joint
• Osteokinematics: General body movements (flexion, extension)
• Convex-Concave Rule: Movement direction changes depending on whether femur or tibia is moving

Role of ACL and PCL

• ACL: Prevents excessive anterior translation of tibia; assists anterior glide during flexion
• PCL: Prevents excessive posterior translation; assists posterior glide during extension

Role of Menisci

• Structure: Wedge-shaped (thicker peripherally)
• Function: Provides congruency; assists in movement by deforming under load
• Flexion: Menisci facilitate anterior translation with uphill motion
• Extension: Menisci compression ensures femoral contact

Functional Implications

• Normal Range of Motion: 130º flexion, up to 160º in deep squatting
• Knee Flexion Needs:
  • 60-70º for walking
  • 80º for stair climbing
• Hyperextension (Genu Recurvatum): More than 5-10º extension

Influence of Two-Joint Muscles

• Passive Insufficiency: Rectus femoris limits knee flexion if hip is flexed

Conclusion

• Next session: Medial/lateral rotation, pathomechanics
• Importance of understanding mechanics for preventing injuries