Knee Flexion Anatomy and Function

Knee Flexion

• Objectives: Students will be able to identify the primary muscles involved in knee flexion, the range of motion for knee flexion, the effect of weakness in knee flexion, and the effect of contracture in knee flexion.

Primary Muscles

• Hamstring Muscle:
  • Lateral hamstring: biceps femoris
  • Medial hamstring: semimembranosus, semitendinosus

Range of Motion

• The range of motion of complete knee flexion, is 0 to 120-130 degrees.

Factors Limiting Motion

• Tension of Quadriceps muscles
• Contact of posterior aspect of thigh with calf muscle of lower leg

Anatomy of Biceps Femoris (Lateral Hamstring)

• Origin: Upper inner quadrant of posterior surface of ischial tuberosity (long head); middle third of linea aspera, lateral supracondylar ridge of femur (short head).
• Insertion: Styloid process of head of fibula, lateral collateral ligament, lateral tibial condyle.

Nerve Supply

• Long Head: Tibial portion of sciatic nerve.
• Short Head: Common peroneal portion of sciatic nerve.

Anatomy of Semimembranosus (Medial Hamstring)

• Origin: Upper outer quadrant of posterior surface of ischial tuberosity.
• Insertion: Medial condyle of tibia below articular margin, over popliteus and oblique popliteal ligament
• Action: Flexes and medially rotates knee; extends hip.
• Nerve: Tibial portion of sciatic nerve.

Anatomy of Semitendinosus (Medial Hamstring)

• Origin: Upper inner quadrant of posterior surface of ischial tuberosity.
• Insertion: Upper medial shaft of tibia below Gracilis.
• Action: Flexes and medially rotates knee; extends hip.
• Nerve: Tibial portion of sciatic nerve.

Effects of Weakness

• Weakness of both medial and lateral hamstrings: Permits hyperextension of the knee
• Bilateral weakness: Leads to anterior tilting of the pelvis and potential lumbar lordosis
• Weakness of lateral hamstrings: Causes tendency toward loss of lateral stability, leading to bowleg position
• Weakness of medial hamstrings: Decreases medial stability, leading to knock-knee position with tendency to lateral rotation of the leg on the femur.

Effects of Shortness or Contracture

• Shortness of hamstrings: Restricts knee extension when hip is flexed, or restricts hip flexion when knee is extended.
• Contracture of both medial and lateral hamstrings: Results in knee flexion and posterior tilting of the pelvis, potentially leading to a flattened lumbar spine.