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COUPLED MOTION
Locking and unlocking of knee
Made by: Tejus Kaur
:BPT 3rd year

Knee joint
• It is the largest joint of the body
...

• Articular surfaces:
The condyles of femur
The patella

The condyles of tibia

• It is condylar synovial joint, incorporating two condylar joints between the condyles
of the femur and tibia, and one saddle joint between the femur and the patella
...

• Ligaments:
knee joint is supported by the following ligaments:
❖ Fibrous capsule

❖ Ligamentum patella
❖ Transverse ligament

❖ Medial collateral ligament

maintains

❖ Lateral collateral ligament

side to side stability

❖ Oblique popliteal ligament

❖ Arcuate popliteal ligament
❖ Anterior cruciate ligament
❖ Posterior cruciate ligament
❖ Medial meniscus
❖ Lateral meniscus

maintains
anteroposterior stability

Muscles producing movements at knee joint
• Extension: Quadriceps femoris
• Flexion: Biceps femoris
Semitendinosus
Semimembranosus

• Medial rotation of flexed leg: Popliteus, Semimembranosus, Semitendinosus
• Lateral rotation of flexed leg: Biceps femoris
• Locking (standing in “attention”): vastus medialis

• Unlocking (standing “at ease”): popliteus

Coupled motions
• The flexion and extension do not occur in pure sagittal plane motions but include frontal
plane components termed as “coupled motion”
...


• With knee flexion around the obliquely oriented axis, the tibia moves from a position

oriented slightly lateral to the femur to a position slightly medial to the femur in full flexion;
that is, the foot approaches the midline of the body with knee flexion
...


Locking mechanism of the knee
• There is an obligatory lateral rotation of the tibia that accompanies the final stages of knee
extension that is not voluntary or produced by muscular forces
...


• The medial articular surfaces of the knee is longer than the lateral articular surfaces
...


• As extension continues, the longer medial plateau continues to roll and glide anteriorly after
the lateral side of the plateau has halted
...


• Increasing tension in the knee joint ligaments as the knee approaches full extension may also
contribute to the obligatory rotational motion, bringing the knee into its closed-packed or
locked position
...


• Automatic rotation is also known as locking or screw home mechanism of the knee
...


• A flexion force will automatically result in medial rotation of the tibia because the longer
medial side will move before the shorter lateral compartment
...


• This automatic rotation or locking of the knee occurs in both weight-bearing
and non weight-bearing knee joint function
...


• Unlocking, consequently, is brought about by lateral rotation of the femur on
the tibia before flexion can proceed

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