Knee Joint: Primary Movements, Accessory Rotation, and Biomechanics

What is the movement of the knee joint?

The knee joint, a complex hinge joint, primarily facilitates flexion and extension, allowing for bending and straightening of the leg. It also permits limited degrees of internal and external rotation, particularly when the knee is flexed, contributing to its remarkable versatility and stability.

Introduction to the Knee Joint

The knee is one of the largest and most intricate joints in the human body, crucial for locomotion, weight-bearing, and athletic performance. Structurally, it is often described as a modified hinge joint, formed by the articulation of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). While its primary function is to allow the lower leg to move relative to the thigh, its complex array of ligaments, menisci, and surrounding musculature enables a nuanced range of motion essential for daily activities and demanding physical tasks. Understanding these movements is fundamental for anyone involved in exercise, rehabilitation, or human movement science.

Primary Movements of the Knee

The two most prominent and powerful movements of the knee joint occur in the sagittal plane.

• Flexion:

  • Description: This movement decreases the angle between the posterior surfaces of the thigh and leg, essentially "bending" the knee. It brings the heel closer to the buttocks.
  • Range of Motion: Typically ranges from 0 degrees (full extension) to 130-150 degrees, depending on individual flexibility and soft tissue approximation.
  • Primary Muscles Involved: The hamstring group (biceps femoris, semitendinosus, semimembranosus) are the primary movers. Other significant contributors include the gastrocnemius (calf muscle), sartorius, gracilis, and popliteus.
  • Functional Significance: Essential for walking, running, sitting, squatting, and climbing stairs.

• Extension:

  • Description: This movement increases the angle between the posterior surfaces of the thigh and leg, effectively "straightening" the knee. It moves the lower leg away from the buttocks.
  • Range of Motion: From a flexed position back to 0 degrees, where the leg is fully straight. Hyperextension (beyond 0 degrees) can occur in some individuals but is often limited by ligaments and joint structures.
  • Primary Muscles Involved: The quadriceps femoris group (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) are the sole extensors of the knee.
  • Functional Significance: Crucial for standing, kicking, jumping, and propelling the body forward during gait.

Secondary (Accessory) Movements: Rotation

While flexion and extension are the primary actions, the knee also permits a limited degree of rotation, particularly when it is in a flexed position (approximately 20-30 degrees or more). When the knee is fully extended, the joint is "locked" by various structures, severely restricting rotation.

• Internal (Medial) Rotation:

  • Description: The tibia rotates inward relative to the femur, meaning the toes turn inward if the foot is off the ground, or the femur rotates outward relative to the tibia if the foot is fixed.
  • Range of Motion: Approximately 10-15 degrees.
  • Primary Muscles Involved: Popliteus, semitendinosus, semimembranosus, sartorius, gracilis.
  • Functional Significance: Plays a role in changing direction, pivoting, and the "screw-home mechanism" during terminal extension (see below).

• External (Lateral) Rotation:

  • Description: The tibia rotates outward relative to the femur, meaning the toes turn outward if the foot is off the ground, or the femur rotates inward relative to the tibia if the foot is fixed.
  • Range of Motion: Approximately 20-30 degrees.
  • Primary Muscles Involved: Biceps femoris (long head), assisted by the tensor fasciae latae (via the iliotibial band).
  • Functional Significance: Also involved in pivoting and the screw-home mechanism.

The Role of Key Structures in Knee Movement

The ability of the knee to move efficiently and safely relies on the coordinated action of its bony, ligamentous, and muscular components.

• Bones:

  • Femur: The distal end forms two condyles that articulate with the tibia.
  • Tibia: The proximal end (tibial plateau) provides a relatively flat surface for articulation.
  • Patella: A sesamoid bone embedded within the quadriceps tendon, which glides over the femoral condyles, improving the mechanical advantage of the quadriceps.
  • Fibula: Though part of the lower leg, it does not directly articulate with the femur and is not a primary component of the knee joint itself, but serves as an attachment point for ligaments and muscles.

• Ligaments: Provide static stability and guide movements.

  • Cruciate Ligaments (ACL & PCL): The Anterior Cruciate Ligament (ACL) prevents anterior translation of the tibia on the femur and hyperextension. The Posterior Cruciate Ligament (PCL) prevents posterior translation of the tibia on the femur. They also limit rotation.
  • Collateral Ligaments (MCL & LCL): The Medial Collateral Ligament (MCL) resists valgus (knock-kneed) forces. The Lateral Collateral Ligament (LCL) resists varus (bow-legged) forces. They also tighten in extension, limiting rotation.

• Menisci: Two C-shaped fibrocartilaginous pads (medial and lateral menisci) situated between the femoral condyles and the tibial plateau.

  • Function: They enhance joint congruity (fit), absorb shock, distribute forces, and aid in joint lubrication. They also play a role in guiding rotational movements.

• Muscles: The surrounding musculature generates the force for movement and provides dynamic stability.

  • Quadriceps Femoris: Primary knee extensors.
  • Hamstrings: Primary knee flexors and internal/external rotators.
  • Gastrocnemius: Assists with knee flexion (as it crosses the knee joint).
  • Popliteus: Initiates knee flexion from full extension and internally rotates the tibia to "unlock" the knee.

Biomechanics of Knee Movement: The Screw-Home Mechanism

A unique and crucial aspect of knee biomechanics is the "screw-home mechanism." As the knee extends during the last 20-30 degrees, the tibia externally rotates relative to the femur (or the femur internally rotates on the tibia if the foot is fixed). This slight rotation, facilitated by the shape of the femoral condyles, tightens the collateral and cruciate ligaments, effectively "locking" the knee in full extension. This locking provides greater stability for standing with minimal muscular effort. To initiate flexion from a fully extended position, the popliteus muscle must first internally rotate the tibia (or externally rotate the femur) to "unlock" the knee.

Functional Implications and Practical Application

Understanding the intricate movements of the knee is paramount for:

• Exercise Prescription: Designing effective and safe exercises that target specific muscles and respect joint mechanics (e.g., proper squat depth, appropriate resistance for hamstring curls).
• Injury Prevention: Recognizing movements and forces that place undue stress on ligaments, menisci, or cartilage (e.g., avoiding excessive knee valgus during landing, limiting high-impact activities with rotational forces on a fully extended knee).
• Rehabilitation: Guiding recovery from knee injuries by restoring range of motion, strengthening supporting musculature, and retraining proprioception.
• Performance Enhancement: Optimizing athletic movements by improving knee stability, strength, and coordination.

Conclusion

The knee joint, while often simplified as a hinge, is a marvel of biomechanical engineering. Its primary movements of flexion and extension, coupled with critical accessory rotation, enable a vast array of human movements. This complex interplay of bones, ligaments, menisci, and muscles ensures both mobility and stability, making the knee indispensable for daily life and athletic pursuits. A comprehensive understanding of its kinematics and kinetics is essential for anyone seeking to optimize human movement, prevent injury, or facilitate recovery.