Knee Joint

The knee is the largest and one of the most complex joints in the human body, serving as the primary link between the thigh and lower leg. As a modified hinge joint, it allows flexion and extension while also permitting rotation when flexed, making it essential for walking, running, jumping, and maintaining stability during weight-bearing activities.

Quick Reference

Attribute	Details
Joint Type	Modified hinge (synovial) joint
Bones	Femur, tibia, patella (fibula indirectly)
Articulations	Tibiofemoral (main), patellofemoral
Degrees of Freedom	1 primary (flexion/extension) + rotation when flexed
Key Ligaments	ACL, PCL, MCL, LCL
Key Structures	Medial meniscus, lateral meniscus, patellar tendon
Primary Actions	Flexion, extension, internal/external rotation (when flexed)
Innervation	Femoral, tibial, common fibular nerves

Anatomy

The knee joint complex consists of two main articulations: the tibiofemoral joint (between the femur and tibia) and the patellofemoral joint (between the patella and femur). The tibiofemoral joint is where most of the knee's weight-bearing occurs, while the patellofemoral joint helps improve the mechanical advantage of the quadriceps during knee extension.

Bony Structures

The femur articulates with the tibia through two convex femoral condyles (medial and lateral) that sit on the relatively flat tibial plateau. This incongruent joint surface is stabilized by the menisci. The patella, the largest sesamoid bone in the body, sits within the quadriceps tendon and glides through the trochlear groove of the femur during knee flexion and extension.

Menisci

The medial and lateral menisci are C-shaped fibrocartilaginous structures that sit between the femoral condyles and tibial plateau. They increase joint congruency, distribute load across the joint surface, provide shock absorption, and contribute to joint stability. The medial meniscus is more firmly attached and less mobile than the lateral meniscus, making it more susceptible to injury.

Ligaments

Cruciate Ligaments: The anterior cruciate ligament (ACL) prevents anterior translation of the tibia on the femur and controls rotational stability. The posterior cruciate ligament (PCL) prevents posterior translation of the tibia and is the strongest knee ligament.

Collateral Ligaments: The medial collateral ligament (MCL) resists valgus stress and is attached to the medial meniscus. The lateral collateral ligament (LCL) resists varus stress and is not attached to the lateral meniscus, allowing greater mobility.

Range of Motion

Normal knee range of motion varies with age, sex, and activity level. The knee's primary motion is flexion and extension, with additional rotational capacity when the knee is flexed.

Motion	Normal Range	Notes
Flexion	0-135°	Can reach 150-160° with passive force or heel-to-buttock
Extension	0°	Neutral position; some individuals have hyperextension up to 10°
Internal Rotation	10-30°	Only available when knee is flexed beyond 30°
External Rotation	30-40°	Only available when knee is flexed beyond 30°

info

Knee rotation is "locked out" in full extension due to the screw-home mechanism, where the tibia externally rotates on the femur during the final degrees of extension, providing stability in standing.

Joint Actions

The knee's actions are primarily flexion and extension, with rotational movements available when the knee is in a flexed position.

Flexion

Flexion decreases the angle between the posterior thigh and calf, bringing the heel toward the buttock.

Prime Movers:

• Hamstrings (biceps femoris, semitendinosus, semimembranosus)
• Gastrocnemius

Function: Essential for walking, running, sitting, squatting, and clearing the foot during swing phase of gait.

Extension

Extension increases the angle between the thigh and calf, straightening the knee.

Prime Movers:

• Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius)

Function: Critical for standing, walking, climbing stairs, jumping, and maintaining upright posture.

Rotation (when flexed)

Internal Rotation rotates the tibia medially on the femur when the knee is flexed.

Prime Movers:

• Popliteus
• Semitendinosus
• Semimembranosus
• Sartorius
• Gracilis

External Rotation rotates the tibia laterally on the femur when the knee is flexed.

Prime Movers:

• Biceps femoris

Function: Important for changing direction, pivoting movements, and the screw-home mechanism during terminal extension.

Muscles Acting On It

The knee joint is controlled by muscles from both above and below, with the quadriceps and hamstrings serving as the primary movers.

Muscle	Primary Action	Role	Notes
Rectus Femoris	Extension	Prime mover	Also flexes hip
Vastus Lateralis	Extension	Prime mover	Largest quadriceps component
Vastus Medialis	Extension	Prime mover	Important for patellar tracking
Vastus Intermedius	Extension	Prime mover	Deep to rectus femoris
Biceps Femoris	Flexion, external rotation	Prime mover	Long and short heads
Semitendinosus	Flexion, internal rotation	Prime mover	Medial hamstring
Semimembranosus	Flexion, internal rotation	Prime mover	Medial hamstring
Gastrocnemius	Flexion	Synergist	Also plantarflexes ankle
Popliteus	Flexion, internal rotation	Stabilizer	"Unlocks" the knee from extension
Sartorius	Flexion, internal rotation	Synergist	Also flexes and externally rotates hip
Gracilis	Flexion, internal rotation	Synergist	Also adducts hip
Tensor Fasciae Latae	Stabilization via IT band	Stabilizer	Maintains lateral knee stability

tip

The VMO (vastus medialis obliquus) is often emphasized in rehabilitation due to its role in proper patellar tracking and preventing lateral patellar displacement.

Mobility Work

Quadriceps Stretches

Standing Quad Stretch

1. Stand on one leg (use wall for balance if needed)
2. Bend opposite knee and grasp ankle/foot behind you
3. Pull heel toward buttock while keeping knees together
4. Hold 30-60 seconds each side
5. Keep pelvis neutral (avoid arching low back)

Couch Stretch

1. Place one knee against wall or couch with foot elevated
2. Other foot flat on ground in lunge position
3. Squeeze glutes and drive hips forward
4. Hold 2 minutes each side
5. Progress by bringing front foot closer to wall

Hamstring Stretches

Standing Hamstring Stretch

1. Place one heel on elevated surface (bench, chair)
2. Keep both legs straight, hips square
3. Hinge at hips, reach toward elevated foot
4. Hold 30-60 seconds each side

90/90 Hamstring Stretch

1. Lie on back near doorway
2. Place one leg up doorframe, other leg through doorway
3. Keep both knees straight, pelvis neutral
4. Hold 2-3 minutes each side

Patellar Mobilizations

Superior/Inferior Glides

1. Sit with leg extended, quadriceps relaxed
2. Use thumbs to push patella superiorly (toward hip)
3. Hold 10 seconds, release
4. Push patella inferiorly (toward foot)
5. Hold 10 seconds, release
6. Repeat 10-15 times

Medial/Lateral Glides

1. Sit with leg extended, quadriceps relaxed
2. Push patella medially (toward midline)
3. Hold 10 seconds, release
4. Push patella laterally (away from midline)
5. Hold 10 seconds, release
6. Repeat 10-15 times

IT Band Work

Foam Roll IT Band

1. Lie on side with foam roller under lateral thigh
2. Support upper body on forearm
3. Roll from hip to just above knee
4. Pause on tender spots for 30-60 seconds
5. Perform 2-3 minutes each side

Common Issues

ACL Tears

The anterior cruciate ligament is one of the most commonly injured knee ligaments, particularly in sports involving cutting, pivoting, and sudden deceleration.

Mechanism: Non-contact pivoting, sudden deceleration, or direct blow to knee. Often accompanied by a "pop" sensation.

Symptoms: Immediate swelling, instability, positive Lachman test, inability to continue activity.

Management: Grade I-II may be conservative with physical therapy. Grade III tears often require surgical reconstruction in active individuals.

Meniscus Tears

Meniscal injuries can result from acute trauma (particularly with rotation on a loaded knee) or degenerative changes with aging.

Mechanism: Twisting motion on planted foot, deep squatting, or degenerative wear.

Symptoms: Joint line pain, catching/locking sensation, swelling, difficulty fully extending knee.

Management: Conservative treatment for degenerative tears. Surgical repair or partial meniscectomy for mechanical symptoms or unstable tears.

Patellofemoral Pain Syndrome (PFPS)

Anterior knee pain around or behind the patella, often related to tracking issues, muscle imbalances, or overuse.

Common Causes: VMO weakness, tight lateral structures, hip weakness, training errors, biomechanical factors.

Symptoms: Pain with stairs (especially descending), squatting, prolonged sitting ("theater sign"), running.

Management: Strengthen VMO and hip muscles (especially glute medius), address tight lateral structures, modify training loads, correct movement patterns.

IT Band Syndrome

Lateral knee pain caused by friction of the iliotibial band over the lateral femoral epicondyle, common in runners and cyclists.

Common Causes: Overuse, hip muscle weakness (glute medius), tight TFL/IT band, training errors (sudden mileage increase).

Symptoms: Sharp or burning lateral knee pain, worse with repetitive flexion/extension (especially around 30°).

Management: Reduce aggravating activity, strengthen hip abductors and external rotators, foam roll/massage TFL, address biomechanical factors.

Patellar Tendinopathy (Jumper's Knee)

Overuse injury of the patellar tendon, common in jumping and running sports.

Common Causes: Repetitive jumping/landing, training volume spikes, inadequate recovery, quad/hamstring imbalances.

Symptoms: Pain at inferior pole of patella, worse with jumping/squatting/stairs, morning stiffness.

Management: Progressive loading program (eccentric exercises), address training loads, strengthen entire kinetic chain, consider tissue treatment (dry needling, etc.).

Related Joints

Hip Joint

The hip and knee work together as primary movers of the lower extremity. Hip muscle weakness (particularly glute medius) can lead to dynamic knee valgus and increased knee injury risk. Several muscles cross both joints (rectus femoris, hamstrings, sartorius, TFL), creating interdependence in movement patterns.

Clinical Relevance: Address hip strength and mobility when treating knee pain. Femoral internal rotation and adduction during loading activities increases knee valgus stress.

Ankle Joint

The ankle provides the foundation for knee mechanics. Limited ankle dorsiflexion forces compensatory knee valgus during squatting movements. The gastrocnemius crosses both joints, linking ankle and knee function.

Clinical Relevance: Adequate ankle mobility (especially dorsiflexion) is essential for proper knee mechanics during squats, lunges, and landing from jumps.

Foot Complex

Foot pronation and arch collapse can contribute to internal tibial rotation and dynamic knee valgus. Foot position influences the entire kinetic chain up through the knee to the hip.

Clinical Relevance: Address foot mechanics and consider orthotics when appropriate for patients with knee pain and excessive pronation.

Sources

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4. Dutton M. Dutton's Orthopaedic Examination, Evaluation, and Intervention. 5th ed. McGraw-Hill Education; 2020.

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6. Amis AA. The functions of the knee ligaments. In: Fanelli GC, ed. The Multiple Ligament Injured Knee. 2nd ed. Springer; 2013:19-36.

7. Powers CM. The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Orthop Sports Phys Ther. 2003;33(11):639-646.

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