A weak knee can come from ligament injury, weak thigh muscles, arthritis, or nerve problems. See a doctor if it gives way.
Common causes
- Ligament injury (ACL, MCL).
- Weak quadriceps or hip muscles.
- Meniscus tear.
- Osteoarthritis.
- Patellofemoral pain.
- Nerve problems (femoral nerve).
Clues
Giving way on stairs points to muscle weakness or ligament injury.
Care
Strengthen quads and hips. Use RICE.
See a doctor
If the knee gives out, swells or locks.
Brace
A brace can help short term.
Causes of a weak knee
Knee weakness or a feeling of giving way can arise from quadriceps weakness, ligament injury (ACL, MCL), meniscus tear, patellofemoral problems, arthritis, nerve problems such as femoral nerve compression or lumbar radiculopathy, or deconditioning.
- An ACL tear often comes with a pop, swelling within hours and instability when pivoting.
- Osteoarthritis affects roughly 32.5 million US adults, per the CDC.
- Giving way with back pain and leg numbness may point to a spine cause.
- Diabetes and low B12 may also lead to neuropathy with weakness.
Care and when to see a doctor
Strengthen quadriceps, hamstrings and hips, use RICE for acute injuries and consider a brace for support during activity. A doctor can examine ligaments and order MRI. See a doctor for swelling, locking, repeated giving way, an inability to bear weight, fever with a hot joint or sudden weakness in the leg. A hot red swollen knee may be infection and is urgent.
Causes of a weak knee
A weak or buckling knee can come from quadriceps weakness, ligament injury (ACL, MCL), meniscus tear, patellofemoral pain, arthritis or nerve problems. ACL tears affect about 200,000 people yearly in the US.
Care and when to see a doctor
Strengthen quads, hamstrings and hips, use supportive shoes and rest after injury. See a doctor for swelling, locking, giving way or inability to bear weight. MRI can show soft tissue damage.
Common structural causes
Knee instability is caused by ACL tears, which are more common in female athletes at 2 to 8 times the rate of males in some sports. Meniscus tears cause locking and giving way. Quadriceps weakness after injury or surgery reduces stability. Patellar instability occurs in 6 per 100,000 people.
Rehabilitation
Physical therapy to strengthen quadriceps, hamstrings and hip muscles, along with balance training, is first line. A brace may help for confidence. MRI shows ligament and meniscus damage. Surgery is considered for ACL tear with instability in active people. Seek medical care after twisting injuries with swelling, or for repeated giving way.
Based on data from the American Academy of Orthopaedic Surgeons
