Knee Weakness and Knees Giving Way

What knee weakness feels like

People describe several different things as knee weakness, and the description points toward the cause:

• Giving way or buckling: the knee suddenly collapses, often when pivoting or on stairs. Suggests a ligament tear, meniscus tear or slipping kneecap.
• Unable to trust the knee: a constant sense of instability. Common after ligament injury and with arthritis.
• True muscle weakness: difficulty getting up from a chair, climbing stairs or straightening the leg. Suggests muscle or nerve causes.
• Catching or locking: the knee jams and you have to wiggle it free. Suggests a loose fragment or torn meniscus.

The phrase "weak in the knees" in everyday speech means feeling suddenly shaky with strong emotion, such as love, fear or shock. That is a surge of adrenaline and a drop in blood pressure, not a knee problem. Feeling faint and wobbly in both legs can also come from low blood sugar, dehydration or low blood pressure.

Mechanical causes

Ligament injuries

An ACL tear is the classic cause of a knee that gives way. It happens during sudden stops, pivots or awkward landings in sports. Many people hear a pop, the knee swells within hours, and afterwards it buckles on turning. Injuries to the medial collateral ligament (MCL) and posterior cruciate ligament (PCL) can also cause instability.

Meniscus tear

A meniscus tear damages the shock-absorbing cartilage. It causes pain along the joint line, swelling over a day or two, catching, locking and buckling. In people over 40, tears often happen gradually from wear with minor twists.

Kneecap instability

With patellar instability, the kneecap slips partly or fully out of its groove, usually toward the outside. The knee gives way with a sickening shift, often in teenage girls and people with loose joints. After a first dislocation, recurrence is common.

Patellofemoral pain

Patellofemoral pain syndrome causes aching around the kneecap on stairs, squatting and after sitting. Pain can cause the muscles to switch off briefly, so the knee buckles without any structural damage.

Arthritis and muscle weakness

Osteoarthritis

Knee osteoarthritis is a common cause of weak, unstable knees after age 45. Pain and swelling inhibit the quadriceps, the large muscle at the front of the thigh, which stabilizes the knee. Worn cartilage and loosened ligaments add to instability. Buckling raises fall risk in older adults.

Quadriceps weakness

The quadriceps do most of the work of keeping the knee straight under load. They weaken quickly after injury, surgery, bed rest or illness, and with age (sarcopenia). Quadriceps weakness by itself can make the knee buckle on stairs and slopes.

Tendon rupture

A fall onto a bent knee can rupture the quadriceps or patellar tendon. You cannot straighten the knee or lift the straight leg, and there may be a gap above or below the kneecap. This needs surgery within days.

Nerve causes

• Pinched nerve in the lower back: an L3 or L4 nerve root irritated by a disc or arthritis (lumbar radiculopathy) weakens the quadriceps, with pain or numbness in the front of the thigh and shin.
• Femoral nerve injury: after hip surgery, pelvic surgery, a groin catheter or bleeding into the muscle, often in people on blood thinners.
• Diabetic amyotrophy: painful thigh weakness in people with diabetes.
• Central nervous system causes: stroke, multiple sclerosis and spinal cord problems usually also affect other parts of the body.
• Muscle diseases: inclusion body myositis characteristically weakens the quadriceps in people over 50, causing falls.

Causes compared

How doctors find the cause

A doctor or physical therapist will ask when the weakness started, whether there was an injury, what makes the knee give way, and about back pain, numbness and other joints. They will:

• look for swelling, wasting of the thigh muscle and alignment
• test the ligaments (for example the Lachman test for the ACL)
• check the menisci and kneecap stability
• test leg strength, reflexes and sensation

Tests may include:

• X-ray: for arthritis, fractures and kneecap position
• MRI: for ligament, meniscus, cartilage and tendon injuries
• Spine MRI or nerve conduction studies: if a nerve cause is suspected
• Blood tests: for inflammation, diabetes or muscle enzymes when appropriate

Treatment for weak knees

Exercise and physical therapy

Strengthening is the core treatment for almost every cause. A program usually includes:

• quadriceps exercises such as straight leg raises, wall sits, step-ups and mini squats
• hamstring and hip strengthening, because weak hip muscles let the knee collapse inward
• balance and proprioception training, such as single-leg standing
• sport-specific training for return to activity

Most people notice improvement in 6 to 12 weeks.

Other measures

• Bracing: hinged braces or kneecap stabilizing braces for instability.
• Weight loss: each 1 lb (0.45 kg) of body weight puts roughly 4 lb of load across the knee when walking.
• Pain relief: acetaminophen (paracetamol) or NSAIDs such as ibuprofen, following the label dose. Topical NSAID gels are a good option for knee arthritis. Avoid long-term oral NSAIDs if you have kidney, stomach or heart problems.
• Walking aids: a cane in the opposite hand reduces load on an arthritic knee.

Surgery

Surgery is considered for ACL tears in active people, meniscus tears that lock the knee, recurrent kneecap dislocation, tendon ruptures, and advanced arthritis that has not responded to other treatment (knee replacement).

Frequently asked questions

What does it mean when your knee gives out?

A knee that gives out usually has a mechanical problem, such as a torn ACL or meniscus, or a kneecap that slips out of its groove. Arthritis and weak thigh muscles can also cause buckling. Occasionally a pinched nerve in the back is responsible.

What does "weak in the knees" mean?

In everyday language, feeling weak in the knees means feeling suddenly shaky or faint from strong emotion such as attraction, fear or shock. Medically, true knee weakness is different: a knee that buckles or cannot support your weight, from a joint, muscle or nerve problem.

Can weak knees be strengthened?

Yes. Targeted exercises for the quadriceps, hamstrings and hips, plus balance training, improve knee stability for most causes, including arthritis and after ligament injuries. A physical therapist can tailor a program. Gains usually appear within 6 to 12 weeks of regular work.

Should I see a doctor if my knee keeps buckling?

Yes. Repeated buckling can damage cartilage and the meniscus further and raises your risk of falls. A doctor can check for a ligament or meniscus tear or kneecap instability and decide whether you need imaging, physical therapy or surgery.

Can a back problem cause knee weakness?

Yes. A disc or bone spur pressing on the L3 or L4 nerve root in the lower back can weaken the thigh muscles that hold the knee straight. It usually comes with back pain or numbness in the front of the thigh or shin.