Knee stability is tested through a combination of hands-on physical exams and functional movement tests, each targeting a different ligament or structure. Some tests require a trained examiner, while others you can do at home to gauge whether your knee needs professional evaluation. The specific test used depends on which part of the knee is suspected to be injured.
Signs You Can Check at Home
Before seeing a specialist, pay attention to what your knee does during everyday movement. A knee that buckles or gives way when you walk, climb stairs, or pivot is the most telling sign of instability. If your kneecap feels like it’s sliding out of place when you bend or straighten your leg, that points to patellar instability, where the kneecap isn’t tracking properly in its groove.
Other red flags include popping or cracking sounds during stair climbing, a sensation of your knee “catching” on something mid-motion, pain and swelling that worsen with activity, or your leg simply not supporting your weight. If your knee feels loose or unreliable during light activities like walking or housework, that’s a strong signal that the ligaments or tendons holding things together have been compromised.
A simple functional check: stand on the affected leg for 30 seconds. If you can’t maintain balance, or if your knee feels like it might buckle, that asymmetry is worth investigating. Compare it to your other leg. These home observations won’t tell you which structure is damaged, but they help you communicate clearly with a clinician and determine urgency.
The Lachman Test for ACL Injuries
The Lachman test is the most reliable hands-on exam for the anterior cruciate ligament, the ligament that prevents your shinbone from sliding forward under your thighbone. It has roughly 89% sensitivity for detecting ACL tears, making it nearly as accurate as an MRI (94%).
To perform it, the examiner bends your knee to about 20 to 30 degrees. One hand stabilizes your thighbone while the other grips the top of your shinbone and pulls it forward. The examiner is feeling for how far the shinbone shifts and whether it hits a firm stopping point. Hand placement matters: gripping closer to the knee joint produces more accurate results than grabbing lower on the shin.
ACL laxity is graded on a three-point scale based on how many millimeters the shinbone shifts forward compared to the uninjured knee. Grade I (mild) is up to 5 mm of extra movement. Grade II (moderate) is 6 to 10 mm. Grade III (severe) is 11 to 15 mm, typically indicating a complete tear. A second test, the anterior drawer test, works on a similar principle but is slightly less sensitive at about 78%.
The Pivot Shift Test for Rotational Instability
The pivot shift test detects a different kind of ACL-related instability: the rotational kind that makes your knee give out during cutting or pivoting movements. The examiner starts with your knee fully straight, then slowly bends it to 90 degrees while simultaneously rotating your shin outward and pushing the knee inward. In a positive test, the shinbone visibly shifts forward at around 20 to 30 degrees of bending, then snaps back into place.
The International Knee Documentation Committee grades pivot shift results from 0 to 3. Grade 0 is normal. Grade 1 is a subtle glide. Grade 2 produces a distinct clunk, the kind you can feel and sometimes hear. Grade 3 is a locked subluxation, where the bones temporarily get stuck out of position. This test closely mimics the actual giving-way episode that happens during sports, which is why many surgeons consider it the most functionally meaningful ACL test, even though it can be harder to perform on a tense or guarded patient.
Collateral Ligament Stress Tests
The medial and lateral collateral ligaments run along the inner and outer edges of your knee, preventing it from bowing inward or outward. Each is tested with a stress test that pushes the knee in the direction the ligament is supposed to resist.
For the medial collateral ligament (MCL), the examiner bends your knee to 20 to 30 degrees and pushes the knee inward while pulling the ankle outward. This is called the valgus stress test, and it’s considered the workhorse exam for medial knee injuries. The examiner places fingers directly over the joint line to feel for gapping between the bones.
Results break down into three grades. A grade I tear produces pain but no significant gapping, meaning the ligament is partially torn but still intact enough to hold. Grade II shows increased gapping with a definite endpoint, like a rubber band that stretches further than normal but still catches. Grade III means the gap has no endpoint at all: the joint just keeps opening, indicating a complete tear. Research from orthopedic surgeon Robert LaPrade’s group found that 3.2 mm of extra medial gapping indicates a complete superficial MCL tear, while 9.8 mm suggests a complete injury to the entire medial knee complex.
The lateral collateral ligament (LCL) is tested the same way but in reverse, pushing the knee outward (varus stress) at the same 20 to 30 degrees of flexion.
Testing the Posterior Cruciate Ligament
The posterior cruciate ligament (PCL) prevents the shinbone from sliding backward. The posterior drawer test checks for this. You lie on your back with your knee bent to 90 degrees and your foot flat on the table. The examiner pushes your shinbone straight backward and measures how far it moves.
In a healthy knee, clinical studies show about 1 to 2 mm of backward translation during this test. Cadaver studies under more controlled conditions find around 5 mm of natural laxity, so some movement is normal. The critical threshold is 10 mm: patients with backward displacement at or above 10 mm typically need surgical repair, while those below 10 mm can often be treated with rehabilitation alone. Your examiner will compare the injured knee to your healthy side to determine how much extra movement exists.
Functional Hop Tests
Physical exams tell you which structure is loose. Functional tests tell you whether your knee is actually stable enough to handle real-world demands. The single-leg hop test is the most widely used: you hop forward as far as you can on one leg and measure the distance, then repeat on the other leg.
The result is expressed as a limb symmetry index (LSI), which is the distance on your injured leg divided by the distance on your healthy leg, expressed as a percentage. An LSI of 85% to 90% or higher on hop tests is the standard cutoff for returning to sport after knee surgery. If your injured leg hops 85 cm and your healthy leg hops 100 cm, your LSI is 85%, right at the lower boundary. Research published in the Orthopaedic Journal of Sports Medicine found that a more symmetrical hop distance also predicted better quadriceps strength recovery, meaning the hop test serves as a proxy for overall knee function, not just stability.
These tests matter because a ligament can be surgically repaired but the knee still feels unstable if the surrounding muscles aren’t strong enough to compensate.
Why Muscle Strength Matters for Stability
Ligaments are passive stabilizers. They hold bones in place like cables. But the muscles around your knee, especially your quadriceps in front and hamstrings in back, act as active stabilizers that can compensate for ligament damage or protect against future injury.
The key metric is the hamstring-to-quadriceps strength ratio. A normal ratio falls between 50% and 80%, meaning your hamstrings produce at least half the force of your quadriceps. As that ratio approaches 100%, the hamstrings gain a greater capacity to stabilize the knee dynamically, particularly by resisting the forward-and-outward sliding motion that happens when the ACL is compromised. This is why post-injury rehabilitation emphasizes hamstring strengthening even when the quadriceps are the more visible muscle group.
Subjective Stability Questionnaires
Clinicians also use standardized questionnaires to quantify how stable your knee feels to you. The IKDC Subjective Knee Evaluation Form asks you to rate the highest level of activity you can perform without your knee giving way. The scale runs from very strenuous activities like jumping and pivoting (basketball, soccer) down through heavy physical work, moderate running, light walking, and finally inability to perform any activity due to giving way.
This matters because two people with the same amount of ligament laxity on a physical exam can have very different functional outcomes. One might feel perfectly stable during daily life while the other’s knee buckles on stairs. Your subjective experience of stability is a critical piece of the diagnostic picture, and it directly influences whether conservative treatment or surgery makes more sense for your situation.