Why Your Knee Still Hurts 6 Months After ACL Surgery

Key Takeaways

• Approximately 86% of ACL reconstruction patients still have greater than 10% quadriceps strength deficit at 6 months, making quad weakness the most common driver of persistent knee pain
• Arthrogenic muscle inhibition (AMI) affects up to 56% of patients at 6 weeks post-injury and creates a neural shutdown that prevents adequate quad activation, even with consistent exercise
• Patellar tendon (BTB) graft recipients report anterior knee pain at rates of 5% to 48% depending on follow-up timing, with kneeling pain persisting in over 21% at 2 years
• Athletes who return to sport with quad strength below 90% limb symmetry index have a 33% reinjury rate versus 12.5% for those above the threshold
• For every 1% increase in quadriceps strength symmetry, reinjury risk drops by 3%, making targeted quad strengthening the single most impactful intervention for persistent post-surgical knee pain

You did everything right. You had the surgery, showed up to every PT appointment, followed the protocol. Six months later, your knee still hurts. It aches after sitting for too long, flares up during squats, and feels unstable on stairs. Your surgeon says the graft looks fine on imaging. So why does it still hurt?

The answer almost always traces back to what is happening above the joint, not inside it. We see this pattern constantly in our orthopedic PT practice: patients who have a structurally successful reconstruction but persistent pain because the muscles, neural pathways, and movement patterns around the knee haven't caught up to the surgical repair. This guide covers the five most common reasons your knee still hurts at 6 months and what objective testing reveals about each one.

Is Quadriceps Weakness Causing Your Knee Pain?

Quadriceps weakness is the most common and most overlooked cause of persistent knee pain after ACL reconstruction. A systematic review of 37 studies found that the average side-to-side quad strength deficit at 6 months post-ACLR was 23% plus or minus 8%. Only 13.5% of studies showed patients meeting the clinical benchmark of 10% or less asymmetry. By 12 months, the average deficit still sat at 14%.

Those numbers translate directly into pain and function. Research published in the Journal of Athletic Training found that for every 1 N-m/kg increase in quadriceps strength, IKDC scores (a standard knee function measure) improved by 17.6 points. Patients who reached a quad strength threshold of 2.18 to 2.62 N-m/kg were significantly more likely to report acceptable symptom levels. A separate study found that injured-limb quad strength combined with knee pain predicted 74% of the variance in patient-reported knee function at return to activity.

The problem is that quad weakness after ACL surgery is not simply from disuse. Your nervous system is actively suppressing quad activation through a mechanism called arthrogenic muscle inhibition. Standard exercises alone often cannot overcome this neural block, which is why targeted neuromuscular retraining and objective strength testing matter more than just "doing your exercises."

What Is Arthrogenic Muscle Inhibition and Why Does It Matter?

Arthrogenic muscle inhibition (AMI) is a reflexive neural shutdown that prevents your quadriceps from fully activating after knee injury or surgery. It affects up to 56% of patients at 6 weeks post-injury and is not the same as simple muscle atrophy. AMI is a centrally mediated process where damaged or disrupted joint receptors send altered signals to the spinal cord, which responds by inhibiting the motor neurons that drive your quad.

A 2023 study in Frontiers in Sports and Active Living confirmed that AMI affects both the injured and uninjured limb, proving its central neural origin. This bilateral effect also means that limb symmetry index comparisons can overestimate your recovery because both legs are weaker than they should be. JOSPT research documented this directly: of 40 patients who met 90% LSI criteria at 6 months, only 16 had actually reached 90% of their pre-injury performance level.

The encouraging finding is that approximately 80% of AMI cases are reversible with targeted clinical techniques. Sonnery-Cottet and colleagues (2022) developed a grading system for AMI and demonstrated that most patients can restore quad activation during their first clinical session using specific hamstring fatigue protocols and neuromuscular electrical stimulation combined with voluntary exercise. The key is identifying AMI through objective activation testing rather than assuming the quad is simply weak from disuse.

Could Your Graft Type Be Contributing to Anterior Knee Pain?

The type of graft used in your ACL reconstruction significantly influences where and how you experience post-surgical knee pain. Bone-patellar tendon-bone (BTB) grafts harvest tissue from the front of your knee, leaving a structural deficit in the patellar tendon that can produce chronic anterior knee pain and difficulty kneeling.

A systematic review of Level I studies found that anterior knee pain incidence after BTB graft ranges from 5.4% to 48.4% depending on follow-up timing. Kneeling pain is even more common, ranging from 4% to 75.6% across studies. Data from the New Zealand ACL Registry showed that at 2 years, 21.3% of BTB graft recipients reported kneeling difficulty versus 9.4% for hamstring graft recipients. Using a patellar tendon graft increases anterior knee pain risk by 3.4 times compared to hamstring graft.

Hamstring grafts avoid the anterior knee donor site issue but come with their own trade-off: hamstring weakness that can take longer to resolve, particularly in sprinting and deceleration. Neither graft type is universally superior. What matters at 6 months is whether your rehab has addressed the specific deficits your graft type creates. BTB recipients need targeted patellar tendon loading and gradual kneeling desensitization. Hamstring graft recipients need eccentric hamstring strengthening protocols that rebuild the harvested muscle's capacity.

Are You Dealing With Arthrofibrosis or a Cyclops Lesion?

Two structural complications can cause persistent stiffness and pain that mimics other post-surgical issues. Arthrofibrosis, the buildup of excessive scar tissue that restricts knee motion, occurs in approximately 4% to 8% of ACL reconstructions requiring intervention, though reported rates range as high as 35% with broader diagnostic criteria.

Risk factor research shows that female patients face 2.5 times higher risk of arthrofibrosis, and competitive athletes (Tegner activity level 7 or above) have 3.56 times higher odds. The clearest sign is a persistent extension deficit: if you cannot fully straighten your knee by 6 months despite consistent range of motion work, arthrofibrosis should be on the differential.

Cyclops lesions are nodules of fibrous tissue that form in front of the ACL graft and mechanically block full extension. MRI studies detect cyclops lesions in 25% to 47% of reconstructed knees, though approximately 80% are asymptomatic. Symptomatic cyclops syndrome, where the lesion causes a hard block to extension with an audible or palpable clunk, occurs in 1% to 10% of cases. The telling feature is that 93% of symptomatic cases are diagnosed within 6 months of surgery.

If your extension is objectively limited and not improving with manual therapy and progressive stretching, your PT should communicate these findings to your surgeon. Both arthrofibrosis and symptomatic cyclops lesions respond well to arthroscopic intervention when identified.

What Benchmarks Predict Whether Your Knee Will Keep Hurting?

Objective benchmarks predict both persistent pain and reinjury risk with remarkable precision. The Delaware-Oslo ACL Cohort study published in BJSM found that for every 1-month delay in return to sport up to 9 months, reinjury rate dropped by 51%. For every 1% increase in quadriceps limb symmetry index, reinjury risk decreased by 3%. Patients who returned with quad LSI below 90% had a 33.3% reinjury rate compared to 12.5% for those above the threshold.

The standard return-to-sport test battery includes quad strength LSI at or above 90%, single-leg hop for distance LSI at or above 90%, triple hop, crossover hop, and 6-meter timed hop, each at or above 90%. Athletes who did not pass the full battery had a 4 times greater likelihood of graft rupture.

The challenge is that hop tests recover faster than raw strength. At 6 months, the average hop test deficit is only 11% while the average quad strength deficit is 23%. An athlete can pass hop tests while still carrying a significant quad weakness that increases both pain and reinjury risk. This is why force plate testing and isokinetic dynamometry are essential. They measure what hop tests miss: the actual force production capacity of your quadriceps compared to the uninvolved side and compared to pre-injury estimates.

What Supplements Support Knee Recovery After ACL Surgery?

Rebuilding the tissues around a reconstructed knee requires adequate protein intake and targeted supplementation. Protein at 1.6 to 2.2 grams per kilogram of bodyweight daily, distributed across 20 to 40 gram servings per meal, provides the amino acids your muscles need to rebuild the quad mass lost during the post-surgical period. Leucine-rich protein sources (whey, eggs, poultry) are particularly effective because leucine directly triggers muscle protein synthesis.

Creatine monohydrate at 5 grams daily supports the high-intensity quad strengthening that drives recovery from ACL reconstruction. Research consistently shows creatine enhances strength gains during resistance training, which is exactly the training stimulus your quad needs at 6 months. A 20-gram-per-day loading phase for the first 5 to 7 days accelerates saturation, followed by 5 grams daily for maintenance. Take it with a meal containing carbohydrates to enhance uptake.

Conclusion

Persistent knee pain at 6 months after ACL surgery is common, identifiable, and treatable. The evidence points overwhelmingly to quad strength as the primary driver: 86% of patients still carry meaningful quad deficits at this stage, and every percentage point of strength you recover translates to measurably better function and lower reinjury risk.

We test quad strength, neuromuscular activation, hop performance, and movement quality using force plates and isokinetic dynamometry because subjective "feeling good" does not predict outcomes. Objective data does. If your knee still hurts at 6 months, the numbers will tell you why, and the right program will fix it.

Book Your Evaluation

Frequently Asked Questions

Is it normal for your knee to still hurt 6 months after ACL surgery?

It is common but not inevitable. Approximately 15% to 39% of patients report some degree of persistent pain or symptoms at 6 months depending on the measure used. The most frequent cause is quadriceps weakness, which is treatable with targeted strengthening and neuromuscular retraining.

How long does quad weakness last after ACL surgery?

The average quad strength deficit is 23% at 6 months and 14% at 12 months. Most patients do not reach the recommended 10% or less asymmetry threshold until well past 9 months. Targeted strengthening with objective monitoring is the fastest path to closing the gap.

Does the type of ACL graft affect knee pain?

Yes. Patellar tendon (BTB) grafts carry 3.4 times higher risk of anterior knee pain and 21% incidence of kneeling difficulty at 2 years. Hamstring grafts have lower anterior knee pain rates but may produce hamstring weakness that affects sprinting and deceleration.

What is arthrogenic muscle inhibition?

AMI is a neural reflex that suppresses quadriceps activation after knee injury or surgery. It affects up to 56% of patients and cannot be overcome with standard strengthening alone. Neuromuscular electrical stimulation and targeted activation protocols can reverse approximately 80% of cases.

When should I be concerned about stiffness after ACL surgery?

If you cannot achieve full knee extension (0 degrees) by 6 to 8 weeks post-surgery, and the deficit persists despite consistent range of motion work, arthrofibrosis or a cyclops lesion should be evaluated. Both are treatable but require identification through objective measurement and potentially imaging.

Bottom Line

• Quadriceps weakness is the primary driver of persistent knee pain after ACL surgery, with 86% of patients still exceeding the recommended 10% strength deficit threshold at 6 months
• Arthrogenic muscle inhibition is a neural shutdown (not simple weakness) that suppresses quad activation in up to 56% of patients, but 80% of cases are reversible with targeted clinical techniques
• Objective benchmarks predict outcomes: athletes returning with quad LSI below 90% face a 33% reinjury rate, and every 1% improvement in quad symmetry reduces reinjury risk by 3%

Continue Learning

• Complete ACL Recovery Timeline Guide From 3 Months to Competition: what to expect at each stage of ACL rehabilitation
• The Complete Guide to Criterion-Based Return-to-Sport Testing: the objective benchmarks that predict safe return to competition

ACL Masterclass: From Table to Turf: TSU's comprehensive course on criterion-based ACL rehabilitation