If you've been cleared after ACL surgery but your knee still doesn't feel like yours, you're not imagining things, and you're not alone. "Cleared" and "ready" are not the same thing. Here's what the research actually says about the gap between them, including where the evidence is strong and where it isn't.
The short answer
There's no single date that makes you ready. The research points to three things that matter together:
Time. In one well-known study, the knee reinjury rate was lower for each month later athletes returned, up to about 9 months after surgery.
Measured strength and function. In particular, how strong your quad is compared with your other leg (and, if possible, with your pre-surgery numbers), ideally tested on equipment rather than judged by eye.
How you feel about it. Lower psychological readiness, including confidence in the knee, has been linked to second ACL injuries in younger athletes.
None of these is a guarantee. Together, they give you a more complete picture than the calendar alone. Your surgeon and physical therapist make the final call on returning to sport.
Why this matters: second injuries are common
Most people do get back to sport, but fewer get back to the level they were at. A large review of 69 studies (7,556 people) found that on average 81% returned to some sport, 65% returned to their pre-injury level, and 55% returned to competitive sport.¹
Second ACL injuries are also more common than many people expect. A review of 19 studies found an overall second ACL injury rate of 15% (to either knee). For athletes younger than 25 who went back to sport, it was 23%, or nearly 1 in 4.² A 2024 review found a pooled second-injury rate of about 17%.³
That's why the return decision deserves more than a date on the calendar.
What the research says about timing
A key study on timing followed 106 athletes in pivoting sports for two years after ACL reconstruction. Among those who returned to pivoting sports, each month of delay up to 9 months after surgery was associated with a 51% lower rate of knee reinjury (any knee reinjury, not only a second ACL tear). After 9 months, the study found no further significant drop, and about 1 in 5 of those who waited past 9 months still had a knee reinjury.⁴
Two honest caveats:
That was one study of about 100 athletes, and the timing finding came from the 74 who returned to pivoting sports.
A 2024 review pooling 21 studies found that athletes who suffered a second ACL injury had returned about 25 days earlier on average than those who didn't, but the authors rated the certainty of that evidence as very low. They also found no timing difference among professional athletes.³
So "9 months" isn't a magic number. It's a commonly recommended minimum, based mainly on one cohort study, and it's not a finish line.
It's also worth knowing that return-to-sport testing often happens earlier than that. A 2025 review of 143 studies found that, in research studies, testing was done at about 7.3 months after surgery on average, and earlier in North American studies (about 6.8 months) than in Europe or Australia.⁵
What the research says about passing return-to-sport tests
Return-to-sport tests usually combine strength testing (often the quad and hamstring) with hop tests, and compare your injured leg with your healthy one. Many use a target of 90%, and some use 80 to 85%.¹² ¹³
Here's where the evidence gets more complicated than you'll usually hear:
Some individual studies look strong. In 158 male professional athletes, those who did not pass all six discharge tests before returning to team training had about 4 times the risk of tearing their graft.⁶ In the timing study above, 38% of those who failed the criteria had a knee reinjury, compared with 6% of those who passed. However, that difference was not statistically significant.⁴
Reviews of many studies are more mixed. A 2019 review found that only 23% of patients passed their return-to-sport test batteries. Passing was linked to a lower risk of rupturing the graft, but not to a lower risk of all ACL injuries combined, and it was linked to a higher risk of an ACL injury in the other knee.⁷ Another 2019 review, which pooled 4 studies, found no significant link between passing and second ACL injury, and rated the quality of the evidence as very low.⁸ A 2024 review of 9 studies (1,410 people) again found that passing was linked to a lower risk of graft rupture, but not to a lower risk of knee injuries overall, second ACL injuries overall, or ACL injury in the other knee.⁹
What that means for you: passing your tests is linked to a lower risk of re-tearing the graft, but it is not a guarantee, and it hasn't been shown to protect the other knee. Testing is a tool, not a verdict.
One more piece of context: even healthy people with no ACL injury fail some of these tests. A 2025 review found that, depending on the test, between about 5% and 50% of healthy people didn't meet the usual passing standards.¹⁰ That's a reminder that a single number shouldn't decide everything.
The one measure that keeps showing up: quad strength
Across the research, quad strength comes up again and again. In the timing study above, more symmetrical quad strength before returning was significantly associated with a lower knee reinjury rate.⁴ An updated rehab guideline from University of Delaware clinicians calls for frequent and accurate quad strength testing, a delayed return timeline, step-by-step progressions for running, cutting and return to competition, and continued injury prevention work after you return.¹¹
If your quad strength has never been measured on equipment, that's a good question to ask your PT.
Why "90% symmetry" can fool you
Most tests compare your injured leg with your healthy leg. The problem is that your healthy leg can lose strength too while you're less active during rehab, so hitting 90% of it can look better than it really is.¹²
A study of 70 athletes tested them before surgery and again 6 months after.¹² At 6 months:
57% reached 90% symmetry on all their strength and hop tests.
Only 29% reached 90% of what their healthy leg could do before surgery.
Of the 40 athletes who "passed" on symmetry, 24 (60%) fell short when compared with their pre-surgery numbers.
In a small, preliminary analysis (11 second injuries), the pre-surgery comparison flagged more of the athletes who later had a second ACL injury.
The authors concluded that symmetry scores frequently overestimate knee function after ACL reconstruction.
What jump testing can show that hop tests miss
Hop tests usually measure distance. Newer research suggests that vertical jumps, especially when measured on force plates, can show deficits that distance hops hide:
In 26 male athletes who had been cleared to return, jump height on the surgical leg was only 83% of the other leg in a single-leg vertical jump and 77% in a single-leg drop jump, even though they looked symmetrical on standard hop and strength tests.¹³
In 126 athletes tested on force plates at the time of return to sport, compared with 532 healthy athletes, the athletes coming back from ACL surgery were still asymmetrical when pushing off and, mainly in double-leg jumps, when landing. They also jumped lower in most tests, despite having passed traditional discharge criteria.¹⁴
A 2024 review of 42 studies found that people who'd had ACL reconstruction showed lasting deficits in vertical jump performance.¹⁵
An honest limit: these studies show the deficits exist. They don't yet prove that fixing them prevents reinjury. It's still useful information to have before you go back to cutting and landing at full speed.
Your confidence counts
Feeling "not ready" is worth taking seriously. In a study of 329 people who returned to sport, younger athletes (20 and under) who went on to a second ACL injury had scored lower on a psychological readiness questionnaire at 12 months than younger athletes who didn't get reinjured. In older athletes, there was no clear difference.¹⁶ A positive psychological response has also been linked to getting back to your pre-injury level.¹
If you don't trust your knee yet, say so. That's useful information for your medical team, not a weakness.
What to ask before you go back
Use these questions with your surgeon and physical therapist:
How far out from surgery am I? If it's under 9 months, ask how your timeline compares with the roughly 9-month point used in research.
Has my quad strength been measured on equipment? And what was the number?
Is my injured leg being compared only with my other leg, or with where I was before surgery?
Have my jumps and landings been tested, not just hop distance?
Have I gradually worked back through running, sprinting, cutting and practice before full games?
Do I actually feel confident in the knee? Be honest.
How Code 8 fits in
Code 8 is not physical therapy and doesn't replace your medical team. We work with athletes after their surgeon or PT clears them to train, and we stay in contact with that team.
What we add is measurement and coached training in the gap between PT discharge and full competition: force plate testing to see how each leg pushes off and lands, motion capture to look at landing and cutting mechanics, and laser timing for sprints and change of direction. We retest regularly so progress is visible, and those numbers go to you, your family and your medical team to support the return decision.
If you're in San Luis Obispo County and want to talk through where you are, book a free consult or read more about our ACL return-to-sport training.
The bottom line
Being cleared is a milestone. Readiness is something you can partly measure. Give it time (9 months is a common research-based benchmark, though the evidence behind it is limited), get your strength and jumps actually tested, compare against where you were before surgery when possible, and take your own confidence seriously. No test can promise you won't get hurt again, but time and real numbers together beat a date alone.
This article is general information, not medical advice. Always follow your surgeon and physical therapist on when and how to return to activity.
References
Research sourced from PubMed.
Ardern CL, Taylor NF, Feller JA, Webster KE. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014;48(21):1543-52. https://doi.org/10.1136/bjsports-2013-093398
Wiggins AJ, et al. Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: a systematic review and meta-analysis. Am J Sports Med. 2016;44(7):1861-76. https://doi.org/10.1177/0363546515621554
Piussi R, Simonson R, Zsidai B, et al. Better safe than sorry? A systematic review with meta-analysis on time to return to sport after ACL reconstruction as a risk factor for second ACL injury. J Orthop Sports Phys Ther. 2024;54(3):161-175. https://doi.org/10.2519/jospt.2023.11977
Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016;50(13):804-8. https://doi.org/10.1136/bjsports-2016-096031
Pales Taylor ML, Ihn H, Cushman DM, Bodkin SG. Time of administering return to sport testing following anterior cruciate ligament reconstruction: a systematic review. Phys Ther Sport. 2025;76:1-12. https://doi.org/10.1016/j.ptsp.2025.09.001
Kyritsis P, Bahr R, Landreau P, Miladi R, Witvrouw E. Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture. Br J Sports Med. 2016;50(15):946-51. https://doi.org/10.1136/bjsports-2015-095908
Webster KE, Hewett TE. What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis. Sports Med. 2019;49(6):917-929. https://doi.org/10.1007/s40279-019-01093-x
Losciale JM, Zdeb RM, Ledbetter L, Reiman MP, Sell TC. The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis. J Orthop Sports Phys Ther. 2019;49(2):43-54. https://doi.org/10.2519/jospt.2019.8190
Zhou W, Liu X, Hong Q, Wang J, Luo X. Association between passing return-to-sport testing and re-injury risk in patients after anterior cruciate ligament reconstruction surgery: a systematic review and meta-analysis. PeerJ. 2024;12:e17279. https://doi.org/10.7717/peerj.17279
Wood A, Hargreaves M, Manfredi JN, et al. Anterior cruciate ligament reconstruction return to sport testing passing rates for healthy people: a systematic review. Am J Sports Med. 2025;53(13):3291-3297. https://doi.org/10.1177/03635465241313194
Brinlee AW, Dickenson SB, Hunter-Giordano A, Snyder-Mackler L. ACL reconstruction rehabilitation: clinical data, biologic healing, and criterion-based milestones to inform a return-to-sport guideline. Sports Health. 2022;14(5):770-779. https://doi.org/10.1177/19417381211056873
Wellsandt E, Failla MJ, Snyder-Mackler L. Limb symmetry indexes can overestimate knee function after anterior cruciate ligament injury. J Orthop Sports Phys Ther. 2017;47(5):334-338. https://doi.org/10.2519/jospt.2017.7285
Kotsifaki A, et al. Single leg vertical jump performance identifies knee function deficits at return to sport after ACL reconstruction in male athletes. Br J Sports Med. 2022;56(9):490-498. https://doi.org/10.1136/bjsports-2021-104692
Kotsifaki R, Sideris V, King E, Bahr R, Whiteley R. Performance and symmetry measures during vertical jump testing at return to sport after ACL reconstruction. Br J Sports Med. 2023;57(20):1304-1310. https://doi.org/10.1136/bjsports-2022-106588
Dutaillis B, Diamond LE, Lazarczuk SL, Timmins RG, Bourne MN. Vertical jump testing after anterior cruciate ligament reconstruction: a systematic review and meta-analysis. Med Sci Sports Exerc. 2024;56(2):181-192. https://doi.org/10.1249/MSS.0000000000003298
McPherson AL, Feller JA, Hewett TE, Webster KE. Psychological readiness to return to sport is associated with second anterior cruciate ligament injuries. Am J Sports Med. 2019;47(4):857-862. https://doi.org/10.1177/0363546518825258