Ask most people recovering from an ACL reconstruction when they’re “ready” to return to sport, and the honest answer is usually a date on a calendar, nine months, twelve months, whatever their surgeon or coach mentioned early on. The evidence says that date should mean very little on its own. What predicts whether the knee will hold up is how it performs on a battery of hop tests, compared directly against the uninjured leg.
WHAT THE TEST ACTUALLY MEASURES
The standard hop test battery includes four tests: a single hop for distance, a triple hop for distance, a crossover hop for distance, and a six-metre timed hop. Each test is performed on one leg and compared to the other, producing a Limb Symmetry Index, the injured leg’s performance expressed as a percentage of the uninjured leg’s. This four-test battery was first described by Noyes and colleagues in 1991 and remains the backbone of return-to-sport testing after ACL injury today.
The generally accepted target is a Limb Symmetry Index of at least 90% across all four hop tests, alongside adequate quadriceps and hamstring strength. Below that threshold, the evidence is clear that the knee is not mechanically ready for the deceleration, cutting, and landing forces of full sport.
WHY TESTING BEATS GUESSING
This is one of the best-studied areas in all of sports rehabilitation. A 2016 Delaware-Oslo cohort study by Grindem and colleagues, published in the British Journal of Sports Medicine, followed ACL-reconstructed athletes and found that those who passed a battery of objective discharge criteria, including hop testing, had an 84% lower rate of reinjury than those cleared on time alone. A related study by Kyritsis and colleagues found that athletes who returned to sport without meeting all six clinical discharge criteria were four times more likely to rupture their graft.
Despite this, surveys of ACL rehabilitation consistently find that a large proportion of athletes are cleared to return using time since surgery as the primary criterion, with formal hop testing used far less often than the evidence would suggest it should be. Passing the hop battery does not guarantee a reinjury-free return, but failing it is one of the more reliable predictors we have that the graft or the other knee is at meaningfully higher risk.
THE KINETIC CHAIN COST OF CUTTING CORNERS
Perhaps the most sobering statistic in this area comes from a 2016 systematic review and meta-analysis by Wiggins and colleagues, which found an overall second ACL injury rate of around 15% in athletes who returned to sport after reconstruction, roughly split between the same knee (7%) and the opposite knee (8%). For athletes under 25 who returned to a high level of sport, that combined risk rose to around 23%. That contralateral figure is exactly the kinetic chain effect hop testing is designed to catch: an athlete compensating for a knee that isn’t fully ready places disproportionate load on the other leg, and it is often the other leg that fails first.
THE LONG-TERM PICTURE
The honest, harder-to-hear evidence concerns long-term joint health. A widely cited 2009 systematic review by Øiestad and colleagues, published in the American Journal of Sports Medicine, found that osteoarthritis develops in a substantial proportion of knees within 10 to 15 years of an ACL injury, whether or not the ligament is surgically reconstructed. Reconstruction reduces the chance of the knee giving way and needing a total knee replacement later, but it does not, by itself, prevent osteoarthritis. What does appear to matter is minimising further trauma to the joint, meaning every additional graft rupture, meniscus tear, or cartilage injury adds to that long-term risk. Passing objective hop testing before return to sport is one of the few modifiable factors that reduces the odds of exactly that kind of additional trauma.
WHAT WE DO AT PRAXIS
Our sports physiotherapists run the full four-test hop battery alongside strength testing and movement quality screening before clearing anyone back to cutting, pivoting, or contact sport after an ACL injury. If you’re approaching return to sport after ACL reconstruction, book a return-to-sport assessment with our team and test the knee properly, rather than guessing from the calendar.
Until next time, Praxis What You Preach…
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References
Noyes FR, Barber SD, Mangine RE. Abnormal lower limb symmetry determined by function hop tests after anterior cruciate ligament rupture. American Journal of Sports Medicine. 1991;19(5):513-518.
Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after anterior cruciate ligament reconstruction: the Delaware-Oslo ACL cohort study. British Journal of Sports Medicine. 2016;50(13):804-808.
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. British Journal of Sports Medicine. 2016;50(15):946-951.
Wiggins AJ, Grandhi RK, Schneider DK, Stanfield D, Webster KE, Myer GD. Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: a systematic review and meta-analysis. American Journal of Sports Medicine. 2016;44(7):1861-1876.
Øiestad BE, Engebretsen L, Storheim K, Risberg MA. Knee osteoarthritis after anterior cruciate ligament injury: a systematic review. American Journal of Sports Medicine. 2009;37(7):1434-1443.