Physio · 7 minute read

After an ACL reconstruction, the date you go back to sport matters less than whether your leg is ready. Strength testing measures that by comparing the operated leg with the other one in force, jumping and landing.

Most people coming back from an ACL reconstruction ask their physio the same thing: when can I play? Time since surgery is part of the answer. The rest depends on how strong the operated leg is, how well it absorbs a landing, and how it compares with the other side. You can’t judge those by feel, because a knee can feel fine long before the muscles around it have caught up.

ACL injuries and reinjuries in numbers

ACL reconstructions are becoming more common in Australia. Over 15 years, the yearly rate rose by 43%, and by 74% in people under 25, according to an Australian study. The people most at risk were men aged 20 to 24 and women aged 15 to 19.

Surgery is only the start. A review of 69 studies found 81% of people returned to some kind of sport, 65% got back to their previous level, and 55% returned to competitive sport.

A second injury is the bigger worry. Researchers at a Melbourne knee clinic followed patients who had their ACL reconstructed before age 20. Over about five years, 35% had another ACL injury, split almost equally between the new graft and the other knee.

Time, strength and reinjury

Two findings come up again and again. The first is time. In the Delaware-Oslo study, which followed people in Norway after ACL surgery, each month they waited cut the reinjury rate by 51%, up to about 9 months after the operation. After that, waiting longer made little difference.

The second is strength, in particular the quadriceps at the front of the thigh. The same study found that more even quad strength between the legs before returning was linked with fewer reinjuries. People who returned without passing the study’s tests reinjured their knee far more often (38%) than those who passed (under 6%), though the group was small enough that this gap fell just short of statistical certainty.

A study of 158 male professional athletes found something similar. Those who went back without meeting all six discharge tests were four times as likely to rupture the new graft, and weaker hamstrings compared with the quads was also linked to rupture. These were elite players, so the exact figure may not apply to recreational players.

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What a strength test measures

We use VALD force plates and a VALD ForceFrame at our Yarraville and Hampton East studios. The force plates read how much force each leg makes as you jump, land and hop, and how quickly it makes it. The ForceFrame isolates single muscle groups, so your physio can compare your left quads with your right, and your hamstrings against your quads.

The result is shown as a percentage. If your operated leg produces 85% of the force of your other leg, the gap is 15%. Many physios look for that gap to be within about 10% before a return to sport, alongside the hop tests, time since surgery and how confident you feel in the knee.

Why one test isn’t enough

Comparing one leg with the other has a catch. After surgery, the good leg loses some strength too, because you have been doing less on it. That can make the operated leg look closer to normal than it is. One study found 57% of people passed when their operated leg was compared with the other leg, but only 29% passed when it was compared with what both legs could do before the injury.

Hop tests can also hide differences. In one study of male athletes who had been cleared to play, hop distance and strength looked even between the legs, yet the operated leg jumped clearly lower in a vertical jump. That’s why your physio will look at several measures together, and why a retest every few weeks during rehab tells you more than one test at the end.

Who makes the call

An international consensus on return to sport describes it as a gradual process that runs alongside rehab, ideally decided together by you, your physio and, where there is one, your coach. Strength testing doesn’t make the decision. It gives you all the same numbers to look at. If you’re still in rehab with us, your physiotherapist can build the testing into your plan.

Getting tested

1. 1Book in, no referral neededStrength testing is a 40-minute physiotherapy appointment, claimable on private health extras.
2. 2Bring your surgery detailsTell us the date of your operation and what your surgeon has said about returning to sport.
3. 3Do the testsYour physio picks tests that suit where you are in rehab, from strength checks early on to jumps and hops later.
4. 4Go through your reportYou’ll see each measure for both legs, the gap between them, and what to work on next.
5. 5RetestYour physio will suggest when to come back so you can see the gap closing.

The takeaway

After an ACL reconstruction, enough time and even strength between your legs are both linked with fewer reinjuries. Strength testing measures the second one, so you and your physio can decide on your return based on what your leg can do.