Working out after ACL surgery: how much exercise is too much?

If you’re facing ACL reconstruction – or have recently had the operation – you probably know that ACL rehab is a bit of a balancing act.

On the one hand, you want to keep your knee active so you can maximise your recovery timeline. But on the other hand, there are dangers in overdoing it – especially in the early stages.

So what does “overdoing it” look like, how would you know if you were putting your ACL under excessive strain, and what does a well-balanced ACL rehab look like? Here’s our take…

What’s actually happening in your knee after ACL surgery?

An ACL reconstruction is a significant operation. The surgeon will remove your torn ligament and create a series of tiny holes in your femur and tibia bones. They will then fashion a graft (usually a piece of your hamstring), stretch it tight across the joint space and secure it to those holes.

The end result is a good, strong substitute ACL. But it will take time, rest and a carefully-controlled programme of physiotherapy to bring it to its full strength. We generally say it takes 6 months for the graft to heal into the bone tunnels and develop a healthy blood supply, and 9 months or more to reach full strength.

Where do patients typically go wrong?

On the whole, people who have ACL reconstructions tend to be young and active. If they make a mistake with their rehab, it’s rarely because they fail to keep up with their physio exercises. It’s usually the opposite: they tend to push too hard and fast in the early stages because they want to rebuild strength as soon as possible.

But this risks irritating the graft just as the tissues are healing and remodelling themselves, generating fresh inflammation that will actually break down the collagen growth you’re trying to encourage. It also runs the risk of loosening that crucial fixation to the bone. If you’re ambitious and sporty, putting your body to the test comes naturally. But for the first three months after ACL surgery, and certainly for the first few weeks, rest and protection are more important than pushing your limits.

A measured ACL recovery will look more like this…

The first 3 months: keep it in-line

Immediately after surgery, you’ll focus on keeping your leg rested and straight (see Bethany’s rehab journey, below). From there, you’ll progress to crutches and gentle mobility exercises – movements like lifting your heel up to your bottom or raising your leg while straight. The key is to keep the movement “in line”, meaning a straight trajectory (rather than from side to side or in rotation). Your early-stage ACL graft won’t tolerate extremes of movement, like twisting, turning and over-bending. You should also avoid exercising with heavy loads, for instance doing lunges with weights.

Months 3 to 6: progression without surprises

Around the three-month mark, most patients are able to progress to some running exercises. But it will be slow and gentle at first. We usually recommend a treadmill, where the going is flat and there are no unexpected bumps to contend with. By six months, you should be able to transition to light outdoor running (again, prioritising flat, predictable surfaces where there’s less risk of tripping). You want to avoid sudden, out-of-the-blue movements and activities. Start small and build up gradually. Your knee should begin to feel a lot stronger; however, even if it feels perfect, keep in mind that it hasn’t yet fully healed. Pushing yourself at this stage might feel like commitment. Biologically, it isn’t.

Months 9 and beyond: reaching full strength

Nine months is generally when we would expect your ACL to be fully healed (though for some patients, it can take longer). Your physiotherapist will give you guidance here, but this is usually the safest point to return to regular sporting activities, high-level training and heavier-loading exercises (like box jumps, pivoting, bounding). A rule of thumb is for your new knee to reach 90% of the strength and functional capacity of the uninjured knee. This is known as a Limb Symmetry Index (LSI) of 90% or greater. This is a good measure of how your new knee will stay secure, stable and injury free.

Red flags: how your knee tells you to stop

The good thing about our bodies is that they’re generally very good at signalling their limits – and when we might have overreached them. As you work on your post-operation rehab, these are some common red flags to be conscious of:

Sharp or lingering pain – mild muscle soreness that fades with rest is a normal part of post-exercise recovery. Sharp stabbing pain – or a throbbing ache that lingers – is a warning to stop the activity and rest before you do serious damage.
Significant swelling (effusion) – Major swelling around your knee (as opposed to some mild tightness after exercise) is a sign that the tissues are irritated and the knee is acting to protect itself with cushioning fluid. This creates a hostile environment for the graft.
Lingering Stiffness: Excess fluid buildup doesn’t just cause discomfort; it can also physically limit your range of motion, making the joint feel stiff and tight. You want to be encouraging smooth overall movement in your knee, not recurrent stiffness.
Instability – Overdoing a workout can fatigue the nearby muscles, such as the quadriceps and hamstrings, that provide crucial support to your joint as it heals. This can potentially put extra strain on the new ligament, causing it to feel wobbly or unsteady.

Bethany’s rehab journey

After fully rupturing her ACL during a football match, 20-year-old striker and sports coach Bethany Norman had ACL reconstruction surgery with Chris Bailey Orthopaedics. Here, she explains what the first 3 months of rehab were like.

“For the first few days [after surgery] it’s quite difficult to move. It wasn’t so much painful as very weak around the knee. To begin with you have to keep your leg straight, without bending, and lie completely flat when you sleep. You have to throw a dressing gown cord round your foot and raise it with your hands when you want to lift the leg. After that I was on to crutches for about two weeks.

“You have regular physio, and they give you mobility exercises to start off with. They’re not very intense; it’s just things like lifting your heel up to your bottom, or raising your leg straight up, holding it there and then putting it back down again.

“After that you go on to things like strengthening and cardio exercises and eventually squats and lunges. Ten weeks after the operation, the physio did a test to see if I could walk without a limp (I could). He also had me do a leg press to see whether my recovering leg could bear 70% of the weight that my other leg could bear. At that point it could only do about 20%! But by the 12-week mark I had made it to 70%.”

Read the full story of Bethany’s ACL injury and recovery