The Brain-Bandwidth Problem in ACL Rehab
Can your ACL-er move well when they are reacting, scanning, deciding and dealing with pressure?
For years, ACL rehabilitation has rightly focused on the things we can measure.
Range of motion.
Effusion.
Quadriceps strength.
Hamstring strength.
Hop tests.
Change of direction.
Psychological readiness.
Time since surgery.
And to be very clear from the outset, none of these things are suddenly unimportant. Please do not read this blog and throw your dynamometer into the bin like it has personally betrayed you.
Strength still matters. A lot.
Hop performance still matters. Movement quality still matters. Confidence still matters. Time and biological healing still matter. The boring basics are still the boring basics for a very good reason.
But over the last few years, a growing body of ACL research has been nudging us toward a slightly bigger question:
What if the athlete looks physically ready… but their nervous system is still working overtime just to keep the knee under control?
That is where things get really interesting.
And, if I am being honest, a little bit brain-hurty.
ACL injury is not just a knee event
When someone ruptures their ACL, we often think about the obvious local consequences: pain, swelling, loss of range, quadriceps inhibition, altered loading, and reduced confidence. Then, after ACL reconstruction, we add the surgical insult, graft healing, donor-site morbidity, and the long road of progressive rehab.
But ACL injury is not just a local knee problem. It can also change the way the central nervous system contributes to movement control.
Gokeler and colleagues argued that ACL injury can result in changes in the brain that may not be sufficiently targeted by traditional rehabilitation approaches. Their review highlighted that many current ACL rehab programs do a good job of addressing strength, endurance and basic neuromuscular function, but may not fully address motor learning, neuroplasticity, and the athlete’s ability to transfer clinic-based movement control into sport (Gokeler et al., 2019).
That last bit is the key.
Because our athletes do not return to a quiet gym, perfect lighting, predictable surfaces, and a lovely physio saying, “Whenever you’re ready, mate.”
They return to sport.
Sport has opponents. Teammates. Balls. Fatigue. Noise. Pressure. Contact. Bad decisions. Worse decisions. And occasionally a 16-year-old winger who treats a Tuesday night training drill like it is the World Cup final.
The question, then, is not just: Can they perform the task?
It is also:
Can they perform the task when their brain is busy doing sport things?
Looking ready is not always the same as being ready
One of the tricky things about ACL rehab is that an athlete can look excellent in controlled testing environments.
They can jump well.
They can hop well.
They can pass strength targets.
They can move beautifully when the task is predictable.
And again, that is good. We want that.
But in sport, athletes do not get to focus all of their attention on making their knee behave. They need to scan the field, track the ball, avoid defenders, make decisions, anticipate movement, regulate speed, and respond to constantly changing information.
This is where the concept of neurocognitive reliance becomes really important.
Grooms and colleagues proposed that after ACL reconstruction, some athletes may rely more heavily on cognitive and visual processing to maintain knee control. In other words, the knee might look stable, but the brain may be working harder than usual to create that stability (Grooms et al., 2023).
That is fine when the task is simple.
It becomes more problematic when the athlete returns to an environment where attention is being pulled in ten different directions at once.
A simple way to think about this is bandwidth.
If an athlete needs too much brain bandwidth just to control their knee, what happens when sport starts demanding that same bandwidth for perception, decision-making, reaction, communication, and chaos management?
Something has to give.
And sometimes, what gives is movement quality.
The brain-body connection in landing mechanics
This is where the work from Criss and colleagues is fascinating.
In a group of 30 female high school soccer players, the researchers explored the relationship between brain activity during a motor task and landing biomechanics during a drop vertical jump. Their key biomechanical variable was peak knee abduction moment; essentially a frontal-plane loading variable often discussed in relation to ACL injury-risk mechanics.
They found that greater knee abduction moment during landing was associated with greater activation in brain regions involved in sensory, spatial and attentional processing, including the lingual gyrus, intracalcarine cortex, posterior cingulate cortex and precuneus (Criss et al., 2021).
Now, we need to be sensible here.
This does not mean we can look at a brain scan and predict someone’s ACL destiny like a very niche and expensive fortune teller.
This was a small study. It was associative. It does not prove causation. And it certainly does not mean that every athlete with “dynamic valgus” needs to be sent for neuroimaging between their Copenhagen planks and calf raises.
But it does support a broader point that I think is clinically important:
High-risk movement patterns may not simply be a strength problem or a technique problem. They may also reflect how the athlete is processing sensory, spatial and attentional information.
That is a big deal.
Because for a long time, we have been quite comfortable watching a landing and saying, “They need more glute strength,” or “They need to control their knee better.”
And sometimes that is true.
But perhaps sometimes, the issue is not just whether they have the physical capacity to control the movement. It may also be whether their nervous system can organise that movement efficiently under the demands of sport.
Motor learning matters
This is where the Gokeler paper becomes practically useful.
One of the main messages from their review is that ACL rehab should better incorporate principles of motor learning. They discuss several concepts that are very relevant clinically, including external focus of attention, implicit learning, differential learning, self-controlled learning and contextual interference (Gokeler et al., 2019).
That sounds like a lot of academic words, so let’s make it more practical.
An external focus of attention means cueing the outcome of the movement, rather than obsessively cueing body parts. So instead of constantly saying, “Keep your knee over your second toe,” you might say, “Push the ground away,” “land quietly,” or “stick the landing like you’re landing on glass.”
An implicit learning approach uses analogies and simple cues, rather than overloading the athlete with technical instructions. Think: “Land like you’re landing on eggs and don’t want to crack them.” Slightly ridiculous? Yes. Potentially useful? Also yes.
Differential learning means giving athletes movement variability. Different directions, different speeds, different starts, different landings, different visual conditions, different levels of fatigue. Because sport rarely asks athletes to perform the exact same movement, under the exact same conditions, for three sets of ten while their physio nods approvingly in activewear.
And contextual interference is about making practice more variable and less predictable over time. Blocked practice may look great in the session, but variable and random practice may help transfer better into real-world performance.
That is a really important distinction.
Because sometimes rehab can look beautiful in the clinic but not transfer well enough to sport.
And unfortunately, sport does not care how beautiful your clinic session looked.
Rude, but true.
The problem with standard return-to-sport testing
Most ACL return-to-sport test batteries still heavily favour physical performance in controlled settings.
Again, that is not wrong. It is necessary.
But it is probably incomplete.
Grooms and colleagues argue that standard RTS tests may fail to detect neural compensation and neurocognitive reliance. Their point is not that we should abandon strength or hop testing, but that we may need to augment these tests with neurocognitive challenges that better reflect the attentional demands of sport (Grooms et al., 2023).
This is where dual-task testing becomes useful.
A dual-task test compares performance under a standard condition with performance under a more cognitively demanding condition. For example, an athlete might complete a hop test normally, and then repeat a similar task while responding to visual cues, making decisions, avoiding distractors, or reacting to changing information.
The difference between those performances is often described as the dual-task cost.
In very simple terms:
How much does their performance drop when their brain has to do more than just focus on the knee?
Grooms and colleagues suggested that, depending on the athlete and task complexity, a dual-task cost greater than 10% may indicate excessive neurocognitive reliance. But they also rightly acknowledge that more ACL-specific work is needed to refine these recommendations (Grooms et al., 2023).
So I would not treat 10% as a magical ACL number handed down from the rehab gods.
It is better viewed as a useful clinical flag.
A prompt to look closer.
A reason to ask: “Does this athlete still perform well when the task becomes more reactive, more distracting, and more sport-like?”
This does not need to be fancy
One of the nicest things about this area is that you do not necessarily need expensive technology to start thinking differently.
Yes, light systems, force plates, motion capture, and reaction-time tools can be fantastic if you have access to them. I am certainly not against technology. I am very much in favour of objective testing and making our rehab more measurable.
But the absence of fancy equipment does not mean the absence of good clinical reasoning.
Grooms and colleagues specifically discuss low-tech options, including PowerPoint slides, flashcards, colours, numbers, or clinician hand signals to add cognitive and reactive demands to movement tasks (Grooms et al., 2023).
That is encouraging.
Because you can start simple.
You can ask the athlete to hop only when they see a certain colour. You can call out numbers and have them react only to odd numbers. You can use cones, balls, visual targets, hand signals, or decision-making games. You can progress from planned movement to reactive movement. From predictable tasks to unpredictable tasks. From quiet gym reps to controlled chaos.
The goal is not to make rehab random for the sake of it.
The goal is to ask better questions.
Can they still land well when they are reacting?
Can they still decelerate when they are scanning?
Can they still cut when they are making a decision?
Can they still control the knee when attention is directed elsewhere?
Can they still move like an athlete when the task stops looking like a physio test?
That, to me, is where the next layer of ACL rehab becomes really exciting.
Training versus testing
One thing I think is important here is to separate training from testing.
When we are training, we are trying to progressively expose the athlete to the demands they will face in sport. That means gradually adding speed, fatigue, reaction, decision-making, visual scanning, perturbation, and sport-specific complexity.
When we are testing, we are trying to see what happens when the task changes.
Does their hop distance drop?
Does their landing stiffen?
Does their knee control worsen?
Does their reaction time slow?
Do they make more cognitive errors?
Do they preserve performance by becoming slower, safer and less athletic?
That last one is important.
Sometimes athletes maintain movement quality by quietly sacrificing speed, intent or reactivity. On paper, the landing might still look neat. But the athlete has solved the problem by slowing everything down.
That might be okay early.
But it is not the final destination.
Because sport is not kind enough to wait while your athlete chooses the safest possible landing option.
My take-home message
The point of all this is not to make ACL rehab more complicated just for the sake of it.
We already have enough complexity. And enough acronyms. And enough people on the internet yelling about knees going over toes.
The point is to make ACL rehab more representative of what athletes are actually returning to.
For me, the key message is this:
Passing strength and hop tests should probably be seen as the minimum standard, not the final green light.
An athlete needs the physical capacity. They need the strength. They need the power. They need the confidence. They need the biological time. They need the sport-specific conditioning.
But eventually, they also need to show that their movement system can cope when the brain is busy.
Because that is sport.
Sport is not just force production. It is force production under perception, decision-making, pressure, fatigue and chaos.
And the more we can prepare athletes for that, the better our ACL rehab will become.
Want to go deeper?
So, if you have made it this far and are now quietly questioning every hop test you have ever done… welcome to the club.
The goal here is not to throw out strength testing, hop testing or good old-fashioned clinical reasoning.
The goal is to add another layer.
Because our athletes are not returning to a quiet clinic room. They are returning to opponents, balls, teammates, fatigue, pressure, decisions, and chaos.
If you want to go deeper on how to actually assess and train this neurocognitive side of ACL rehab, I strongly recommend watching Dustin Grooms and Meredith Chaput’s neurocognitive ACLR masterclass on Learn.Physio.
They are two of the best people in the world to learn this from, and they do a brilliant job of turning a complex topic into something clinicians can actually start using.
You can watch the full masterclass here now inside Learn.Physio.
References
Criss, C. R., Grooms, D. R., Diekfuss, J. A., Anand, M., Slutsky-Ganesh, A. B., DiCesare, C. A., & Myer, G. D. (2021). Neural activity and landing biomechanics: Exploring the relationships between the brain, body, and ACL injury-risk. Orthopaedic Journal of Sports Medicine, 9(7 Suppl. 3). https://doi.org/10.1177/2325967121S00151
Gokeler, A., Neuhaus, D., Benjaminse, A., Grooms, D. R., & Baumeister, J. (2019). Principles of motor learning to support neuroplasticity after ACL injury: Implications for optimizing performance and reducing risk of second ACL injury. Sports Medicine, 49, 853–865. https://doi.org/10.1007/s40279-019-01058-0
Grooms, D. R., Chaput, M., Simon, J. E., Criss, C. R., Myer, G. D., & Diekfuss, J. A. (2023). Combining neurocognitive and functional tests to improve return-to-sport decisions following ACL reconstruction. Journal of Orthopaedic & Sports Physical Therapy, 53(8), 415–419. https://doi.org/10.2519/jospt.2023.11489