Return to Running after ACLR
There is a growing push in sports medicine to define “return to running” (RTR) using objective criteria. Strength thresholds. Limb symmetry. Time from surgery.
It sounds scientific. It feels structured. But when you actually read the literature closely, a different story emerges. Below, I summarize two different research papers:
Return to running (RTR) after knee joint surgery: A narrative review (2026).
Relevant strength parameters to allow return to running after primary anterior cruciate ligament reconstruction with hamstring tendon autograft (2022).
The illusion of criteria-based decision making
Did you know, there is no consistent, validated and agreed upon set of criteria to determine ‘readiness to run.’
Key Findings for Return to Running (RTR) in 2026:
Rehabilitation Shift: Moving away from solely time-based milestones to passing functional criteria, such as pain-free activity, no effusion, and ‘adequate strength’
Timeline: Most experts recommend a minimum of 12–16 weeks post-surgery (specifically for ACLR) before initiating running.
Criteria-Based Milestones: Common criteria include minimal swelling, good range of motion, and specific quadriceps strength benchmarks.
ACL Focus: Current literature, such as the 2026 review, is heavily focused on ACL reconstruction, with limited high-quality evidence for other types of knee surgeries.
Strength Requirement: A quadriceps limb symmetry index (Q-LSI) greater than 60% is suggested to reduce re-rupture risk during the return to running phase.
Psychosocial Factors: Fear of reinjury and poor confidence are significant barriers for patients, suggesting that psychological readiness is as important as physical readiness.
Common Criteria to Assess Readiness:
Pain & Swelling: No pain or effusion during daily activity.
Strength: A Q-LSI > 60-70% (often tested via isokinetic testing).
Movement Quality: Proper biomechanics during single-leg hop tests.
Functional Mobility: Full range of motion (ROM).
But these are inconsistently applied and rarely validated. Some do not even make any sense as they are not even true pre-requisites for high impact activity!
The field has moved from time-based rehab → criteria-based rehab according to the narrative review, but without using criteria that would actually predict effective performance outcomes.
Criteria exist. Ecological validity does not.
A closer look: strength-based “cutoffs”
A commonly cited study I reviewed, attempts to solve this problem by identifying strength thresholds for RTR after ACL reconstruction (Grondin et al., 2022).
Their conclusion:
Quadriceps Limb Symmetry Index (LSI) ≈ 65%
Hamstring LSI ≈ 80%
Quadriceps strength ≈ 1.60 Nm/kg
These are presented as: “objective parameters to allow a return to running”
At first glance, this appears useful.
But when you dissect the study, the cracks are obvious. Below are the flaws in this line of thinking….
Crack #1: They predicted behavior—not readiness
The study did not show that these thresholds:
reduce reinjury risk
reflect tissue capacity
ensure safe loading
Instead, they showed: Athletes who returned to running had higher strength values.
That is a correlation, not a readiness model.
Running did not become safe at 65%.
People with higher strength were simply more likely to run.
No kidding.
Crack #2: The outcome is weak
Return to running was defined as:
Completing ≥50% of a running program by 6 months
Not:
no pain; no swelling
biomechanical quality
long-term joint health
Just: Did they run… at least a bit?
This is not a physiological endpoint.
It is a compliance metric.
Crack #3: Statistical cutoffs ≠ biological thresholds
The famous “65% LSI” was derived using:
ROC curves
Youden Index optimization
This means: The cutoff simply best separates two groups:
those who ran
those who didn’t
It does NOT represent:
a healing threshold
a safe loading capacity
a meaningful biological marker
It is a statistical convenience—not a physiological truth.
Crack #4: Limb symmetry is fundamentally flawed
Even the authors acknowledge:
The contralateral limb weakens after ACL reconstruction
This can artificially inflate LSI values
So what happens? An athlete can “pass” a symmetry test because both legs are weak
Which leads to the uncomfortable reality: Symmetry ≠ capacity
Crack #5: The most telling finding
One of the study’s models found: Lower strength in the non-injured limb increased the likelihood of returning to running
Think about that. Being weaker improved your chances of “passing.”
Why?
Because it improves symmetry.
This is not a small flaw.
This is a system-level problem with the metric itself.
Crack #6: Circular reasoning in decision-making
The study used a 60% LSI cutoff to allow running in the first place.
Then later concluded: ~65% predicts return to running
This is circular:
The system determines who runs
Then the system “proves” its own criteria
Wait, what??
This is not validation. It is self-confirmation.
Crack #7: Running is treated as a yes/no event
Across both the narrative review and strength-based studies, one issue persists:
Running is reduced to a binary outcome:
Returned / Did not return
But running is not an event.
Running is:
thousands of ground contacts
repeated joint loading cycles
progressive exposure to speed and force
Yet no study meaningfully tracks:
weekly running volume
acceleration demands
surface variation
load progression rates
We are trying to solve a load problem with a checkbox.
Crack #8: Strength does not equal movement readiness
Even within the same study:
Quadriceps deficits persist well beyond 4 months
Altered biomechanics are still present at 12 months
So we allow running based on:
👉 strength measures
While knowing:
👉 movement quality is still impaired
This disconnect matters.
What these papers actually prove
The narrative review [Return to Running (RTR) after Knee Joint Surgery: a Narrative Review Diemer et al.] shows - We don’t have valid RTR criteria
The strength study (Relevant Strength Parameters to Allow Return to Running after Primary Anterior Cruciate Ligament Reconstruction with Hamstring Tendon Autograft Jérôme Grondin) shows - We are trying to create them using markers an S&C would never use.
Together, they reveal:
The criteria are built on:
weak outcomes
flawed metrics
circular logic
The real issue: we’re asking the wrong question
The research field keeps asking:
❌ “What score allows running?”
Instead of:
✔ “What load can this athlete tolerate—and how do we progress it?”
What’s missing from the conversation
No current model adequately accounts for:
load progression
cumulative tissue stress
mechanical quality under fatigue
coaching and movement skill
psychological readiness
Yet these are the variables that actually determine success.
Maybe, just maybe, I cover this in my ACL REHAB FROM A-Z online course
A better way to think about return to running
Return to running should not be:
❌ a test
❌ a threshold
❌ a single decision point
It should be:
✔ a progressive exposure strategy
✔ a load management process
✔ a coached skill reintroduction
Again, I cover this in my course….
Bottom line
You don’t “pass” return to running because you hit 65% of blah blah blah…
You start running because:
the load is appropriate and you did thing to progress you to this!!!!!!!!!!!!!!!
the progression is controlled
References
Diemer, F., Schoch, W., Sutor, V., & Zebisch, J. (2025). Return to running (RTR) after knee joint surgery: A narrative review. https://doi.org/10.1055/a-2808-1311
Grondin, J., Crenn, V., Gernigon, M., Quinette, Y., Louguet, B., Menu, P., Fouasson-Chailloux, A., & Dauty, M. (2022). Relevant strength parameters to allow return to running after primary anterior cruciate ligament reconstruction with hamstring tendon autograft. https://doi.org/10.3390/ijerph19148245
