After ACL reconstruction the quad stayed smaller but 1RM matched
In short
In 12 participants averaging 2.6 years after ACL reconstruction, MRI showed the reconstructed leg had 7.5% less total knee-extensor volume, 7.7% less vastus lateralis, and 5.8% less rectus femoris than the contralateral control leg. Despite that, single-leg press 1RM did not differ significantly between legs. Muscle size deficits and strength recovery do not move at the same rate.
Anyone who has compared thigh circumference in the mirror after knee surgery knows the picture. What this study establishes is that the difference persists years after reconstruction — and that it does not show up in 1RM.
What was measured?
Twelve participants (7 female, 5 male, average age 26.7) who were on average 2.6 years past ACL injury underwent MRI for muscle volume and 1RM testing for unilateral leg-press strength, comparing the reconstructed leg against the contralateral control leg.
After a standardised meal and overnight fast, they performed 6 sets of 8 reps at 80% of 1RM on a unilateral leg press. Vastus lateralis biopsies were taken from both legs at rest and immediately post-exercise, and immunoblotting assessed phosphorylation of hypertrophic signalling proteins including the mTORC1 pathway.
The size deficit was still there
The reconstructed leg showed clear deficits: total knee-extensor volume down 7.5% (p=0.037), vastus lateralis down 7.7% (p=0.031), and rectus femoris down 5.8% (p=0.022).
The deficits did shrink over time. Knee-extensor (R=0.37) and vastus lateralis (R=0.36) deficits correlated with time since injury — recovery is slow but ongoing.
Strength showed no difference
This is the result worth carrying: unilateral leg-press 1RM did not differ significantly between legs (p=0.192). A leg with 7.5% less muscle pressed the same weight.
That fits what is already known — strength is not a function of cross-sectional area alone. Neural drive, motor learning, and leverage offset a substantial part of a size deficit.
No detectable signalling difference
The exercise itself moved the mTORC1 pathway as expected: p-TSC2 Thr1462 and p-4E-BP1 Thr37/46 fell while p-S6 Ser240/244 rose. But no between-leg differences were detected in mTORC1 signalling, either at rest or immediately post-exercise.
Twelve participants is small, and biopsies were taken at a single post-exercise timepoint. The authors call for further investigation closer to reconstruction and at later post-exercise timepoints. The absence of a detectable difference is not proof that signalling is not involved.
How a returning lifter should read it
Watching only the left-right size difference on the way back from knee surgery makes recovery feel stalled. This study says strength may already have caught up during that stretch. The reverse also holds: matching 1RM does not mean the muscle has come back.
That is where Muscle Index being a strength measure matters practically. It recovers relatively fast after a return, but that is not a signal that tissue has fully recovered. Read with the ACL adapts to training load and how muscle size and strength correlate.
Frequently asked questions
How long do quadriceps deficits persist after ACL reconstruction?
In participants averaging 2.6 years post-injury, the reconstructed leg still showed 7.5% less total knee-extensor volume, 7.7% less vastus lateralis, and 5.8% less rectus femoris. Deficit size correlated with time since injury, indicating slow ongoing recovery.
Does a smaller muscle mean less strength?
Not in this study. Despite a 7.5% muscle volume deficit, single-leg press 1RM showed no significant difference between legs (p=0.192). Strength is not a function of cross-sectional area alone.
Is hypertrophic signalling blunted in the reconstructed leg?
No difference was detected. Resistance exercise moved the mTORC1 pathway as expected — p-TSC2 and p-4E-BP1 fell while p-S6 rose — but no between-leg differences in mTORC1 signalling appeared at rest or immediately post-exercise.
So why does muscle size not fully return?
This study did not answer that. The authors concluded only that no between-leg difference was detectable under the specific conditions tested, and that timepoints closer to reconstruction and later after exercise warrant further investigation.
What are the limitations of this study?
It included 12 participants and sampled muscle biopsies at a single post-exercise timepoint. Participants were already an average of 2.6 years past injury, so the signalling state in the period immediately after surgery was outside its scope.
Source: PubMed