Hamstring autografts, familiar, reliable, and backed by decades of outcomes data. Strength comes back. Limb symmetry improves. Return-to-sport (RTS) boxes get checked.
A new study suggests, however, that something subtler may be happening beneath those reassuring numbers — specifically how the grafted medial hamstring behaves when athletes are cleared to return.
The knee may look ready. The hamstrings may not be acting like they used to.
The Question Behind the Study
Return-to-sport testing after anterior cruciate ligament (ACL) reconstruction is heavily strength-based. If quadriceps and hamstrings torque normalize and limb asymmetry falls below accepted thresholds, clearance often follows.
What’s less commonly evaluated is how those muscles are firing.
This study asked a focused question: Does harvesting the medial hamstrings for ACL reconstruction alter neuromuscular behavior at the time of return to sport — even when strength looks normal?
How the Study Was Set Up
Researchers compared 46 patients who underwent primary ACL reconstruction with a medial hamstring autograft to 46 matched healthy controls.
All participants met commonly accepted RTS criteria: Hamstring-to-quadriceps ratio ≥60% and ≤10% interlimb torque differences at multiple speeds.
Testing included: Isokinetic knee flexion at 60, 180, and 300 deg/sec, Surface EMG of the medial and lateral hamstrings and Analysis of activation amplitude and time to peak activation.
Everyone was “cleared” — at least by conventional standards.
Strength Looked Normal
On paper, the ACL reconstruction group checked out. No differences in peak torque between reconstructed knees and controls, no differences in EMG amplitude for medial or lateral hamstrings and no change in medial-to-lateral activation ratios.
If the evaluation stopped there, the conclusion would be straightforward: hamstrings recovered.
But timing told a different story.
The Medial Hamstring Fired Faster – Consistently
Across all tested speeds, the medial hamstring in the ACL reconstruction group reached peak activation earlier than in healthy controls.
This faster time-to-peak was: Present at 60, 180, and 300 deg/sec, specific to the grafted medial hamstring but absent in the lateral hamstrings.
The result suggests a persistent neuromuscular adaptation — not a weakness, but a change in coordination strategy.
Why Earlier Activation Matters
Earlier firing isn’t inherently good or bad. But when it’s isolated to a graft-harvested muscle and persists at RTS, it raises important questions.
A medial hamstring that activates sooner may be: compensating for altered tendon structure or responding to changes in neural input after graft harvest or also carrying a higher neuromuscular demand during dynamic tasks.
Over time, that altered coordination could influence tibial control during cutting and deceleration, load sharing across the knee, and reinjury risk — particularly in high-speed or fatigued states.
Highlighting a Blind Spot
This study doesn’t argue against hamstring autografts. Strength recovers. Athletes meet RTS criteria. Performance tests pass.
But it does highlight a blind spot.
Normal strength does not guarantee normal neuromuscular behavior. At the moment athletes return to sport, the grafted medial hamstring may already be working differently — earlier, faster, and potentially harder.
For surgeons, the message isn’t to change graft choice overnight. It’s to recognize that return-to-sport clearance based solely on torque and symmetry may miss meaningful neuromuscular adaptations that persist well beyond rehab milestones.
The knee may be ready. The hamstrings may still be rewriting the playbook.
Origin Study Title Link: Medial Hamstrings Used as Autograft for ACL Reconstruction Show an Abnormal Neuromuscular Pattern at the Time of Return to Sport
Authors: Sebastiano Nutarelli, MS, Alessandro Sangiorgio, MD, Thomas Legrand, PhD, Federico Monzoni, MS, Luca Deabate, MD, Marco Delcogliano, MD, PD, Eamonn Delahunt, PhD, Giuseppe Filardo, MD, PhD, MBA
