Recommendations for hamstring function recovery after ACL reconstruction

Key points

1. 01Knee flexor strength deficits of 0-20% are common at return to sport after ACLR, with hamstring graft patients showing more pronounced deficits than patellar tendon graft patients
2. 02In a sample of more than 4000 patients, only 47% achieved a knee flexor limb symmetry index of 90% or greater
3. 03Each additional 10% deficit in the knee flexor-to-extensor strength ratio was associated with a 10.6-fold increased risk of ACL re-injury in professional football players
4. 04A periodised programme progressing from isometric and low-load exercises to eccentric, high-speed, and functional tasks is recommended across four stages: early, mid, late, and return-to-sport training
5. 05Both knee-dominant (e.g., Nordic hamstring exercise) and hip-dominant (e.g., stiff-leg deadlift) exercises are needed to target all hamstring muscles and restore multiplanar function

How it was conducted

Design

Narrative review / expert consensus recommendations

Topic

Hamstring function recovery after anterior cruciate ligament reconstruction (ACLR)

Scope

Synthesises research on hamstring injury prevention, HSI rehabilitation, and performance training and translates it to the ACLR athlete

Framework

Criterion-based rehabilitation across four stages: early (weeks 0-4), mid (weeks 5-12), late (weeks 13-18), and return-to-sport training (weeks 19-24+)

Population

Athletes undergoing ACLR, with particular focus on hamstring tendon autograft (HG) recipients

Primary outcome

Recommendations for hamstring strengthening, exercise selection, and return-to-sport criteria

What they found

• In a sample of more than 4000 ACLR patients, only 47% achieved a knee flexor limb symmetry index (LSI) of 90% or more
• After ACLR with bone-patellar tendon-bone graft (305 patients), average knee flexor LSI was 97% at 6 months; two-thirds achieved 90% LSI; after HG, average LSI was 89% with less than half (46%) achieving the 90% cut-off
• Isometric knee flexor LSI at 4, 8, and 12 weeks after ACLR with HG was 54%, 70%, and 76% respectively
• Rate of force development (RFD) LSI during knee flexion after ACLR averaged 55%, lower than maximal force LSI of 66%
• Selective semitendinosus (ST) muscle atrophy of 10-28% is typical with HG, commonly accompanied by gracilis atrophy of approximately 20-30%
• An 18% deficit in medial hamstring volume was reported after HG ACLR, driven by large deficits in ST volume (30%), despite compensatory semimembranosus hypertrophy
• Eccentric knee flexor strength improvement of 13-19% is typical after 6-10 weeks of knee-based eccentric hamstring strengthening
• Sprint running achieves the highest hamstring neuromuscular activation; the Nordic hamstring exercise achieves only 8-75% of the EMG activation seen during sprinting
• High-speed running hamstring work was shown to double from 7 to 9 m/s running speed
• A 10.6-fold increased risk of ACL re-injury for each additional 10% deficit in knee flexor-to-extensor strength ratio was reported in professional football players
• Return-to-sport criteria recommended: knee flexor LSI 90%, knee extensor/flexor ratio 60%, absolute isokinetic peak torque 1.5 Nm/kg at 60-90 degrees/s, NordBord LSI 90% and peak torque reaching target values

Limitations

• This is a narrative review with no systematic search or meta-analytic pooling; recommendations are based on expert opinion and indirect evidence translated from non-ACLR populations
• There is a recognised lack of high-quality original research specifically on hamstring rehabilitation in ACLR patients; most evidence comes from HSI prevention and performance literature
• The recommended exercise progressions and stage timelines are based largely on clinical experience rather than RCT evidence in ACLR cohorts
• Individual variation in ST tendon regeneration (which may not occur in 10-50% of HG patients) is acknowledged but specific evidence to guide management in non-regenerators is absent

Why it matters

For patients

Patients who have had ACL reconstruction, especially with a hamstring tendon graft, should expect a structured multi-month programme targeting both knee and hip strength before returning to sport, as persistent weakness significantly raises the risk of re-injury.

For clinicians

Clinicians should screen hamstring strength at return to sport using both LSI and absolute torque thresholds, prioritise eccentric and long-length strengthening from mid-stage onward, and incorporate high-speed running in the final stages of rehabilitation.

For readers

This review provides a practical framework synthesising indirect evidence for hamstring reconditioning after ACLR, highlighting that current rehabilitation is often insufficient and that more rigorously designed studies in this population are needed.