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Recommendations for Hamstring Function Recovery After ACL Reconstruction

Buckthorpe, M., Danelon, F., La Rosa, G., Nanni, G., Stride, M. and Della Villa, F., 2021. Recommendations for hamstring function recovery after ACL reconstruction. Sports Medicine, 51(4), pp.607-624.

In today’s letter

• Overview of the hamstring deficits after ACL reconstruction and the reasons why they occur

• 6 clinical tips

• 3 resources to check out to further your knowledge about hamstring rehab

• Meme of the week: Happens way too often than I would like to admit 😄 

• Rapid Results =

Hamstring deficits after ACL reconstruction are widespread and persistent, especially in patients with hamstring tendon grafts, leading to long-term strength loss, neuromuscular inhibition, and increased reinjury risk.

• Professional takeaway =

Prioritise eccentric training (safely) and progressive overload to rebuild hamstring strength, especially at shorter muscle lengths and slower contraction speeds. Exercises like Nordic curls and single-leg deadlifts (when the time is right) while monitoring limb symmetry and rate of force development to optimise return-to-sport readiness.

Take a Look at What Others Have To Say About It

Aim of the study

This paper looks into hamstring deficits after ACLR, discussing how to restore strength and function while aligning rehab strategies with the overall recovery process

Did you know?

Many athletes dream of returning to their pre-injury performance levels after ACL reconstruction (ACLR), but the reality is often far from ideal.

Return-to-sport (RTS) rates are discouragingly low, and even when athletes do make it back, they rarely perform at the same level 🤕. Giving another insult to injury, the risk of reinjury is alarmingly high, with nearly 30% of young athletes suffering another ACL or knee injury post-RTS.

So, why are these outcomes so poor despite advancements in surgical techniques and rehab protocols?

The big factor is unfortunately the gap between research and clinical practice, where evidence-based strategies are not always fully implemented. Additionally, the rehab process often focuses too heavily on physical recovery while neglecting psychological and social aspects.

A more well rounded, biopsychosocial approach is needed to optimise RTS success.

The “Mighty” Hamstrings

One of the most important—yet frequently underestimated—components of ACL rehabilitation is hamstring function. As knee stabilisers, the hamstrings act as an agonist to the ACL, helping to reduce anterior tibial shear forces during movement.

The protective role is particularly importnat during “high-risk” movements such as cutting and pivoting, where excessive or unproductive strain on the ACL can lead to reinjury.

The hamstrings also co-activate with the quadriceps to maintain balanced knee mechanics not just during sports, but in normal day to day movements like stepping off a curb.

And thus…

Their strength is fundamental for joint stability and long-term knee health.

However, sadly hamstring deficits are a very common and significant problem post-ACLR, especially in patients who undergo hamstring tendon autografts.

These people can experience up to a 50% loss in hamstring strength within the first month post-op, and deficits often persist at RTS—sometimes for years.

Even in cases without graft-related weakening, strength imbalances remain a concern. Athletes with just a 10% reduction in their hamstring-to-quadriceps strength ratio are over ten times more likely to suffer another ACL injury.

Additionally, residual weakness contributes to a higher risk of hamstring strain injuries, with reinjured athletes demonstrating an average of 14% lower hamstring strength compared to those who remain injury-free.

Long-term, yes the deficits injury risk big time; But from a strength and conditioning perspective they also compromise:

1. Athletic performance

2. Movement efficiency

3. Joint stability.

Struggles of Hamstring Recovery After ACLR

Similarly to quads, hamstring deficits following ACL reconstruction are widespread, regardless of the surgical approach used, and they often persist well beyond return to sport.

Knee flexor strength deficits can range from 0–20% at RTS, only 47% of patients manage to achieve the recommended ≥90% limb symmetry index (LSI).

That is NOT GOOD 🩼

These deficits are particularly pronounced in people who choose to go the hamstring tendon autografts route, with only 46% reaching the ≥90% LSI threshold compared to 75% of bone-patellar tendon-bone patients (for the hamstrings - these people have their own struggles with quad activation 😄) .

Lets just highlight that a little more so it sinks it,

Early post-op deficits in hamstring graft patients are large, with knee flexor strength reduced to just 54% LSI at four weeks and only improving to 76% by 12 weeks.

The struggled don’t stop with overall strength. Specific hamstring function is also impaired.

1. Weakness tends to be more pronounced at shorter muscle lengths, particularly at greater knee flexion angles.

2. Deficits are also more evident at slower contraction speeds, which has implications for controlled, eccentric loading drills like landing and deceleration.

But perhaps the most concerning factor is the significant reduction in the rate of force development (RFD), which reflects how quickly the hamstrings can generate force—a major factor for high-speed movements.

Post-ACLR, RFD deficits are more severe than maximal strength deficits, with LSI dropping to 55% for RFD compared to 66% for maximal strength. The knee flexor-to-extensor RFD ratio is also significantly altered, largely due to extensor (quadriceps) deficits, which further compromise knee stability.

“Currently, the most accepted ratio is that hamstring should be 60% of the quadriceps strength. But this is only relevant if the quadriceps is 2.5/3 x peak torque to bodyweight”

Andreas Bjerregaard

Stability Of The Knee

Looking past the knee flexion (one of the actions of hamstrings), this powerful muscle also helps in hip movement and delicate knee control.

• Hip extension

• Knee stability

• Pelvic control

Medial hamstrings like semimembranosus and semitendinosus help prevent medial condyle lift-off and excessive knee valgus—the mechanism in ACL injuries.

Additionally, hamstring deficits impact knee internal rotation strength, particularly after hamstring graft ACLR, contributing to excessive external tibial rotation and dynamic knee valgus, which increases reinjury risk.

Even at a muscular level, architectural changes post-ACLR increase injury risk. The biceps femoris long head, one of the main muscle in sprinting and high-intensity movements, often develops shorter fascicle lengths and greater pennation angles after ACLR when using the hamstring graft, similar to what’s seen in athletes with a history of hamstring strain injuries.

These changes matter because shorter Bicep femoris (long head) fascicles (<10.56 cm) are associated with a fourfold increase in hamstring strain risk, while each additional centimeter in fascicle length reduces that risk by about 20%.

Guys, 4 fold…

4 times more likely. Not 4% but 400%. Just think about this for a minute.

Now,

Given that shorter fascicles are more prone to eccentric-induced muscle damage, running-related hamstring injuries become a real concern for post-ACLR athletes.

Unfortunately, muscle fascicle length can only be measured using ultrasound

Why Do Hamstring Deficits Linger After ACLR?

The hard to swallow pill is that hamstring deficit often begin before the injury even occurs. Pre-existing knee flexor weakness can contribute to ACL tears in the first place and may persist throughout the rehab process.

This is why both relative strength (like limb symmetry index) and absolute strength values need to be considered when making return-to-sport decisions.

Unfortunately, rehab programs don’t always address these deficits effectively.

Inadequate

1. Volume

2. Intensity

3. Frequency

Of strengthening exercises can leave athletes with lingering weakness (this is accelerated with poor compliance)

The Toughest Choice

Beyond rehab deficits, the choice of surgical technique may play a bigger part.

Hamstring tendon autografts tend to result in more severe and long-lasting deficits due to donor site morbidity.

In simple terms,

Using the hamstring tendon as a graft creates a grade 4 muscle-tendon lesion, leading to prolonged weakness and a higher risk of hamstring strain injuries.

One major issue with hamstring graft is the proximal migration of the semitendinosus muscle-tendon junction, which can disrupt muscle function.

While semitendinosus tendon regeneration may take around 18 months, it doesn’t always happen—studies show that 10–50% of patients never regenerate their donor tendon.

I bet you didn’t know the number is that high… I didn’t either!

Shrinkage

Muscle atrophy is another consequence, with selective semitendinosus atrophy ranging from 10–28% and gracilis atrophy reaching about 30%.

If compensatory hypertrophy of other hamstring muscles, like the biceps femoris or semimembranosus, doesn’t occur, overall hamstring volume decreases, compromising knee flexion strength.

This can also create imbalances in the transverse plane, where a lack of semimembranosus hypertrophy weakens medial hamstring function and affects knee control.

Neuromuscular changes add another layer to these deficits.

Many ACLR patients experience chronic neuromuscular inhibition, which limits voluntary hamstring activation, especially during eccentric contractions.

Muscle activation and size deficits are even more pronounced in patients who fail to regenerate their semitendinosus tendon. Some may compensate with biceps femoris hypertrophy, but if semimembranosus doesn’t follow suit, this can actually worsen the medial-to-lateral strength imbalance, increasing the risk of reinjury.

In The Clinic

You may guess where this is going. Clinically there is a need for targeted and individualised rehab.

Pre-screening (if possible) for hamstring strength deficits before surgery can help guide prehab, while imaging like ultrasound can track semitendinosus tendon regeneration post-hamstring graft.

Muscle volume measurements should also be considered to identify imbalances between the medial (semitendinosus, semimembranosus) and lateral (biceps femoris) hamstrings.

Clinical Tips

Individualisation: Every person has unique challenges that they present with, so rehab programs should be customised based on their specific deficits, sport demands, and psychological barriers.

Load management: A structured, gradual progression in training volume and intensity is essential to prevent both underloading (which can prolong deficits) and overloading (which increases reinjury risk). Regular strength assessments help fine-tune loading parameters to match recovery timelines.

Lumbo-pelvic integration: The hamstrings don’t work in isolation—they rely on a stable core and strong glutes for best function. Incorporating exercises like bridges, planks, and bird dogs increases neuromuscular control and reduces strain on the knee.

Program Design

Early activation: Hamstring strengthening should begin as soon as possible post-op to prevent excessive atrophy and neuromuscular inhibition. Isometric exercises and low-load resistance work (such as prone leg curls or banded hamstring holds) can safely engage the muscle early in recovery.

Progressive overload: Eccentric exercises like Nordic hamstring curls and Romanian deadlifts should be introduced progressively to rebuild strength & increase tendon resilience. The key is controlled exposure to higher loads over time.

Bilateral & unilateral work: Addressing strength asymmetries is critical, and combining bilateral (e.g., Romanian deadlifts) with unilateral movements (e.g., single-leg deadlifts, Nordic curls) makes sure that a balanced and functional recovery is taking plave. Unilateral exercises are especially important for restoring limb symmetry

Top 3 Resources to Check Out

And learn more about the hamstring post ACLR

1. Rehab protacol for hamstrings post ACLR (PDF) - LINK

2. Hamstring to quad ratio (article) - LINK

3. Strengthening (short VIDEO)

Meme of The Week

When You’re Ready to Learn More

We have great resources for you to devourer

1. Killer Articles —> Literally everything you need to know about a specific injury

2. Tendinopathy 4 phase rehab frame work - My book that walks you through a step by step process of treating tendons

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