Updated May 2026. Written by the Upwell Health Collective clinical team. Clinically reviewed May 2026. Next review due November 2026. For educational purposes only.
Most ACL rehabilitation programmes are built around the quad. And they should be — arthrogenic muscle inhibition targets the quadriceps first and hardest, the quad LSI is the most commonly tested strength metric at return to sport, and the quadriceps-cartilage relationship means quad strength has direct implications for long-term joint health.
But here is what is happening in clinics across Australia right now, every day: athletes are being cleared for return to sport with their quad strength tested, their hop tests completed, their psychological readiness assessed — and their hamstring function is either not specifically tested, or tested in a way that dramatically underestimates the actual deficit.
A study of patients at 8 and 12 months after ACL reconstruction with hamstring tendon autograft found that at 1 year post-surgery — when patients are typically advised to return to sport — the passing rate of 90% or above on eccentric hamstring strength testing was 41%. On conventional concentric isokinetic testing, 73% passed. Same patients. Same timepoint. Entirely different story depending on which test you used (PMC11184683).
41%. At 12 months. In athletes being cleared for pivoting sport.
The hamstring is not the secondary structure in this story. It is one of the primary dynamic protectors of the ACL, the most directly impacted muscle group in hamstring tendon autograft reconstruction, and the structure whose specific deficits are most commonly missed by the standard testing that determines whether athletes are cleared to return to the sport that just required 12 months of rehabilitation to come back from.
This article is the complete guide to hamstring function after ACL reconstruction: why it matters, what the biology of harvest does to the muscle, what standard testing misses, why eccentric strength is the variable that counts, and what evidence-based hamstring rehabilitation actually looks like.
Before getting into the deficit, let’s be clear about the mechanism. The hamstring is not just a knee flexor. In the context of ACL protection, it performs a specific and critical dynamic stabilisation role that directly parallels the function of the ligament itself.
The ACL’s primary mechanical role is to restrain anterior tibial translation — the forward movement of the tibia relative to the femur that occurs under quadriceps-dominant loading. The hamstrings produce posterior shear force on the tibia through their knee flexion moment — directly opposing anterior tibial translation. This is why the hamstrings are called dynamic co-protectors of the ACL.
During deceleration, landing, and change of direction — the exact movements during which ACL injuries occur — the hamstrings must activate pre-emptively, before ground contact, to load the posterior chain and protect the ACL from the quadriceps-dominant anterior shear that the landing event will generate. This pre-activation timing is a neuromuscular event, not a strength event. It happens in the 200 milliseconds before the foot hits the ground. And it depends on both adequate hamstring strength and intact proprioceptive signalling from the ACL itself — which, post-reconstruction, is significantly disrupted.
Add to this the specific deficit created by hamstring tendon harvest — the most commonly used graft source for ACL reconstruction in Australia — and the hamstring story becomes clinically urgent.
When the semitendinosus (and typically the gracilis) are harvested for ACL graft, the tendinous portion of the muscle is removed. What remains is the muscle belly, separated from its distal attachment point. The body subsequently undergoes a process of muscle-tendon regeneration — but this process is incomplete, variable, and frequently underestimated in its clinical impact.
Key findings from the research on hamstring tendon graft (HTG) harvest effects:
The hamstring deficit after HTG is not a temporary side effect of surgery that resolves with time and general rehabilitation. For a meaningful proportion of patients, it is a persistent structural and neuromuscular problem that requires specific, targeted intervention to address.
Here is the clinical problem in one number: 41% versus 73%.
At 12 months post-ACLR with HTG, 73% of patients pass eccentric isokinetic concentric knee flexion strength testing at 90% LSI. 41% pass eccentric NordBord testing at the same threshold at the same timepoint (PMC11184683).
These are not different patients. They are the same patients, tested with two different tools. The concentric test tells a story of reasonable recovery. The eccentric test tells a story of profound, persistent deficit in exactly the movement quality that determines ACL protection during sport.
Why the gap? Because concentric and eccentric muscle function are physiologically distinct. Concentric force — the muscle shortening during knee flexion curl — is the function most commonly tested and most commonly recovered. Eccentric force — the muscle lengthening under load during knee extension, the function that decelerates the limb and loads the posterior chain before ground contact — is the function that is most impaired by the proximal migration of the muscle-tendon junction, by the neuromuscular inhibition following harvest, and by the persistent atrophy documented in the long-term literature.
Sport does not load the hamstrings concentrically. The ACL-protective function of the hamstring is eccentric. Decelerating from a sprint. Landing from a jump. Loading the posterior chain before a cut. These are all eccentric demands. And the standard concentric test used by most rehabilitation practitioners and sports medicine physicians at the return-to-sport milestone is measuring the wrong function.
The NordBord — a specialised eccentric testing device that measures force through the Nordic hamstring exercise — is now considered the appropriate standard for capturing the eccentric deficit that matters for ACL protection. At Upwell, eccentric hamstring testing is part of our MRSS 2.0 assessment, not an optional extra.
Strength deficit is only part of the hamstring story after ACLR. The neuromuscular timing deficit may be equally important — and it is almost entirely invisible to standard clinical testing.
As described earlier, the ACL is richly innervated with mechanoreceptors that provide the central nervous system with joint position sense, load feedback, and reflexive muscle activation signals. These signals are the trigger for the pre-emptive hamstring co-activation that protects the ACL before ground contact during landing and cutting. When the ACL ruptures — and when the reconstruction disrupts the mechanoreceptive architecture — this afferent signal is compromised.
Research on hamstring neuromuscular timing after ACLR documents measurable delays in hamstring pre-activation relative to the uninjured limb. These delays — measured in milliseconds — translate into a window during which the tibia is not adequately decelerated by the hamstring before the ACL graft reaches peak load. The graft is being asked to absorb loads it should have shared with the hamstring. And if the hamstring is both weak and late to activate, the cumulative effect on graft loading during sport is significantly greater than strength numbers alone would suggest.
The Nordic hamstring exercise, and more broadly eccentric training, appears to address both components. The mechanical stimulus of eccentric loading drives hypertrophic and neural adaptations. And specific reactive hamstring training — drop landings, reactive agility with posterior chain loading, plyometric deceleration drills — targets the timing and pre-activation deficits that strength training alone cannot reach.
The Nordic hamstring exercise (NHE) is a bodyweight eccentric hamstring loading exercise performed by kneeling on a pad, anchoring the ankles, and slowly lowering the trunk forward under hamstring control. It is simple, requires no equipment beyond a partner or ankle anchor, and has one of the most robust evidence bases of any exercise intervention in sports medicine.
In uninjured populations, NHE protocols consistently produce increases in hamstring muscle volume, eccentric strength, and fascicle length — and have been shown to reduce hamstring strain injury rates significantly in team sport populations. The relevance to ACLR populations is direct: the same eccentric overload that produces hypertrophic and neural adaptations in healthy athletes is precisely the stimulus needed to address the atrophy and neuromuscular inhibition following HTG harvest.
The emerging ACLR-specific evidence is compelling:
Beyond the Nordic exercise specifically, the broader evidence for eccentric-biased hamstring training in the ACLR population points in a consistent direction:
Training at longer muscle lengths. Evidence suggests that training at longer muscle lengths — where the hamstring is under greater stretch during eccentric loading — produces similar strength gains and muscle architecture changes to dedicated eccentric training, indicating utility for longer muscle length hamstring programming during ACLR rehabilitation (Physio Network, 2025). Exercises like the Romanian deadlift, hip extension with knee flexion at end range, and seated leg curl variations at long muscle length specifically target this architectural adaptation.
The hip extension versus knee flexion split. Hamstring strengthening during ACL rehab needs to include both short and long muscle length exercises, and a mixed approach of hip extension and knee flexion exercises. Deficits in knee flexion and hip extension strength can both be detrimental to injury risk at return to sport (Physio Network, 2025). Programmes that exclusively target knee flexion are addressing only part of the hamstring’s functional role.
High-speed running. The hamstrings are heavily loaded during the late swing phase of running — the point of maximum hip flexion and knee extension, when the hamstring decelerates the extending knee under maximum stretch and velocity. This is one of the highest demands placed on the hamstring in any athletic context. Programmes that do not include progressive high-speed running — building from jogging through striding to maximal sprint efforts — leave this loading parameter entirely unaddressed. Return to sport without adequate exposure to high-speed running is return to sport with an untested hamstring in the context it is most at risk.
Flywheel training. Flywheel (inertial) training provides a variable resistance that increases as movement velocity increases — the opposite of conventional weight training, which is hardest at the point of greatest mechanical disadvantage. During the deceleration phase of flywheel exercises, the hamstring is loaded eccentrically at higher velocities than standard weight training produces. Multiple studies of flywheel training in ACLR populations have documented superior gains in eccentric strength and functional performance compared to conventional training alone.
Based on the current evidence, here is the framework for hamstring rehabilitation that should be embedded in every ACL programme from Phase 1 through return to sport and beyond.
In the immediate post-operative period, the primary hamstring goal is preventing the inhibition and disuse atrophy that begins within days of surgery. Active hamstring exercises are commenced from week 1:
Note: for HTG patients, the semitendinosus harvest site is in early healing in Phase 1. High-load eccentric hamstring exercise is deferred. The goal is activation and early hypertrophic stimulus, not eccentric loading.
As tissue healing allows, progressive loading of the hamstring in both its concentric and eccentric roles begins:
The focus shifts toward the eccentric loading patterns that most directly relate to ACL protection and sport performance:
At Upwell, hamstring rehabilitation is not an afterthought in the ACL programme. It is a parallel stream to quadriceps rehabilitation from Phase 1, with its own progressive targets, specific exercise selection based on graft type, and formal eccentric testing at the return-to-sport milestone.
Key elements of our approach:
If you are in ACL rehabilitation and your programme does not include specific eccentric hamstring training, NordBord testing at return to sport, and progressive sprint work, these are conversations worth having with your clinical team. Not because those things are the full programme — but because without them, a significant and measurable gap in the return-to-sport assessment is being left unaddressed.
Contact our team or book an assessment if you want a programme that addresses the full picture — including the posterior chain that too many programmes overlook.
If you have had a hamstring tendon graft ACL reconstruction, here are the most important questions to raise with your rehabilitation team:
Is my hamstring being specifically tested as well as my quad? If only quad LSI is being tracked, the eccentric deficit is invisible. Ask for hamstring LSI data, and ask whether it is being measured concentrically or eccentrically.
Is eccentric hamstring training in my programme? Romanian deadlifts, Nordic preparation or full Nordics, flywheel training. If the answer is “we do some leg curls,” that is not a complete answer.
When am I going to do sprint training? High-speed running is a hamstring training requirement, not just a cardio milestone. If sprint progressions are not in the programme plan, ask when they will be introduced and what criteria gate their introduction.
Will my hamstring be tested at return to sport? And if so, how? A concentric isokinetic or hand-held dynamometry assessment is not the same as eccentric NordBord testing. Understand what is being tested and what the pass criteria are.
The hamstring is a primary dynamic protector of the ACL. Hamstring tendon harvest directly disrupts the muscle’s architecture, producing eccentric deficits that persist beyond 10 years in some patients and are significantly underestimated by standard concentric testing. At 12 months post-ACLR with HTG — the standard return-to-sport timepoint — only 41% of patients pass eccentric hamstring testing at the 90% LSI threshold. 59% are being cleared to pivoting sport with a functionally significant posterior chain deficit that their quad testing does not reveal.
The solution is specific, progressive, eccentric-biased hamstring rehabilitation from Phase 1 onward — including BFR for early hypertrophic stimulus, Nordic hamstring and flywheel training for eccentric-specific loading, and high-speed running for late-swing exposure. Combined with eccentric NordBord testing at return to sport.
The quad story in ACL rehab is well told. It’s time the hamstring got the same attention.
This article is for educational purposes only. It does not substitute for individual clinical assessment. If you are currently in ACL rehabilitation, please work with a qualified physiotherapist and exercise physiologist for personalised guidance. Information last reviewed May 2026.