Using the hamstrings to better assess ACL return to sport readiness

Anterior cruciate ligament (ACL) rupture is a common sports-related injury. While conservative treatments are available, the most common treatment for sports population is the surgical reconstruction of the ligament and it is increasingly common to use hamstrings tendons autograft is such operations. 1,2,3,4,5

Despite the major impact of such surgeries on the hamstrings, the most common methods of return-to-sport readiness often do not look at the hamstring muscle group. This article aims to look at the impact ACL surgery has on the hamstrings, evaluate current return-to-sport readiness, and recommend how processes could be improved to better detect the readiness of the hamstrings.

Post-surgery impact on the hamstrings

Functional impact: Although ACL reconstruction is a very common surgery, the current postoperative protocols seem not to be sufficient to ensure the complete recovery of the hamstrings after this kind of operation in the long term. Indeed, it has been shown that subjects reconstructed with ST/GR show hamstring strength deficits and biomechanical alterations even long after surgery.5–8 Specifically, several authors9,10 reported that significant hamstrings strength and power deficits persist despite the completion of rehabilitation in reconstructed patients7, compared with control subjects6 or with their preoperative symmetry levels.8 Other studies demonstrated the existence of altered biomechanical strategies in these patients during functional tests such as hopping and jumping11–14 , gait9,15 or during simple functional16 and squat movements.14,17 Moreover, some authors reported that in these patients measurable kinematic and kinetic inter-limb differences can exist, despite the symmetrical lower limb performance upon functional testing2,14,18 or despite adequate muscle strength symmetries.18 In general, it seems that these weaknesses are related to the long term negative adaptations of the harvested tendons (ST/GR), that in many cases fail to completely regenerate.19

Morpholical/structural impact: Recent image studies20–23 demonstrated that subjects who underwent this kind of surgery showed morphological deficits of the harvested hamstrings that in most cases remained smaller than those of the non operated limb. In particular, the authors found that ST and GR show reduced muscle volume and cross section area (CSA) and increased tendons length. The combined effect of this modification in hamstring muscle architecture is a decreased force producing capacity of the ST and GR, driven by the shorter lever arm participating in torque output, since smaller muscle fibers operate at shorter lengths.24,25 In addition to these findings, the same image studies reported an increase in the volume and CSA of the non harvested hamstrings (mainly the biceps femoris) reflecting a subsequent adaptation of the no-donor hamstring muscles of the operated leg (see below, right) compared to the healthy one (see below, left).20


These findings led authors to conclude that the not directly involved hamstrings, try to compensate for the weakness of the ST and GR by showing hypertrophy, thereby altering the balance between the medial and the lateral compartment of the thigh. Hence, we could suppose that this compensatory hypertrophy is a consequence of the increased activation of the lateral hamstrings (BF) and a decreased activation of the medial hamstring (ST) during knee flexion movements.

Analyzing current returning to play screening

These changes not only impact the ability of the hamstring to return to normal function, but also have a further impact on the repaired ACL. The medial hamstrings compartment may play a more significant role than the other hamstrings in unloading the ACL26 since it ensures dynamic knee joint stability during movements with high risk of non-contact ACL injury27–29 and it has the potential to counter the external outward rotating knee moments associated with ACL rupture.30 Thus, given that the majority of secondary ACL injuries are caused by no-contact mechanisms,31,32 identifying specific alterations of the ST and GR muscles is of primary importance for the success of the surgery in the long term and to avoid secondary re-ruptures.

In order to evaluate patient’s return-to-sport readiness, some months after surgery the functionality of the lower limb is assessed with a battery of functional performance tests (FPTs). The final goal of the rehabilitation is to show high inter-limb symmetry in the tests, expressed with the limb symmetry index (LSI = [involved limb/uninvolved limb] × 100). When a patient reaches the minimum score of 85-90% of LSI among the tests, he is generally considered ready to return to sport activities.33

In general the tests batteries are standardized protocols focused on assessing lower limb function as a whole, designed regardless of the surgical technique. For this reason the screening protocols could be weak in detecting deficits related to specific muscles, such as the medial hamstrings. One example of a test battery might include:

Example ACL return-to-sport test battery
Test Protocol
Maximal voluntary isometric contraction (MVIC) of the knee extensor and flexor muscles Tests at 30°, 60° and 90° of knee flexion
Isokinetic maximal strength of the knee extensor and flexor muscles Tests at 60°/s and 180°/s
Hop tests Single hop for distance, triple hop for distance, crossover hop for distance
Jump tests Vertical jump, countermovement jump, single leg jump, drop jump
Field tests Shuttle run test, Carioca test, agility test

If we analyze the most common FPTs used as criteria for the return to sport readiness after ACL reconstruction, we find that these tests mainly involve the knee extensors muscles,34,35 reflecting a low sensitivity for the detection of deficits due to a weakness of the hamstrings. In fact, although the hamstrings are synergists in multi-joint exercises, electromyographic (EMG) research shows that their activity is relatively modest during several movements such as squat36, leg press37, cutting maneuvers38, or other common movements used to assess lower limb function (e.g. hops, jumps). On the contrary, when the surgery involves directly the hamstrings tendons, the recovery of these muscles should be the primary goal and then, one of the crucial aspects to specifically assess.

Moreover, we don’t know which is the association of the scores of these tests and the neuromuscular activity of the involved muscles.39 In fact, with a standard test battery, it is impossible to quantify the contribution of each muscle compartment in performing the request task. For example, performing a maximal voluntary isometric contraction (MVIC) of the hamstrings in a leg curl machine, our final outcomes will be the expressed Newtons, but we will never know the partial contribution of each hamstrings muscles in performing that task. Hence, in the light of the previous findings about morphological hamstrings imbalance and BF compensation after the harvesting of the ST and GR, we could potentially see a patient that in the return-to-sport screening shows symmetrical hamstrings maximal strength and symmetrical performance in the FPTs, but hides a critical pathological weakness of the ST and GR muscles due to an hyperactivity of the not harvested hamstrings.

Thus, we could decide that our patient is ready to come back to train, whereas he is at high risk to ACL re-rupture due to the weakness of one of the most important ACL agonist (ST). These suggestions could potentially explain why some authors40 found that graft failure after ST/GR autograft is significantly higher than for patellar tendon autograft despite similar rates of return to competition. In other word it means that patients operated with hamstrings graft that had a secondary ACL re-rupture, had passed the return to sport screening within the normal range of time. So, we could hypothesize that maybe the screening was not able to detect those asymmetries due to medial hamstrings deficits.

Improving return to play screens

How could we improve the standard test battery in order to increase its sensibility to these possible hidden compensations? The first step to answer this question is to study the biomechanics of the lower limb, searching for those tasks that facilitate the activation of the medial compartment of the hamstrings and decrease the activity of the lateral compartment. In this way we could potentially increase the contribution of the medial hamstrings, thus improving the sensibility of the test in detecting possible deficits.

The following are some reflections that could help in the reasoning about how we could improve the specificity of our screening:

Recommendation 1: changing the angle of foot rotation. Some authors,41–43 measuring the changes in the medial and lateral hamstrings within several grades of tibial rotation in healthy subjects, found that the EMG activity of the lateral hamstrings decrease when the tibia is medially rotated. Therefore, developing a set of tests in which the foot is internally rotated could, at least to some extent, limit the effects of the lateral hamstrings compensatory mechanism on the final result of the test.

Recommendation 2: assessing lower limb strength at deep angle of knee flexion. Another clear example of how a common strength test could be improved in order to be more specific for the ST changes, concerns the angle at the knee during a maximal voluntary isometric contraction (MVIC) of the hamstrings. Indeed, from EMG studies we know that the main contribution of the ST (and the medial hamstrings compartment in general) in the knee flexion maximal strength is at high angles of knee flexion,44,45 and that the contribution of the biceps femoris decreases as the angle at the knee increases.46 Thus, with a hamstrings MVIC performed at deep knee flexion or hyperflexion47, we could have a more specific information about the recovery of the medial compartment of the hamstrings, avoiding the compensatory strategy of the lateral hamstrings, more activated at low angles of knee flexion.

Recommendation 3: rethinking the isokinetic strength test. The isokinetic strength test is one of the most common tests used to assess lower limb muscle strength after ACL reconstruction, and it is considered the gold standard for assessing muscle endurance. It measures knee extension/flexion torque at different speeds of movement, which remain constant during the test. This test is performed using the full range of motion of the knee joint (normally 0°-120°)46, and the final outcome of the test is the maximal quadriceps and hamstrings peak torque achieved during the repetitions. However, the maximal hamstrings peak torque occurs in general between 20° and 30° of knee flexion22,46,48, at a knee angle that mainly activate the BF with less contribution of the ST. In the light of the previous findings a simple question arises: could this test really detect our deficits? Is this test really useful in detecting hamstrings strength weaknesses if it mainly requires BF action that is not directly affected by the surgery? Decreasing the ROM of the movement could we get a more specific test for detecting weaknesses in the medial hamstrings compartment?

Final thoughts: are we measuring what we intended to measure?

These reasonings, arising from a literature review, open a series of new questions that every therapist should take into account at the moment of assessing ACL return-to-sport readiness. There exist a lot of protocols and guidelines regarding ACL reconstruction and RTP evaluation that aim to standardize the assessment phase. However, standardization is not always the key to understand what is better to do. Thus, before we analyze our patients or athletes and decide what they should do on the basis of general guidelines, we should ask us a very important question which applies to many areas of sport and medicine: are we measuring what we intended to measure?


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