Recommendations for hamstring function recovery after ACL reconstruction

Key Points

1. Early emphasis on quadriceps strengthening in ACL rehabilitation has potential to overshadow the need for hamstring strengthening.
2. Deficits in knee flexion strength can play a large role in ACL re-injury and should be more carefully measured during the return to sport decision-making process.
3. Hamstring strengthening during ACL rehab should include both short and long muscle length exercises, a mixed approach of hip extension and knee flexion, a specific focus on eccentric strength at long muscle lengths, and high-speed running.

BACKGROUND & OBJECTIVE

Considerable deficits have been reported in hamstring function in the early post-operative period following anterior cruciate ligament (ACL) repair, as well as in the return to sport (RTS) period (1). Although hamstring strength deficits are larger in those with hamstring tendon autograft (HTG), they are still of high concern in those with patellar tendon autograft (PTG).

Early emphasis on quadriceps strengthening in ACL rehabilitation has potential to overshadow the need for concomitant hamstring strengthening. Deficits in knee flexion and hip extension strength can be detrimental to injury risk upon RTS, highlighting the need for a continual focus on the posterior chain during ACL rehabilitation (2).

The aim of this paper was to discuss important alterations of the hamstring muscles after ACL reconstruction (ACLR), consider their implications for program design, and guide clinicians on how to apply this information.

Considerable deficits have been reported in hamstring function in the early post-operative period following ACL repair.

Hamstring strengthening during ACL rehabilitation should be periodized and carefully planned, exposing patients to increasing muscle lengths, intensities and velocities over time.

METHODS

The authors wrote a review article utilizing 217 papers on a variety of topics on ACLR, hamstring injury, and the lower extremity. The paper provided a sample template for ACLR rehab, an example of periodized progression through exercise prescription, and extensive citations from a range of papers with varying levels of evidence.

RESULTS

Knee flexor deficits after ACLR are typically more pronounced at shorter muscle lengths. As a two-joint muscle, the hamstrings are maximally shortened when the hip is extended and knee is flexed, as in a standing hamstring curl (3). At 6 months post-op, 67% of patients with PTG had an average knee flexor limb symmetry index (LSI) > 90%, while only 46% of patients with HTG achieved this symmetry (4). The use of HTG can result in chronic neuromuscular inhibition and persistent hamstring strength deficits more pronounced during eccentric than concentric activation (5). After ACLR with HTG there also appears to be a proximal migration of the semitendinosus muscle-tendon junction, that can take up to 18 months to repair, and may not ever fully repair in up to 50% of patients (6).

Evidence suggests training at longer muscle lengths can produce similar strength gains and muscle architecture changes to eccentric training, indicating potential utility for longer muscle length hamstring training during ACLR rehabilitation (7). Hamstring exercises performed with the hip flexed are thought to have greater dynamic correspondence to the functional requirements of the hamstring in locomotion. During the swing phase of sprinting, internal moments at the hip are double that at the knee, indicating a need for proximal hip, pelvis, and trunk strengthening and coordination in order to help ensure proper hamstring length-tension relationship is maintained (8).

LIMITATIONS

This paper serves as a generalized review. It lacks a systematic approach to article selection and does not carry any statistical power. Although informative, the paper represents a low level of evidence and was not intended to serve as a clinical practice guideline or sweeping update of our current understanding.

CLINICAL IMPLICATIONS

Functional hamstring strengthening should be a key component of all ACL rehabilitation, with extra focus given to those with HTG. Earning the title, “the muscular ACL of the knee”, contraction of the hamstrings contributes to knee stability by resisting forward translation and excessive rotation of the tibia (9). The combined role of the hamstrings in both hip extension and knee flexion, and their need for rapid eccentric strength at longer muscle lengths during sprinting and functional tasks, should help guide clinicians as they program and progress their patient’s hamstring exercises.

Hamstring exercises can be categorized by their predominant contraction type, contraction speed, muscle length, and joint position. Shorter muscle length exercises might be preferable during the early post-operative period, such as the prone leg curl, standing leg curl, and bridge. During later stages, the Nordic hamstring exercise represents a short muscle length hamstring exercise focused on maximal eccentric strength. Longer muscle length exercises that incorporate hip hinge patterns train the proximal hamstring fibers to a greater degree, such as the Romanian deadlift, 45-degree back extension, and kettlebell swing. Longer muscle length exercises that incorporate both hip extension and knee flexion, such as the straight leg bridge and seated leg curl have the capacity to improve muscle fascicle length and contribute to eccentric strength gains at longer muscle lengths.

Sprinting is arguably the most effective hamstring exercise there is, as it incorporates rapid angular velocities and high demands of both hip extension and knee flexion and has the greatest carryover to sport performance for field athletes. A two-fold increase in hamstring activation has been reported between sprinting at 7m/s and 9m/s, further indicating the need for maximal velocity sprinting for specific hamstring adaptations.

Hamstring strengthening during ACL rehabilitation should be periodized and carefully planned, exposing patients to increasing muscle lengths, intensities and velocities over time. A wide range of hamstring exercises and high-speed running should be implemented during ACL rehabilitation in order to reduce the likelihood of re-injury and optimize RTS performance.

+STUDY REFERENCE

Buckthorpe M, Danelon F, La Rosa G (2020) Recommendations for Hamstring Function Recovery After ACL Reconstruction. Sports Med.

SUPPORTING REFERENCE

1. Cristiani R, Mikkelsen C, Forssblad M, Engström B, Stålman A. (2019). Only one patient out of five achieves symmetrical knee function 6 months after primary anterior cruciate ligament reconstruction. Knee surgery, sports traumatology, arthroscopy.
2. Kim HJ, Lee JH, Ahn SE, Park MJ, Lee DH. (2016). Influence of Anterior Cruciate Ligament Tear on Thigh Muscle Strength and Hamstring-to-Quadriceps Ratio: A Meta-Analysis. PloS one.
3. Baumgart C, Welling W, Hoppe MW. (2018). Angle-specific analysis of isokinetic quadriceps and hamstring torques and ratios in patients after ACL-reconstruction. BMC Sports Sci Med Rehabil.
4. Harput G, Kilinc HE, Ozer H, Baltaci G, Mattacola CG. (2015). Quadriceps and Hamstring Strength Recovery During Early Neuromuscular Rehabilitation After ACL Hamstring-Tendon Autograft Reconstruction. Journal of sport rehabilitation
5. Opar DA, Williams MD, Timmins RG, Hickey J, Duhig SJ, Shield AJ. (2015). Eccentric hamstring strength and hamstring injury risk in Australian footballers. Med Sci Sports Exerc.
6. Tengman E, Brax Olofsson L, Stensdotter AK, Nilsson KG, Häger CK. (2014). Anterior cruciate ligament injury after more than 20 years. II. Concentric and eccentric knee muscle strength. Scand J Med Sci Sports.
7. Bourne, MN, Duhig SJ, Timmins RG, Williams MD, Opar DA, Al Najjar A, Kerr GK, Shield AJ. (2017). Impact of the Nordic hamstring and hip extension exercises on hamstring architecture and morphology: implications for injury prevention. BJSM
8. Mendiguchia J, Alentorn-Geli E, Brughelli M. (2012). Hamstring strain injuries: are we heading in the right direction? BJSM.
9. Shin CS, Chaudhari AM, Andriacchi TP. (2011). Valgus plus internal rotation moments increase anterior cruciate ligament strain more than either alone. Med Sci Sports Exerc.