- The hamstrings appear to be protective against anterior cruciate ligament (ACL) loading, while the quadriceps induce ACL loading and anterior shear force.
- The soleus and gluteus medius appear to be protective of ACL loading by applying posterior shear force to the tibia, and keeping the knee in optimal alignment, respectively.
- Movement strategies are extremely varied and individual, and it is unknown whether focusing on training specific muscles will reduce the incidence of ACL tears.
BACKGROUND & OBJECTIVE
Anterior cruciate ligament (ACL) tears are one of the most common knee injuries sustained during sports, and they are characterized by a costly and lengthy rehabilitation (1). ACL re-injury rates have been reported as high as 30%, with many athletes losing multiple competitive seasons to injury rehabilitation (2). ACL injuries occur most often during non-contact dynamic tasks, shortly after initial contact, where the knee experiences high mechanical loads, compression, rapid loading into flexion, and large degrees of valgus and rotation.
It is thought that effective training of specific muscle groups and movement patterns may mitigate ACL loads during injurious scenarios, thus protecting the ACL from injury. An important prerequisite for clinicians in administering rehabilitation and preventative exercise for the ACL rehabilitation patient is understanding how individual muscles contribute to ACL loading.
The authors sought out to summarize the existing evidence on how specific lower limb muscles contribute to ACL load.
The hamstrings, soleus, and gluteus medius have the greatest ability to oppose ACL loading.
The authors included a variety of studies for their narrative review.
In research there are 3 primary methods to assess how muscle force may contribute to ACL loading: in vitro (or in situ) experiments involving cadavers, in silico experiments involving computer simulation, and in vivo experiments involving living organisms.
Each method comes with distinct advantages and limitations.
In vitro experiments often apply static forces using robotics to manipulate cadaveric joints. They do not account for full body kinematics. Their distinct advantage however is they can load tissue to failure and quantify ultimate tissue strength.
In silico experiments use data collected from healthy organisms and computer modeling to simulate forces. Results of these studies should be interpreted cautiously, as these methods carry low validity and rely on assumptions and uncertainties.
In vivo experiments offer more dynamic loading options, as surgical implantation of a differential variable reluctance transducer on the ACL fibers, can quantify ACL strain in real life movement. Less invasive methods including EMG and fluoroscopy provide a rough estimate at best. In vivo methods can only assess subthreshold injury loads.
Lastly, data analyzing movement patterns and muscle activation lacks generalizability, as it does not account for individual variability.
The quadriceps muscle group significantly contributes to ACL load by producing an anterior shear force at the tibia.
At knee flexion <30-50°, the quadriceps induces the greatest ACL loading, contributing to tibial anterior translation, tibial internal rotation, knee valgus rotation and knee valgus moment.
At knee flexion >80°, the quadriceps helps to unload the ACL due to the changing angle of pull of the patellar tendon relative to the longitudinal axis of the tibia.
Most ACL injuries occur with knee flexion <70° however, making the quadriceps an antagonist to the ACL during injury scenarios.
The hamstrings produce a posterior shear force at the tibia, working to unload the ACL beyond 20-30 degrees of flexion.
Due to the line of action and small mechanical advantage, the hamstrings are not able to produce a strong posterior force when the knee is near full extension.
In vitro and in silico studies demonstrate how hamstring activation can reduce ACL strain during hamstrings-quadriceps co-contraction.
The biceps femoris is thought to provide the greatest protective ACL unloading of the 3 hamstring muscles.
The role of the gastrocnemius in ACL loading remains inconclusive, as many studies have published conflicting results.
The soleus is a single joint muscle. Although it does not cross the knee joint, its influence on the ankle directly affects the knee.
Passive tension from the soleus in dorsiflexion resists anterior tibial translation, and activation of the soleus causes posterior translation of the tibia.
Gluteal Muscle Group
Our understanding of gluteal muscle force and its association with knee joint loading is limited to in silico investigations.
Modeling studies have shown how decreased gluteus medius force can lead to increased knee valgus moments and greater loading on the ACL.
Other modeling studies have shown inconclusive results at best.
See Figure 1 for a visual illustration of the force vectors acting on the tibiofemoral joint from different muscle groups.
The hamstrings, soleus, and gluteus medius appear to have the greatest ability to oppose ACL loading, while the quadriceps and gastrocnemius appear to have the greatest ability to induce ACL loading. In a functional movement scenario however, it is difficult to separate out soleus versus gastrocnemius contraction, and to influence co-contraction of the hamstrings and quadriceps. This information can be more readily applied in a controlled rehabilitation environment and is most useful for clinicians who are prescribing exercise in early to mid-stage rehabilitation.
Compensatory task strategies adopted by individual organisms during dynamic movement including valgus, varus and rotational forces at the knee are extremely variable and individual. Studying movement is helpful to better understand potential kinetic chain patterns and strategies, however the data cannot be generalized or widely applied.
We must take what we learn from in silico and in vivo studies with a grain of salt, understanding they are simply one more contribution to a large body of research on movement variability. In vitro studies are helpful to understand arthrokinematics, however they have significant drawbacks.
Despite the limitations, future research will continue to investigate ACL loading with the hopes of increasing our understanding of ACL injuries and improving our prevention and treatment techniques.
- Majewski M, Susanne H, Klaus S. 2006. Epidemiology of athletic knee injuries: a 10-year study. Knee.
- Grindem, H., Snyder-Mackler, L., Moksnes, H., Engebretsen, L., & Risberg, M. A. 2016. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. BJSM.