Muscle Force Contributions to Anterior Cruciate Ligament Loading

Abstract

Anterior cruciate ligament (ACL) injuries are one of the most common knee pathologies sustained during athletic participation and are characterised by long convalescence periods and associated financial burden. Muscles have the ability to increase or decrease the mechanical loads on the ACL, and thus are viable targets for preventative interventions. However, the relationship between muscle forces and ACL loading has been investigated by many different studies, often with differing methods and conclusions. Subsequently, this review aimed to summarise the evidence of the relationship between muscle force and ACL loading. A range of studies were found that investigated muscle and ACL loading during controlled knee flexion, as well as a range of weightbearing tasks such as walking, lunging, sidestep cutting, landing and jumping. The quadriceps and the gastrocnemius were found to increase load on the ACL by inducing anterior shear forces at the tibia, particularly when the knee is extended. The hamstrings and soleus appeared to unload the ACL by generating posterior tibial shear force; however, for the hamstrings, this effect was contingent on the knee being flexed greater than ~ 20° to 30°. The gluteus medius was consistently shown to oppose the knee valgus moment (thus unloading the ACL) to a magnitude greater than any other muscle. Very little evidence was found for other muscle groups with respect to their contribution to the loading or unloading of the ACL. It is recommended that interventions aiming to reduce the risk of ACL injury consider specifically targeting the function of the hamstrings, soleus and gluteus medius.

Fig. 1

Illustration of knee joint degrees of freedom using a previously described musculoskeletal model [30] in OpenSim [31]

Fig. 2

Illustration of force unit vectors acting at the tibiofemoral joint of knee-spanning muscles from knee flexion angle of 0° (full extension) to 120° of flexion. Unit vectors were derived from a previously published and validated musculoskeletal model [30] using a previously described tool [107] in OpenSim [31]. Note that unit vectors are visualised at their effective attachment points (as opposed to their anatomical attachment points), which accounts for wrapping surfaces and can therefore change as a function of the knee flexion angle. For muscles with multiple actuator components (quadriceps, hamstrings and gastrocnemius), the average attachment point and unit force vector was visualised. As the soleus did not span the knee, its unit vector was invariable due to the changing knee flexion angle. All illustrated unit vectors represent attachments to the shank, with the exception of the gastrocnemius which attaches to the femur

Fig. 3

Frontal (x-axis) and transverse (y-axis) plane tibiofemoral moment arms of knee-spanning muscles derived from a previously validated musculoskeletal model [30]. Circles indicate the moment arm of each muscle at 30° of knee flexion; circle size is proportional to the muscle’s corresponding physiological cross-sectional area derived from Ward et al. [111]; faded lines show the change in moment arm across the knee flexion range (0°–120°)