ACL Rehab: Weight Bearing v. Non-Weight Bearing Exercise Selection
An estimated 200,000 anterior cruciate ligament (ACL) injuries are sustained annually in the United States. There is a need for sports medicine clinicians to understand the mechanical properties of the ACL, the ACL reconstructed knee and how different exercises and technique variations play a role in ACL loading.
While examining ACL loading and the potential for the surrounding musculature to load or unload the ACL, the quadriceps and hamstrings are the primary muscles involved. Contraction of the quadriceps exerts an anteriorly directed force on the proximal tibia when the knee is flexed between 0-60 degrees, increasing ACL loading. In knee flexion positions greater than 60 degrees the quadriceps exerts a posteriorly directed force unloading the ACL. The antagonist to the quadriceps is the hamstrings. When contracted the hamstrings exerts a posteriorly directed force on the proximal tibia throughout the entire knee range of motion. This helps to unload the ACL.
While most ACL rehabilitation protocols incorporate both weight-bearing (WB) and non-weight-bearing (NWB) exercises, there is evidence to suggest that individuals who perform predominantly WB exercises tend to have less knee pain and more stability.
One of the first exercises typically implemented towards the beginning of ACL rehabilitation is quad sets. A progression of this exercise is to place an ankle weight on the patient’s ankle to provide increased resistance upon VMO contraction, making the task more difficult. However, the tensile force on the newly reconstructed ACL is almost two times greater when the resistive force is placed near the ankle as compared to the middle of the lower leg. As soon as a patient is able to fully bear weight on a reconstructed knee, close-chained exercises are recommended to allow strengthening of the surrounding musculature without increased tensile forces placed on the ACL. For example, performing a double-leg mini-squat does not seem to produce an increase in ACL strain compared to a seated knee extension while in a knee-flexed position. The WB squat may actually help to recruit hamstring musculature to a greater extent which may potentially unload the ACL.
The primary factor associated with increased ACL strain with WB exercises is trunk positioning. Maintaining a vertical trunk position during WB exercises moves the center of mass posteriorly. This causes the knees to move anteriorly over the toes to maintain balance, increasing ACL strain. Therefore, while in a WB position, a slightly forward trunk lean acts to decrease ACL loading by increasing hamstring activity.
Because ACL loading is less with WB exercises compared to NWB exercises and because ACL loading is relatively low with WB exercises performed in lower angles of knee flexion, early after surgery a rehabilitation protocol implementing more WB exercises may be more beneficial to a patient long term than a protocol implementing both WB and NWB exercises throughout the process.
This article refers to the 2012 publication, "Anterior Cruciate Ligament Strain and Tensile Forces for Weight-Bearing and Non-Weight-Bearing Exercises: A Guide to Exercise Selection." from the Journal of Orthopaedic & Sports Physical Therapy, 42(3), 209.