The Influence of Muscle Forces and External Loads on Cruciate Ligament Strain

We know it is important to avoid excessive strain on reconstructed ligaments, but we do not know how in dividual muscles affect cruciate ligament strain. To an swer this, we studied the effect of muscle forces and external loads on cruciate ligament strain. Nine cadav eric knee joints were tested in an apparatus that allowed unconstrained knee joint motion. Quadriceps, ham string, and gastrocnemius muscle forces were simu lated. Additionally, external loads were applied such as varus-internal or valgus-external rotation forces. Cru ciate ligament strain was recorded at different knee flex ion angles. Activation of the gastrocnemius muscle sig nificantly (P < 0.05) strained the posterior cruciate ligament at flexion angles larger than 40°. Quadriceps muscle activation significantly strained the anterior cruciate ligament when the knee was flexed 20° to 60° (P < 0.01) and reduced the strain on the posterior cruciate ligament in the same flexion range ( P< 0.05). Activation of the hamstring muscles strained the pos terior cruciate ligament when the knee was flexed 70° to 110° (P< 0.05). Combined varus and internal rotation forces significantly increased anterior cruciate ligament strain throughout the flexion range (P < 0.05). The re sults suggest that to minimize strain on the ligament after posterior cruciate ligament surgery, strong gas trocnemius muscle contractions should be avoided be yond 30° of knee flexion. The study also calls into ques tion the use of vigorous quadriceps exercises in the range of 20° to 60° of knee flexion after anterior cruciate ligament reconstruction.