Changes of the Patellar Tendon Moment Arm Length in Different Knee Angles: A Biomechanical in Vivo Study

Patellar tendon moment arm length (PTma) changes at different knee flexion angles have not been determined in invivo studies. We aimed to determine PTma in four different knee angles using Magnetic Resonance Imaging (MRI) topredict in vivo changes in the moment arm length from different knee angles during running.PTma was measured as the perpendicular distance from muscle–tendon line of action to the knee joint axis of rotationat 0° (full extension), 20°, 40°, and 60° flexion of knee by using MRI method. Repeated measure ANOVA method wasapplied to compare the moment arm length among four degrees of knee flexion (p <0.05). A regression analysis wasused to predict the PTma during different knee joint angles.The PTma in the four angles at 0°, 20°, 40°, and 60° of knee flexion were 42.55±4.20, 39.91±2.98, 37.73±2.87, and36.18±2.90 mm, respectively (p <0.05). The regression analysis provided an equation to predict the PTma from differentknee joint angles during running.PTma values decreased from knee extension to flexion in a linear manner. These findings have important implicationsfor estimating PTma using a regression equation model from different knee joint angles.Level of evidence: V