Is it necessary to include biarticular effects within joint torque representations of knee flexion and knee extension?

The purpose of this study was to consider whether it is necessary for biarticular effects to be accounted for in subject-specific representations of maximal voluntary knee extension and knee flexion torques. Isovelocity and isometric knee torques were measured on a single participant at three different hip angles using a Contrex MJ dynamometer. Maximal voluntary torque was represented by a 19-parameter two-joint function of knee and hip joint angles and angular velocities with the parameters determined by minimising a weighted root mean square difference between measured torques and the two-joint function. The weighted root mean square difference between the two-joint function and the measured knee flexion torques was 14 Nm or 9% of maximum torque, whilst for knee extension the difference was 26 Nm or 9% of maximum torque. The two-joint representation was shown to be more accurate than an existing single-joint representation for torques measured at hip angles other than those used to derive the single-joint function parameter values. The differences between the traditionally used single-joint representation and the measured knee flexion and knee extension torques were largest for the most extended hip joint angle (15% and 18% of maximum torque respectively) while the corresponding differences for the two-joint function were 9% and 8% of maximum torque. It is concluded that a two-joint function can account for changes in knee flexion and knee extension joint torques due to both monoarticular and biarticular muscles over a range of both hip and knee angles, and this has the potential to improve the biofidelity of whole body subject-specific torque-driven simulation models.