The Influence of Patellofemoral Kinematics on the Effective Quadriceps Moment Arm: a Dynamic in Vivo Study

INTRODUCTION The moment arm (ma) of a tendon is a crucial quantity for musculoskeletal dynamics as it defines how the linear force within a muscle is transformed into a torque. For the quadriceps this relationship is complicated by the fact that the patella serves as an intermediary (a dynamic fulcrum) between the quadriceps and tibia. Thus, the term effective quadriceps moment arm (EQma) was coined to define the quadriceps indirect ability to generate a torque on the tibia [1]. To date, no study has quantified the EQma during dynamic in vivo movement. Furthermore, the critical question of how patella-femoral (PF) kinematics may influence the EQma remains unresolved. Specifically, patella alta has been widely implicated as the source of a reduced EQma in patients with cerebral palsy, contributing to an already weakened extensor mechanism [2]. Yet, a recent study demonstrated that individuals with patella alta had a significantly larger EQma [3]. Therefore the purpose of this study was to determine the EQma during a knee extension exercise in a large group of asymptomatic controls, using in vivo dynamic MRI. A secondary goal of the study was to determine if the EQma was correlated with PF or tibiofemoral (TF) kinematics.