Is the Effective Quadriceps Moment Arm Influenced by Patella Alta (a Dynamic in Vivo Study)?

INTRODUCTION The moment arm (ma) of a tendon is a crucial quantity for understanding musculoskeletal dynamics as it defines how the linear force within a muscle is transformed into a torque. For most tendons this is a fairly well established parameter as it can be defined from the kinematics of the tendons’ origins, insertions, and/or wrapping points. Yet, 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 transfer of the quadriceps force into a torque on the tibia cannot be directly calculated. Therefore, the term effective quadriceps moment arm (EQma) was coined to define the ability of the quadriceps to generate a torque on the tibia [1]. To date, no study has quantified this parameter during dynamic in vivo movement. Furthermore, the critical question of how patellofemoral (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 [2,3]. In contrast, a recent study demonstrated that individuals with patella alta had a significantly larger EQma [4]. Therefore the purpose of this study was to determine, using in vivo dynamic MRI, the EQma during a knee extension exercise in a large group of asymptomatic controls. A secondary goal of the study was to determine if the EQma was correlated with PF or tibiofemoral (TF) kinematics.