Dynamic In Vivo Moment Arms of the Individual Quadriceps Components

INTRODUCTION: Patellofemoral pain (PFP) is closely related to static patellar malalignment and patellar maltracking. The etiology of patellar maltracking is multifactorial; relating to femoral shape, passive soft-tissue restraints; and/or an imbalance of forces on the patella. A number of structures, both passive and active, contribute to the force balance on the patella. Previous studies have investigated the moment arm of the patellar tendon and the quadriceps muscle as a whole. However, the presence of increased patellar spin (positive spin or varus results in the superior patellar pole moving laterally) in PFP [1,2] suggests that the relative moment contributions of the individual quadriceps components play an important role in stabilizing the patella. Therefore, in order to understand the relative contribution of each quadriceps component to the balance of forces on the patella, the individual moment arms of the quadriceps tendons must be known. Previous studies have relied on in vitro methods at static knee flexion angles or modeling. No study has characterized the moment arms of the individual quadriceps components in vivo. Thus, the purpose of this study was to quantify the moment arms of the quadriceps tendons in vivo and noninvasively during dynamic flexion/extension of the knee. The hypothesis of this study was that the tendons associated with the medial and lateral quadriceps components will have significant off-axis moment arms (resulting in patellar spin) compared to the central quadriceps components. A secondary purpose was to determine if increasing the magnitude of quadriceps contraction influences the dynamic moment arms of the quadriceps components.