Rectus Femoris Knee Muscle Moment Arms Measured in Vivo During Dynamic Motion With Real Time Magnetic Resonance Imaging

Abstract

Moment arms represent a muscle's ability to generate a moment about a joint for a given muscle force. The goal of this study was to develop a method to measure muscle moment arms in vivo over a large range of motion using realtime magnetic resonance (MR) imaging. Rectus femoris muscletendon lengths and knee joint angles of healthy subjects (N = 4) were measured during dynamic knee joint flexion and extension in a largebore magnetic resonance imaging (MRI) scanner. Muscletendon moment arms were determined at the knee using the tendonexcursion method by differentiating measured muscletendon length with respect to joint angle. Rectus femoris moment arms were averaged across a group of healthy subjects and were found to vary similarly during knee joint flexion (mean: 3.0 (SD 0.5) cm, maximum: 3.5 cm) and extension (mean: 2.8 (SD 0.4) cm, maximum: 3.6 cm). These moment arms compare favorably with previously published dynamic tendonexcursion measurements in cadaveric specimens but were relatively smaller than moment arms from centerofrotation studies. The method presented here provides a new approach to measure muscletendon moment arms in vivo and has the potential to be a powerful resource for characterizing musculoskeletal geometry during dynamic joint motion.