Quantifying dynamic muscle lengths and moment arms of musculoskeletal joints using FEBio studio: A demonstration in the glenohumeral joint.

Muscle lengths and moment arms are necessary to quantify musculoskeletal joint torques and subsequent rotations created via muscle contractions. High-precision morphologic and kinematic data collected with dynamic stereoradiography allow semi-automatic calculation of muscle lengths and moment arms using computational methods coded into FEBio Studio. A verification/validation model of known geometry was created, simulating glenohumeral anatomy and motion, where anatomic landmarks and muscle lines of action vectors were used to calculate muscle length and moment arms dynamically throughout a motion. Moment arms were also decomposed about anatomic coordinate systems to describe the muscle's relative contributions to physiologic planes of motion. A use case of pre- and post-operative reverse total shoulder arthroplasty was then demonstrated, where resultant changes in muscle lengths and moment arms due to the non-anatomic joint replacement agreed with prior studies. These methods, now integrated into FEBio Studio, will enable researchers and clinicians to study the effects of changing morphology and kinematics on dynamic musculoskeletal function related to muscle length and moment arms in an open-source platform.