Testing and application of a novel hybrid combination of ultrasound and motion analysis for estimation of Achilles tendon moment arms in vivo

Muscle-tendon parameters, including the moment arm (MA), are of importance in biomechanical studies for use in determining muscle forces and in musculoskeletal modeling and treatment of muscle pathologies. Prior methods (such as the center of rotation method and the tendon excursion method) for determining the Achilles tendon MA have not addressed the effect of muscle contraction level on Achilles tendon MA length and utilize protocols that have functional limitations which require numerous measurements for a single MA estimation. Hence, a novel method for estimating the MA of the Achilles tendon was designed to overcome the limitations of currently used methods. The method is able to characterize MAs at rest and maximum voluntary contraction (MVC) as well as a function of ankle joint angle. The hybrid method combines motion capture and ultrasound to require only two simple measurements for MA estimation. In this study, a hybrid method was developed and validated for Achilles tendon MA estimation using an ovine model. It was then applied in vivo to ten subjects. On average, Achilles tendon MA lengths increased from dorsiflexion to plantar flexion, as seen in previous studies. Small increases in MA length were seen from rest to MVC, averaging 3.4%. This result directly addressed the effect of muscle contraction on Achilles tendon MA length and shows that there is relatively no change in MA from rest to MVC, which was not distinctly answered in previous studies. Also, hybrid MA estimates were markedly smaller in magnitude than those previously published in literature and the magnitude difference between studies lies mainly in the methodologies used. In summary, the hybrid method was successfully developed on an ovine model in which it was able to reduce necessary measurements, estimate MAs at rest and MVC and range of motion limits, and characterize human Achilles tendon MAs, showing lower overall magnitudes and relatively no differences in MA length with muscle contraction Implications of the MA magnitudes observed in this study can have research and clinical applications in terms muscle force estimates as well as in planning of surgical procedures of patients with muscle pathologies.