The musculotendon moment arm (MA) is the perpendicular distance from a muscle’s line of action to the rotational center of a joint. Moment arms are important in muscle modeling [1], and thus their accuracy is of great importance. Current in vivo techniques for computing MAs include the center of rotation and tendon excursion methods [1, 2]. The tendon excursion (TE) method relates the change in musculotendon length to an angular change in joint position [3]. This requires two measurements of musculotendon length for each MA computed. Similarly, the center of rotation method requires multiple image-based geometry measurements to compute the MA for a specific joint angle. The TE and center of rotation methods are both prone to measurement error, and thus it is difficult to ascertain the accuracy of the resulting MA. In this paper we present a novel hybrid methodology combining ultrasound (US) and video-based motion capture to compute the Achilles tendon moment arm. An advantage of this approach is that data used to derive the MA are acquired using highly accurate and reliable measurement instruments (i.e, US & motion capture), which may improve the accuracy of the MA estimate. The purpose of this paper is to present the hybrid methodology and validation results based on testing the method using an animal model.

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