Meeting Abstract

34.1  Tuesday, Jan. 5  Muscle-tendon architecture shapes conditions for economical force production SAWICKI, G. S.*; ROBERTS, T. J.; Univ. North Carolina at Chapel Hill and NC State University; Brown University greg_sawicki@ncsu.edu

During cyclic movements (e.g. running, walking), muscle fibers at distal joints save metabolic energy by producing force nearly isometrically, avoiding costly mechanical work. Series elastic structures (e.g. tendon and aponeurosis) within the muscle-tendon unit are crucial for economical force production because they can stretch, storing strain energy while limiting the length changes applied directly to the muscle fibers. The goal of this study was to determine how muscle-tendon architecture (i.e. free tendon length) influences the conditions required to maintain fibers isometric during cyclic force production. We used an optimally tuned real-time feedback controller to drive frog plantaris muscle-tendon through the length change pattern that minimized plantaris fiber length change (as monitored from implanted sonomicrometry crystals). To study the effects of architecture, the feedback protocol was performed with the muscle-tendon unit clamped at (1) the distal free tendon (DFT) and (2) the distal aponeurosis (DA). In both DFT and DA conditions, the muscle-tendon unit length change trajectory required for isometric force production was highly asymmetric, displaying rapid lengthening followed by much slower shortening. The rate of muscle-tendon stretch required to keep fibers isometric was greater in the DFT versus DA condition. However, in both cases, the muscle-tendon stretch rate exceeded the maximum shortening velocity of the fibers. These results indicate that much of the compliance in a muscle-tendon unit may arise from the aponeurosis and not the free-tendon. As a consequence, even in muscle-tendons with relatively short free-tendons, extremely high rates of stretch may be required to maintain isometric force production.