Meeting Abstract
Elastic structures associated with muscle play a key role in energy cycling during cyclical movement. While the role of tendons in energy storage and recovery is well-established, the role of intramuscular connective tissues (IMCTs) is less clear. The arrangement of Hill-type muscle models dictates that length changes in muscle fibers and the elastic elements within muscle are equal. In this model, when muscles undergo isometric contractions in locomotion, IMCTs are not stretched, and therefore cannot store elastic energy. We developed a model with an alternative arrangement to reflect the strains that IMCTs may undergo during deformation of pennate muscles during contraction. In the model, the IMCT is represented as a transverse elastic element (TEE), perpendicular to the muscle’s line of action. A component of fiber force compresses the TEE, influencing muscle thickness and pennation angle. To explore the role of a TEE in locomotion, we drove the model with a constant fiber length and sinusoidal force pattern and calculated pennation angle, muscle length, and thickness. Modeling results were tested against in vivo biplanar fluoroscopy recordings of locomoting turkeys whose lateral gastrocnemius muscles were implanted with radiopaque beads. Both the model and in vivo data showed muscle belly lengthening and shortening, indicating energy cycling. In the model, muscle length changes were accommodated solely by changes in fiber rotation and muscle thickness. Fibers changed length in vivo, but energy storage and recovery was also accommodated by changes in muscle thickness, consistent with energy cycling in the thickness direction. Contrary to predictions from Hill-type models, the TEE model suggests that energy cycling in the IMCT is potentially substantial.
