S2-7 Thursday, Jan. 4 11:30 - 12:00 The battle of the bulge: structural determinants of muscle gearing during dynamic contractions ENG, Carolyn M. *; AZIZI, Emanuel; ROBERTS, Thomas J.; Brown University; Univ. of California, Irvine; Brown University firstname.lastname@example.org
There are many phenomena in muscle that occur as a result of interactions between contractile and structural elements at multiple scales. One of these phenomena is architectural gearing, which is quantified as the ratio of muscle velocity to muscle fiber velocity. Many pennate muscles operate with a gear ratio greater than one because muscles shorten through a combination of muscle fiber shortening and fiber rotation. Within a muscle, gearing is variable across contractions. During low force contractions, muscles operate at high gear while muscles operate at low gear during high force contractions. This variable gearing has a significant impact on muscle performance by allowing for faster contractions for high-speed movements and more forceful contractions at low speeds. We hypothesize that gearing in any given contraction is determined by the dynamic interaction of fiber-generated forces, fluid force transmission, and the elastic behavior of intramuscular connective tissues. Because muscle is isovolumetric, muscle fibers must bulge radially when they shorten. This radial bulging exerts forces on the surrounding fluid at angles orthogonal to the fibers, providing a pathway to load connective tissues that ensheath fibers, fascicles and the whole muscle. The nature of how fluid pressures and fiber forces interact to load connective tissues in three-dimensions remains poorly understood. To date, architectural gearing has primarily been explored under controlled conditions with constant muscle force and maximal muscle activation. We combine modeling and experimental approaches to understand the fundamental interactions that determine gearing during more realistic conditions where muscles are submaximally activated and forces vary dynamically.