Meeting Abstract
Mallard ducks are capable of nearly vertical takeoffs from both aquatic and terrestrial environments. The leg provides propulsive force for takeoffs in both media, in part, through ankle extension; however, force production differs between these media, creating an interesting challenge for leg muscle function. On land, force production depends on ground reaction forces; while in water, hydrodynamic force depends on the square of propulsive movement velocity. Correspondingly, we previously found angular velocity at the ankle is twice as fast for aquatic (991 ± 335 °/s) than terrestrial (487 ± 211 °/s) takeoffs. The gastrocnemius is a pennate muscle that drives ankle extension in ducks. Pennate muscles can change overall shortening velocity by changing velocity along the fascicle and through pennation angle changes in response to changes in load (i.e., variable gearing). We used sonomicrometry to measure fascicle length changes in the gastrocnemius during aquatic (n=5) and terrestrial (n=9) takeoffs in 4 mallard ducks. Gastrocnemius fascicle shortening velocity is nearly 3 times higher during aquatic (4.2 L/s) vs. terrestrial takeoffs (1.4 L/s), due to both larger excursions and shorter durations. We calculated pennation angle for a subset of the trials (aquatic: n=3, terrestrial: n=7) and found aquatic takeoffs involve similar pennation angle changes (4.0 ± 0.9 °) compared to terrestrial takeoffs (4.1 ± 1.9 °). These data demonstrate that shortening velocity changes substantially depending on the takeoff medium, highlighting the challenge for muscles to generate movement in multiple media. Fascicle shortening velocity, not changes in pennation angle excursion, drives the gastrocnemius’s contribution to faster leg motions during aquatic takeoffs.
