Meeting Abstract
Mallard ducks use different strategies for increasing speed when moving over land vs. in the water. Notably, they increase leg cycle frequency in order to go faster on land but prefer to use a constant cycle frequency (2.6 Hz) across their full range of speeds during surface swimming. Stride frequency and stride length interact to determine where leg muscles act on the length-tension and force-velocity curves. The lateral gastrocnemius (LG) is a large ankle extensor in ducks that is important for powering surface swimming. We wanted to test whether the LG was tuned to operate at 2.6 Hz; specifically, does it produce maximal work or power at 2.6 Hz? We used the work loop technique to examine in situ LG work and power production at 3 frequencies: 2.6 Hz, the in vivo condition, as well as a lower (1 Hz) and higher (4 Hz) frequency. We performed muscle work loops at submaximal stimulation and used a constant length excursion, stimulation duty factor, and phase at stimulation onset that mimicked literature values for in vivo muscle excursion and activation patterns. For all ducks we examined (n=6), work decreased with increasing frequency because the muscle had time to develop higher forces at lower frequencies. In 3 ducks, power increased with frequency, and in the other 3, power was higher at 2.6 Hz than at lower or higher frequencies, suggesting that 2.6 Hz might permit maximal LG power production for some animals. Alternatively, a 2.6 Hz cycle frequency may represent a compromise between work and power production, at least for the LG. Additional studies are needed to examine the effect of cycle frequency on work and power production in other mallard leg muscles and to determine whether 2.6 Hz has a hydrodynamic significance.
