Muscle mechanics and hydrodynamics of swimming anurans

Using swimming frogs, my dissertation explores how muscles, the nervous system, and the fluid environment interact to control swimming performance. Many studies address how neuromuscular control relates to movement and performance. Separately, much work focuses on interactions between fluid and limbs to control swimming speed and efficiency. However, very few studies explicitly address the integration of neuromuscular control, muscle mechanics, hydrodynamics and performance. Yet, knowing how these parameters interact is critical for understanding how the coupled design of muscles and propulsory enables effective control of swimming performance. To approach this problem I use anurans as models because of the vast literature available on frog muscle physiology, in addition to a growing body of work addressing frog swimming hydrodynamics. I address four major questions: (1) How does the nervous system control muscular work and power output to modulate swimming performance (e.g. speed and acceleration)? To address this question I used in vivo techniques to directly measure muscle force, length change and activation to develop a statistical model explaining interrelationships between the magnitude of muscle recruitment, muscle force, muscle strain, work and power output. This model proposes a mechanism for how the nervous system modulates muscle mechanics in order to control power generated against fluid dynamic loads. (2) How do joint and foot movement patterns relate to the hydrodynamic forces required for swimming? To address this question I developed a forward-inverse dynamic model to simulate swimming performance output from joint kinematics input. Findings from the model suggest that for Xenopus laevis (a purely aquatic frog), foot rotational motions (powered mainly by ankle joint rotation) are much more important than foot translational motions (driven by hip and knee extension) for generating thrust. These results are contrary to the established understanding of frog swimming mechanics where proximal joints were thought to power swimming in frogs. (3) Do fully aquatic species employ unique propulsive strategies that differ from general patterns observed in more 'generalized' semi-aquatic frogs? To address this question, I measured kinematics and used hydrodynamic modeling to compare time-varying joint and foot motions in two fully aquatic species (Xenopus laevis and Hymenochirus boettgeri ) and two semi-aquatic species (Rana pipiens and Bufo americanus). Instead of revealing a sharp difference between both groups, I found that the four species examined represent a continuum between propulsion driven exclusively by rotational motion (X. laevis ), mostly rotational motion (H. boettgeri), rotational and translational motion (B. americanus), and mostly translational motion (R. pipiens). (4) How do forces from a network of linked biarticular muscles interact with hydrodynamic forces to produce observed hind limb motions? To address this question, I used EMG combined with mechanical modeling. I found that at low speeds, frogs employ many different kinematics 'strategies' (driven by unique underlying muscle coordination patterns) to achieve the same range of performance. However as speed increases, variation in joint kinematics and EMG patterns decrease, suggesting that there are limited muscle control strategies that enable high speed swimming. Overall, these four studies contribute to an understanding of how aquatic animals control internal mechanics (muscle force, work and power) to generate the limb movements necessary to modulate swimming performance.