Meeting Abstract
Fishes need to swim long distances efficiently and accelerate quickly to escape predators or capture prey. To swim, fish contract the muscles on either side of their bodies to propel forward. Faster swimming requires higher muscle forces, but may also require an overall stiffer body because the reaction forces from the environment are also higher. How do fish modulate force production and body stiffness over a range of swimming speeds? One strategy fish may use is to increase the effective stiffness of their bodies while accelerating by co-contracting antagonistic muscles or activating more muscle during lengthening (eccentric contractions). To measure the muscle activity, we implanted bipolar electromyographic electrodes in the superficial red axial muscle of bluegill sunfish (Lepomis macrochirus) and recorded muscle activation during forward accelerations and steady swimming between 0.5-2.5 body lengths/second. We used a new digital inertial measurement unit, containing three-axis accelerometers and gyroscopes, to quantify the acceleration and 3D orientation of a fish’s body. We also quantified swimming kinematics using high speed video. In forward accelerations, the muscles were active for a larger portion of the tail beat cycle and came on at a different time compared to that in steady swimming. Further, duty cycles (percentage of strain cycle period) was greater in forward accelerations than in steady swimming and varied by location on the body. These results suggest that muscle on both sides of a fish’s body may co-contract during accelerations but not in steady swimming. Fishes likely change their effective body stiffness by shifting the timing and duration of muscle activity. These changes in muscle activity may allow a shift between impulsive, high force movements and efficient, low force, steady movements.
