Meeting Abstract
Carangiform swimmers, such as the bluegill sunfish (Lepomis macrochirus), engage in an undulatory swimming motion that propels them forward primarily via the movement of the caudal region. However, counterintuitively, the majority of the musculature in a bluegill sunfish is located in its anterior body, not near the tail. For effective transmission of the force from the anterior muscle to the tail, ideally the peduncle region just anterior would be relatively stiff. However, bluegill, like many fishes, have relatively flexible bodies. One way they may be able to resolve this problem is by actively stiffening their body by co-contraction of muscles on opposite sides of the body, particularly during high speed swimming or rapid linear accelerations, when fluid dynamic forces are large. To test this hypothesis, we measured muscle activity, acceleration, and kinematics of bluegill sunfish swimming steadily and performing accelerations at flow speeds of 1-2.5 body lengths per second. To measure accelerations and body kinematics, we attached a small six-axis inertial measurement unit and filmed each fish using high speed cameras. We measured muscle activity using standard fine wire electromyographic electrodes implanted in the red muscle and measured duty cycle, or the fraction of the cycle period during which the muscle is active. Duty cycle increases with speed and during acceleration, sometimes exceeding 0.5. Duty cycles greater than 0.5 indicate that muscle activity on opposite sides of the body is overlapping, and thus that the fish is actively stiffening its body during swimming.
