In their natural habitat, fish rarely swim steadily. Instead they frequently accelerate and decelerate. Relatively little is known about how fish produce extra force for acceleration in routine swimming behavior. In this study, we examined the flow around bluegill sunfish Lepomis macrochirus during steady swimming and during forward acceleration, starting at a range of initial swimming speeds. We found that bluegill produce vortices with higher circulation during acceleration, indicating a higher force per tail beat, but they do not substantially redirect the force. We quantified the flow patterns using high speed video and particle image velocimetry and measured acceleration with small inertial measurement units attached to each fish. Even in steady tail beats, the fish accelerates slightly during each tail beat, and the magnitude of the acceleration varies. In steady tail beats, however, a high acceleration is followed by a lower acceleration or a deceleration, so that the swimming speed is maintained; in unsteady tail beats, the fish maintains the acceleration over several tail beats, so that the swimming speed increases. We can thus compare the wake and kinematics during single steady and unsteady tail beats that have the same peak acceleration. During unsteady tail beats when the fish accelerates forward for several tail beats, the wake vortex forces are much higher than those at the same acceleration during single tail beats in steady swimming. The fish also undulates its body at higher amplitude and frequency during unsteady tail beats. These kinematic changes likely increase the fluid dynamic added mass of the body, increasing the forces required to sustain acceleration over several tail beats. The high amplitude and high frequency movements are also likely required to generate the higher forces needed for acceleration. Thus, it appears that bluegill sunfish face a trade-off during acceleration: the body movements required for acceleration also make it harder to accelerate.

STATEMENT1 Bluegill sunfish accelerate primarily by increasing the total amount of force produced in each tail 2 beat but not by substantially redirecting forces. 3ABSTRACT 4 In their natural habitat, fish rarely swim steadily. Instead they frequently 5 accelerate and decelerate. Relatively little is known about how fish produce extra force 6 for acceleration in routine swimming behavior. In this study, we examined the flow 7 around bluegill sunfish Lepomis macrochirus during steady swimming and during 8 forward acceleration, starting at a range of initial swimming speeds. We found that 9 bluegill produce vortices with higher circulation during acceleration, indicating a higher 10 force per tail beat, but do not substantially redirect the force. We quantified the flow 11 patterns using high speed video and particle image velocimetry and measured 12 acceleration with small inertial measurement units attached to each fish. Even in steady 13 tail beats, the fish accelerates slightly during each tail beat, and the magnitude of the 14 acceleration varies. In steady tail beats, however, a high acceleration is followed by a 15 lower acceleration or a deceleration, so that the swimming speed is maintained; in 16 unsteady tail beats, the fish maintains the acceleration over several tailbeats, so that the 17 swimming speed increases. We can thus compare the wake and kinematics during 18 single steady and unsteady tailbeats that have the same peak acceleration. During 19 unsteady tailbeats when the fish accelerates forward for several tailbeats, the wake 20 vortex forces are much higher than those at the same acceleration during single 21 tailbeats in steady swimming. The fish also undulates its body at higher amplitude and 22 frequency during unsteady tailbeats. These kinematic changes likely increase the fluid 23 dynamic added mass of the body, increasing the forces required to sustain acceleration 24 over several tailbeats. The high amplitude and high frequency movements are also 25 likely required to generate the higher forces needed for acceleration. Thus, it appears 26 that bluegill sunfish face a tradeoff during acceleration: the body movements required 27 for acceleration also make it harder to accelerate. 28

Fishes generate force to swim by activating muscles on either side of their flexible bodies. To accelerate, they must produce higher muscle forces, which leads to higher reaction forces back on their bodies from the environment. If their bodies are too flexible, the forces during acceleration could not be transmitted effectively to the environment, but fish can potentially use their muscles to increase the effective stiffness of their body. Here, we quantified red muscle activity during acceleration and steady swimming, looking for patterns that would be consistent with the hypothesis of body stiffening. We used high-speed video, electromyographic recordings, and a new digital inertial measurement unit to quantify body kinematics, red muscle activity, and 3D orientation and centre of mass acceleration during forward accelerations and steady swimming over several speeds. During acceleration, fish co-activated anterior muscle on the left and right side, and activated all muscle sooner and kept it active for a larger fraction of the tail beat cycle. These activity patterns are both known to increase effective stiffness for muscle tissue in vitro , which is consistent with our hypothesis that fish use their red muscle to stiffen their bodies during acceleration. We suggest that during impulsive movements, flexible organisms like fishes can use their muscles not only to generate propulsive power but to tune the effective mechanical properties of their bodies, increasing performance during rapid movements and maintaining flexibility for slow, steady movements.

27Fishes generate force to swim by activating muscles on either side of their flexible bodies. To 28 accelerate, they must produce higher muscle forces, which leads to higher reaction forces back 29 on their bodies from the environment. If their bodies are too flexible, the forces during 30 acceleration cannot be transmitted effectively to the environment. Here, we investigate whether 31 fish can use their red muscle to stiffen their bodies during acceleration. We used high-speed 32 video, electromyographic recordings, and a new digital inertial measurement unit to quantify 33 body kinematics, red muscle activity, and 3D orientation and centre of mass acceleration during 34 forward accelerations and steady swimming over several speeds. During acceleration, fish co-35 activated anterior muscle on the left and right side, and activated all muscle sooner and kept it 36 active for a larger fraction of the tail beat cycle. These activity patterns are consistent with our 37 hypothesis that fish use their red muscle to stiffen their bodies during acceleration. We suggest 38 that during impulsive movements, flexible organisms like fishes can use their muscles not only 39 to generate propulsive power but to tune the effective mechanical properties of their bodies, 40 increasing performance during rapid movements and maintaining flexibility for slow, steady 41 movements. 42

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