Bending moments in swimming fish

Most fish swim with body undulations that result from fluid-structure
interactions between the fish's internal tissues and the surrounding
water. Gaining insight into these complex fluid structure interactions is
essential to understand how fish swim. To this end, we developed a
dedicated experimental-numerical inverse dynamics approach to calculate
the lateral bending moment distributions for a large-amplitude undulatory
swimmer that moves freely in 3D space. We combined automated motion
tracking from multiple synchronised high-speed video sequences,
computation of fluid-dynamic stresses on the swimmer’s body from
computational fluid dynamics, and bending-moment calculations using these
stresses as input for a novel beam model of the body. The bending moment,
which represent the system’s net actuation, varies over time and along the
fish’s central axis due to muscle actions, passive tissues, inertia, and
fluid dynamics. Our 3D analysis of 113 swimming events of zebrafish larvae
ranging in age from 3 to 12 days after fertilisation shows that these
bending moment patterns are not only relatively simple but also strikingly
similar throughout early development, and from fast starts to periodic
swimming. This suggests that fish larvae may produce and adjust swimming
movements relatively simply, yet effectively, while restructuring their
neuromuscular control system throughout their rapid development.